Semiconductors based on Electrical resistance

Unit - III

Semiconductors and Transport Physics

Intrinsic Semiconductors - Energy band diagram – direct and indirect band gap semiconductors - Carrier concentration in intrinsic semiconductors - extrinsic semiconductors - Carrier concentration in N-type & P-type semiconductors - Variation of carrier concentration with temperature - Carrier transport in Semiconductors: Drift, mobility and diffusion - Hall effect and devices - Ohmic contacts - Schottky diode.

Introduction

Semiconducting material has electrical conductivity between a good conductor and a good insulator. It is simply called semiconductor. It is a special class of material which is very small in size and sensitive to heat, light and electricity.

Semiconducting materials behave as insulators at at low temperature and as conductors at high temperature. Moreover, these materials have two types of charge carriers i.c., electrons and holes.

Germanium and silicon are two important elemental semiconductors. They are used in diodes and transistors.

Gallium arsenide (GaAs) and indium phosphide (InP) are two important compound semiconductors. They are used in LEDs and Laser diodes.

The study of semiconducting materials is essential for engineers due to their wide applications in semiconductor devices in engineering and technology.

The invention of semiconductors opened a new branch of technology called solid state electronics.

It leads to the development of ICs, microprocessors, computers and supercomputers.

In short, semiconductors play a vital role in almost all advanced electronic devices.

Definition

Based on Electrical resistance

Semiconductor has electrical resistance which is lesser than an insulator but more than that of a conductor. Its electrical resistivity is in the order of 10-4 to 0.5 ohm metre.

Based on Energy band

A semiconductor has nearly an empty conduction band and almost filled valence band with a very small energy gap (= 1 eV). (Fig 3.1)

General properties of the semiconductors

• They have crystalline structure.

• Bonding between the atoms is formed by covalent bond.

• They have empty conduction band at OK.

• They have almost filled valence band.

• The energy gap is small.

• They exhibit a negative temperature coefficient of resistance. ie., increase in temperature leads to decrease in resistance.

• If impurities are added to a semiconductor, its electrical conductivity increases. Further, if temperature of the semiconductor is increased, its electrical conductivity increases.

This property is in contrary to that of metals in which if temperature is increased or impurities are added, their electrical conductivity decreases.

Classification of semiconductors

The semiconductors are classified mainly into two types based on the composition of materials. They are

(i) Elemental semiconductors

(ii) Compound semiconductors

Elemental Semiconductors (Indirect band gap semiconductor)

The semiconductors which are made from a single element of fourth group elements in periodic table are known as elemental semiconductors.

They are also called as indirect band gap semiconductors. 

Example

Two important elemental semiconductors and their energy band gaps are given in table 3.1.

Compound semiconductors (Direct band gap semiconductors)

Semiconductors which are formed by combining third and fifth group or second and sixth group elements in the periodic table are known as compound semiconductors.

They are also called as direct bandgap semiconductors.

Characteristics

• The compound semiconductor have large forbidden gap and carrier mobility.

• They are formed by both ionic and covalent bonds.

• The recombination of electron and hole takes place directly. During this process, the light photons are emitted in visible or infrared region.

Some important compound semiconductors are given in table 3.2.

Uses

The compound semiconductors are used in photovoltaic cell, photoconductive cell, LEDs [Light Emitting Diode] and Laser diodes.

Table 3.3

Differences between elemental and semiconductors (indirect and direct semiconductors)