What are semiconductors?

Semiconductors

What Are Semiconductors?

Semiconductors are materials whose conductivity lies between conductors (like metals) and insulators (like ceramics). They may be made of pure elements, such as silicon (Si) and germanium (Ge), or compounds like gallium arsenide (GaAs). Semiconductors are the backbone of modern electronics.

Examples of Semiconductors

• Silicon (Si) - widely used in electronic circuits.
• Germanium (Ge) - used in transistors.
• Gallium Arsenide (GaAs) - used in solar cells and laser diodes.

Table of Contents

• Holes and Electrons
• Band Theory
• Properties of Semiconductors
• Types of Semiconductors
• Intrinsic Semiconductor
• Extrinsic Semiconductor
• N-Type Semiconductor
• P-Type Semiconductor
• Difference Between Intrinsic and Extrinsic
• Applications

Holes and Electrons

In semiconductors, the current flows due to two types of charge carriers - electrons (negatively charged) and holes (positively charged). Holes are formed when an electron leaves a position, leaving behind a vacancy, which behaves like a positive charge.

Mobility of Electrons and Holes

Electrons move faster than holes because electrons travel in the conduction band while holes move in the valence band where motion is more restricted.

Mobility is higher if:

• The particle has less effective mass.
• The time between collisions is longer.

For pure silicon at 300K:

• Electron mobility = 1500 cm² V^-1s^-1
• Hole mobility = 475 cm² V^-1s^-1

Band Theory of Semiconductors

Band theory explains how energy levels form bands in solids. When atoms come together, their energy levels split into closely packed bands. The gap between these bands where no electrons are present is called the band gap.

Conduction and Valence Bands

• Valence Band: The highest occupied energy band containing valence electrons.
• Conduction Band: The lowest unoccupied band where free electrons move freely.

Fermi Level

The Fermi level (E_F) is the highest filled energy level at absolute zero. It lies between the conduction and valence bands. In n-type semiconductors, it is closer to the conduction band; in p-type semiconductors, it is closer to the valence band.

Properties of Semiconductors

• Resistivity = 10^-5 to 10⁶ Ωm
• Conductivity = 10⁵ to 10^-6 mho/m
• Temperature Coefficient of Resistance = Negative
• Current is due to both electrons and holes.

Why Does Resistivity Decrease with Temperature?

When temperature increases, more electrons gain energy to jump from the valence band to the conduction band, increasing charge carriers and reducing resistivity.

Key Features of Semiconductors

• Act as insulators at 0 K, and conductors at higher temperatures.
• Electrical properties can be controlled by doping.
• Low power loss and compact size.
• Resistivity is higher than conductors but lower than insulators.
• Resistivity decreases as temperature increases.

Types of Semiconductors

• Intrinsic Semiconductor
• Extrinsic Semiconductor

Intrinsic Semiconductor

These are pure semiconductors like silicon or germanium, containing no impurities. At 0 K, they behave like insulators. With temperature rise, some valence electrons gain energy and move to the conduction band, creating free electrons and holes in equal numbers.

Current Formula

Total current: I = I_e + I_h

Carrier concentration relation: n = n₀ e^{-E_g / 2 K_B T}

• E_g = Energy band gap
• K_B = Boltzmann constant

Extrinsic Semiconductor

When impurities are added to intrinsic semiconductors, they become extrinsic. This process is called doping. It creates two types:

N-Type Semiconductor

• Formed by doping with pentavalent atoms (P, As, Sb).
• Provides extra free electrons (majority carriers).
• Holes act as minority carriers.
• Overall, the material remains electrically neutral.

P-Type Semiconductor

• Formed by doping with trivalent atoms (B, Al, In).
• Creates holes (majority carriers).
• Electrons are minority carriers.
• Crystal is still neutral overall.

Difference Between Intrinsic and Extrinsic Semiconductors

Intrinsic Semiconductor	Extrinsic Semiconductor
Pure semiconductor	Impure (doped) semiconductor
ne = nh	ne ≠ nh
Low conductivity	High conductivity
Conductivity depends only on temperature	Conductivity depends on temperature and doping
No impurities	Doped with trivalent or pentavalent atoms

Applications of Semiconductors

Everyday Uses

• Used in temperature sensors.
• Present in 3D printers.
• Integrated into microchips, computers, calculators, and self-driving cars.
• Make up transistors, diodes, and MOSFETs used in electronic circuits.

Industrial Uses

• Used in LEDs, solar cells, transistors, and microprocessors.
• Vital for control systems in space vehicles, robots, and trains.

Importance of Semiconductors

• Portable and lightweight.
• Require low power input.
• Shockproof and reliable.
• Long life and free from noise during operation.