Solids and Semiconductor

Band theory of metals:

In an atom, there are fixed energy level. If two atoms are brought closer due to repulsion between the valence electron, their energy levels are changed. Similarly, if three atoms are brought closer due to mutual repulsion between valence electrons, the energy levels are further changed. Thus, due to mutual between valence electron, in a small space a range of energy levels are seen forming continuous structure called energy bond. It is divided into three regions:

(i) Valence band (VB): The region of energy band where valence electrons are present is called valence bond.

(ii) Conduction bond (C.B): The region of energy bond where free electrons are present is called conduction bond.

(iii) Energy gap (Eg): The region of energy bond which separates valence bond and conduction bond is called energy gas.

Difference between conductor, semi conductor and Bad conductor:

Conductor	Semi conductor	Bad conductor
1. Those substance whose energy gap of energy bond is zero are called conductor. Eg: Copper, Iron, etc.	Those substances whose energy gap of energy gap of energy band is small is called semi – conductor. Eg: Silicon, germanium, etc.	Those substance whose energy gap of energy bond is wide are called bad conductor. Eg: Diamond, glass, etc.
2. It has positive temperature coefficient. Ie. αt = + ve	It has – ve temperature coefficient. Ie. αt = -ve	It has – ve temperature coefficient. Αt = - ve.
3. At normal condition V.B. and C.B. are partially filled.	At normal condition V.B. and C.B are partially filled.	At normal condition V.B. is completely filled but C.B. is completely empty.

Hole and electron current:

In semiconductor, the valence electron moves from the end of negative potential to the end of positive potential and the hole moves from the end at positive potential to the end at negative potential is called hole current.

The electric current set up in the semi – conductor due to the flow of the free electron is called electron current. Now, let us consider the semiconductor is connected to a battery. The free electron in the conduction bond will be attracted by the anode of battery, it neutralizes a positive charge on it. At the same time, an electron leaves the cathode of the battery and enters into the semi conductor to take the place of the previous electron. In this way, if a large number of electrons an electric current is set up in the circuit.

Consider atoms (Say A, B, C, D, etc) in the semi conductor. Also consider the electron of A has jumped from valence bond to conduction bond. As a result a hole has been created in A. The valence electron in B and C are still in valence bond. If the battery is connected, the valence electron of atom C is pushed towards atom B and that of B is pushes towards the atom A. The result is that the valence electron of the atom B moves to the hole in the atom A and a hole is created in B. The valence electron in C jumps into the hole in B and a hole is created in atom C.

This way is a hole is created of the extreme and of the semiconductor which is connected to the cathode of the battery; an electron from the cathode enters into the hole neutralizes the atom. At the same time, the anode of the battery detaches an electron from an atom which lies at the other extreme end of the semiconductor. As a result, a new hole is created which moves towards the cathode.

Semiconductor:

A semiconductor is a material whose electrical conductivity lies between those of conductors and insulator.The semi – conductor having tetravalent atoms and found in nature is called Intrinsic or pure semi conductors. E.g: Silicon, Germanium, etc. The semi conductor formed by addition of impurities like trivalent or pentavalent atoms to the intrinsic semi conductor is called extrinsic semi conductor. Eg: SiB, GeP, etc. Doping (semiconductor), intentionally introducing impurities into an extremely pure semiconductor to change its electrical properties.The process of adding a measured quantity of a trivalent or a pentavalent impurity to a pure semi conductor is called doping in a semi – conductor.

In semiconductor production, doping intentionally introduces impurities into an extremely pure (also referred to as intrinsic) semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as degenerate. In the context of phosphors and scintillates, doping is better known as activation.

P-type of semiconductor:

 When trivalent impurities like boron is mixed with pure semiconductor. P-types of semiconductor is formed. In the case the 3-valence electron share the electron with 3e of the semiconductor i.e. Si. And formed covalent band but as the semiconductor has 4-bond among which 3es is completely formed bond and remain one formed incomplete covalent band. And the incomplete covalent bond has only one electron and the space of one another electron is vacant due to storage so, that vacant space is called hole or positive charge particle. In p-type of semiconductor the majority charge carrier is hole is hole and minority charge carrier is electro. As the current is pass vacant space or hole is filled by a electron as electron is moving opposite of the electrified apply or from negative to positive hence the hole shift from positive side to negative side.

N-type semiconductor:

when we mixed pent valence atom like arsenic with the pure semiconductor then N-type semiconductor is formed and the valance electron of silicon combine with arsenic and formed 4-covelent bond and one electron of arsenic remain and revolving or moving in them as free electron. In n-type of the semiconductor free electron are majority charge carrier and hole are minority charge carrier.

Difference between P – type and N – type semi conductor are:

P – type	N – type
(i) The semiconductor formed by addition of trivalent atom is called P – type semi conductor.	(i) The semi conductor formed by addition of pentavalent atoms is called N – type .semiconductor.
(ii) Eg: SiB, Ge ,Ga, etc.	(ii) Eg: SiP, Ge,As, etc.

Difference between conductor and semi – conductor

Conductor	Semiconductor
(i) Those substance whose energy gap of energy band is called zero are called conductor.	(i) Those substance whose energy gap of energy bond is called (narrow) is called semi – conductor.
(ii) The structure of energy band for conductor shown as below: Figure	(ii) The structure of energy bond for semi – conductor as shown below: Figure
(iii) Eg: Copper, Iron, etc.	(iii) Eg: Silicon, germanium etc.

p–n junction diode:

A p–n junction diode is made of a crystal of semiconductor, usually silicon, but germanium and gallium arsenide are also used. Impurities are added to it to create a region on one side that contains negative charge carriers (electrons), called n-type semiconductor, and a region on the other side that contains positive charge carriers (holes), called p-type semiconductor. When two materials i.e. n-type and p-type are attached together, a momentary flow of electrons occur from n to p side resulting in a third region where no charge carriers are present. This region is called the depletion region due to the absence of charge carriers (electrons and holes in this case). The diode's terminals are attached to the n-type and p-type regions. The boundary between these two regions, called a p–n junction, is where the action of the diode takes place.

When P – type and N – type semi conductor, they are placed in contact, the diffusion of majority charge carries being through the junction. Such arrangement of P and N – type semi conductor is called PN junction. In this arrangement P region acts as one electrode and H region acts as other electrode. So, PN function is called function diode or semiconductor or crystal diode.

Due to the immobile charge carries of depletation layer a potential difference which acts as the barrier of diffusion of charge carries though the junction, is called potential barrier (Vb).

When P – type and N – type semiconductor are placed in contact, diffusion of charge carries through it begins. On both sides of junction, a layer of immobile charge carrier (electrons in P – refion and holes in N – region] is formed. This layer is called depleation layer.

p-n junction as forward biased:

When a battery is connected to the diode with p-section connected to positive pole and n-sectin to the negative pole, the junction diode is said to be forward biased as shown in figure if the forward bias is greater than the potential barrier,the majority carrier move towards the junction and across it. The current which flow due to majority carrier is called forward current. Forward biased p-n diode acts as closed circuit.

Reverse biased: when the battery is connected to junction diode with p-section connected to negative pole and n-section to the positive pole, the junction is said to be reverse based.

Figure of both forward and reverse diode

When revere bias is applied, the majority charge carrier does not cross the junction. However a little amount of current flows due to the motion of minority carriers. This current is called reverse current which is increase in temperature.

PN diode as a half wave rectifier:

This type of junction diode is called half wave rectifier because it only used half positive cycle of a.c. input and give out half positive cycle of a.c.

As shown in fig. 1 the experimental arrangement of half wave rectifier. As it consist of an a.c source with step down transformer. And the secondary coil of the transformer consist of junction diode connected along with load resistance

During the positive half cycle of a.c. input point C become positive and D become negative so, junction diode acts as forward biasing. Hence output voltage is produce across load resistance

During negative half cycle of a.c. input point C become negative and D become positive. So the junction diode acts as reverse biasing and it also acts as switch off. Hence, no output voltage is produce across the load resistance. This process is repeated for other half cycle. The input and output voltage are shown in

fig. 2

PN diode as a full wave rectifier:

The junction diode which is used as full wave rectified. The rectifier which has both the circle i.e. positive half cycle and negative half cycle of a.c input is called full wave rectifier. Show in graph of d.c.output.

Figure a. show the arrangement of full wave rectifier which consist of central trapped transformer whose primary coil is connected with a.c. source and the secondary coil of the transformer connected with D1 and D2 diode along the load resistance from fig. a

Now from fig. b we have for positive half cycle of a.c. input, dueling this case the point A become positive and B become negative and the junction diode D1acts as forward biased where as D2 acts as reverse bias and hence the output voltage across the load resistance produce.

Again from fig. c we have fro negative half cycle of a.c. input during this case the point A become negative and B become positive and in that case junction diode acts as reverse bias and D2 acts as forward bias and the output voltage across the load resistance is produce and this is continuous for other half cycle.

Zener diode:

Zener diode is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but also permits it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage or "Zener knee voltage" or "Zener voltage" or "avalanche point" or "peak inverse voltage"

Zenereffects :

It is a type of electrical breakdown in a reverse biased p-n diode in which the electric field enables tunneling of electrons from the valence to the conduction band of a semiconductor and leading to a large no. of free minority carriers which suddenly increase the reverse current.

Figure

Mechanism.

At the high reverse-bias voltage the p-n junction's depletion region expands and leading to a high strength electric field across the junction. A sufficiently strong electric field enables tunneling of electrons from the valence to the conduction band of a semiconductor leading to a large no. of free charge carriers. This sudden generation of carriers rapidly increases the reverse current and gives rise to the high slope conductance of the Zener diode

Zener diode as a voltage regulator:

The function of a regulator is to provide a constant output voltage to a load connected in parallel with it in spite of the ripples in the supply voltage or the variation in the load current and the zener diode will continue to regulate the voltage until the diodes current falls below the minimum IZ(min) value in the reverse breakdown region. It permits current to flow in the forward direction as normal, but will also allow it to flow in the reverse direction when the voltage is above a certain value - the breakdown voltage known as the Zener voltage. The Zener diode specially made to have a reverse voltage breakdown at a specific voltage. In breakdown the voltage across the Zener diode is close to constant over a wide range of currents thus making it useful as a shunt voltage regulator.

The purpose of a voltage regulator is to maintain a constant voltage across a load regardless of variations in the applied input voltage and variations in the load current. A typical Zener diode shunt regulator is shown in Figure 3. The resistor is selected so that when the input voltage is at VIN(min) and the load current is at IL(max) that the current through the Zener diode is at least Iz(min). Then for all other combinations of input voltage and load current the Zener diode conducts the excess current thus maintaining a constant voltage across the load. The Zener conducts the least current when the load current is the highest and it conducts the most current when the load current is the lowest.

If there is no load resistance, shunt regulators can be used to dissipate total power through the series resistance and the Zener diode. Shunt regulators have an inherent current limiting advantage under load fault conditions because the series resistor limits excess current. 

 A zener diode of break down voltage Vz is reverse connected to an input voltage source Vi across a load resistance RL and a series resistor RS. The voltage across the zener will remain steady at its break down voltage VZ for all the values of zener current IZ as long as the current remains in the break down region. Hence a regulated DC output voltage V0 = VZ is obtained across RL, whenever the input voltage remains within a minimum and maximum voltage.

A transistor:

A transistor is a three terminal semiconductor device mostly used in electronics for amplification purposes. A Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions. In an NPN transistor, a thin and lightly doped P-type base is sandwiched between a heavily doped N-type emitter and another N-type collector; while in a PNP transistor, a thin and lightly doped N-type base is sandwiched between a heavily doped P-type emitter and another P-type collector. . A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

Proper biasing of a transistor:

Biasing is external energy applied across base terminal to activate the transistor. it is necessary because of following reason.

1. To work transistor is properly.

2. Stabilization is for maintaining a Q-point stable.

There are different methods for providing bias to the transistor some of them are:

 Base bias

If we bias the transistor by using the proper value of RB ,then the biasing is calledbase biasing.

Figure

Base and collector should be connected to +ve potential with respect to the emitter. Emitter is grounded

Base current is calculated by IB=Vcc−VBERB${\mathrm{I}}_{\mathrm{B}}=\frac{{\mathrm{V}}_{\mathrm{c}\mathrm{c}}-{\mathrm{V}}_{\mathrm{B}\mathrm{E}}}{{\mathrm{R}}_{\mathrm{B}}}$

Collector current is calculated by Ic=Vcc−VCERC${\mathrm{I}}_{\mathrm{c}}=\frac{{\mathrm{V}}_{\mathrm{c}\mathrm{c}}-{\mathrm{V}}_{\mathrm{C}\mathrm{E}}}{{\mathrm{R}}_{\mathrm{C}}}$

Voltage divider bias

If the transistor is biased by using the voltage divider as shown below then it is called voltage divider bias.  The resistance R1 and R2 forms the voltage divider.

Thevinineqv. Resistance = R1 // R2

Thevinineqv. Voltage =  R2R1+R2$\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1+{\mathrm{R}}_2}$ * Vcc

Transistor as a Switch:

If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the “ I-V ” characteristics curves The areas of operation for a Transistor Switch are known as the Saturation Region and the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown below.

Operating Regions

The pink shaded area at the bottom of the curves represents the “Cut-off” region while the blue area to the left represents the “Saturation” region of the transistor. Both these transistor regions are defined as:

Cut-off Region

Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.

Cut-off Characteristics

Figure: 

 

 

Saturation Region

Here the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched “Fully-ON”.

Saturation Characteristics

Then we can define the “saturation region” or “ON mode” when using a bipolar transistor as a switch as being, both junctions forward biased, VB > 0.7v and IC = Maximum. For a PNP transistor, the Emitter potential must be positive with respect to the base.