Intrinsic and Extrinsic Semiconductors in Basic Electronics

INTRODUCTION :

Different materials have different properties we shall consider the property of conduction of electricity, by any material. Certain materials like copper, silver and gold are good conductors of electricity; whereas materials like glass, wood, paper, etc., are bad conductors or insulators for electricity. Both the conductors and the insulators find wide applications in engineering industry. 

There is another category of material whose conductivity lies in between conductors and insulators. Such materials are called semi conductors. Germanium and silicon are two of the well- known semi conductor materials. Further, in this chapter, we shall study the intrinsic and extrinsic semiconductors, PN junction diode zener diode and transistor operation and characteristics.

CONDUCTORS, INSULATORS & SEMICONDUCTORS :

The electrical properties of different materials can be explained in terms of the electrons having energies in the valence band and conduction band, because the electrons lying in the lower energy bands which are normally filled, play no role in the conduction process.

(i) Insulators : Insulators are the solid materials which are bad conductors of electricity. ex : Wood, Plastic, rubber. In terms of energy bands, Insulators

(a) have full valence band.
(b) have an empty conduction band
(c) have a wide forbidden energy gap.

So in case of insulators a very large amount of energy must be supplied to cause an electron to cross from the valence band to the conduction band.

(ii) Conductors: Conductors are the solid materials which are good conductors of electricity. ex: Copper, Aluminium, gold. In terms of energy bands conductors have –

(a) No forbidden energy gap between valence band and conduction band i.e., Valence band and conduction band are overlapped.
So in case of conductors very large number of free electrons are available even at extremely low temperatures.



(iii) Semi Conductors : Semi conductors are the solid materials which are neither good conductors nor bad conductors of electricity. ex : Silicon, Germanium.
In terms of energy band diagrams semiconductors have
(i) Narrow forbidden energy gap EG 1eV.
(ii) Partially filled valance and conduction bands at room temperature.

INTRINSIC AND EXTRINSIC SEMICONDUCTORS:

Semiconductors can be classified into two types : (1) Intrinsic semiconductors and (2) Extrinsic semiconductors.

1. Intrinsic Semiconductors :

A semiconductor in an extremely pureform of state is known as intrinsic semiconductor. Even at room temperature, some of the valence electrons acquire sufficient energy to enter the CB. To form free electrons. Under the influence of the electric field, these electrons contribute to electric current.
A missing electron in the VB leaves a vacant space there, called a hole . Holes also contribute to electric current.
Creation and Conduction through Semiconductor
In an intrinsic semiconductors, event at room temperature, electron-hole pairs are created. When electric field is applied in an intrinsic semiconductor, the current conduction occurs by two courses, specifically by free electrons and holes. Under the influence of electric field, conduction through the semiconductor is both by free electrons and holes. The band gap energy for Ge is 0.7 eV and Si is 1.1 eV.

2. Extrinsic Semiconductors:

In order to increase the conductivity of a semiconductor, an impurity is added to it and this process is called doping, The impurity called doping material or dopant.

The resultant material after doping is called extrinsic semiconductor. The amount of impurity added is extremely small, say 1 or 2 impurity atoms for 108 atoms of pure semiconductors.

The usual dopants are :

(i) Pentavalent atoms having five valence electrons. Arsenic (As) Antimony (Sb), phosphorus (P).

(ii) Trivalent atoms having three valence electrons.   gallium (Ga), indium (In), aluminium (Al), boron (B).

Pentavalent impurities are called donor impurities because they donate free electrons to the semiconductor.

Trivalent impurities are called acceptor impurities because the holes created can accept the electrons. 

Based on the kind of impurity added, extrinsic semiconductors are divided into :
(i) N- type semiconductor and (ii) P- type semiconductor.

DIFFERENCES BETWEEN INTRINSIC AND EXTRINSIC SEMICONDUCTORS

No:

Intrinsic Semiconductor
Extrinsic Semiconductor
1
It is a pure semiconductor. It could be either Ge or Si
These are obtained by adding impurities to pure semiconductor. These may be p-type or n-type.
2
At zero temperature, it becomes insulator.
Such is not a case here.
3
Conductivity increases with temperature.
Conduction is possible at room temperature also.
4
Conductivity is in between metals and insulators; it is less compared to extrinsic semiconductors.
Conductivity is much higher, than intrinsic semiconductors (but less than metals).
5
No. of electrons and holes are equal.
No. of electrons and holes are not equal. In n-type, electrons > > holes. In p-type, holes > > electron
6
Fermi level is exactly midway between conduction band and valence band.
Fermi level is close to conduction band in n-type and is close to valence band in p-type.



Self and Mutual Inductance

Self Inductance:


The ability of an inductor to induce voltage in itself when the current changes is called its self inductance or simply inductance. It is denoted by the letter L and its units are Henrys (H).

Illustration of Self Inductance
Self Inductance Coil


It can be expressed as, L = VL/(di/dt)

Where VL is the induced voltage in volts
di/dt is the rate of change of current in amperes per second.

If di/dt = 1 A/s and VL = 1v, then L = 1H

Hence a coil has an inductance of one henry if an emf of one volt is induced in it when current through it changes at the rate of 1 A/s.

Physical factors affecting the value of inductance (L):

It is found that whenever current through an inductor changes (i.e. increases or decreases), a counter emf is induced in it which tends to oppose this change. This property of the coil due to which it opposes any change of current through it is called inductance (L).

In terms of physical factors
L = μ0μrAN2/l Henrys

Where μ0 = absolute permeability of air or vacuum.
μr = relative permeability of the core material.
A = cross-sectional area of core
N = number of turns of the coil
l = core length

Mutual Inductance (M):


The ability of varying current in one coil to induce voltage in a nearby coil is known as the mutual inductance. It is denoted by the letter M or LM and its units are Henrys (H).
Illustration of Mutual Inductance

It can be expressed as M= VL2/(di1/dt)

Where VL2 is the mutually-induced emf produced in the second coil (di1/dt) is the rate of change of current through the first coil.

Physical Factors Affecting the Value of Mutual Inductance (M):

When two coils are placed so close to each other that the expanding and collapsing magnetic flux of one coil links with the other, an induced emf is produced in the other coil. These two coils are then said to have mutual inductance (M).

In terms of physical factors,

M = μ0μrAN1N2/l
Where ‘l’ is the length of the magnetic path.

Coefficient of Coupling (K):


The degree of magnet coupling is known as the coefficient of coupling, denoted by the letter k. If L1 and L2 are the inductances of two inductors then the coefficient of coupling is expressed as
k = Flux linkages between L1 and L2 / Total flux produced by L1
(or)
K = M / (L1L2)

If all the flux due to one coil (L1) links with the other then k = 1. If the flux of one coil (L1) does not at all link with the other coil (L2) then k = 0. Thus k is a constant whose value lies between 0 and 1.

A high value of k = 1, called tight coupling allows the current in one coil to induce more voltage in the other coil. A low value of k called loose coupling induces less voltage. If the coils are kept for apart or kept perpendicular, the coefficient of coupling is zero and hence there is no mutual inductance.



Types of Inductors:


Inductors are classified in many ways. Some of them are as follows.

1. According to the core material used,
(a) air core inductors
(b) iron core inductors
(c) ferrite core inductors

2. According to the frequency of operation
(a) audio frequency (AF) coils
(b) radio frequency (RF) coils.

3. According to the method of winding
(a) single layer coils
(b) multi layer coils

4. According to the operation
(a) fixed coils
(b) variable coils

Transformer - Construction and Working Principle

The transformer is a static device which transfers the electrical energy from one circuit to another without change in its frequency. This transformation is done due to the Faraday's law of electromagnetic induction between the two circuits. Usually it has two windings, namely primary and secondary.

The primary winding receives the energy from a source where as the secondary winding delivers the induced voltage to a load. Both the primary and secondary winding coils are wound on a former or core materials. The use of core reduces the losses, such as hysteresis and eddy currents. The core may be the same or different for primary and secondary windings placed close to each other enough so that the magnetic lines of force from one coil will cut the turns of the other coil.
Diagram of a Transformer
The prime function of transformers is:

1. To transfer the power from one circuit to another acting as a coupling device.
2. The high or low output is obtained across the secondary winding.

It has several advantages, such as

1. No frictional losses.
2. Low maintenance cost because it does not requires much attention.

Transformer - Construction:


As explained in preceding section, commonly transformer consists of two windings, namely primary winding and secondary winding. These windings are wounded on a core material to reduce the losses such as eddy currents and hysteresis.

The core material used in transformer construction is of two types (Iron. Silicon.....). The core material is made of thin sheets of 0.35 mm to 0.5 mm thick. The sheets are available in different shapes, such as E, I, L, O, U and T which are laminated to minimize the eddy currents. The laminations are separated by an insulating materials, such as varnish, oxide layer etc.



The primary and secondary windings are wounded on the two limbs or on the central limb. According to the type of winding, there are two types of constructional methods [shown in Figure] in transformers. i.e.. shell type transformer and core type transformer. In core type construction most of the core is enclosed by the windings. However in the shell type transformer most of the windings are enclosed by the core. The core is made from laminations, usually the yoke is built up from a stack of laminations and the windings are formed around this. Once the windings have been formed extra laminations are added to form complete the core.

(a) Core Laminations
(b) Core Type Transformer
(c) Shell Type Transformer

Working Principle of Transformer:


The transformer works on the principle of "Electromagnetic Induction", Consider an ideal transformer (having no losses) consists of two stationary windings, namely primary and secondary.
The number of turns in primary is NP and NS is the number of turns in secondary. The primary windings are supplied with an ac voltage 'Vi' at a frequency 'f" hertz. It causes to flow an alternating current 'Ii' of same frequency of 'Vi'. As the current is alternating, it will produce an alternating flux (Φ) in the core which will be linked by both the primary and secondary windings. The varying flux causes to generate emf in two windings. The emf 'Ep' across the primary is the same magnitude of the same magnitude of source supply but opposite in phase.

This emf is also known back emf (or) 'counter emf' of the primary. The induced emf across secondary winding is known as 'mutually induced emf (Es). This emf is antiphase with supply voltage 'vi' and its magnitude is proportional to the rate of change of flux and the number of secondary turns. Further, this emf can be utilized to deliver power to any load connected across the secondary.

If a transformer has the number of turns in secondary is higher than primary winding. it is called step-up transformer and its emf is of high magnitude. Otherwise, if number of turns is less than primary, the transformer is said to be step-down transformer and the secondary emf is of smaller magnitude. The instantaneous values of applied voltage, number of turns, emfs are shown in the blog posts.

APPLICATIONS OF TRANSFORMERS:


A transformer can be used for different purposes in various electronic and electrical circuits. The details are as follows:

(a) As a power transformer to provide desired ac voltage either single valued or multiple values.
(b) As matching device between amplifiers and speakers or-as inter stage coupling device between any two circuits to provide impedance matching for having maximum energy transfer.
(c) As a tuner in all communication devices for selection of proper signal in conjuction with capacitor (s).
(d) As an isolating transformer to reduce the change of electronic shock from one stage to another.

Specifications and Uses of Capacitors

Specifications of Capacitors:


The specifications of capacitors are:

1. Capacitor Value:

It provides the value of a capacitor C as farads moreover printed or colour coded over the body of the capacitor. It's units are Farads. Practical capacitors are available from 1pf to 1000 mF.

2. Tolerance:

The variation in capacitance value from the indicated value.

3. Dielectric Constant:

It is defined as the ratio of capacitance of a capacitor containing the dielectric material, to the capacitance of the same capacitor with air or vaccum as the dielectric. It is denoted by the letter K. It may be expressed as

K = Capacitance of the capacitor with dielectric/ Capacitance of the capacitor with air dielectric.

The dielectric constant is also known as relative permittivity Ɛr.

The relative permittivities of some dielectric media are listed in the Table.

Material
Dielectric constant or relative permittivity (Ɛr)
Air or vacuum
1
Bakelite
4.5 – 5.5
Ceramic
50 - 300
Fibre
5 – 8
Glass
7 – 8
Mica
3 – 6
Paper (dry)
2 – 2.5
Paper (waxed)
3 – 5
Polystyrene
2.6
Porcelain
5 – 6
Rubber
2 – 3.5

4. Dielectric Strength:

 It is very important specification of a capacitor (insulator or dielectric medium), which gives the maximum voltage gradient that a unit thickness of the medium can withstand without breaking down. It’s unit is Volt/metre (V/m) although it is usually expressed in KV/m.m.

Factors affecting the Dielectric Strength:

a. It decreases with increase in the thickness of dielectric material.
b. It decreases with increase in frequency.
c. It decreases with humidity and temperature
d. It decreases with increasing time of application of electric current.

5. Power Factor:

It gives the fraction of input power dissipated as heat loss in the capacitor. The quality of a capacitor in terms of minimum loss is often indicated by its power factor. The lower the numerical value of the power factor the better is the quality of the capacitor.
There are some other specifications that are to be considered while selecting a capacitor. They are.

6. Temperature Coefficient: Variation of capacitance value with the temperature.

7. Voltage Rating: Maximum voltage that can be applied across a capacitor.

8. Leakage Resistance/Leakage Current: Reciprocal terms that denote leakage property of capacitor.



Uses of Capacitors:


No
Type
Max Working Voltage in Volts
Typical Uses/ Applications
1
Tubular Rolled paper
1000
Coupling and decoupling AF circuit
2
Metallised paper
400
Coupling and decoupling AF circuit
3
Polystyrene
125 – 500
Resonant circuits, coupling measuring circuit
4
Polyester
500
Coupling Decoupling and Smoothing
5
Stacked Mica
2000
RF Coupling Bypassing circuit
6
Silvered Mica
1000
RF Resonant circuits and Measuring circuits.
7
Silvered Ceramic
1000
RF amplifiers, RF Bypass Circuits
8
Ceramic with high dielectric constant
350

9
Aluminium Electrolytic
600
Rectifier filters and smoothing
10
Tantalum Electrolytic
6 – 150
Coupling, decoupling Bypassing in transistor amplifiers and PCB’s
11
Air/ Polystyrene variable capacitance
1000
Tuning circuits in receivers and transmitters
12
Mica Trimmers and padders
500
Tracking and alignment of receivers
13
Air Trimmers
500
Tracking and alignment of receivers



Capacitors - Classification, Applications and Properties

Capacitors are classified in to different types based on various factors as given below :

1. According to the Type of Dielectric Used :

Ex : Mica, Paper, Ceramic, Air, Electrolytic Capacitors.

2. According to the Physical Construction :

(i) Fixed Capacitors : Whose capacitance value cannot be varied mechanically or by any other external means.
Ex : Mica, Paper, Ceramic, Electrolytic capacitors.

(ii) Variable capacitors : Whose capacitance value can be varied quite frequently or less frequently.
Ex : Tuning capacitors, and Trimming capacitors.

3. According to the Polarization :

(i) Polarized : Used in d.c applications
Ex : Aluminium, Tantalum Electrolytic Capacitors

(ii) Non polarized : Used in a.c applications.
Ex : Aluminium, Tantalum Electrolytic Capacitors

4. According to Voltage Rating :

(i) Low voltage capacitors (< 50V)
Ex : Ceramic, Electrolytic capacitors
(ii) High voltage capacitors (> 100V)
Ex : Mica, Glass, Ceramic capacitors.

FIXED CAPACITORS :


The Capacitance depends on the following factors :
(i) Directly proportional to the area of the plates (A) in square metres.
(ii) Inversely proportional to the distance (d) between the plates in metres.
(iii) Depends on to the permittivity of the medium between the plates (Ɛ).

Or C = ƐA/d  (since, Ɛ = Ɛ0 Ɛr)

Therefore, C = Ɛ0 Ɛr A/d Farad
where Ɛ0 = absolute permittivity of air = 8.8540 x 10-12 F/m
and Ɛr = relative permittivity of medium.

VARIABLE CAPACITORS:


Capacitors whose capacitance value cannot be varied mechanically or by any other external means. These capacitors usually have the fixed value with tolerance bearing from ± 1 to ± 20%. They are of various types depending on the use of dielectric between the plates.

1. Paper Capacitors :


In this type of capacitors paper is used as the dielectric medium. Based on the construction paper capacitors are divided into two types namely :

1. Impregnated paper capacitors
2. Metalised paper capacitors

Properties of Paper Capacitors :

1. They are usually high-voltage (> 100V) capacitors.
2. Their capacitance value is usually between 0.002 µF and 0.05 µF.
4. They are mechanically very strong.
4. They are very cheap.
5. They are quite bulky.
6. They have poor high frequency characteristics.

Applications of Paper Capacitors:

• Used as RF-suppression capacitors in circuits where noise interference from RF sources can occur.
• Used as bypass capacitors in amplifiers.
• Used in high voltage DC circuits.
• Used in commutating circuits of silicon-controlled rectifiers.

2. Mica Capacitors :


In this type of capacitors mica is used as the dielectric medium. Based on the construction mica capacitors are divided into two types namely :

1. Stacked mica capacitors
2. Silvered mica capacitors.

Properties of Mica Capacitors :

1. Mica capacitors have good mechincal strength.
2. They can be operated to temperatures as high as 900°C.
3. They can with stand very high voltages (Thousand of volts).
4. They are suitable for very high-frequency operation.
5. The capacitance value is generally between 5 to 3300 PE
6. The capacitance value is highly stable.
7. They are cheaper than polyester capacitors.

Applications of Mica Capacitors:

• Used as high-voltage capacitors in low-frequency power applications.
• Used as high-voltage RF capacitors.
• Used as high-voltage transmitter-capacitors.




3. Glass Capacitors:


In this type of capacitors Boro silicate glass or glass with potassium, barium or lead oxides is used as the dielectric medium.

Properties of Glass Capacitors:

1. Stability and frequency characteristics are better than mica capacitors.
2. Cost is higher.

Applications of Glass Capacitors:

These are used in medium power transmitters.

4. Polyester Capacitors :


These capacitors are made of polyethelene terapthalate (Mylar, Melinex, Terelyne etc).

Properties of Polyester Capacitors :

1. They have high capacitance (a few ,tF) in small volume.
2. They are very stable.
3. They have good high temperature (up to 250oC) properties.
4. They have good mechanical strength.
5. They have very low leakage.
6. Their cost is much higher than that of paper, ceramic and mica capacitors.

Applications of Polyester Capacitors:

• Used as coupling capacitors.
• Used as stable capacitors where capacitance stability over long years of operation is a must.
• They can replace leaky electrolytic capacitors in many R.F applications.

5. Polystyrene Capacitors :


These capacitors are made by rolling the polystyrene film with aluminium foil.
Properties of Polystyrene Capacitors
1. Excellent stability.
2. Low moisture pick-up.
3. Slightly negative temperature co-efficient.
4. Maximum operating temperature is only about 85°C.
5. Comparatively bigger in size.

Applications of Polysterene Capacitors :

• Used in precision timing circuits.
• Used in ICs.
• Used in high tuned circuits and sub standards.

6. Ceramic Capacitors :


These are the latest type capacitors. Ceramic is used as the dielectric material. Depending on the construction capacitors are of four types:

1. Tubular type
2. Disc type
3. Monolithic type
4. Barrier layer type.

1. Tubular Type:

These are in tubular form capacitance ranges from 5 to 1000 PF voltage ranges up to 5 KV and up to 10,000 PF in the lower voltage ranges.

2. Disc Type :

These are made up of thin ceramic film operated at voltage gradients up to 4000 V/mm. The disc types of capacitors have high capacitance per unit volume and are very economical. These capacitors are available from a few PF to 20,000 PF working voltage is 750V dc or 350V dc. These are low cost and small size capacitors used for coupling and by pass use in IF and RF circuits.

3. Monolithic Capacitors:

It consists of interleaved thin layers of ceramic - platinum electrodes fused. These capacitors are available from 10 PF to 0.47 μF at voltage ratings of 40 and 100 V dc. Insulation resistance is 1000 or 1 giga ohm. The size of the capacitor reduces by 40%.

4. Barrier Layer Type : 

These capacitors are made from barium titanate dielectric. These have much lower insulation resistance and are useful only for low voltage types. Operating temperature ranges from -40 to 80°C and insulation resistance values are about 1.5 MΩ for low capacitance values and 100 KΩ at 0.22 μF.

Applications:

Ceramic capacitors are used for by pass, decoupling and bias applications, used in scan correction circuits in TV receivers, these capacitors with prescribed temperature capacitance variations are used to compensate for impedance temperature changes in circuits. They are used in hybrid ICs.

Electrolytic Capacitors :

Electrolytic capacitors are of two types depending on the electrode material used
1. Aluminium Electrolytic capacitors
2. Tantalum Electrolytic capacitors

Both are available in wet and dry types.

1. Aluminium Electrolytic Capacitors :

Aluminium electrode is used in these capacitors. These capacitors have relatively high leakage resistance. The capacitance values are ranging from 1 to 2,70,000 µF and voltages ranging from 3 to 450 V with tolerance 50%. These capacitors are low cost and polarised.

2. Tantalum Electrolytic Capacitors: 

Tantalum Electrode is used in these capacitors. The features of these capacitors are rugged, low leakage resistance, small size and high capacity. Good for high reliability and critical industrial use. Generally polarised, but are also supplied as non polarized types and can be operated up to 175°C. These have the same uses as aluminium electrolytics but are superior in performance. Values are ranging from 1 to 1200 μF and voltage ratings ranging from 3 to 300V tolerance from 5 to 20%.

Applications of Electrolytic Capacitors:

These are used as by pass capacitors in amplifers, for inter-stage coupling between two amplifiers stages, as smoothing capacitors in D.C. Power supplies, as phase-shifting capacitors in single phase induction motors.