Semiconcuctors and & 1

Chapter-2

Semiconcuctors Diodes & Transistors

Contents  Semiconcuctors and Diodes: Energy Band of Insulators, Conductors and Semi Conductors Intrinsic and extrinsic .  PN junction , barrier potential, V-I characteristics.  Special Purpose Diodes: , Varactor diodes ,Light Emitting Diodes (LEDs),photo diodes, Solar cell. . Specification parameters of diodes and numbering. . Bipolar Junction Transistors: Structure, typical doping, Principle of operation,Detailed study of input and output characteristics of common emitter configuration, specifications.

2.1 Introduction

A material is one whose electrical properties lie in between those of insulators and good conductors. Examples are: and . In terms of energy bands, semiconductors can be defined as those materials which have almost an empty conduction band and almost filled valence band with a very narrow energy gap (of the order of 1 eV) separating the two. Some materials are intrinsic semiconductors. An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form. The semiconducting properties occur in these materials naturally. However, most of the semiconducting materials used in are extrinsic. Those intrinsic semiconductors to which some suitable impurity or doping agent has been added in extremely small amounts are called extrinsic or impurity semiconductors. Depending on the type of doping material used, extrinsic semiconductors can be sub-divided in to N-type and P-type semiconductors.

The p-n junction is a homojunction between a p-type and an n-type semiconductor. It acts as a diode, which can serve in electronics as a , logic gate, (Zener diode), switching or (varactor diode); and in optoelectronics as a light-emitting diode (LED), or solar cell. In a relatively simplified view of semiconductor materials, we can envision a semiconductor as having two types of charge carriers- holes and free electrons which travel in opposite directions when the semiconductor is subject to an external electric field, giving rise to a net flow of current in the direction of the electric field.

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2.2 Semi Conductors and Junction Diodes

The electrical properties of a material depend largely upon how tightly outer electrons with in the atoms of that material are bound to the central nucleus. On the basis of this, materials can be classified in to the following three groups.

 Conductors

 Insulators

 Semi conductors

Material in which the electrons are loosely bound to the central nucleus is called conductor.In the conductor electrons are free to drift around the material at random from one atom to another.

Examples: Copper, Aluminium, Silver etc.

Material in which the outer electrons are tightly bound to the nucleus is called insulator. There are no free electrons in insulator to move around the material.

Examples: PVC, Rubber, Wood etc.

Semi conductors are those materials their conductivity lies in between the conductivity of conductors and insulators and are called Semi conductors.

Examples: Germanium, Silicon, Carbon etc.

As per the rule of octate, the electrical properties of materials can again be defined on the basis of valance electrons (the electrons in the outer most orbits) numbers.

If the number of valance electrons is less than 4, the material is generally called conductor. Instead of accepting electrons, it is easier to donate electrons to fill the outer sub shell as 8. If the number of valance electrons is more than 4, the material is generally called insulator. Instead of donating electrons, it is easier to accept lesser electrons to fill the outer sub shell. If the number of valance electrons is equal to 4, the material is generally called semi conductor. Here the probability of donating and accepting electrons is equal.

2.2.1 Energy Bands:

In a single isolated atom, the electrons in the any orbit possess a definite energy. However an atom in solid is greatly influenced by the closely packed neighboring atoms. The electrons of the outer sub shell are shared by more than one atom in solid, the energy levels of outer shell electrons are changed considerably.

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Because of this, electrons in the same shell have a range of energies rather than a single energy. This range of energy is known as energy bands.

The figure shows the basic concept of energy bands in solid. The electrons in the first orbit have range of energy and form the 1st energy band. In the same way second orbit electrons form the 2nd energy band; third orbit electrons form the 3rd energy band and so on. The following are the more important energy bands.

. Valance Band

Valance band in a solid is the energy band possessed by the valance electrons. Under normal condition valance band has the electrons of highest energy. Depending on materials this band may be filled completely or partially.

. Conduction Band

The energy band which possesses the conduction electrons in a solid is called Conduction Band. In metals, the valance electrons are loosely attached to the nucleus and they can be easily detached. These electrons are called free electrons or conduction electrons. They are responsible for the conduction of current through the material. The current conduction is not possible, if there are no free electrons in the conduction band.

The gap between the valance band and conduction band is called forbidden energy gap. The width of energy gap represent, how stronger the valance electrons are bonded to the nucleus. The greater the gap more tightly the valance electrons are bonded to the nucleus. To make valance electron free, an external energy equal to the forbidden energy gap must be supplied to lift the electrons from the valance band to the conduction band. The forbidden energy gap is usually expressed in terms of electron volt (eV).

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2.2.1.2 Energy Band of Insulators, Conductors and Semi Conductors:

The electrical behavior of solid can be explained with the help of energy bands.

. Insulators

Fig (a) shows the energy band diagram of insulators. Here the valance band is full while the conduction band is empty. More over the energy gap between valance band and conduction band is very large (15 eV).Therefore a very high electric field is required to lift the valance electrons to the conduction band. Due to this reason the electrical conductivity of insulator is extremely small and can be regarded as zero under normal condition.

. Conductors

In the energy band diagram of conductors, there is no forbidden energy gap between the valance band and the conduction band .The two bands actually overlap as shown in fig(b).It indicates that, the valance band energies are the same as the conduction band energies and it is very easy for a valance electron to become a conduction electron. Therefore without supplying additional energy these materials can have a large number of free electrons and act as good conductors.

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. Semi Conductors

In the case of semi conductors, the valance band is almost filled and conduction band is empty. But the forbidden energy gap is very small (1 eV) as shown in fig(c).There fore comparatively a smaller electric field (smaller than required in the case of insulator but greater than conductor) is required to lift the valance electrons to the conduction band. Thus the conductivity of semiconductor lies between a conductor and insulator.

2.2.2 Intrinsic Semi Conductor:

A semi conductor in its purest form is known as intrinsic semi conductor. To form molecules of matters, the atoms in every element are held together by the bonding action of valance electrons. Each atom has the tendency to fill its outer most shell by acquiring eight electrons in it. In the case of an intrinsic semiconductor such as Ge or Si, it has only four electrons in its outer shell of its atom. To fill the shell as eight it requires four electrons more. This is acquired by forming bond through sharing one valance electron from each of the neighboring atoms. Such bonds are called Co-valent bond. Thus in semi-conductors the atoms are

Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 6 arranged themselves in a uniform three dimensional pattern, so that each atom is surrounded by four atoms. This orderly pattern is known as crystal.

Figure shows a two dimensional symbolic representation of silicon crystal. Here each of the valance electrons of silicon atom is shared by one of its four nearest neighbors to form covalent bond. At this state all the valance electron within the crystal are tightly bond to the parent atoms and no free electrons are available to cause electrical conduction. Therefore at absolute zero temperature, intrinsic semi conductor act as a perfect insulator.

Due to temperature, covalent bond with in an intrinsic semi conductor will break and free electrons and holes are produced. This process is called pair generation. The number of free electrons is equal to the number of holes. These free electrons and holes moves in the crystal in a random manner. If an electron meeting a hole in a broken covalent bond and covalent bond is re-established. This process is called electron hole recombination.

2.2.3 Extrinsic Semi Conductor:

The conductivity of the intrinsic semiconductor can be increased by adding small amount of impurities. The process of adding impurities to the intrinsic (pure) semiconductor is called doping. The doped semiconductor is then called extrinsic (impure) semi conductor.

Depending on the dopant (impurity) used, extrinsic semi conductor can be divided in to two classes.

 N-type Semi conductor.

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 P-type Semi conductor.

. N-type Semi conductor

N-type semi conductor is an extrinsic semi conductor doped with a pentavalent impurity like Antimony, Phosphorus and Arsenic etc.The fig(a) shows the crystal structure obtained when a silicon is doped with a pentavalent impurity.

Here four of the five valance electrons of impurity atom form covalent bonds with the surrounding four silicon atoms and the fifth will be nominally unbounded and is free to move about the crystal. This electron can be easily exited from the valance band to the conduction band by applying negligible amount of energy. Here each impurity atom provides one free electron into the silicon crystal. This type of impurity provides millions of free electrons and hence fifth valent elements are called donors. In N-type semi conductor, the number of free electrons provided by the pentavalent impurity is far exceeding the number of holes (thermally generated) in the crystal. Thus N-type semi conductor has a relatively large number of free electrons called majority carriers and few thermally generated holes called minority carriers. Due to the predominance of negative charged electrons over positive charged holes, this type of semiconductor is called N-type semi conductor. The N-type semi conductor can be represented as shown in fig (b).It consists of

 Free electrons (Majority carriers).

 Holes (Minority Carriers).

 Immobile positive ions.

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. P-type Semi conductor

P-type semi conductor is an extrinsic semi conductor doped with a trivalent impurity like Gallium, indium and Boron etc.The fig (a) shows the crystal structure obtained when silicon is doped with a trivalent impurity.

Here three valance electrons of impurity atom form covalent bonds with the surrounding three silicon atoms. The fourth neighboring atom of silicon is unable to form a covalent bond with the impurity atom, because the impurity atom does not have the fourth electron in its valence orbit. Hence the fourth covalent bond is incomplete because of shortage of one electron. This vacancy of electron existing in the fourth bond constitutes a hole with the positive charge associated with it. Hole has a tendency to snatch the electron from the neighboring atom. Here each atom of trivalent impurity gives one free hole to the crystal .Hence this type of impurity is called accepter. Thus P-type semi conductor has a relatively large number of holes called majority carriers and few thermally generated free electrons called minority carriers.

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Due to the predominance of positive charged holes over negative charged electrons, this type of semiconductor is called P-type semi conductor. The P-type semi conductor can be represented as shown in fig (b).

It consists of

 Holes (Majority carriers).

 Free electrons (Minority Carriers).

 Immobile negative ions.

2.2.4 PN Junction:

When P-type and N-type semi conductor is suitably joined to an N-type semi conductor, PN junction is formed. Such a PN junction is the basic building block on which the operation of all semi conductor devices depends.PN junction is cannot be made by simply pushing the pieces together but fabricated by special techniques such as growing, allowing diffusing etc.

The P-region has holes and acceptor ions and N-region has electrons and doner ions. Here electrons and holes are mobile and ions are immobile. The electrons in the N-type material diffuse into the P-type and combine with holes in P-type material, creating negatively charged ions in the P-type material nearby junction. Similarly holes from P-type material diffuse into the N-type material and combine with electrons in the N-type material, creating positively charged ions particularly in the region close to the junction in N-type material.

After a few recombinations of electrons and holes, a narrow width of fixed positive charge on N-side of the junction and fixed negative charge on P-side of the junction formed as shown in figure. This region is known as . This region has immobile ions which are electrically charged, hence the region is also called space charge region. Due to this region further diffusion is prevented, because now positive charge on N-side repels holes to cross from P-type to N-type and negative charge on P-type repels electrons to enter from N-type to P-type. Thus barrier is setup against further movement of charge carriers and is called potential

Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 10 barrier or junction barrier. For Silicon PN junction barrier potential is about .7 volt where as for Germanium, it is .3 volt.

PN Junction with Forward Bias:

When an external voltage is applied to the PN junction in such a way that positive terminal of the battery is connected to the P-type and negative terminal of the battery is connected to the N-type. This arrangement is called forward biased. At this arrangement holes in P-type will be repelled by the positive terminal of the battery and moves towards the junction. Similarly electrons in N-type are repelled by the negative terminal of the battery and moves towards the junction. As a result potential barrier is weakened and width of the depletion region is reduced. Thus majority carriers diffuse across the junction. If the forward voltage is greater than the potential barrier voltage, the depletion region will be completely eliminated and as a result current will increases through the junction. This current is called forward current. Current is carried by free electrons in the N-region and holes in P-region. But through the external circuit, current is carried by only free electrons.

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PN Junction with Reverse Bias:

When an external voltage is applied to the PN junction in such a way that positive terminal of the battery is connected to the N-type and negative terminal of the battery is connected to the P-type. This arrangement is called reverse biased. At this arrangement holes in P-type will be attracted by the negative terminal of the battery and moves away from the junction. Similarly electrons in N-type are attracted by the positive terminal of the battery and moves away from the junction. As a result potential barrier is increased and depletion region is widened. The increased potential barrier prevents the flow of majority charge carriers across the junction. Thus a high resistance is established by the junction and practically no current will flow through the junction. But this potential barrier helps the minority carriers to cross the junction. Minority carriers are thermally generated and are temperature dependent but independent of reverse bias voltage up to certain limit. The current due to the flow of minority carriers is known as reverse saturation current.

Break down in PN Junction:

If the reverse bias voltage is increased beyond a certain limit, a new phenomenon called break down occurs. In this region high current may be passed through the junction. This high current may generate large amount of heat to destroy the junction. The two processes are responsible for junction break down in reverse biased condition namely,

 Avalanche break down

 Zener break down

Avalanche Break Down:

When a very large negative bias is applied to the p-n junction, sufficient energy is imparted to charge carriers that reverse current can flow, well beyond the normal reverse saturation current. In addition, because of the large electric field, electrons are energized to such levels that if they collide with other charge carriers at a lower energy level, some of their energy is transferred to the carriers with low energy, and these can now

Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 12 contribute to the reverse conduction process, as well. This process is called impact ionization. Now, these new carriers may also have enough energy to energize other low energy electrons by impact ionization, so that once a sufficiently high reverse bias is provided, this process of conduction takes place very much like an avalanche: a single electron can ionize several others. This phenomenon is known as avalanche break down.

Zener Break Down:

When increasing reverse bias voltage across the junction, the electric field at the junction also increases. This high electric field causes covalent bonds within the crystal to break. Thus a large number of charge carriers become available. Thus a large current to flow through the junction, This phenomenon is called zener break down.

2.2.5 Semi conductor Diodes:

Diode is a two terminal device consisting of a PN junction formed either in Ge or Si crystal. Here the terminal on the P-side is called the and the terminal on the N-side is called the . In the symbol of the diode anode is identified by large arrow. The forward current direction in the diode is in the direction of the arrow(ie,from P to N).The PN junction conducts the current only when it is in forward biased and no current flows through it when it is in reverse biased(i.e. ,current flows in only one direction). Thus the diode is called uni directional device.

2.2.5.1 VI characteristics of junction Diode:

VI characteristics of a junction Diode represents the relation between the applied voltage across the diode and the current that flows through it. The circuit arrangement to plot the VI characteristic of a diode is shown in fig (a).

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In the circuit the P can be varied to set the bias across the diode at different levels, the corresponding voltage and current can be noted from the voltmeter and ammeter connected in the circuit. Fig (b) shows a typical VI characteristic of a Silicon junction Diode. From the curve it is clear that, when no external voltage is applied across the diode, no current flow through the circuit (point ‘o’ in the graph).

During forward bias condition (anode is connected to positive terminal and cathode is connected to negative terminal of the supply), the diode current is very small and increases very slowly till external voltage exceeds the barrier voltage (.3V in Ge and .7V in Si). The reason for slow increase of current in this region is that the external voltage applied is used to overcome the potential barrier. Above this voltage even a small increase of forward voltage produces a sharp increase in current. This voltage at which current starts to increase rapidly is called the cut in voltage or knee voltage.

The reverse characteristic of the diode can be obtained by the same circuit arrangement shown in fig (a).In a reverse bias state a very small current known as leakage current or reverse saturation current flows through the diode. If the reverse bias is increased continuously, a stage reaches when the kinetic energy of electrons (minority carriers) become so high that they knock out electrons from the covalent bonds. At this stage break down occurs and high current will be passed through the diode. The voltage at which break down occur is called break down voltage.

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Ideal Diode An ideal diode is a diode that acts like a perfect conductor when voltage is applied forward biased and like a perfect insulator when voltage is applied reverse biased. When the anode is more positive than the cathode, the diode conducts and acts as a short circuit. When the cathode is more positive than the cathode, the diode does not conduct and act as an open circuit. The v-I characteristics of an ideal diode is shown below. From the characteristics, it is clear that the diode is conducting, or “on,” in the forward bias region and the current is flowing in the direction of the arrow in the diode symbol. Conversely, the diode is not conducting, or “off,” in the reverse bias region.

Fig: Ideal Diode Characteristics

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2.2.5.2 Diode Equation The ideal diode characteristic equation is known as the Shockley equation, or simply the diode equation.It gives an expression for the current through a diode as a function of voltage.

i = is [exp ( )-1] Where,V is the applied voltage across the terminals of the diode.

VT is called thermal voltage and is given by VT = kT/q. Therefore the equation become,

i = i [exp ( )-1] s k = Boltzmann’s constant = 1.38 x 10-23 J/K T= Temperature in Kelvin and q = magnitude of charge on an electron = 1.6 x 10-19 C

2.2.5.3 Diode specifications characteristics and parameters The following are the different diode parameters.

 Semiconductor material: The semiconductor material used in the PN junction diode is of paramount importance because the material used affects many of the major diode characteristics and properties. Silicon is the most widely used material as if offers high levels of performance for most applications and it offers low manufacturing costs. The other material that is used is germanium. Other materials are generally reserved for more specialist diodes. The semiconductor material choice is of particular importance as it governs the turn on voltage for the diode - around 0.6volts for silicon and 0.3 volts for germanium, etc..

 Forward voltage drop (Vf): Any electronics device passing current will develop a resulting voltage across it and this diode characteristic is of great importance, especially for power rectification where power losses will be higher for a high forward voltage drop. Also RF diodes often need a small forward

voltage drop as signals may be small but still need to overcome it.

The voltage across a PN junction diode arise for two reasons. The first of the nature of the semiconductor PN junction and results from the turn-on voltage mentioned above. This voltage enables the depletion layer to be overcome and for current to flow. The second arises from the normal resistive losses in the device. As a result a figure for the forward voltage drop are a specified current level will be given. This figure is particularly important for rectifier diodes where significant levels of current may be passed.

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 Peak Inverse Voltage (PIV): This diode characteristics is the maximum voltage a diode can withstand in the reverse direction. This voltage must not be exceeded otherwise the device may fail. This voltage is not simply the RMS voltage of the incoming waveform. Each circuit needs to be considered on its own merits, but for a simple single diode half wave rectifier with some form of smoothing afterwards, it should be remembered that the capacitor will hold a voltage equal to the peak of the incoming voltage waveform. The diode will then also see the peak of the incoming waveform in the reverse direction and therefore under these circumstances it will see a peak inverse voltage equal to the peak to peak value of the waveform.

 Maximum forward current: When designing a circuit that passes any levels of current it is necessary to ensure that the maximum current levels for the diode are not exceeded. As the current levels rise, so additional heat is dissipated and this needs to be removed.

 Leakage current: If a perfect diode were available, then no current would flow when it was reverse biased. It is found that for a real PN junction diode, a very small amount of current flow in the reverse direction as a result of the minority carriers in the semiconductor. The level of leakage current is dependent upon three main factors. The reverse voltage is obviously significant. It is also temperature dependent, rising appreciably with temperature. It is also found that it is very dependent upon the type of semiconductor material used - silicon is very much better than germanium.

The leakage current characteristic or specification for a PN junction diode is specified at a certain reverse voltage and particular temperature. The specification is normally defined in terms of in microamps, μA or picoamps, pA.

 Junction capacitance: All PN junction diodes exhibit a junction capacitance. The depletion region is the dielectric spacing between the two plates which are effectively formed at the edge of the depletion region and the area with majority carriers. The actual value of capacitance being dependent upon the reverse voltage which causes the depletion region to change (increasing reverse voltage increases the size of the depletion region and hence decreases the capacitance). This fact is used in varactor or diodes to good effect, but for many other applications, especially RF applications this needs to be minimised. As the capacitance is of importance it is specified. The parameter is normally detailed as a given capacitance (in pF) at a given voltage or voltages. Also special low capacitance diodes are available for many RF applications.

 Package type: Diodes can be mounted in a variety of packages according to their applications, and in some circumstances, especially RF applications, the package is a key element in defining the overall RF diode characteristics. Also for power applications where heat dissipation is important, the package can define many of the overall diode parameters because high power diodes may require packages that can be bolted to heatsinks, whereas small signal diodes may be available in leaded formats or as surface mount devices.

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Excessive forward current and reverse are the most common causes of diode failure. In both cases the diode gets very hot, what destroys the pn junction. Occasional peaks of voltage or current exceeding these rates for very short times (few milliseconds)may not overheat the junction, but repeated peaks may fatigue the junction. By design, diodes are selected with ratings that exceed two or three times the expected peaks in the circuit.

2.3 Types of diodes

We can distinguish the following types of diodes:

• Rectifier diodes are typically used for power supply applications. Within the power supply, you will see diodes as elements that convert AC power to DC power.

• Switching diodes have lower power ratings than rectifier diodes, but can function better in high frequency application and in clipping and clamping operations that deal with short-duration pulse waveforms.

A number of special purpose diodes for specific applications in this fast developing world. Some of the more common special-purpose diodes are (a) Zener diode (b) Light-emitting diode (LED) (c) Photo-diode (d) (e) Varactor diode and solar sells.

Applications The main applications of semiconductor diodes in modern electronic circuitry are as under : 1. As power or rectifier diodes. They convert ac current into dc current for dc power supplies of electronic circuits. 2. As signal diodes in communication circuits for modulation and demodulation of small signals. 3. As Zener diodes in voltage stabilizing circuits. 4. As varactor diodes–for use in voltage-controlled tuning circuits as may be found in radio and TV receivers.

2.3.1 Zener Diode:

Suppose that the reverse bias on the diode is greatly increased. When the voltage is sufficient, valence electrons will be freed from their positions around the nuclei that bind them. Since these electrons possess excess energy, their collisions with other atoms will knock loose additional electrons; these, in turn, will knock loose more electrons, and the reverse current becomes an "avalanche." This effect was first noted and utilized by Clarence Zener, for whom the phenomenon is named. Note that, for reverse voltages greater than the breakdown voltage, it takes only a very small change in voltage to cause a large change in current; essentially, the diode goes from OFF to ON as the voltage becomes more negative. The breakdown or Zener voltage of a diode can be controlled in the manufacturing process.

Zener diodes are also called breakdown diodes. These are specially doped PN junction diodes to produce controlled break down characteristics without damage and are operated in the break down region.

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The break down in zener diode is influenced by two phenomenon, zener effect and avalanche effect. Here zener effect is predominant for break down voltages less than about 4V and avalanche break down is predominant for voltages greater than 6V.Between 4V and 6V, both effects are present. Because of high temperature and current capability, Silicon is usually preferred for the manufacture of zener diodes. The break down voltage Vz and resistance Rz of zener diode is controlled by varying the doping level of the PN junction. Increasing the impurity will decrease both break down voltage and resistance.

Fig (b) shows the VI characteristics curve of a zener diode. Here the forward characteristic is similar to an ordinary junction diode. But in reverse characteristic, if the voltage is increased a small but subsequently constant leakage current begins to flow as for other diodes. This current remains constant until certain voltage is reached. Beyond this voltage the reverse current increases rapidly to a high value Izmax.This voltage is called break down voltage.The current corresponding to this voltage is called break down current. This is the minimum current required to sustain break down. When zener diode operates in this region the voltage Vz across it remains fairly constant even though the current flowing through it is varied.

Applications

 Voltage regulator

 Fixed reference voltage source

 Over voltage protection circuit.

2.3.2 Light Emitting Diode (LED):

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Light emitting diode is a PN junction that emits optical radiation generated by the recombination of electrons and holes, when the junction is forward biased. Most of the commercial LEDs are realized using a highly doped N and a P Junction.

Principle of Operation

The figure (a) shows the energy band diagram of a pn+ (heavily n-type doped) junction without any bias. Here,EC is the conduction band energy,EV is the valance band energy and EF indicates the fermi level. Eg is the forbidden energy gap between the valance band and the conduction band. There is a potential barrier from EC on the n-side to the EC on the p-side, called the built-in potential, Vo. This built-in potential Vo prevents the electrons diffusing from n+ to p side.

Fig:The energy band diagram of a pn+ junction under unbiased and biased conditions

In the figure (b), forward bias voltage V is applied across the junction.This forward bias voltage V reduces Vo and thereby allows electrons to diffuse or be injected into the p-side.Since electrons are the minority carriers in the p-side, this process is called minority carrier injection. But the hole injection from the p side to n+ side is very less and so the current is primarily due to the flow of electrons into the p-side. These electrons injected into the p-side recombine with the holes. This recombination causes a release of energy.

The released energy can appear in the form of heat (non radiative release of energy) or in the form of photon (radiative release of energy). Both type of release of energy occur in any semi conductor material. But certain special type of materials has a high degree of probability of radiative release of energy. Junction made of such semi conductor materials act as light-emitting diodes. (GaAs) is one of such materials

Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 20 and the optical radiation emitted has a wave length of 885nm (infrared).Gallium phosphide (GaP) and Gallium Arsenide Phosphide (GaAsP) are also used for constructing LEDs. In the case of GaAsP, the actual wave length emitted can be controlled by varying the proportion of Phosphorus to Arsenic, and visible radiation of different colors can be obtained. Red, yellow, and green LEDs are commercially available in a variety of sizes and shapes.

Advantages:

 High reliability

 Fast response

 Low cost

 Low power consumption

Disadvantages:

 Temperature dependence of radiation

 Sensitivity to over voltage damage

Applications:

 Indicator lamp and displays in equipments such as digital watches, calculators etc.

 Optical communication system

2.3.2 Photo-diode

A photo-diode is a reverse-biased silicon or germanium pn junction in which reverse current increases when the junction is exposed to light. The reverse current in a photo-diode is directly proportional to the intensity of light falling on its pn junction. This means that greater the intensity of light falling on the pn junction of photo-diode, the greater will be the reverse current.

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Working principle

When light (photons) falls on the pn junction, the energy is imparted by the photons to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current. As the intensity of light incident on the pn junction increases, the reverse current also increases. In other words, as the incident light intensity increases, the resistance of the device (photo- diode) decreases.

The figure below shows the basic photo-diode circuit. The circuit has reverse biased photo-diode, R and D.C supply V.

Fig: Basic Photo diode Circuit

When no light is incident on the pn junction of photo-diode, the reverse current Ir is extremely small.

This is called dark current. The resistance of photo-diode with no incident light is called dark resistance (RR).

When light is incident on the pn junction of the photo-diode, there is a transfer of energy from the incident light (photons) to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current. As the intensity of light increases, the reverse current IR goes on increasing till it becomes maximum. This is called saturation current.

Modes of Operation

Photodiodes can be operated in different modes, which are as follows:

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 Photovoltaic mode – It is also known as zero bias mode, in which a voltage is generated by the illuminated photodiode. It provides a very small dynamic range and non-linear dependence of the voltage produced.

 Photoconductive mode - The diode used in this mode is more commonly reverse biased. The application of reverse voltage increases the width of the depletion layer, which in turn reduces the response time and capacitance of the junction. This mode is very fast, and exhibits electronic noise

mode - Avalanche are operated in a high reverse bias condition, which allow multiplication of an avalanche breakdown to each photo-generated electron-hole pair. This results in internal gain within the photodiode, which gradually increases the responsivity of the device.

Photodiode Applications

Photodiodes find application in the following:  Cameras  Medical devices  Safety equipment  Optical communication devices  Automotive devices 2.4 Solar Cells

In recent years, there has been increasing interest in the solar cell as an alternative source of energy. When we consider that the power density received from the sun at sea level is about 100 mW/cm2 (1 kW/m2), it is certainly an energy source that requires further research and development to maximize the conversion efficiency from solar to electrical energy. The basic construction of a silicon p-n junction solar cell appears in Figure. The metallic conductor connected to the p-type material and the thickness of the p-type material are such that they ensure that a maximum number of photons of light energy will reach the junction. A photon of light energy in this region may collide with a valence electron and impart to it sufficient energy to leave the parent atom. The result is a generation of free electrons and holes. This phenomenon will occur on each side of the junction. In the p-type material, the newly generated electrons are minority carriers and will move rather freely across the junction as explained for the basic p-n junction with no applied bias.

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Fig: Structure of a Solar Cell A similar discussion is true for the holes generated in the n-type material. The result is an increase in the minority-carrier flow, which is opposite in direction to the conventional forward current of a p-n junction. This increase in reverse current. Since V = 0 anywhere on the vertical axis and represents a short-circuit condition, the current at this intersection is called the short-circuit current and is represented by the notation

ISC. Under open-circuit conditions (id =0), the photovoltaic voltage VOC will result. This is a logarithmic function of the illumination as shown in figure. VOC is the terminal voltage of a battery under no-load (open-circuit) conditions. Note, however, in the same figure that the short-circuit current is a linear function of the illumination. That is, it will double for the same increase in illumination ( fC1 and 2fC1 in Figure) while the change in VOC is less for this region. The major increase in VOC occurs for lower-level increases in illumination.

Eventually, a further increase in illumination will have very little effect on VOC, although ISC will increase, causing the power capabilities to increase.

Fig: Vsc and Isc versus illumination for a solar cell

Selenium and silicon are the most widely used materials for solar cells, although gallium arsenide, indium arsenide, and cadmium sulfide, among others, are also used. The efficiency of operation of a solar cell is determined by the electrical power output divided by the power provided by the light source. Ƞ = () ×100% ()

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1.5 Numbering and coding schemes for diodes There are a number of common, standard and manufacturer-driven numbering and coding schemes for diodes; the two most common being the EIA/JEDEC standard and the European standard: EIA/JEDEC A standardized 1N-series numbering system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Among the most popular in this series were: 1N34A/1N270 (Germanium signal), 1N914/1N4148 (Silicon signal), 1N4001-1N4007 (Silicon 1A power rectifier) and 1N54xx (Silicon 3A power rectifier) Pro Electron The European Pro Electron coding system for active components was introduced in 1966 and comprises two letters followed by the part code. The first letter represents the semiconductor material used for the component (A = Germanium and B = Silicon) and the second letter represents the general function of the part (for diodes: A = low-power/signal, B = Variable capacitance, X = Multiplier, Y = Rectifier and Z = Voltage reference), for example: . AA-series germanium low-power/signal diodes (e.g.: AA119) . BA-series silicon low-power/signal diodes (e.g.: BAT18 Silicon RF Switching Diode) . BY-series silicon rectifier diodes (e.g.: BY127 1250V, 1A rectifier diode) . BZ-series silicon zener diodes (e.g.: BZY88C4V7 4.7V zener diode) Other common numbering / coding systems (generally manufacturer-driven) include:

. GD-series germanium diodes (ed: GD9) — this is a very old coding system . OA-series germanium diodes (e.g.: OA47) — a coding sequence developed by Mullard, a UK company Introduction

The invention of the BJT in 1948 at the Bell Telephone Laboratories ushered in the era of solid-state circuits, which led to electronics changing the way we work, play, and indeed, live. The invention of the BJT also eventually led to the dominance of information technology and the emergence of the knowledge-based economy.

The transistor is the main building block “element” of electronics. It is a and it comes in two general types: the Bipolar Junction Transistor (BJT) and the Field Effect Transistor (FET). Here we will discuss the structure and operation of the BJT and also describe the different BJT configurations.We also explain amplifying and switching action of BJT.

3.2 Structure and Principle of Operation

Transistor is a three terminal active device which transforms current flow from low resistance path to high resistance path. This transfer of current through resistance path, given the name to the device ‘transfer resistor’ as transistor. Transistors consists of junctions within it, are called junction transistors. The bipolar junction transistor (BJT) is a three terminal device consists of two P-N junctions connected back to back.

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Emitter junction Collector junction Region of operation

Reverse biased Reverse biased Cut-off region

Forward biased Reverse biased Active region

Forward biased Forward biased Saturation region

Reverse biased Forward biased Inverse action Current carries inside is by two opposite polarities of charge carriers (electrons and holes), hence the name bipolar junction transistor.

If a P-type material is sandwiched between two N-type materials as shown in fig (a), the resulting structure is called NPN transistor. Similarly when N-type material is sandwiched between the two P-type materials as shown in fig (b), the resulting structure is called PNP transistor. In both cases, the first layer where the emission or injection of the carriers starts is called emitter. The second layer through which carriers passes is called the base and the third layer which collects the injected carriers is called collector. In the symbol, the emitter has an arrowed head; it points the direction of the conventional emitter current (from P to N region).

Although the emitter and collector are same type of material, they have different physical and electrical properties. The collector section is physically larger than the emitter section, since it has to collect all injected carriers and to withstand the large reverse bias voltage. The base is very thin and lightly doped. The size of the emitter falls between the base and collector region and is heavily doped. The doping level of collector region is between heavily doped emitter and lightly doped base.

A transistor has two junctions namely emitter base junction(emitter junction) and collector base junction(collector junction).These two junctions can be biased in four different ways, so that transistor operates in four different regions as stated in the following table.

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3.3 Operation of NPN Transistor:

In the case of NPN transistor, forward biased applied across the emitter junction (by using VEE) lowers the emitter-base potential barrier, where reverse biased applied across the collector-base junction increases the collector base potential barrier. Due to this, electron in the emitter moves into the base and holes in the base moves into the emitter. The injected electrons, which are minority carriers in the base, diffuse across the base layer and gives rise to a current InE. Injected holes from base to emitter constitute hole current IpE. The InE is very greater than IpE. As the base is thin lightly doped P-material, the number of holes in the base is small and only few electrons will recombine with the holes at the base constitute base current. Due to high reverse bias at the collector junction (by using VCC). The remaining electrons reaches the collector constitute collector current InC.

3.4 Transistor Configurations:

The most common application of a transistor is as . An amplifier requires two input terminals and two output terminals. But while using a three terminal device such as transistor as amplifier, one of its terminals has to be common to the input and output circuits. A transistor can be arranged to have any one of its terminal is common to both input and output. Thus it can be connected in following three configurations.  Common Base (CB).  Common Emitter (CE).  Common Collector (CC). Input and Output Characteristics of Bipolar Transistors

To describe the behavior of a three terminal device, it requires two sets of characteristics. The relation between the input voltage and input current for different values of the output voltage is called the input characteristics and output characteristics show the relation between the output current and output voltage for different values of input current.

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3.4.1 Common Base Configuration

The common base configuration for NPN transistor is shown in fig (a). In this arrangement input is given between emitter and base, while output is taken across the collector and base. Here base is common to both input and output. The forward-biased emitter –base voltage is regarded as the input voltage and emitter current is considered to be the input current where as the reversed-biased collector -base voltage is regarded as the output voltage and collector current is the output current.

Fig(b) shows the circuit arrangement for determining CB characteristics of a NPN transistor. Here the emitter-base voltage can be varied by means of rheostat R1. The collector voltage can be varied by adjusting the rheostat R2.The required currents and voltages can read from the milliammeters and voltmeters connected in the circuit.

 Input Characteristics:

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Input characteristics of a CB configuration is the plot of emitter current IE as a function of emitter base voltage VEB at constant collector-base voltage VCB. Different curves can be plotted for different values of

VCB. Fig (c) shows a set of typical input characteristic curves for a NPN transistor, plotted at different VCB (0, 10,and 20 volts).

For a given value of VCB (for eg: 0volt) the input characteristic curve of a CB configuration is just like the forward characteristic of a junction diode (because the emitter base is a PN junction and is forward biased).However due to early effect an increase in the magnitude of the collector-base voltage VCB, slightly increases the emitter current for a given VEB.

Input Resistance:

Dynamic input resistance is the ratio of change in emitter base voltage to resulting change in emitter current at a constant collector-base voltage.

= ∆ ⁄∆ |

EARLY EFFECT

Because of reverse bias at collector junction, the depletion layer is wide and it penetrates both in to the base region and collector region. But the doping of the base region is much smaller than that of collector region. Hence the penetration of the depletion layer into the base region is much greater than the penetration into the collector region. Therefore the effective width of the base gets reduced. As the magnitude of the reverse bias at the junction increases, the effective base width decreases. This phenomenon is known as early effect or Base width modulation.

 Output Characteristics:

The plot of the collector current Ic as a function of the collector to the base voltage VCB with constant emitter current IE is referred to as common base output characteristics. A set of typical output characteristics of NPN transistor is shown in fig(d).

The output characteristics can be divided in to three distinct regions.

 Active region

 Saturation region

 Cut off region

Active region:

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This is the normal operating region of the transistor, when used as an amplifier. In this region the emitter junction will be in forward bias and collector junction will be in reverse bias. When the emitter current is zero, collector current Ic is not zero and there will be a very small current in the collector. This is the reverse leakage current ICBO. When the emitter-base junction is forward biased to get emitter current IE: Note that as the emitter current increases above zero, the collector current increases to a magnitude essentially equal to that of the emitter current as determined by the basic transistor-current relations. Note also that the collector current will raise even when VCB is zero. As the collector voltage VCB is increased, there will no much change in collector current. But an increase in emitter current, there will be a corresponding change in Ic.Thus collector current is almost independent of collector to base voltage VCB and is dependent only on emitter current. Therefore the curve appears to be almost flat. The curves clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by

IC≅IE Saturation region: The region, where both the emitter-base and collector-base junctions remain forward biased is known as the saturation region of the transistor. The region is located to the left of the line VCB=0.When VCB is slightly negative, the collector to base becomes forward biased. Here collector current decreases sharply for a small increase in forward bias across collector junction.

Cut off region:

The region to the right of the line VCB=0 and below the characteristics for IE=0 is the cut off region of the transistor. In this region both junctions of the transistor are reverse biased and hence only leakage current will flow through the transistor.

Output Resistance:

The dynamic output resistance is the ratio of change in collector to base voltage to the corresponding change in collector current at constant emitter current.

= ∆⁄∆ |

Common Base Current Gain

The current gain for a common base configuration is denoted by ‘α’ and is given by

= ⁄

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Fig(d) : Common Base output characteristics

3.4.2 Common Emitter Configuration

The common emitter configuration for NPN transistor is shown in fig(a). In this arrangement input is given between base and emitter, while output is taken across the collector and emitter. Here emitter is common to both input and output. The forward-biased emitter –base voltage is regarded as the input voltage and base current is considered to be the input current where as the reversed-biased collector –emitter voltage is regarded as the output voltage and collector current is the output current.

 Input Characteristics:

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Input characteristics of a CE configuration is the plot of emitter current IB as a function of emitter base voltage VBE at constant collector-emitter voltage VCE. Different curves can be plotted for different values of

VCE.Fig (c) shows a set of typical input characteristic curves for a NPN transistor, plotted at different VCE(2,6,and 10 volts).

For a given value of VCE (for eg: 2volt) the input characteristic curve of a CE configuration is just like the forward characteristic of a junction diode (because the emitter base is a PN junction and is forward biased).How ever due to early effect an increase in the magnitude of the collector-emitter voltage VCE, slightly decreases the base current for a given VBE.

Input Resistance:

Dynamic input resistance in CE configuration is the ratio of change in emitter base voltage to resulting change in base current at a constant collector-emitter voltage.

= ∆ ⁄∆ | =

 Output Characteristics:

The plot of the collector current Ic as a function of the collector to the emitter voltage VCE with constant emitter current IB is referred to as common emitter output characteristics. A set of typical output characteristics of NPN transistor is shown in fig(d). Note that on the characteristics, the magnitude of IB is in microamperes, compared to milliamperes of IC. Consider also that the curves of IB are not as horizontal as those obtained for IE in the common-base configuration, indicating that the collector-to-emitter voltage will influence the magnitude of the collector current.

The output characteristics can be divided into three distinct regions.

 Active region

 Saturation region

 Cut off region

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Active region: This is the normal operating region of the transistor, when used as an amplifier. In this region the base-emitter junction will be in forward bias and collector-emitter junction will be in reverse bias. When the base current is zero, collector current Ic is not zero and there will be a very small current in the collector. This is the reverse leakage current ICEO. When the emitter-base junction is forward biased to get base current IB. The collector current will raise even when VCE is zero. As the collector voltage VCE is increased, the collector current first increases and then rate of increase is quite small and Ic become nearly constant. But the curves are not horizontal, because for a fixed values of base current IB the magnitude of collector current increases slightly with increase in VCE. The active region of the common-emitter configuration can be employed for voltage, current, or power amplification.

Saturation region:

The region, where both the base- emitter and collector-emitter junctions remain forward biased is known as the saturation region of the transistor. Under this condition, the collector current is independent and it doesn’t depend upon the input current IB.

Cut off region:

The region to the right of the line VCE=0 and below the characteristics for IB=0 is the cut off region of the transistor. In this region both junctions of the transistor are reverse biased and hence only leakage current will flow through the transistor.

Output Resistance:

The dynamic output resistance in CE configuration is the ratio of change in collector to emitter voltage to the corresponding change in collector current at constant base current.

= ∆ ⁄∆ | =

Common Emitter Current Gain

The current gain for a common emitter configuration is denoted by ‘β’ and is given by

= ⁄

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3.4.3 Common Collector Configuration

In common collector configuration, collector terminal is common to both input and output. Circuit arrangement in CC configuration for NPN transistor is shown in figure below. Here input is applied between base and collector, while output is taken from emitter.CC configuration provides very high input resistance and very low output resistance. Due to this reason the voltage gain of this configuration is less than unity. Therefore this circuit is not used for amplification. It can be used for impedance matching for driving a low impedance load from a high impedance source.

Comparison between Three Transistor Configurations Properties Common Base Common Emitter Common Collector

Input Impedance Low Medium High

Output Impedance Very High High Low

Voltage Gain High Medium Low

Current Gain Low Medium High

Power Gain Low Very High Medium

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Relation between and

= ⁄ = ⁄ We know that = + = = − + Dividing numerator and denominator by Dividing numerator and denominator by Therefore, Therefore, ⁄ ⁄ = = + +

= = − +

Transistor Specification Parameters

The type number of the device is a unique identifier given to each type of Type transistor. There are three international schemes that are widely used: number European Pro-Electron scheme; US JEDEC (numbers start with 2N for transistors); and the Japanese system (numbers start with 2S).

There are two types of transistor: NPN and PNP. It is important to choose Polarity the correct type otherwise all the circuit polarities will be wrong.

The two main types of material used for transistors are germanium and silicon. Other materials are used, but in very specialised transistors. A Material knowledge of the type of material used is important because it affects many properties, e.g. forward bias for the base emitter junction is 0.2 - 0.3 V for germanium and ~0.6 V for silicon.

Collector to Emitter breakdown voltage. This is the maximum voltage that can be placed from the collector to the emitter. It is normally measured

VCEO with the base open circuit - hence the letter "0" in the abbreviation. The value should not be exceeded in the operation of the circuit otherwise damage may occur.

Collector to base breakdown voltage. This is the maximum collector base

VCBO voltage - gain it is generally measured with the emitter left open circuit. This value should not be exceeded in the operation of the circuit.

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Collector current, normally defined in milliamps, but high power transistors may be quoted in amps. The important parameter is the IC maximum level of collector current. This figure should not be exceeded otherwise the transistor may be subject to damage.

The collector emitter saturation voltage, i.e. the voltage across the

VCEsat transistor (collector to emitter) when the transistor is turned hard on. It is normally quoted for a particular base and collector current values.

hfe (common-emitter current gain or static forward current transfer ratio) is defined as the ratio of the input dc current and the output dc current of the transistor (static current gain). This parameter is also known as β. The ratio of collector current to base current, β is the fundamental parameter characterizing the amplifying ability of a bipolar transistor. β is usually assumed to be a constant figure in circuit calculations, but unfortunately hfe this is far from true in practice. As such, manufacturers provide a set of β (or "hfe") figures for a given transistor over a wide range of operating conditions, usually in the form of maximum/minimum/typical ratings. One popular small-signal transistor, the 2N3903, is advertised as having a β ranging from 15 to 150 depending on the amount of collector current. Generally, β is highest for medium collector currents, decreasing for very low and very high collector currents.

Transistor Numbering There are two main numbering systems for semiconductor diodes, transistors. One numbering or code system is used more widely in Europe and the other in the USA. The European based system is known as the Pro- electron system, sometimes also written as Pro-Electron system, and the one used more widely in North America is the JEDEC coding system.

Joint Electron Device Engineering Council (JEDEC)

These part numbers take the form: digit, letter, sequential number, [suffix] The letter is always 'N', and the first digit is 1 for diodes, 2 for transistors, 3 for four-leaded devices, and so forth. But 4N and 5N are reserved for opto-couplers. The sequential numbers run from 100 to 9999 and indicate the approximate time the device was first made. If present, a suffix could indicate various things. For example, a 2N2222A is an enhanced version of a 2N2222. It has higher gain, frequency, and voltage ratings. Always check the data sheet.

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Examples: 1N914 (diode), 2N2222, 2N2222A, 2N904 (transistors). NOTE: When a metal-can version of a JEDEC transistor is remade in a plastic package, it is often given a number such as PN2222A which is a 2N2222A in a plastic case.

Pro-Electron

These part numbers take the form: two letters, [letter], sequential number, [suffix] The first letter indicates the material: A = Ge B = Si C = GaAs R = compound materials. The second letter indicates the device type and intended application: A: diode, RF B: diode,varactor C: transistor, AF, small signal D: transistor, AF, power E: Tunnel diode F: transistor, HF, small signal K: Hall effect device L: Transistor, HF, power N: Opto-coupler P: Radiation sensitive device Q: Radiation producing device R: , Low power T: Thyristor, Power U: Transistor, power, switching Y: Rectifier Z: Zener, or voltage regulator diode The third letter indicates if the device is intended for industrial or commercial applications. It's usually a W, X, Y, or Z. The sequential numbers run from 100-9999. Examples: BC108A, BAW68, BF239, BFY51. Instead of 2N and so forth, some manufacturers use their own system of designations. Some common prefixes are: MJ: Motorola power, metal case MJE: Motorola power, plastic case MPS: Motorola low power, plastic case MRF: Motorola HF, VHF and transistor

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RCA: RCA device TIP: Texas Instruments (TI) power transistor, plastic case TIPL: TI planar power transistor TIS: TI small signal transistor (plastic case) ZT: Ferranti ZTX: Ferranti

Examples: ZTX302, TIP31A, MJE3055.

Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta