Revision: Electronic Devices >> Semiconductor Electronics - Materials, Devices and Simple Circuits Physics Science (English Medium) Class 12 CBSE

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 semiconductor diode's depletion zone is the area surrounding the p-n junction where there are no mobile charge carriers, this area generates an electric field that allows the diode to conduct in one direction while blocking in another.

The barrier that the repelling forces use to stop the mobile charge carriers (at the PN junction) is known as the potential barrier.

This results from the concentration of immobile charges close to the junction after electrons and holes diffuse across the function.

(i) Rectifier: It is a device which converts alternating current into direct current.

(ii) Amplifier: An amplifier is a device which increases the energy of a weak signal by supplying energy from an external source. An amplifier increases the amplitude of a input signal.

(iii) Oscillator: An oscillator is a device which produces electrical oscillations of adjustable frequency and constant amplitude. An oscillator is basically an amplifier. A part of the output energy is fed back into the L-C circuit to produce sustained oscillations.

It is a semiconductor device used to convert photons of solar light into electricity. It generates emf when solar radiation falls on the p-n junction. A p-type silicon wafer of about 300 μm is taken over which a thin layer of n-type silicon is grown on one side by the diffusion process.

A special purpose junction diode that converts light energy into electrical current, works on the principle of the photoelectric effect, operates in reverse bias, and generates a current when exposed to light (proportional to the intensity of incident light), is called a Photodiode.

A unique form of a bipolar device which permits the current flow in the reverse direction when the voltage applied is above a certain characteristic value called Zener voltage or breakdown voltage, most commonly used in voltage regulators to protect other semiconductor devices from fluctuations in voltage, is called a Zener Diode.

A material in which the valence band is completely filled and the conduction band is empty, separated by a large energy gap (a few eV), so electrons cannot move freely.

The highest occupied energy band containing valence electrons.

OR

Valence band is the wide range of energies possessed by the valence electrons. Valence band is the highest energy band, occupied by the valence electrons. It is completely filled for inert gases, but partially filled for other materials. 

An energy band is the wide range of energies possessed by an electron in a solid.

A material with a small energy gap (about 1 eV) between valence and conduction bands, allowing limited conduction at room temperature.

An empty or partially filled band above the valence band in which electrons can move freely and conduct current.

OR

Conduction band is the wide range of energies possessed by the conduction band electrons. It is the lowest unfilled band, for insulators. But it is partially filled for conductors. Current conduction is due to the electrons in this band. 

A material having a partially filled valence band (or overlapping valence and conduction bands), allowing electrons to move easily and conduct electricity.

r_a = \[\frac {ΔV}{ΔI}\]

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

• 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.

• Conductors have many free electrons, so electric current flows easily; insulators have almost no free electrons, so current does not flow easily.
• In conductors, resistance increases with temperature, whereas in semiconductors it decreases.
• Semiconductors have properties between conductors and insulators, and at absolute zero, they behave like insulators.

• An intrinsic semiconductor is a pure semiconductor (like silicon or germanium) without impurities.
• At low temperatures, it behaves like an insulator because all electrons are bound in covalent bonds.
• At room temperature, some bonds break and create electron–hole pairs.
• In an intrinsic semiconductor, the number of electrons equals the number of holes (ne = nh = ni).
• Its conductivity increases with temperature because more electron–hole pairs are produced.

• Semiconductors have a small energy gap (about 1 eV) between the valence band and conduction band.
• At absolute zero, they act like insulators because the valence band is full and the conduction band is empty.
• At room temperature, some electrons move into the conduction band, leaving holes, and both contribute to conduction; conductivity increases with temperature.

• A p–n junction is formed by joining p-type and n-type semiconductors.

• Electrons diffuse from n → p.

• Holes diffuse from p → n.

• Recombination occurs near the junction.

• Immobile ions are left behind.

• A depletion region is formed (no free charge carriers).

• An electric field develops across the junction.

• A barrier potential is established.

• At equilibrium:
  Diffusion current = Drift current
  Net current = 0

• Applied voltage reduces barrier potential.

• Depletion region width decreases.

• The majority of carriers cross the junction.

• Current increases rapidly after the threshold voltage.

• Current is in the mA range.

• Barrier potential increases.

• Depletion region widens.

• Diffusion current decreases.

• Small reverse current flows (minority carriers).

• The reverse current is almost independent of voltage (until breakdown).

• At breakdown → current increases sharply.