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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

Some special types of diodes

Zener diode is designed to operate under reverse bias in the breakdown region and used as a voltage regulator.

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Zener diode as voltage regulator

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The unregulated dc voltage (filtered output of a rectifier) is connected to the Zener diode through a series resistance Rs such that the Zener diode is reverse biased.

If the input voltage increases, the current through Rs and Zener diode also increases. This increases the voltage drop across Rs without any change in the voltage across the Zener diode. This is because in the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes.

Similarly, if the input voltage decreases, the current through Rs and Zener diode also decreases. The voltage drop across Rs decreases without any change in the voltage across the Zener diode. Thus any increase/decrease in the input voltage results in, increase/decrease of the voltage drop across Rs without any change in voltage across the Zener diode.

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Thus the Zener diode acts as a voltage regulator.

Optoelectronic devices

Semiconductor diodes in which carriers are generated by photons (photo-excitation are called optoelectronic devices.

  1. Photodiode is used for detecting optical signal (photo-detectors). It works under reverse bias.

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  1. Light emitting diode (LED) converts electrical energy into light and emits light.

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LEDs have the following advantages over conventional incandescent low power lamps:

  1. Low operational voltage and less power.

  2. Fast action and no warm-up time required.

  3. The bandwidth of emitted light is 100 Å to 500 Å or in other words it is nearly (but not exactly) monochromatic.

  4. Long life and ruggedness.

  5. Fast on-off switching capability.
  1. Photovoltaic devices (solar cells) convert optical radiation into electricity.

    A solar cell is basically a p-n junction which generates emf when solar radiation falls on the p-n junction. It works on the same principle as the photodiode, except that no external bias is applied and the junction area is kept much larger for solar radiation to get more power.

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The generation of emf by a solar cell, when light falls on, is due to three basic processes: generation, separation and collection:

  1. Generation of e-h pairs due to light (with hν > Eg ) close to the junction;

  2. Separation of electrons and holes due to electric field of the depletion region. Electrons are swept to n-side and holes to p-side;

  3. The electrons reaching the n-side are collected by the front contact and holes reaching p-side are collected by the back contact. Thus p-side becomes positive and n-side becomes negative giving rise to photo-voltage.
    • The important criteria for the selection of a material for solar cell fabrication are
      1. band gap (~1.0 to 1.8 eV),

      2. high optical absorption (~104 cm–1),

      3. electrical conductivity,

      4. availability of the raw material, and

      5. cost

    • Solar cells are used to power electronic devices in satellites and space vehicles and also as power supply to some calculators.
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