Revision: 11th Std >> Semiconductors MAH-MHT CET (PCM/PCB) Semiconductors

The material with electrical conductivity between that of a conductor and an insulator, whose number of charge carriers can be controlled as per requirement, is called a semiconductor. (e.g. Silicon, Germanium)

The different energy levels with continuous energy variation are called energy bands.

The range of energies possessed by valence electrons is called valence band.

The range of energies possessed by conduction electrons is called conduction band.

The energy difference between the valence band and the conduction band is called forbidden energy gap.

The solids which have a large number of free electrons are called conductors. (e.g. Iron, Aluminium)

The solids which have very small number of free electrons are called insulators. (e.g. Glass, Wood)

A pure semiconductor such as pure silicon or pure germanium is called an intrinsic semiconductor.

The semiconductor with impurity added to it is called a doped semiconductor or extrinsic semiconductor.

The semiconductor in which silicon or germanium crystal is doped with pentavalent impurity (donor), making electrons the majority charge carriers, is called an n-type semiconductor.

The semiconductor in which silicon or germanium crystal is doped with trivalent impurity (acceptor), making holes the majority charge carriers, is called a p-type semiconductor.

The resistance offered by a p-n junction diode when it is in forward biased condition is called static (DC) resistance.

The resistance of a diode at a particular applied voltage is called dynamic (AC) resistance.

When a high reverse voltage causes a sudden and uncontrollable increase in current, the phenomenon is called avalanche breakdown.

A p-n junction when provided with metallic connectors on each side is called a junction diode.

When n-type and p-type semiconductor materials are fused together, the junction formed is called a p-n junction.

The formation of a narrow region on either side of the junction which becomes free from mobile charge carriers is called depletion region.

The difference in potential that prevents charge carriers from moving across the p-n junction is called the potential barrier.

The current flowing from p-side to n-side due to diffusion of electrons and holes because of concentration difference is called diffusion current.

The current flowing from n-side to p-side due to holes and electrons created in the depletion region is called drift current.

A device that changes its resistance when light is incident on it is called a photoresistor.

A device with two junctions and three terminals is called a Bi-polar Junction Transistor.

A device that conducts electricity when illuminated with light is called a photodiode.

A device that emits light when current passes through it is called a Light Emitting Diode (LED).

A device that emits light of specific frequency is called a solid state laser.

A small device having hundreds of diodes and transistors is called an integrated circuit.

A device that converts light energy into electric energy is called a solar cell.

A temperature sensitive resistor whose resistance changes with change in its temperature is called a thermistor.

A thermistor whose resistance decreases with increase in temperature and has a negative temperature coefficient is called an NTC thermistor.

A thermistor whose resistance increases with increase in temperature and has a positive temperature coefficient is called a PTC thermistor.

r_a = \[\frac {ΔV}{ΔI}\]

It is the reciprocal of the slope of the I-V characteristics at that point.

R_g = \[\frac {V}{I}\]

r_g = \[\frac {ΔV}{ΔI}\]

• Conductors → E_g = 0 - bands overlap, electrons flow freely.
• Semiconductors → E_g < 3 eV — small gap, conducts at room temperature.
• Insulators → E_g > 5 eV — large gap, no conduction.
• Ge = 0.72 eV, Si = 1.1 eV — both semiconductors.
• Metal conductivity decreases with temp. Semiconductor conductivity increases with temp. 

• Pure substance - Intrinsic semiconductors are pure semiconductors (e.g. pure Si, pure Ge).
• Conduction: They conduct electricity due to both charge carriers — electrons and holes.
• Equal carriers — Number of holes = Number of free electrons per unit volume, i.e., n_i = n_e = n_h​, where n = number density of charge carriers.

1. Conductivity: Extrinsic semiconductors contain added impurities; conductivity increases depending on the valency of the impurity (p-type or n-type).

2. n-type vs p-type carriers & formula

• Potential Barrier — Silicon = 0.6–0.7 V, Germanium = 0.3–0.35 V; barrier is developed due to diffusion of electrons and holes in unbiased condition. 
• Forward Biasing — p-side → +ve terminal; diode is ON; depletion region decreases; knee voltage seen in I-V characteristics.
• Reverse Biasing — n-side → +ve terminal; diode is OFF; depletion region increases; breakdown voltage seen in reverse I-V characteristics.

• Size & Weight: Semiconductor devices are smaller in size and lightweight, which also enables faster speed of operation.
• Power Consumption: They operate at small voltages (few mV) and require very less current (µA or mA), hence consume lesser power and produce almost no heating effects — making them thermally stable.
• Controllability: The electronic properties of semiconductors can be controlled to suit our requirement, and fabrication of ICs is possible.
• Sensitivity: They are sensitive to electrostatic charges, radiation, and fluctuations in temperature — making them fragile in harsh environments.
• Limitations: They are not useful for controlling high power, require controlled conditions for manufacturing, and very few materials are semiconductors.

• It is a temperature sensitive resistor.
• They can measure temperature variations of a small area due to their small size.
• A small change in surrounding temperature causes a large change in resistance.