Semiconcuctors and Diodes & Transistors 1
Chapter-2
Semiconcuctors Diodes & Transistors
Contents Semiconcuctors and Diodes: Energy Band of Insulators, Conductors and Semi Conductors Intrinsic and extrinsic semiconductors. PN junction diode, barrier potential, V-I characteristics. Special Purpose Diodes: Zener diode, 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,Transistor specifications.
2.1 Introduction
A semiconductor material is one whose electrical properties lie in between those of insulators and good conductors. Examples are: germanium and silicon. 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 electronics 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 rectifier, logic gate, voltage regulator (Zener diode), switching or tuner (varactor diode); and in optoelectronics as a light-emitting diode (LED), photodiode 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 electron hole 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 depletion region. 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 anode and the terminal on the N-side is called the cathode. 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 potentiometer 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.