Semiconductor Electronics - Materials, Devices and Simple Circuits

Introduction 

The building blocks of the electronic circuits are devices in which a controlled flow of electrons can be achieved. These devices were mostly vacuum tubes (also called valves) like the vacuum diode which had two electrodes, viz., anode (often called plate) and cathode; triode which has three electrodes – cathode, plate and grid; tetrode and pentode (respectively with 4 and 5 electrodes) in the 1940s and 1950s. These devices were high on weight, consumed a high amount of power, operated mostly at high voltages (100 V) and had limited life and low reliability. In the 1930s, the development of modern solid-state semiconductors was started. It was found that some solid state semiconductors and their junctions offered the possibility of controlling the number and the direction of flow of charge carriers through them. Simple excitations like light, heat or small applied voltage can change the number of mobile charges in a semiconductor and flow of charge carriers in the semiconductor devices are within the solid itself.

In case of semiconductor devices, no external heating or large evacuated space is required. Semiconductor electronics are smaller in size, consume less power, operate at lower voltages and have long life and high reliability. Students have Semiconductor class 12 chapter in Physics, they can go through this article to understand in a simplified manner.

Semiconductor Material 

If a material has an electrical conductivity between that of a conductor and an insulator, it can be classified as a semiconductor material. Its resistance decreases as its temperature rises while the metals are the opposite. Its conducting properties can be changed by introducing impurities or doping into the crystal structure. When two differently-doped parts are present in the same crystal, a semiconductor junction is created. Silicon, germanium, gallium arsenide, and elements near the so-called "metalloid staircase" on the periodic table are some of the examples of semiconductor materials. 

Silicon is the most commonly used material in the devices and also it is the most critical element to fabricate the electronic circuits. Gallium arsenide is the most common semiconductor after Silicon and is used in laser diodes, solar cells, microwave-frequency integrated circuits and others. By addition of a small amount (of the order of 1 in 108) of pentavalent (antimony, phosphorus, or arsenic) or trivalent (boron, gallium, indium) atoms, conductivity of silicon is increased. This process is called doping and the resultant semiconductors are known as doped or extrinsic semiconductors. The conductivity of a semiconductor can equally be improved by increasing its temperature apart from doping.

Doping process highly increases the number of charge carriers within the crystal. If a doped semiconductor material contains more free holes it is called "p-type", and when it contains more free electrons it is known as "n-type".

N-type (e.g. Doped with Antimony)

 In N-type semiconductors the characteristics are as follows:

1. The Donors are positively charged.

2. A large number of free electrons.

3. A small number of holes in relation to the number of free electrons.

4. Doping gives positively charged donors and negatively charged free electrons.

5. Supply of energy gives negatively charged free electrons and positively charged holes.

P-type (e.g. Doped with Boron) 

In these, types of materials have characteristics as follows:

1. The Acceptors are negatively charged.

2. There are a large number of holes.

3. A small number of free electrons in relation to the number of holes.

4. Doping gives negatively charged acceptors and-positively charged holes.

5. Supply of energy gives positively charged holes and negatively charged free electrons.

Intrinsic or Pure Semiconductors

Un-doped semiconductors are called intrinsic or pure semiconductors. There are no dopant species present. The number of charge carriers is determined by properties of the material itself and not by the amount of impurities. Number of excited electrons and number of holes are in equal amounts i.e. n=p. There can be electrical conductivity in intrinsic semiconductors and it can be due to crystallographic defects or electron excitation. A silicon crystal is not like an insulator. At a temperature above zero, there is a chance that an electron in the lattice will be removed from its position, leaving behind an electron deficiency called a "hole". At that stage, when a voltage is applied, then both the electron and the hole can contribute to a small current flow.

Types of Semiconductor Devices

• These devices are differentiated into two-terminal or three- terminal devices and sometimes for terminal devices. The examples of two-terminal devices include Diode, Zener diode, Laser diode, Schottky diode, Light-emitting diode (LED), Photocell, Phototransistor, Solar cell, etc.

• Some of the examples of three terminal semiconductor devices include bipolar transistor, IGBT, Field-effect transistor, Silicon-controlled rectifier, TRIAC, Thyristor, etc.

Diode

A diode is a semiconductor device that comprises a single p-n junction. P-n junctions are usually formed by joining up of p-type and n-type semiconductor materials. This formation is due to the reason that n-type region has the higher number of electron concentrations whereas the p-type region has a higher number of hole concentration, hence, the electrons get diffused from the n-type region to the p-type region. Hence, this phenomenon is used in generating light.

Transistors

Transistors are of two types: bipolar junction transistor and field effect transistor. The bipolar junction transistor is achieved by the formation of two p-n junctions in two different configurations like n-p-n or p-n-p.

The field effect transistor works on the principle of conductivity and the conductivity can be altered by the presence of an electric field.