Semiconducto2

SEMICONDUCTOR DIODE

CONDUCTIVITY

FORMATION OF DEPLETION LAYER

HOTS (HIGHER ORDER THINKING SKILLS

Q.1 Two semiconductor materials X and Y are made by doping a germanium crystal with indium and arsenic respectively. The two are joined end to end and connected to a battery.
(1)Will the junction be forward biased or reverse biased?
(2)Sketch a V-I graph for this arrangement.

Q.2 What is the function of base region of a transistor. Why is this region made thin and slightly doped?

Q.3 Why we use common emitter amplifier?

VALUE BASED QUESTIONS

Q1. A child uses a semi conductor device in listening radio & seeing pictures on TV He was asked to suggest the techniques as the cost of LPG/CNG is going up, to cope up with future situations.
* What are the values developed by the child?
* What may be the suitable semi conductor material used for utilization of maximum solar energy with reasons ?

Q2.Raju was enjoying TV programme at his home with his family at night.Suddenly the light went off causing darkness all over. Mother asked Raju to bring candle along with matchstick from kitchen to put the TV switch off. Raju at once picked the mobile phone and pressed the button lighting up the bunding. Her mother was surprised and asked where from the light was coming. Raju proudly showed her the mobile.

*• Which valueis displayed by Raju ?
*• Which material is used in LED

Q3.Garima and Gaurav want to purchase a new TV set. They visited electronic shops to look for some TV. The dealer showed them LCD and LED TV. Now they were confused which set to buy.Finally after discussing with friends, reading relevant literature and searching the internet, they decided to purchase LED.

* • Which value is being highlighted I by Garima and Gaurav?

*• What is the difference between LED and LCD?

GIST OF CHAPTER

Semiconductors :
Semiconductors are the substances which are insulators at zero kelvin, but starts conducting as the temperature is increased. The size of forbidden energy gap is much smaller than that for insulators. Due to smaller forbidden gap electrons can easily shift from valence band to the conduction band.

semiconductors are divided into two categories:

(i) Intrinsic Semiconductors.
(ii) Extrinsic Semiconductors

Intrinsic Semiconductors:

Pure germanium and silicon crystals in their natural states are called Intrinsic semiconductors. In germanium crystals atoms are arranged on the corners of a regular tetrahedron. Each of the four with valence electron is shared a nearest atom to form covalent bond. No free electrons are available in pure semiconductors. However, some bonds are broken due to thermal agitation and electron is released. The vacant space left by electron is called hole. Only 1 bond is broken for 109 germanium atoms. So number of free electrons available for conduction are very small. As the crystal is electrically neutral so number of free electrons is equal to number of holes.

If electric potential difference is applied across the semiconductors the electrons will move opposite to field and hole moves in the direction of field, thus forming current. But due to small number of electrons and holes the magnitude of current is very small.



Doping:
The process of addition of impurities to increase the conductivity of Silicon or Germanium crystal is called doping and the impurity atom added is called dopant.
While doping following points should be noted:

(1) The dopant atom should take the position of semiconductor atom.
(2) The presence of dopant should not distort the crystal.
(3)The size of dopant should be almost same a semiconductor atom

(iv) Dopant atoms should not be more than 1% of the crystal atoms.


Methods of Doping;


Heat the crystalline semiconductor in anatmosphere having dopant atoms so that they can diffuse into semiconductor. Implant dopants by bombarding the semiconductor with their ions.

Extrinsic Semiconductors:
The impurity atoms added are of two types:

Pentavalent impurity atoms i.e. atoms having 5 valence electrons such as Antimony or Arsenic. Such atoms added will create excess of free electrons. This type of doped semiconductors is called n-type semiconductor.
Trivalent impurity atoms i.e. atoms having 3 valence electrons, such as Indium or Gallium. Such atoms on being added to germanium crystal make the crystal deficient in electrons and holes will be produced. This type of doped semiconductors are called p-type semiconductors.

Type Semiconductors:

When an impurity atom with 5 valence electrons is added to germanium crystal, it replaces one of the germanium atoms. Four of the five valence electrons form covalent bonds with neighboring atoms and fifth electron becomes free to move in the crystal structure. Thus by adding impurity atom we are increasing the number of electrons and hence conductivity increases. Since charge carriers are negatively charged electrons they are called n-type semiconductors. In n-type semiconductor, some covalent bonds are also broken resulting in formation of electrons and hole.


In N type semiconductors, electrons are thus majority carriers and holes are minority carriers. An N type semiconductor has freely moving electrons and an equal number of stationary positively charged donor atoms. The crystal as a whole is neutral. The fifth valence electron is in fact bound with the parent atom with a very small energy of the order of 0.05 eV.


P – Type Semiconductors:


If we add trivalent impurity to germanium crystal it will replace germanium atom in the crystal structure. The three valence electron will form three covalent bond with neighboring Ge atoms and fourth space is left vacant which is called a hole. Thus, for every trivalent impurity added a hole is created. The Ge crystal so formed is called p-type semiconductor as it contains free holes. Each hole is equivalent to positive charge. The Ge crystal also contains few electrons which are present due to breakage of covalent bonds. For each bond breakage an electron and a hole are released. Thus, p-type semiconductor contains holes in majority and electrons are present in minority.

Formatting of P-N junction diode:

It is formed by placing a P-type crystal in contact with N type crystal and subjecting it to high pressure so that it becomes single piece. The assembly so obtained is called P-N junction or junction diode or crystal diode.

The surface of contact of P and N type crystal is called junction. During the formation of junction diode, holes from P region diffuse into N region and electrons from N region diffuse into P region. In both cases, when an electron meets a hole, they cancel the effect of each other and as a result a thin layer at the junction becomes free from any of charge carriers. This is called depletion layer. The thickness of depletion layer is of the order of 10—6 m.

Due to movement of electrons from N type it gets positive potential and similarly P type gets negative potential. Due to this, there is potential gradient in the depletion layer, negative on P side and positive on N side. In other words, it appears as if some fictitious battery is connected across the junction with its negative pole connected to p-type and positive pole to N type. This potential difference is called potential barrier.


Forward Biasing:


When P type is connected to positive pole of battery and N type with negative pole it is called forward biasing.
When the forward bias is more than the barrier voltage then majority carrier i.e. electrons in N type and holes in P type moves towards junction and cross it. As the forward bias voltage is increased, the flow of majority carriers increases. Because of attraction between electrons and holes, they rush towards each other and recombine at the junction. For each combination a covalent bond in P region near positive terminal of battery breaks and electron hole pair is used. Hole moves forward and electrons moves to the positive of the battery. Thus, in external circuit current passes due to flow of electrons.

When the forward bias voltage is less than the barrier voltage then the charge carriers experience higher resistance and cannot cross this region. As forward bias is increased the current increases with increase in forward bias.
Beyond a certain forward bias the electrons passing from the junction gain sufficient kinetic energy to expel valence electrons from atoms, resulting in large increase in current.


Reverse Biasing:

When P type is connected with negative terminal and N type with positive terminal of battery then it is said to be reverse biased.

In this case, majority carriers move away from the junction. The depletion region width increases and it offers high resistance to charge carriers. However, a few minority charge carriers on being accelerated by reverse bias cross the junction and result in small current in reverse direction. This current is called Leakage current. When reverse voltage is 25V the excessive high temperature destroys the covalent bond strucure in germanium and reverse current rises sharply. This voltage is called Breakdown Voltage.

Diode As a Rectifier:

Rectifier is a device which changes A.C. to D.C.
Principle:

It is based on the principle that junction diode offers low resistance path when forward biased and high resistance when reversed biased.

Half wave Rectifier:
A single diode is used as a half wave rectifier. The a.c. to be rectified is connected across the primary coil P of step down transformer. The terminal A of the secondary coil S of transformer is connected to the junction diode and load resistance RL as shown.
During first half cycle, diode gets forward biased, the conventional current flows in the direction of arrow heads. During second half cycle diode get reverse biased and hence no output is obtained across RL. A small current will although be present due to minority carriers. Thus, we shall get the discontinuous and pulsating output across the load resistance.

Full Wave Rectifier:
In case of full wave rectifier, both halves of input a.c. are rectified. The P region of two diodes are connected to two extreme ends of secondary. A load resistance RL is connected across the common N region and central tapping of the transformer. Output is taken across the load resistance RL.

During the first half cycle of input a.c.let the upper end of secondary be at positive potential with respect to the central tapping. The upper junction diode will be forward biased and diode will be reversed biased. Current will pass through in direction as shown. Similarly, during negative half cycle the diode will be forward biased, the current passes through diodes D2 and the direction remains the same as in the first case. As the direction of current (in both the cases) in load resistance remains the same, we call it d.c.
Types of Diodes:

Solar Cell:
A solar cell is a diode used to convert light energy into electrical energy. A p-n junction with p or n region made so thin that light energy entering is not absorbed appreciably before reaching the junction constitutes solar cell. The thin region is the emitter and other region is the base.
Working: When light is incident on pn junction each photon absorbed creates an electron and a hole. If is because the electron acquires sufficient energy to move from valence to the conduction band. Due to barrier voltage electrons moves towards n region and holes towards the p region.As a result the two regions gets opposite potential and emf is developed across the terminals fo the diode.
This photovoltaic emf can be used as ordinary cell in the electrical circuits.
Applications:[1] Solar cells are used in wrist watches and calculators .

[2] they are used to produce power in artificial satellites and space crafts.
(3)Zener diode:

These are specially designed junction diodes which can operate in the reverse breakdown voltage region without being damaged are called Zener diode.
An important application of zener diode is that it can be used as voltage regulator. The regulating action takes place because of the fact that in reverse breakdown region, a very small change in voltage produces large change in current. This causes a sufficient increase in voltage drop across the resistance to lower voltage back to normal. Similarly, when the voltage across the diode tends to decrease, the current through diode goes down out of proportion so that voltage drop across the resistor is much less and it raises voltage back to normal.

Transistor:
It is a three layer semiconductor device. It can be either pnp or npn. In p-n-p, n layer is sandwiched between two p layers and in n-p-n, p layer is sandwiched between two n layers. The three layers are called emitter, base and collector. The base of transistor is lightly doped and is thin in comparison with emitter and collector. The emitter supplies the charge barriers and collector collects them. Input circuit is generally forward biased and output circuit is reverse biased.
Transistor Actions:
N-P-N Transistor (Common Base):
To understand the action of common base transistor, the base emitter junction is forward biased and collector base junction is reverse biased. The electrons being majority carriers in the emitter are repelled by the negative potential of the emitter junction towards the base. The base has holes as majority carriers and some holes and electrons recombine in the base region and the base is lightly doped. Due to this, the probability of the electron-hole combination in the base region is very small (about 5%). The remaining electrons enter the collector region due to positive potential at the collector terminal. For each electron entering the collector terminal an electron from -ve terminal of emitter base battery enter s the emitter junction maintaining the number of electrons in emitter junction. Thus in NPN transistor, current inside as well as outside is carried by electrons. For transistor,
IE = IC + IB

P-N-P Transistor (Common Base):
In this case also the emitter base junction is forward biased and base collector junction is reversed biased with batteries VEB and VCB respectively. The holes being in majority in P type emitter are repelled towards the N type base region by positive potential of emitter base battery. In the base region some of the holes recombine with electrons in the N type semiconductor. Remaining holes move into the collector region due to -ve terminal of the collector base battery. For each hole that reaches collector terminal an electron leaves the negative pole of the battery VCB and neutralise it. At the same time, an electron from some covalent bond in emitter enters into the positive terminal of battery VEB, creating a hole in the emitter. Thus current inside the semiconductor is carried by holes and outside the semiconductor it is carried by electrons. In this case also
IE = IC + IB

Concept of Amplifier:
Amplifier is a device which produces enlarged version of the input signal. It is used for increasing the amplitude of variation of alternating voltage or current or power.


Common Emitter Amplifier:

Here also the input circuit is forward biased and the output circuit is reverse biased. When no a.c. signal is applied the potential differene VC between the collector and emitter is given by,
VC = VCE - IC RL

When an a.c. signal is fed to the input circuit, the forward bias increases during positive half cycle of the input. This results in increase in IC and consequent decrease in VC , thus during positive half cycle of the input, the collector becomes less positive.
During negative half cycle of the input, forward bias decreases, therefore, the value of IE and
Emitter Amplifier:
Here also the input circuit is forward biased and the output circuit is reverse biased. When no a.c. signal is applied the potential differene VC between the collector and emitter is given by,

VC = VCE - IC RL
When an a.c. signal is fed to the input circuit, the forward bias increases during positive half cycle of the input. This results in increase in IC and consequent decrease in VC , thus during positive half cycle of the input, the collector becomes less positive.
During negative half cycle of the input, forward bias decreases, therefore, the value of IE and IC also decreases and VC would increase making the collector more positive. In common emitter amplifier, thus there is 180ºout of phase amplification.

Current Gain:
It is defined as the ratio of collector current to the base current.
A.C. Current Gain:

It is defined as the change in collector current to the change in base current,

Voltage Gain:


It is defined as the change in output to the change in input voltage,

Power Gain:
It is defined as the change in output power to the change in input power.
Transistor as an Oscillator:

Oscillator converts direct current into alternating current and produces high frequency undamped oscillations.
The base oscillatory circuit consists of an inductance and capacitance called tank circuit. Due to resistance of circuit, a part of energy is dissipated, therefore, amplitude of oscillations goes on decreasing with time and damped oscillations are produced.

In order to maintain these oscillations, energy is supplied to circuit at the right moment and in the right direction using a feedback arrangement. The feedback arrangement consists of primary P and secondary with variable capacitor C of suitable range. The secondary coil of inductance L. The inductance L and capacitance C constitute tank circuit.
Working:

When key is inserted collector current in primary increases. The flow of electrons will be in upward direction. Induced current is produced in secondary in opposite direction. The lower plate of the capacitor will become negatively charged and upper plate positively charged. The forward bias in emitter base circuit increases. This further increases the collector current. When collector current attains maximum value, induced emf supporting the forward bias decreases. Thus collector current decreases and emf will be induced in such a way that it opposes the forward bias. Thus, collector current reaches a certain minimum value. In this way the oscillations in tank circuit will be maintained.

DIGITAL ELECTRONICS
In these electronics circuits, the current or voltages will have only two values, High (1) and Low (0). In digital circuits, the electrical pulses of two levels only are used as signal voltages.

Logic Gates:
A gate is a digital circuit which is used to perform certain specific function. The three basic logic gates are:
a. OR gate
b. AND gate
c.NOT gate
All other logic gates can be formed by combination of these three gates.
Truth Table:
It is table that indicate all possible combinations of input signals and their output.

Boolean Algebra:
This is the algebra which can be applied to logic gates based on Binary number system.