Electric Circuit and Electron Device - SEMICONDUCTOR DIODES (Unit 3)–Lecture Notes

Anna University

Electric Circuit and Electron Device

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Subject Code : EC2151

Subject Name : Electric Circuit and Electron Device

Year : 1st yr

Semester : 2nd Sem

Department : CSE & ECE

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UNIT-III

SEMICONDUCTOR DIODES

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Energy Bands

The range of energies possessed by an electron in a solid is known as Energy band.

Classification of Energy Band

1. Conduction band

2. Forbidden band

3. Valence band

Classification of semiconductors

1. Intrinsic Semiconductors.

2. Extrinsic Semiconductors.

Intrinsic Semiconductors

· A Semiconductor which is in its extremely pure form is known as an intrinsic semiconductor.

· If potential difference is applied across intrinsic semiconductor, the electrons will move towards the positive terminal while the holes will drift towards the negative terminal.

· The total current inside the semiconductor is the sum of currents due to free electrons and holes.

Extrinsic Semiconductors

· The conductivity can be increased by the addition of a small amount of suitable metallic impurity. It is also known as impurity semiconductor.

· The process of adding impurity atoms to the intrinsic semiconductor is called doping.

· The purpose of adding impurity is to increase either the number of free electrons or holes in a semiconductor.

· Two types of impurity atoms are added to the semiconductor.

Pentavalent impurity atoms - containing five valance electrons

Trivalent impurity atoms - containing three valance electrons

· Extrinsic semiconductors are classified as,

N-type Semiconductor

P-type Semiconductor

N-type Semiconductor

· A small amount of pentavalent impurity is added to a pure semiconductor is known as N-type Semiconductor.

· When a pentavalent impurity is added to a pure semiconductor, it displaces some of its atoms. E.g. ARSENIC (As), ANTIMONY (Sb)

· In N-type semiconductor, major part of the current flows due to the movement of Electrons. Therefore electrons in an N-type semiconductor are known as majority carriers and holes as minority carriers.

P-type Semiconductor

· A small amount of trivalent impurity is added to a pure semiconductor is known as P-type Semiconductor.

· E.g. Gallium (Ga), Indium (In), Aluminium (Al), Boron (B) etc.

· In P-type semiconductor, major part of the current flows due to the movement of holes. Therefore holes in a P-type semiconductor are known as majority carriers and electrons as minority carriers.

Conductivity of a semiconductor

Each hole-electron pair created two charge carrying particles is formed. One is negative of mobility µ_n (free electron) and the other is positive of mobility µ_p (hole). These particles move in opposite direction in an electric field.

Hence the current density J = σE

Where n , p = magnitude of free electrons & holes respectively.

σ = Conductivity of semiconductor

E= applied electric field

q = Charge of electron or hole.

Hence σ = (nµ_n + pµ_p )E

For a pure semiconductor n = p = n_i ; where n_i – intrinsic semiconductor

CARRIER CONCENTRATIONS IN AN INTRINSIC SEMICONDUCTOR

In order to calculate the conductivity of a semiconductor, it is necessary to know the concentration of free electrons n and the concentration of holes p.

Concentration of Electrons (n)

The number of electrons in the conduction band,

Concentration of holes (p)

The number of holes in the conduction band,

Energy Gap (E_G)

The energy required to break a covalent bond in a semiconductor is known as energy gap. The Energy gap at any temperature is given by, E_G = E_GO – βT

Drift Current

Drift current is defined as the flow of electric current due to the motion of the charge carriers under the influence of an external electric field applied across the semiconductor material.

Diffusion Current

In a semiconductor material, the charge carriers have the tendency to move from the region of higher concentration to that of lower concentration of the same type of charge carriers. This movement of charge carriers takes place resulting in a current called diffusion current.

Diffusion current density due to holes, Jp = - q D_p dp/dx A/cm²

Diffusion current density due to electrons, Jn = - q D_n dn/dx A/cm²

Total Current Density

Total current is the sum of drift current and diffusion current.

The total current density for P-type semiconductor Jp = - qpµ_pE- q D_p dp/dx

The total current density for N-type semiconductor Jn = - qnµ_nE- q D_n dn/dx

Diffusion Length (L)

The average distance that a charge carrier can diffuse during its lifetime is called as diffusion Length L.

Theory of PN Junction Diode

When a P-type semiconductor s joined to a N-type semiconductor the contact surface is called PN junction or PN diode.

The voltage across PN junction can be applied in two ways.

(i) Forward biasing

(ii) Reverse biasing

The N-type material has high concentration of free electrons and, P-type material has high concentration of holes. At the junction, there is a tendency for the free N-type of diffuse over to the P-side and holes from the P-side to the N-side. This process is called diffusion.

Thus a barrier is set up against further movement of charge carriers. This is called Potential barrier or Junction barrier (V_B).The potential barrier is of the order of 0.1V to 0.3 V.

The mobile charges have been depleted in this region. It is known as depletion layer.

ZENER DIODE

A zener diode is a special purpose diode that is operated in reverse-biased conditions. Its operation depends on the zener breakdown phenomenon.

Symbol

Anode Cathode

V-I Characteristics of Zener diode

The operation of zener diode is same as that of ordinary p-n diode order forward biased condition, whereas under reverse biased condition breakdown of the junction occurs.

Breakdown voltage depends upon the amount of doping. If the diode is heavily doped, depletion layer will be thin and consequently breakdown occurs at lower reverse voltage and further, the breakdown voltage is sharp. The breakdown voltage can be selected with the amount of doping.

The sharp increase in current under breakdown condition is due to the following two mechanisms.

Ø Avalanche breakdown

Ø Zener breakdown

Avalanche breakdown

When doping concentration is less like in ordinary diode then under reverse biased condition a small amount of reverse saturation current flows and is constant as long as the temperature is constant.

When the reverse voltage is increased width of the depletion layer increases at the same time the electrons due to force of attraction by the plates acquire some high velocity and during their motion inside the diode they collide with the electrons in covalent bonds and bring them out.

Due to this multiplication process a large current flows and this kind of breakdown is called Avalanche multiplication or breakdown. Once when this breakdown occurs the diode gets damaged.

Zener breakdown

When doping is heavy then in reverse bias even-before the minority charge carries acquire sufficient velocity the breakdown occurs and is known as Zener breakdown.

In reverse bias under heavy doping condition the width of the depletion layer will be very thin strong electric field exists inside the diode. When reverse voltage increased at once electric field the electrons which are present in the covalent bond are brought due to strong force of attraction. Now, suddenly a large amount of current flows. Nothing but Breakdown occurs. In Zener diode first zener breakdown occurs and later avalanche breakdown.

Applications of Zener diodes

Ø Voltage regulator

Ø Fixing reference voltages in electronic circuits such as power supplies and transistor biasing.

Ø Clippers in wave-shaping circuits.

Square wave generation.

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