UNIT 14 SEMICONDUCTOR PHYSICS

Structure Introduction Objectives Energy Bands in Solids Intrinsic and Extrinsic Semiconductors Conduction in Intrinsic Semiconductors Extrinsic Semiconductors p-n Junction Diode p-n Junction with no External Voltage Characteristics of Forward and Reverse Bias Diode as a Rectifier Other Applications of Diodes Solar Cell Photodiode Light Emitting Diode Zener Diode Summary Terminal Questions . Solutions and Answers

14.1 INTRODUCTION

We are all familiar with simple applications of electronics like radio, television, calculators, personal computers etc. in, our day-to-day lives. If we look inside any electronic equipment, we will find resistors, capacitors, semiconductor diodes, transistors etc. Semiconductors have contributed immensely to the developments in science and technology. The invention of transistors in 1940s and that of integrated circuits (ICs) later led to great advancements in the capabilities of electronic devices and a wide variety of applications in different walks of life. 'This is because electronic components made up of se~niconductormaterials have advantages such as higher reliability, low power requirement and miniaturisation of devices. With recent semiconductor technology it has been possible to fabricate as many as one million transistors on a single 1 cm2 semiconductor chip.

A unique property of semiconductors is a remarkable increase in their conductivity with doping. Their response to light and other electromagnetic radiations results in a drastic change in electrical and optical properties, leading to many useful applications as solar cell, photodiode, infrared detectors etc. Therefore, we begin this unit by discussing how to teach the physics of semiconductor devices and their uses. We first describe the basic features of semiconductor materials. Our experience shows that the formation of energy bands in solids is a difficult concept for students to understand. In Sec. 14.2 we suggest some ways of explaining this along with differences between insulators, metals and semiconductors on the basis of the energy band diagrams.

Sec. 14.3 is devoted to an explanation of intrinsic semiconductors, the type of charge carriers (and their movement) present in them and the conduction mechanism at different temperatures. We also discuss the concept of doping of semiconductors with different types of impurity atoms to bring out a clear picture of carrier concentration and other properties of extrinsic semiconductors.

Sec. 14.4 deals with the teaching ofp-n junction diode, the most simple but extremely useful electronic device made of semiconductors. By varying the doping levels in a simplep-n junction diodes, we can use them for a wide range of applications. In Sec. 14.5 we take up the application of semiconductorp-n junction diodes as a solar cell, photodiode, light emitting diode and zener diode. Semiconductor Physics Objectives

After studying this unit, you should be able to - : Explain better to your students

- the formation of energy bands in solids and difference in metals, insulators and semiconductors using energy band diagrams, - the fundamentals of semiconductor materials and mechanism for transport of charge carriers in them, - the formation of barrier potential in ap-n junction diode, - the conduction property of forward and reverse biasedp-n junction diode, - the working ofp-n junction diode as rectifiers (half wave and full wave), - construction and working of solar cells, photodiodes,,light emimng diodes and Zener diodes. Devise activities to help students acquire a beher understanding of the physics of semiconductor materials and devices; Assess how well your strategies have worked.

14.2 ENERGY BANDS IN SOLIDS

As you are aware, an atom consists of a positively changed nucleus surrounded by electrons. You have taught your students about the energy levels of electrons in atoms. They have learnt that electrons in an atom can exist only in well defined quantized energy levels.

Fig.14.1: Energy levels of the hydrogen atom

Students find it difficult to understand the formation of energy bands from energy levels of atoms in a crystal. You can explain this as follows:

There are billions of atoms in a crystal. You need to explain that when there are two identical atoms such as HZ,N2, 02,etc., each atomic energy level splits into two. When there are three identical atoms such 03,N3, each energy level splits into 3. And when there are N identical atoms in a solid, each atomic energy level splits into N closely spaced levels. The total energy spread may be only a few eV, so that we have as many as lo2' or 102' energy levels in a few eV. This is called an energy band.

The first energy band is formed by the ground state energy levels. Atomic, Nuclear Physics and Electronics ENERGY t VALENCE BAND

I======I st BAND Fig.14.2: Energy bands

Closely spaced outermost filled energy levels of all atoms in the crystal form the outermost band. In an isolated atom, the outermost energy level is called valence shell or valence orbital. There is an infinite number of energy levels or bands, but a finite number of electrons, so some levels are filled while all the others are empty. The outermost filled levelhand refers to valence. Hence in a crystal the outermost filled band is called valence band.

An important point to be emphasised here is the relation between interatomic distance and arrangement of energy bands in solids as shown in Fig. 14.3.

Fig.14.3: Formation of energy bands

Note from the figure, that when interatomic distance is more (denoted by dmin Fig. 14.3), then we have energy levels corresponding to isolated single atoms.

As the interatomic distance decreases (denoted by distances.d and do in Fig. 14.3), atoms interact with each other and different atomic orbitals overlap each others. Thus, instead of discrete energy levels of single isolated atoms, we get energy bands.

You may further explain the origin of energy bands by taking the example of a crystal consisting of N atoms of a semiconductor, say, silicon (Si). Electrons of an isolated Si atom (ls22s2 2p6 3s2 3p2)exist in discrete energy levels. In a crystal, the atoms are very close to each other (2 to 3 A). Electrons interact with each other and the atomic nuclei. The 3s 3p level in group IV atoms has accommodation for 8 electrons, though there are only 4 electrons. They form sp3 tetrahedral bonds, and the s and p cannot be separately distinguished. When N atoms come together, they form two bands of 4N levels each. So the lower of them is filled and the upper one empty. You can use Fig. 14.4 to show the relation between energy and inter-atomic separation (r) for Si.

When the Si atoms are far apart (say r = dl) then the electrons in the outermost shell of one atom do not interact with those of the other and the energy levels of ls2 2s2, 2p6, 3s2, 3p2 of each of the N atoms remain unchanged.

As the inter-atomic separation or distance is decreased from r = dl to r = d2 then we get two bands having a large number of closely spaced energy levels, with an energy gap between them. Sem;conductor Physics

1 4N states lsolated atoms e'eT\ ?I C.B. I 4Energygap j

' state /*--I 14~electrons + I d~ I + Interatomic separation

Fig.14.4: Diagram showing formation of energy bands from energy levels when silicon atoms are brought closer to form a silicon crystal. [C.B. = CONDUCTION BAND and V.B. = VALENCE BAND)

On further decreasing the inter-atomic distance to a point when Si atoms are at actual inter-atomic separation r = a, the 3s 3p energy levels are apportioned into two bands: a band of 4N energy levels completely filled with electrons and another band of 4N empty levels. Now there are as many energy levels in the band of lower energy as the number of valence electrons in the N atoms.

- The lower energy band of 4N filled levels is composed of outermost energy I levels of all atoms having valence electrons, hence it is named valence band (VB). - The highest valence band energy is denoted by E,. - The electrons fiom the valence band (having" lower enerm)Wd r can aium~ n to umern. empty energy band (of higher energy) on gain of energy and become free (as loosely bound to the nucleus) and contribute to electrical conduction. Hence the upper energy band of 4N empty levels is called conduction band (CB). The lowest energy level in the conduction band is denoted by Ec.

- These two bands are separated from each other by an energy gap, which is the difference between the energy levels Ec and E,. Above E, and below E,, there are a large number of closelv s~acedenerm levels.

If this energy gap is very wide then it becomes almost impossible for electrons to jump from valence band to conduction band. Such materials are called insulators.

I The metals, insulators and semiconductors can be differentiated from each other on the basis of energy band structure in them. The best criterion to distinguish between them is this: Let El be the topmost energy of an electron in a crystal such that all other electrons have energy lower than this at 0 K, and let E2 be the next allowed but empty level. If E2 -+El, that is they are two consecutive energy levels in an allowed band, then it is a conductor. If E2 - El is finite (non-zero), then it is a dielectric. (This definition will also work for semimetals, conducting polymers, etc.)

Incidentally if the outermost atomic shell contains an odd number of electrons, such a solid must become a conductor, because there is accommodation for 2N 14N 16N .. . electrons (due to spin multiplicity), but there are only N / 3N l5N electrons present in the crystal. So we get partially filled outermost band. However, nothing can be said ab initio in the case of even number of electrons per atom, and it depends on the actual material, interatomic distance, overlap of wave functions, etc. For example, Ca, Mg, etc. are conductors whereas S, Si, Ge, etc. are dielectrics. Fig.14.5 below gives the energy band diagrams of insulators, semiconductors and conductors. Atomic, Nuclear Physics and Elcctronics

Rec )nducticx~ Ircrrt) barltl &,; = If Valence

(a ) ib) (C r'ig.14.5: Energy band diagram of a) Insulators, b) Semicond~~ctors;and c) Conductors

You can use the following table to explain the difference between metals, semiconductors and insulators.

\ ~aterial-bl Insulators I Semiconductors I Metals

ResiStivity (p) 1 Very high value 1 Lower than low value 1 Glass = 9 x 10" 0-cm insulators r'CU=1.7~ IO-'R- - Ge = 0.6 R-cm , Bands Valence: Full Valence band : Core: Completely Conduction: empty Full Conduction filled band: empty Conduction band:

" I bands (eV)

You can extend the explanation of band gap to metals and insulators. Why don't you try this as an exercise'?

SAQ 1 II How will you explain the formation of energy bands in a metal? What is the main cause of difference between the energy gap between metals, semiconductors and insulators? f

14.3 INTRINSIC AND EXTRINSIC SEMICONDUCTORS

Intrinsic semiconductor is semiconductor in its purest form such as pure silicon and germanium. You can explain its physics using the valence bond model. A semiconductor being a group IV element has four valence electrons. In a semiconducting crystal, the four valence electrons of each atom form covalent bonds (formed by two electrons) with four neighbouring atoms and fill the valence shell

Whenever a free electron is generated, a hole is created simultaneously. A hole is defined as the absence of an electron in an otherwise filled covalent bond. It is like a bubble in water. Free electrons and holes are always generated in pairs.

Therefore in an intrjnsic semiconductor concentration of free electrons (n)in the CB = concentration of holes (p)in the VB = 1zi (intrinsic carrier concentration). r Semiconductor Physics a You must explain that a hole acts as a charge carrier and both types of charge carriers (electrons and holes) move randomly or haphazardly in the crystal. A free electron moves in a crystal because of thermal energy.

w Its path deviates whenever it collides with a nucleus or other free electrons. This gives rise to a zig-zag or random motion similar to gas molecules moving in gas.

w You should make clear to the students how a hole actually moves in the semiconductor: Its position changes as an electron moves from one place to other. You can explain it with the help of following example. (Fig. 14.6 shows the picture of movement of electrons and consecutive shifting of holes. Consider that an electron - hole pair is generated at point A (Fig. 14.6(a)). The free electron goes elsewhere in the crystal leaving behind a hole at point A (Fig. 14.6(b)). The broken bond now has only one electron and this unpaired electron has a tendency to acquire an electron and complete its pair by forming a covalent bond. Due to thermal energy, the electron from neighbouring bond say at point B may get excited to break its own bond and jumps into the hole at A, due to which the original hole at A vanishes and a new hdle appears at B (Fig. 14.6~).Thus movement of electron from point B and A causes movement of original hole from A to B.

What will happen when the hole at B attracts and captures a valence electron from neighbouring bond at C?

Now again movement of electron from C to B causes movement of hole from B to C. You should explain that the movement of holes is complementary to the movement of electrons.

* F?cc electron

Fig.14.6: Movement of a hole through a semiconductor crystal

You should emphasize to the students that "Conventionally the flow of electric current through the semiconductor is taken in the same direction in which holes are moving".

14.3.1 Conduction in Intrinsic Semiconductors

When a battery is connected across a semiconductor, then electrons experience force due to the external field. Due to this, electrons move towards the positive terminal of battery and holes move towards negative terminal of the battery. This flow of charge carriers due to an applied voltage is called drift current.

When different regions of the semiconductor have different amounts of carrier concentration, then charge carriers from heavy concentration areas re-distribute ' themselves evenly throughout the material by the process of diffusion and constitute diffusion current. You could explain that

w At absolute zero temperature, all valence electrons are tightly bound to their ..n..a..~ -tr\-~ --A tho :I.tr;nn:n onm:,-.r\mAlln+r\r ,... Atomic, Nuclear Physics At room temperature, there is a fair probability that some electrons will be and Electronics excited from the valence band to conduction band, resulting in the formation of holes. This probability decreases with an increase in E,. Thus due to thermal energy, some electron - hole pairs are generated and the semiconductor exhibits a small conductivity. For example, at room temperature, say at 27O C or 300 K, Ge has intrinsic carrier concentration - 2.5 x 10'~ per m3. As the temperature is increased, more electron -hole pairs are generated and thus conductivityincreases with increase ip temperature.

Alternatively, we can say that the resistivity decreases as temperature increases. Therefore, semiconductors have a negative temperature coefficient of resistance.

14.3.2 Extrinsic Semiconductors

We have seen that intrinsic semiconductors have negligible conductivity. You may like to point out here that if a small and measured ampunt of chemical impurity is added to the intrinsic semiconductor, its conductivity increases significantly. Such semiconductors are called extrinsic semiconductors.

Intrinsic Semiconductor -+ Extrinsic Semiconductor

Addition of impurities (doping)

Most charge carriers in extrinsic semiconductors originate from the impurity atoms.

n and p type semiconductors

There are two types of extrinsic semiconductors: n-type and p-type semiconductors. You can use the following diagram to represent the formation of n andp-type semiconductors.

n-type (with electrons as majority carriers) Extrinsic Semiconductor \ P-type (with holes as majority carriers)

As you know, four out of five valence electrons of pentavalent elements form covalent bonds with the neighbouring silicon atoms and the fifth electron is very loosely bound to the nucleus.

Using the energy band model, we can say that at 0 K, the loosely bound electrons from the pentavalent impurity atoms go to new states called donor states, which are new states created in the gap just below the bottom of the Conduction Band. Therefore, it is called donor impurity or simply donors.

At room temperature, with whatever little thermal energy available, it is much easier (more probable) to jump the small distance (in energy) from Ed to EC than from Eyto Ec. So a large number of these ext-aelectrons go to the CB and are free to move throughout the crystal. Of course, electrons from VB will also go to CB, as in an intrinsic semiconductor, but they will be extremely few, and will give rise to electron Semiconductor Physics hole pairs as before.

The impurity atom attains one positive excess charge or becomes a positively charged ion (as held tightly in the crystal by the four covalent bonds).

In n-type semiconductors, above room temperature there are a large number of electrons, and a few holes.

Why are n-type semiconductors named so? in the case of n-type semiconductors, the number of free electrons in the CB is far greater than number of holes in the VB. The negative charge is represented by n. Fig. 14.7 gives the energy band diagram of an n-type semiconductor.

-+ : available / allowed filled state

-e- :allowed but empty state

Fig.14.7: Representation of n-type semiconductor

In case ofp-type semiconductors, all three valence electrons of trivalent impurity form covalent bonds with the neighbouring silicon atoms. The fourth neighbouring silicon atom cannot form a covalent bond with the trivalent atom due to the absence of fourth valence electron in them. This results in the deficiency of an electron around the tetra- valent atom. An electron in the adjacent covalent bond having a very small energy, can move to complete the covalent bond.

If there are N group IV atoms and n of them are replaced by trivalent atoms, then there are 4N - n electrons. While forming the crystal, n states are pulled up slightly above the valence band, as shown above, and give rise to acceptor states, which are empty at 0 K, because there are 4N - n electrons to fill in, just exactly equal to the number of states in the valence band. Now at room temperature, a large number of electrons from valence band jump up to fill the empty available states at E,, while only a small fraction goes up to Ec, again by Boltzmann probability distribution. Each jump of course creates a hole. In the end, we have a large number of holes in the valence band and a very small number of electrons in the conduction band.

Since each of these tetravalent atoms accepts one electron from the crystal so they are called acceptor type impurity or simply acceptors.

In p-type semiconductors, there are a large number of holes, a few electrons and a sufficiently large number of negative ions (Fig. 14.8). Atomic, Nuclear Physics and Electronics

-. : available / allowed filled state -e- : allowed but empty state

Fig.14.8: Representation of p-type semiconductor

While teaching these introductory concepts, do clarify to your students that

Even after the addition of impurities, the semiconductor materials @ or n type) as a whole are electrically neutral.

Electrons are charge carriers.

Irthep-type material, the total charge of holes is equal to the total charge on flee electrons and negative ions.

In the n-type material, the negative charge of electrons is equal to the charge of holes and positive ions.

You must also tell the following points to the students:

The conductivity of extrinsic semiconductor is many times greater-than that ofan intrinsic semiconductor as the addition of small amounts of donor or acceptor impurities produces a large number of charge carriers in them.

In an n-type semiconductor, electrons are the majority charge carriers and holes, the minority charge carriers.

The electrical conductivity (a) of a.semiconductor is determined by the mobility (p) and concentration (n) of both holes and electrons and is represented as

where e is the charge of an electron.

You may like to pause and devise ways of knowing how well these concepts have been understood by your students.

SAQ 2

Formulate a set of problems, including numericals based on these concepts. Evaluate students' performance. Point out the areas of difficulty and the remedial measures reauired. Semiconductor Physics 14.4 p-n JUNCTION DIODE

With the help of special fabrication techniques, when a semiconductor is doped in a manner that part of it is n-type and part of it p-type and the crystal structure is continuous at the boundary, then the interface between the two @ and n-type parts) is called thep-n junction. If we take an n-type semiconductor sample and diffusep-type impurities in it, a'p-n junction is formed. To give an idea of this you could mix oil in water and show the interface to your students. A typical width of such a junction is 1o-~ cm. The symbol of the p-n junction diode is shown in Fig. 14.9 along with the direction for conventional current fromp to n-region.

Direction of conventional current

Fig.14.9: Symbol ofp-n junction diode

This device has properties that enable us to use it for various applications. We put two terminals on the device, one at the end ofp-region and the other at the'end of n-region and call this device ap-n junction diode. You may point out that the word diode means two electrodes, where "di" stands for two and "ode" comes from electrode.

Point out that the p-n junction, as a whole, is electrically neutral (due to the electrical neutrality ofp and n-type semiconducfor materials).

What happens in such a device?

14.4.1 p-n Junction with no External Voltage

The p-region has holes and negatively charged impurity ions and n-region has free electrons and positively charged impurity ions.

As soon as ap-n junction is formed, the majority carriers (having higher concentration) in thep and n regions diffise across the junction. This diffusion of holes and electrons across the junction occurs .for a very short time. After a few recombinations of the holes and electrons in the immediate neighbourhood of the junction, a restraining barrier potential is set up due to electric field between acceptor and donor ions. The barrier potential in silicon and germanium is 0.7 V and 0.3 V, respectively. Each recombination eliminates one hole and one electron.

But the negative acceptor ions in the p-region and positive donor ions in the n-region in the immediate neighbourhood of the junction are left uncompensated or unbalanced and repel additional electrons and holes trying to diffuse into the p-region and n- region, respectively. As a result, further diffusion of holes and electrons from one side to the other is stopped by the barrier. Thus a complete recombination of holes and electrons does not occur. The region containing uncompensated acceptor and donor ions is called depletkn region as there is depletion ofelectrons and holes in it. Since this region has ions, which are electrically charged, it is also called the space-charge region.

In Fig. 14.10, we show the (a) carrier distribution and (b) formation of charge depletion layer in ap-n junction. Atomic, Nuclear Physics Electric field and Electronics Majority carriers (bole) Majority carriers (electron) \ / -H+

ion + P n +Depletion layer

Fig.14.10: a) Carrier distribution; and b) Formation of charge depletion layer in a p-n junction

14.4.2 Characteristics of Forward and Reverse Bias

Forward Bias: You know that the p-n junction diode is said to be forward biased when the positive terminal of battery is connected to the p-side and the negative terminal of the battery is connected to the n-side.

Reverse Bias: Thep-n junction diode is reverse biased when the negative terminal of the battery is connected to thep-region and the positive terminal is connected to the n- region.

You should explain to the students, the forward and reverse bias very clearly with the help of Figs.14.11 (a) and 14.1 1 (b), as you will be using these terms several times while explaining different devices.

Fig.14.11:~-n junction diode in a) Forward bias; and b) Reverse bias

You may like to demonstrate the forward and reverse bias as follows:

Take an LED and a 10 KSL resistor in series with dc power supply (0-2V). Show that the LED glows in when it is forward biased and does not glow on reversing the bias voltage. Replace the dc supply with a 1Hz ac signal (1V magnitude) and show switching on and off of LED to students. Now ask why current flows only in forward bias? In forward bias, the holes are repelled by the positive terminal of the battery and electrons are repelled by the negative terminal. They are compelled to move or drift towards the junction and recombination of electrons and holes takes place. Some of the holes and free electrons penetrate the depletion region, thereby reducing the width of the depletion region or potential barrier. Why is this bias given the name forward bias? A p-n junction connected in this manner is said to be forward biased due to the fact that a large amount of current flows in the directiomof flow of majority carriers or forward direction. Here, more majority carriers diffuse across the junction. It results in movement of charge carriers in the space charge region and hence more number of Semiconductor Physics recombinations take place. Due to forward bias, for each recombination of free electron and hole that occurs, an electron from p-type material flows to the positive terminal and the corresponding hole flows towards the negative terminal. Therefore, there is continuous flow of electric current in the external circuit as long as the battery is in the circuit. What happens when the battery voltage is increased? By increasing the battery voltage, the barrier potential gets further reduced, due to which more majority carriers cross the junction. This results in increased current through the p-n junction. You can have your students obtain the I-Vcharacteristics of p-n junction with forward bias. The circuit diagram along with the I-V characteristics graph in forward bjas is shown in Fig. 14.12.

Voltage (a) (b)

Fig.14.12: a) Circuit to obtain 1-V characteristics of a diode in forward bias; and b) ZV characteristics in forward bias

When the diode is forward biased, the diode current is very small for first tenths of volt, as the diode does not conduct well until the external voltage overcomes the barrier potential. The voltage at which current starts to increase rapidly is called cut in or knee voltage (Vo) of the diode (VO= 0.7 V for Si diode and 0.3 V for Ge diode). In case of a reverse biased p-n junction diode, the holes in the p-region are attracted towards the negative terminal of the battery (or the electrons of negative charge enter from the battery and sit in the holes) and the electrons in the n-region are attracted towards the positive terminal of the battery. Thus in this case the majority carriers are drawn away from the junction, resulting in an increase in the width of depletion region. The barrier potential makes it more difficult for the majority carriers to diffuse across the junction. However, this bamer potential is helpful to minority carriers in crossing the junction. Thus there is a very small current in the circuit.

For constant temperature, the rate of generation of minority carriers remains constant. Thus current due to flow of minority current remains the same whether .the battery voltage is low or high. It is therefore called reverse saturation current (of the order of nano-amperes in Si diodes and microamperes in Ge diodes)

When the reverse bias voltage is made too high, then the current through the p-n junction increases abruptly. At this voltage, electrons are released in large numbers due to field emission. It is called the breakdown voltage.

The circuit diagram for obtaining the I-V characteristics of p-n junction with reverse bias along with the graph of the I-V characteristics of p-n junction is given below: Atomic, Nuclear Physics and Electronics

Fig.14.13: a) Circuit to obtain I-V characteristics of a diode in reverse bias; and b) Reverse bias characteristics

14.4.3 Diode as a Rectifier

You know that ap-n junction diode offers low resistance when it is forward biased and high resistance when reverse biased. This unidirectional conducting property of a diode finds great application in rectifiers.

Rectifier is a device which converts an ac voltage into dc voltage. The basic principle is simple. The two half cycles of alternating input voltage provide opposite kind of bias to the junction diode. If the junction gets forward biased in the first half cycle, then it will get reverse biased in the second half cycle. So. when an alternating voltage signal is applied across the junction diode, it will conduct only during those alternating half cycles which bias it in the forward direction. The current in the negative cycle is a thousand times weaker.

Half wave rectifier

A rectifier which rectifies or converts only one half of each ac supply cycle into dc is called a half wave rectifier. The circuit diagram for p-n junction diode working as half wave rectifier is given in Fig. 14.14.

Diode

Pig.14.14: Half wave rectifier circuit

The ac voltage supply (V) is fed across the primary coil of the step down transformer and the secondary coil is connected to the p-n junction diode and load resistance RL. Suppose during the first half cycle the diode gets forward biased. The upper end of the load resistance RL will be at positive potential with respect to the lower end. The magnitude of output across RL during the first half cycle at any time will be proportional to the magnitude of current through it or the number of majority catriers crossing the junction. This in turn depends exponentially on the magnitude of forward bias, which depends on the value of ac input at that time. Thus during the first half of the input cycle when the diode conducts, the output voltage across RL varies in accordance with the ac input.

During the second half of the cycle, the junction diode will get reverse biased. Hence extremely low current will be obtained across RL as a very small current will flow due to minority carriers and negligible output will be obtained during this half cycle. Semiconductor Physics Fig. 14.1 5 shows the input and the output waveforms for the half wave rectifier.

Fig.14.15: Half wave rectifier (a) Input voltage waveform (b) Output voltage waveform

You can use a 1Hz AC supply and an LED to show the rectified half wave on a CRO.

The ha2fwave rectification involves a lot ofwastage of energy. Hence if is not preferred.

Full wave rectifier

A rectifier which rectifies both halves of the ac input cycle is called a full wave rectifier. There are two types of rectifier Eircuits that are used for full wave rectification, one is the centre tap rectifier using two diodes and the other is the bridge rectifier that uses four diodes.

Centre tap full wave rectifier: In this type, twop-n junction diodes are used to make use of both halves of ac input cycle. The circuit diagram is given in Fig. 14. I 6, The ac supply is fed across the primary coil of a step down transformer. The two ends of secondary coil are connected to thep-sections of the two diodes Dl and D2.The load resistance R,, is connected across the n-sections of diodes and central tapping of the secondary coil of the transformer. The dc output is obtained across the load resistance RL.

Fig,14.16: Centre-tap full wave rectifier

You can use 3 LEDs (one near each diode and one near the load, all in series with the respective element) and 1 Hz ac supply. Each of the LED near the diode glows alternately and the third one glows continuously. You can explain the phenomenon as follows:

Suppose during the first half cycle of input cycle, the upper end of the secondary coil is at positive potential an3 lower end is at negative potential. Then the diode Dlwill get forward biased during the first half cycle and will conduct, whereas the diode D2 is reverse biased and will not conduct. During the second half of the ac input, exactly the 0pp0~1tehappens. Now the diode Dl is reverse biased (does not conduct or is open) and diode D2 is forward biased and conduct.. - Fig. 14.17 (a) shows the input and (b) the output waveforms for the full wave rectifier. .Atomic, Nuclear Physics and Electronics

Fig.14.17: Full wave rectifier a) Input voltage waveform b) Output voltage waveform

Bridge rectifier

This type of full wave rectifier has four diodes placed in the form of a bridge. Fig. 14.18 shows the circuit diagram and output voltage form of the bridge rectifier.

Fig.14.18: Bridge rectifier circuit

During the positive half cycle of the ac input voltage, the diodes D2 and D4 are forward biased or are conducting and diodes Dl and D3 are reverse biased or non- conducting. Therefore, current flows through the secondary winding, diode D2, load resistance R1and diode D4. On the other hand, duringthe negative half cycle of the input voltage diodes, Dl and D3 are forward biased or conducting. Diodes Dz and D4 are reverse biased or will not conduct. In this case, the current flows through the secondary winding, diode D,, lbad resistance R, and diode D3. In both the cases. the current passes through the load resistor R1 in the same direction, therefore a fluctuating unidirectional voltage is developed across the load.

SAQ 3

Did you carry out the demonstrations suggested here, in the classroom? How effective were these in teachinrr the related conceDts to vour students?

14.5 OTHER APPLICATIONS OF DIODES

In this section, we shall discuss four applications of diodes, namely, solar cell, phot~d~iode,light emitting diode and Zener diode.

14.5.1 Solar Cell

Fig. 14.19 shows the photovoltaic action in a solar cell and the I-Vcurve of the solar cell in the d8arkand under irradiation. Semiconductor Physics

Fig.14.19: a) Photovoltaic action in solar cell; and b) I-V characteristics of a solar cell in the dark and under irradiation. Vmand I, are voltage and current at maximum power output

If sunlight made of photons possessing energy less than the band gap energy of the semiconductor material, falls on the solar cell, then it is not absorbed at all.

When the p-n junction is not illuminated or is in dark, then the height of the barrier potential across the open circuit adjusts itself such that net current is zero. When sunlight having energy more than the band gap energy falls on the cell, it results in the production of excess carriers which in turn ionize the semiconductor atoms. The built in potential at the p-n junction separates the positive charges (holes) and negative charges (electrons) within the photovoltaic device. This results in forward bias on the junction. The excess electrons created in the p-type material diffuse to the junction and slide down to the n-type material. Similarly extra holes in the n-type material slip into the p-type material. Thus the holes in the valence band are swept into the p-region and electrons in the conduction band are swept into the n-region. This results in flow of external current.

The current in the device is almost directly proportional to the intensity of sunlight and size of the cell.

The amount of power available from a solar cell depends on the area of the material, and intensity and wavelength of light.

Advantage of a solar cell is that it provides renewable, cost effective, convenient and non-polluting power supply.

14.5.2 Photodiode

The fact that the depletion region has an in-built electric field due to the presence of space charges is important in performance of a photodiode as it provides faster response and converts photons into electron-hole pairs at higher efficiency.

The performance can be improved by enlarging the depth of depletion region, by applying reverse bias to the junction. When light of appropriate frequency falls on the junction and gets absorbed, then electron-hole pairs are Atomic, Nuclear Physics formed which reduce the high resistance offered by reverse bias. Thus a and Electronics substantial amount of reverse current flows (in tens of microamperes).

The current in a photodiode changes almost linearly with the amount of light falling or the light flux.

In darkness these junctions act as an open circuit.

You can make use of the following chart to compare these applications.

Name Canstruction 1 Principle or Main function mechanism

Solar cell p-n junction I Photovoltaic Conversion of 1. Calculators diode in which 1 effect sunlight into 1 2. In satellites to either of the p or electrical energy. n regions is made power electrical very thin to avoid systems. much absorption 3. To charge of light energy batteries. before reaching the junction

Photodiode p-n junction diode made from effect optical input into receivers for light (or photo) electrical current remote controls sensitive in reverse bias. in VCR & TV. semiconductor materials, with 2. T3 measure light intensity in very thin p- industry. region whose thickness is determined by wavelength of radiation to be detected.

LED p-n junction Electroluminis Changes an Used in multimeter, diode made up of cence electrical input to intercoms, digital materials having a light output in watches, instrument band gap forward bias. displays, corresponding to calculators, switch near - infrared boards, burglar region or visible alarm and remote light region control devices. (GaAsP or InP)

Zener p-n junction 1 Zener Provide Used in voltage diode with breakdown continuous stabilization and diode heavily doped p mechanism current in the regulation circuits. & n regions and reverse very narrow breakdown depletion layer voltage region (< 10 nm). without being damaged.

14.5.3 Light Emitting Diode (LED)

When ap-n junction is forward biased, then each recombination of an electron and a hole releases energy in the form of light (or photons), whose wavelength depends on the band gap energy of materials forming thep-n junction. Ordinary diodes made up of silicon (an opaque material that blocks thepassage Semiconductor Physics of light) emit light in the invisible far-infrared region.

LEDs have replaced incandescent lamps in many applications because oftheir low voltage, long life andfast on-offswitching. Their latest use is traffic lights.

14.5.4 Zener Diode

When reverse bias across this diode is increased then due to large electric field across the depletion layer (about 3 x 10' Vim), the electrons are pulled out of the covalent bonds, resulting in formation of large number of electron - hole pairs. Hence reverse current increases rapidly. The voltage at which this phenomenon occurs is called breakdown voltage and the mechanism of breakdown is called zener breakdown. Hence the diode is called the zener diode. The I-Vgraph of zener diode is shown in Fig. 14.20.

Fig.14.20: I-V curve of zener diode

If the applied reveke voltage exceeds the breakdown voltage, a zener diode acts as a constant voltage source. A zener diode is specified by its breakdown voltage and maximum power dissipation.

Zener diode as a regulator

A zener diode has very high resistance (of order of megaohms) at bias potentials below the zener voltage. At zener voltage, the zener diode suddenly shows a very low resistance (value between 5 and 100 ohms). It behaves as a constant voltage source in the zener region of operation as its internal resistance is very low and the current through the zener diode is limited only by the series resistance, whose value is such that the maximum power rating of the zener diode is not exceeded.

The simplest regulator circuit consists of a resistor (Rs) in series w~ththe input voltage (V,) and a zener diode connected in parallel with the load resistor (R,,). As long as the voltage across RI is less tharl the zener breakdown vol~agt. (b':), the zener diode does not conduct. In this condition, the resistors R, and RL make a potential divider across the input voltage VI. The circuit diagram for zener diode as voltage regulator is shown in Fig. 14.21. Atomic, Nuclear Physics and Electronics

Fig.14.21: The zener diode voltage regulator

You can actually show a zener diode's action to your students.

At an increased V,, the voltage across RL becomes greater than V, and the zener diode operates in the breakdown region. The resistor Rs limits the zener current from exceeding its rated maximum I*,. The current from the unregulated power supply (Is) splits at the junction of the zener diode (IZ) and the load resistor (IL) as Is = IZ+ Il,.

When the zener diode operates in its breakdown region, then the voltage VZ across it remains fairly constant, even though the current IZflowing through it may vary considerably. If the load current 11,increases because of the reduction in the load resistance, then the current IZthrough the zener diode falls by the same percentage in order to maintain constant current Is. It keeps the voltage drop across Rs constant. Therefore the output voltage Vo remains constant. When load current decreases, the zener diode passes an extra current IZsuch that the current Is is kept constant. The output voltage of the circuit is thus stabilized.

If the input voltage is increased, then the zener diode passes a larger current so that the extra voltage dropped across Rs is compensated. Conversely if input voltage falls, then the voltage drop across Rs also falls and the zener current also falls. Generally, for voltage regulation, the zener diode is selected in such a way that its breakdown voltage is equal to desired regulating output.

You should point out that zener action is only observed in the reverse bias region and therefore its utility.

We now summarise the contents of this unit.

14.6 SUMMARY

Energy bands are formed in solids due to interaction between closely packed atoms and consecutive splitting of energy levels. The behaviour of insulators, metals and semiconductors can be clearly understood on the basis of energy . bands. Electrical conduction in intrinsic semiconductors can be described in terms of motion of electrons and holes. Doping is a process of adding impurities to an intrinsic semiconductor (pure germanium or silicon) to form p or n type extrinsic semiconductors. The extrinsic semiconductors have much higher conductivity as compared to intrinsic semiconductors. Electrons are majority carriers and holes are minority carriers in n-type semiconductors and vice-versa for p-type semiconductors. The electrical conductivity of a semiconductor is determined by the mobility and Semiconductor Physics concentration of both holes and electrons and is represented as. O=e(ne~e+~h~h). The conductivity of semiconductors increases with temperature.

Holes and electrons in p-n junction diffuse to establish a depletion region or a region devoid of mobile charges. The charges in the region adjacent to the depletion region generate a potential difference across the junction. The unique property ofp-n junction diode of unidirectional flow of current makes them usefill as rectifiers. Solar cells are semiconductor junction devices that directly convert solar energy into electrical energy. Photodiodes arep-n junction devices in which the amount of reverse current depends on the zmount of light falling on it. Light emitting diodes arep-n junction devices that change an electrical input into light output. , Zener diodes are heavily doped p-n junction diodes that are optimized for operation in the breakdown region. These diodes find application in voltage stabilization or regulator circuits.

14.7 TERMINAL QUESTIONS

1. You could administer questions of the following kind to your students:

a) A semiconductor is known to have an electron concentration of 8 x 1013 ~m-~ and the hole concentration of 5 x 1012 ~m-~.(i) Is the sample n-type orp-type? (ii) What is the conductivity of the sample if the electron mobility is 23,000 cm2 V-' and hole mobility is 100 cm2 V-Is-'? b) In a semiconductor diode, the barrier potential offers opposition to what kind of charge carriers? c) What is depletion region in a junction diode made of'? d) Give the ratio of holes and conduction electrons in an intrinsic semiconductor. e) Which type of bias results in very high resistance of ap-n junction diode? f) How will you explain the operation of half wave and fill wave (center tap and bridge) rectifiers and compare their performance with the help of input and output waveforms? g) How is zener diode different froin ap-n junction diode? Explain the concept of zener breakdown. h) Explain the function of zener diode as voltage regulator 2. Evaluate their responses. What additional teaching inputs are required?

14.8 SOLUTION AND ANSWERS

SAQs

1 to 3. Please bring your reports to the ECP. Atomic, Nuclear Physics Terminal Questions a~ndElectronics 1. a) Given

i) As the semiconductor has greater electron concentration as compared to hole concentration therefore it is n-type semiconductor.

ii) Conductivity o = e (nepe+ nh~h)= 1.6~10-l9(8~10~~~2.3 + 5x10~~x0.01)

b. In a semiconductor diode the barrier potential offers opposition to majority carriers in both regions. c. The depletion region in a junction diode contains charges that are fixed donor and acceptor ions. d. As the concentration of holes is equal to concentration of electrons in an intrinsic semiconductor thus the ratio of number of holes and conduction electrons is 1 in an intrinsic semiconductor. e. Reverse bias results in very high resistance ofp-n junction diode. For diagram see Fig. 14.13 of Sec. 14.6 of the text. f. See Sec.14.7 of text. g. In usual p-n junction diodes, the reverse current increases rapidly at large reverse breakdown voltage and due to this these diodes of low power rating are destroyed by reverse breakdown voltage. On the other hand, zener diodes are specially designed junction diodes having heavily dopedp and n regions with very narrow depletion layer (< 10 nm) and can operate in the reverse breakdown voltage region continuously without being damaged. h. For zener diodes as regulator see Sec. 14.1 1 of text.

2. Please bring your report to the ECP.