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.
FAQs on Semiconductor Electronics - Materials, Devices and Simple Circuits
1. What is a semiconductor, and how is it different from conductors and insulators?
A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Unlike conductors, semiconductors have fewer free electrons at room temperature, and their conductivity increases as temperature rises, which is opposite to metals. Insulators, on the other hand, do not conduct electricity under normal conditions. Examples include silicon and germanium.
2. Describe the process of doping in semiconductors and its effect on conductivity.
Doping is the intentional addition of impurities to a pure semiconductor to modify its electrical properties. Doping with pentavalent atoms (like phosphorus) creates n-type semiconductors with more free electrons, while trivalent atoms (like boron) result in p-type semiconductors with more holes. This increases the number of charge carriers, hence significantly increasing conductivity.
3. How does the temperature affect the conductivity of intrinsic semiconductors?
In intrinsic semiconductors, conductivity increases with temperature. As temperature rises, more electrons gain enough energy to jump from the valence band to the conduction band, generating more free charge carriers (electrons and holes), which results in higher electrical conductivity.
4. Why is silicon more widely used in semiconductor devices than germanium?
Silicon is the preferred material for most semiconductor devices because:
- It has a higher melting point, making devices stable at higher temperatures.
- Its oxide forms a better insulating layer, ideal for integrated circuits.
- It is more abundant and less expensive than germanium.
- Devices using silicon have lower leakage current at room temperature compared to germanium devices.
5. What is a p-n junction and how does it form the basis of semiconductor devices?
A p-n junction is formed by joining p-type and n-type semiconductor materials. At the junction, electrons from the n-side diffuse into the p-side and recombine with holes, creating a depletion region. This region acts as a barrier and is the fundamental operating principle in devices like diodes, LEDs, and transistors.
6. List the main differences between intrinsic and extrinsic semiconductors.
Intrinsic semiconductors are pure materials with equal amounts of electrons and holes as charge carriers, with conductivity governed by temperature. Extrinsic semiconductors are doped to introduce additional electrons (n-type) or holes (p-type), resulting in much higher conductivity controlled by impurity concentration.
7. Explain how a diode works and mention two of its applications.
A diode allows current to flow mainly in one direction due to its p-n junction structure. When forward biased, it conducts; when reverse biased, it blocks current. Common applications include rectification (converting AC to DC) and use in LEDs for light emission.
8. What roles do electrons and holes play in the conduction process of semiconductors?
Electrical conduction in semiconductors is due to the movement of electrons (in the conduction band) and holes (in the valence band). When an electric field is applied, electrons move towards the positive terminal, while holes (representing missing electrons) appear to move towards the negative terminal, both contributing to the total current.
9. How do majority and minority charge carriers differ in n-type and p-type semiconductors?
In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. In p-type semiconductors, holes are the majority and electrons are the minority. The type of dopant determines which charge carrier is in excess.
10. Discuss the significance of the depletion region in a p-n junction.
The depletion region is crucial because it prevents the direct flow of charge carriers across the p-n junction in the absence of an external voltage. It acts as a potential barrier which must be overcome for current to flow, thus allowing the device to function as a diode or other semiconductor component.
11. Why do semiconductors exhibit increased conductivity on doping instead of becoming metallic?
Doping adds extra electrons or holes, increasing carrier concentration but not making the material a metallic conductor. The doped levels are within the band gap, not merging conduction and valence bands, so excited carriers can still be controlled—this unique property differentiates semiconductors from metals.
12. What are the common applications of semiconductors in everyday electronics?
Semiconductors are used in almost all modern electronics, including diodes, transistors, LEDs, solar cells, photocells, and integrated circuits. They are essential for signal processing, amplification, switching, data storage, and energy conversion devices.
13. How can you distinguish between a two-terminal and a three-terminal semiconductor device with examples?
Two-terminal devices include diodes, LEDs, and Zener diodes, where current flows through two points. Three-terminal devices like bipolar junction transistors and field-effect transistors have an input terminal that controls current between the other two terminals, enabling amplification and switching.
14. What would happen to the conductivity of a pure semiconductor if it is cooled to absolute zero?
At absolute zero temperature, a pure semiconductor's conductivity drops to zero, as no electrons have enough thermal energy to move from the valence to the conduction band. Thus, it behaves as a perfect insulator at 0 K.
15. Explain how light can be used to increase the conductivity in photoconductive semiconductor devices.
In photoconductive devices, light energy excites electrons from the valence band to the conduction band, increasing the number of free charge carriers. As a result, the material's conductivity rises with the intensity of incident light—this is the principle behind devices like photodiodes and photocells.
