Semiconductor Electronics: Complete Chapter Notes For NEET 2027 by PW

Remembering the difference between intrinsic and extrinsic semiconductors, understanding PN-junction biasing and recalling the working of devices such as Zener diodes, LEDs, and photodiodes can become difficult when several concepts are studied together. The chapter becomes easier when you connect charge carriers, doping, junction formation and device applications.

Revise these concepts with the PW Semiconductor Electronics Complete Chapter One-Shot Video for NEET 2027 and then practise questions to strengthen your understanding and improve application of the formulas.

Definition And Types Of Semiconductors

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Its conductivity is approximately in the range of 10^-6 to 10^5 S m^-1.

Semiconductors are mainly classified according to their composition. Based on composition, semiconductors are mainly of two types:

• Elemental semiconductors: These are made of a single element, mainly Silicon (Si) and Germanium (Ge).

• Compound semiconductors: These are made by combining different elements. Examples include Cadmium sulphide (CdS), Gallium arsenide (GaAs), Cadmium selenide (CdSe) and Indium phosphide (InP).

Silicon and germanium are the most important elemental semiconductors for understanding semiconductor devices.

Energy Band Diagram

In a solid, electrons occupy different energy levels that form energy bands.

• Valence band: Contains valence electrons.

• Conduction band: Electrons in this band can contribute to electrical conduction.

• Forbidden energy gap: Energy difference between the valence band and conduction band.

The nature of the energy gap helps distinguish conductors, semiconductors and insulators.

Important band-gap values:

• Germanium: E_g ≈ 0.7 eV

• Silicon: E_g ≈ 1.1 eV

Intrinsic Semiconductors And Charge Carriers

An intrinsic semiconductor is a pure semiconductor without intentionally added impurities.

When an electron receives sufficient energy, it can break away from a covalent bond and move into the conduction band. This leaves behind a vacancy called a hole.

Thus, one electron-hole pair is produced.

For an intrinsic semiconductor:

n_e = n_h = n_i

where:

• n_e = electron concentration

• n_h = hole concentration

• n_i = intrinsic carrier concentration

At a fixed temperature, the carrier concentrations follow:

n_e × n_h = n_i^2

Both electrons and holes contribute to current in a semiconductor. The semiconductor crystal as a whole remains electrically neutral.

Mobility And Conductivity

Mobility describes how easily a charge carrier moves through a semiconductor under an applied electric field.

The mobility is:

mu = v_d / E

where v_d is the drift velocity and E is the electric field.

The current density due to electrons and holes can be written as:

J = e(n_e v_de + n_h v_dh)

The conductivity of a semiconductor is:

sigma = e(n_e mu_e + n_h mu_h)

Also,

J = I / A

and

J = sigma E

Therefore,

sigma = J / E = I / (A E)

Electron mobility is generally greater than hole mobility.

Extrinsic Semiconductors And Doping

An extrinsic semiconductor is obtained by adding a small, controlled amount of impurity to a pure semiconductor. This process is called doping and increases the conductivity of the material.

There are two main types.

P-Type Semiconductor

A P-type semiconductor is formed by adding a trivalent impurity, such as aluminium, to silicon.

Three impurity electrons form covalent bonds, while one bond remains incomplete. This produces a hole.

• Holes are majority carriers.

• Electrons are minority carriers.

• n_h > n_e

N-Type Semiconductor

An N-type semiconductor is formed by adding a pentavalent impurity, such as phosphorus, to silicon.

Four electrons participate in covalent bonding, while the fifth electron becomes available for conduction.

• Electrons are majority carriers.

• Holes are minority carriers.

• n_e > n_h

Even after doping, the semiconductor crystal remains electrically neutral.

Temperature Dependence Of Conductivity

The conductivity of a semiconductor increases with temperature because more electron-hole pairs are generated.

This is opposite to the usual behavior of metals, where conductivity decreases as temperature increases because increased lattice vibrations cause more resistance to electron motion.

PN Junction And Depletion Region

A PN junction is formed by joining P-type and N-type semiconductor regions.

After the junction is formed:

• Holes diffuse from the P-side towards the N-side.

• Electrons diffuse from the N-side towards the P-side.

• Electrons and holes recombine near the junction.

• Negative fixed ions remain on the P-side.

• Positive fixed ions remain on the N-side.

The region around the junction containing these fixed ions and very few mobile charge carriers is called the depletion region.

The depletion region creates an electric field and a potential difference called the barrier potential.

For a uniform electric field:

V_B = E × d

where E is the electric field, and d is the depletion width.

Diffusion current is mainly associated with majority carriers, while drift current is associated with minority carriers.

PN-Junction Biasing

Applying an external voltage to a PN junction is called biasing. It can be forward bias or reverse bias.

Forward Bias

In forward bias:

• P-side is connected to positive potential.

• N-side is connected to a negative potential.

• Depletion-region width decreases.

• The barrier is reduced.

• Majority carriers cross the junction.

• Significant current flows.

Remember: Forward bias → P to positive, N to negative.

Reverse Bias

In reverse bias:

• P-side is connected to negative potential.

• N-side is connected to positive potential.

• Depletion-region width increases.

• The barrier becomes larger.

• Majority carriers are prevented from crossing the junction.

• Only a small reverse current flows in an ordinary diode.

Remember: Reverse bias → P to negative, N to positive.

At sufficiently high reverse voltage, breakdown occurs. In a Zener diode, operation in the breakdown region is used for voltage regulation.

Increasing impurity concentration reduces the depletion-region width.

Diode Characteristics And Zener Diode

A semiconductor diode allows current to flow much more easily in forward bias than in reverse bias.

The dynamic resistance of a diode is:

r_d = delta V / delta I

It is the reciprocal of the slope of the I-V characteristic at a particular operating point.

Zener Diode

A Zener diode is designed to operate in the reverse-bias breakdown region. It maintains an approximately constant voltage across the load and is therefore used as a voltage regulator.

For an idealised Zener regulator:

V_load ≈ V_Z

The Zener diode is connected in parallel with the load.

The source current divides between the load and the Zener diode:

I_source = I_load + I_Z

Therefore:

I_Z = I_source - I_load

Rectifiers

A rectifier converts AC into pulsating DC by allowing current through the circuit in a controlled direction.

Half-Wave Rectifier

A half-wave rectifier uses one diode and conducts during only one half-cycle of the AC input.

Its output frequency is:

f_output = f_input

Full-Wave Rectifier

A full-wave rectifier uses both half-cycles of the AC input. Its output frequency is twice the input frequency.

f_output = 2f_input

Optoelectronic Devices

Semiconductor devices can also interact with light. The important devices for NEET include photodiodes, LEDs and solar cells.

Photodiode

A photodiode is generally operated in reverse bias.

Incident light generates electron-hole pairs when the photon energy is sufficient:

h nu >= E_g

The electric field separates the generated carriers, producing photocurrent. Increasing light intensity generally increases the reverse photocurrent.

LED

A light-emitting diode (LED) is operated in forward bias.

When electrons and holes recombine near the junction, energy is released in the form of photons, producing light.

Solar Cell

A solar cell converts light energy into electrical energy and operates without an external bias.

Incident light generates electron-hole pairs, and their movement produces electrical current.

Two important quantities associated with its characteristics are:

• I_SC: Short-circuit current

• V_OC: Open-circuit voltage

Boolean Algebra And Logic Gates

Digital logic circuits use two basic values:

• 0: False or LOW

• 1: True or HIGH

Some important Boolean identities are:

A + 0 = A

A × 1 = A

A × 0 = 0

A + A = A

A × A = A

A + A̅ = 1

A × A̅ = 0

De Morgan’s Theorems

The two important De Morgan's theorems are:

(A + B)̅ = A̅ × B̅

(A × B)̅ = A̅ + B̅

Remember that when the complement is distributed, AND changes to OR and OR changes to AND.

Logic Gate Summary

NAND and NOR are universal gates. They can be used to construct other basic logic operations.

Important Semiconductor Formulas For NEET

Semiconductor Electronics becomes easier when you connect the concepts of energy bands, charge carriers, doping, PN junctions and biasing with the working of electronic devices. Revise the important formulas and device applications, then practise NEET questions based on diode characteristics, Zener regulation, rectifiers, optoelectronic devices and logic gates.

For quick revision before practice, use the PW Semiconductor Electronics Complete Chapter One-Shot Video for NEET 2027 and then solve chapter-wise NEET questions.

NEET Resources  You Might Like: