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CBSE NOTES CLASS 12 PHYSICS

CHAPTER 14 SEMICONDUCTORS

Types of substances on the basis of conductivity

Metals on the basis of conductivity

Semiconductors on the basis of conductivity

Insulators on the basis of conductivity

Energy bands in solids

Valence band

Conduction band

Forbidden band

Types of substances on the basis of energy bands

Metals on the basis of energy bands

Insulators on the basis of energy bands

Semiconductors on the basis of energy bands

Types of semiconductors

Elemental semiconductors

Compound semiconductors

Types of semiconductors based on purity

Intrinsic semiconductors

Effect of temperature on conductivity of semiconductors

Extrinsic semiconductor

n-type semiconductor

p-type semiconductor

Conductivity of extrinsic semiconductor

p-n junction

Diffusion of charge

Diffusion current

Depletion region

Drift of charge carriers

Drift current

Potential barrier across p-n junction

Semiconductor diode

Forward bias of p-n junction

Reverse bias of p-n junction

V-I characteristics of a diode

Threshold voltage or cut-in voltage

Dynamic resistance of diode

Application of junction diode as a rectifier

Half wave rectifier

Full-wave rectifier

Centre-tap transformer

Electric filter

Role of capacitor in the filter

Some special type of diodes

Zener diode

Zener diode as voltage regulator

Photodiode

Light emitting diodes (LED)

Photovoltaic devices (Solar cells)

Junction transistor

n-p-n transistor

p-n-p transistor

Transistor emitter

Transistor base

Transistor collector

Transistor in saturation region

Transistor in cut-off region

Transistor in active region

Basic transistor circuit configurations and transistor characteristics

Transistor in common base configuration

Transistor in common emitter configuration

Common emitter transistor characteristics

Input resistance of transistor

Output resistance of transistor

Current amplification factor

Transistor as a device

Transistor as a switch - base-biased CE configuration

Transistor as an amplifier

Amplification of dc voltage

Amplification of ac signal

Feedback amplifier

Transistor oscillator

Working of feedback amplifier

Tank circuit

Digital electronics

Analog signal

Digital signal

Logic gates

NOT gate

OR gate

AND gate

NAND gate

NOR gate

Integrated circuits

Linear or analogue ICs

Digital ICs

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 14 SEMICONDUCTORS

Semiconductor diode

A semiconductor diode is a p-n junction with metallic contacts provided at the ends for the application of an external voltage. It is a two terminal device.

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Forward bias of p-n junction

The p -side is connected to positive terminal and n-side to negative terminal of a battery.

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Forward current flows due to majority charge carriers and the width of depletion layer decreases.

The current under forward bias, first increases very slowly till the voltage across the diode crosses a certain value. After this, the diode current increases significantly (exponentially), even for a very small increase in the diode bias voltage. This voltage is called the threshold voltage or cut-in voltage

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Reverse bias of a p-n junction

The p-side is connected to negative terminal and n-side to positive terminal of a battery.

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Reverse current flows due to minority charge carriers and the width of depletion layer increases.

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The current under reverse bias voltage is independent up to a critical reverse bias voltage, known as breakdown voltage (Vbr). When V = Vbr, the diode reverse current increases sharply. Even a slight increase in the bias voltage causes large change in the current. If the reverse current is not limited by an external circuit, the p-n junction will get destroyed.

V-I characteristics of a diode

A graph between V and I is plotted as shown in the figure. In forward bias measurement, a milliammeter is used, since the expected current is large (as explained in the earlier section) while a micrometer is used in reverse bias to measure the current.

In forward bias, the current first increases negligibly, till the voltage across the diode crosses a certain value. After the characteristic voltage, the diode current increases exponentially, even for a very small increase in the diode bias voltage. This voltage is called the threshold voltage or cut-in voltage (~0.2V for germanium diode and ~0.7 V for silicon diode).

For the diode in reverse bias, the current is very small (~μA) and almost remains constant with change in bias. It is called reverse saturation current. However, for special cases, at very high reverse bias (break down voltage), the current suddenly increases.

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The p-n junction diode primarily allows the flow of current only in one direction (forward bias). The forward bias resistance is low as compared to the reverse bias resistance. This property is used for rectification of ac voltage.

Dynamic resistance

The ratio of small change in voltage ΔV to a small change in current ΔI:

rd =ΔVΔI

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