Transistor | It’s important characteristics | Direct and indirect Band gap


In this article we will discuss about the basic concepts related to transistor and its characteristics. 

What is a Transistor?

Definition of a Transistor:

“Transistor is a semiconductor device with three connection parts. This device is mainly used for amplification to switching electronic signals application”.

Characteristics of a Transistor:

  • A transistor represents the relation between current and voltages.
    • It is a two-port network in general
    • Each of the transistor modes has different input characteristics, output characteristics, and current transfer characteristics.
    • A transistor has three poles, and each of the poles is made of N-type & P-type substrate mainly.

A transistor consists of three terminals

  • Emitter
  • Base
  • Collector

Transistor has divided into two key categories

  • Bipolar Junction Transistor (BJT)
  • Field Effect Transistor (FET)

There also exist three modes in a Transistor

  • Common Emitter or C-E Mode
  • Common Base or C-B Mode
  • Common Collector or C-C Mode

Diagram of PNP and NPN transistor

PNP and NPN transistor
PNP and NPN transistor

To know more about PNP and NPN transistors, first, we have to know about P-type and N-type semiconductors.

What is a P-type Semiconductor?

A P-type semiconductor (link) is a type of semiconductor when some impurity (mainly trivalent) is added to the intrinsic or pure semiconductor. In these types, the holes are majority and electronics are minority carriers. The trivalent impurities can be Boron (B), Gallium (Ga), etc.

What is N-type Semiconductor?

An N-type semiconductor is a type of semiconductor when some impurities (mainly pentavalent) are doped to an extrinsic semiconductor. In this, electrons are majority or primary carriers, and holes are minority or secondary carriers.

Some of the examples are Phosphorus (P), Arsenic (As) etc.

In N-type and P-type semiconductors, we observe different types of ‘energy bands’ which plays an important role in the function of a transistor; they are:-

Image Credit: Tem5psuN and p dopingCC BY-SA 4.0

What is Band Gap ?

Band Gap

“The Band Gap refers to the energy difference between the top of the valance band and the bottom of the conduction band in an insulator and semiconductor.”

This is an energy range for solid basically where no electron states can be existent.

Band Gap Diagram

Forbidden Gap

In a solid, the range of energies than an electron within solid may have an energy band, and a range of energy that it may not have is called the forbidden gap.

Forbidden Gap Diagram
Image Credit: S-keiBandGap-Comparison-withfermi-ECC BY-SA 2.5

Valance Band and Conduction Band

In solid states, valance band and conduction bands are the bands closest to the Fermi level (a thermodynamic quantity denoted by µ) and determine the solids’ electrical conductivity.

Valance and conduction Band

To build up a transistor, we need two types of semiconductors, which are:

1. Intrinsic semiconductor

Intrinsic semiconductor
  • – Materials are in pure form
  • – Low electrical conductivity
  • – No. of free electrons in conduction band = No. of the holes in the valance band
  • – Electrical conductivity be influenced by on the temperature.

2. Extrinsic semiconductor

Extrinsic semiconductor

Extrinsic semiconductors are divided into further two types

  • n-type
  • p-type
  • – Impure material doped with p-type and n-type dopants
  • – Numbers of holes and electrons are not equal
  • – High electrical conductivity
  • – Impurities like Sb, P, ln, Bi are doped with Silicon and Germanium atoms.

Direct and indirect bandgap

In semiconductor electronics, a semiconductor’s bandgap can be classified in basic forms as follow:

  • Direct bandgap
  • Indirect bandgap.
Direct Bandgap

Indirect bandgap

Dependent on the band structures, substances have a direct bandgap or indirect bandgap.

  • The direct band-gap occurs when the momentum of the low-energy level from conductive region and high-energy level from valence region are similar.
  • The in-direct band-gap occurs when the momentum of the low-energy level from conductive region and high-energy level from valence region are not similar.
  • When an electron has sufficient energy, they can reach to the conductive band. In this process, photons are being emitted.  
  • For an indirect bandgap material, both photon and phonon has be included in a transition from upper valence band top to the lower conduction band.

The max-energy state in the valence band and the min-energy state in the conduction band are distinguished by the Brillouin zones k-vector or a particular crystal momentum. In the event the k-vectors are distinct, the substance has an “indirect gap”. The bandgap is known as direct if the crystal movement of holes and electrons is the equal in the conduction and valence bands; an e could emit a photon. A photon can’t be emitted within an “indirect” gap since the electron has to pass through an intermediate one and transfer momentum into the crystal lattice.

What is semimetal materials?

In certain substances with a direct gap, the value of the difference is negative. Such substances are called semimetals.

Moss–Burstein Effect

The Moss-Burstein effect or Burstein-Moss shift is the prodigy where the bandgap of a semiconductor may increase.

  • This is witnessed for a degenerate electron distribution or in some variant of semiconductors.  
  • As per Moss-Burstein shift the Band Gap is
Moss–Burstein Effect

Apparent Band Gap = Actual Band Gap + Moss-Burstein shift

In ostensibly doped semiconductor, the Fermi level is to be found between the valence and conduction bands.

For example, in an n-type semiconductor, as the doping concentration increases, electrons populate in the conduction regions that compels the Fermi level to higher energy label.

The Fermi level is located in the conduction band for degenerate amount of doping. Pauli’s exclusion principle prohibits excitation for these pre occupied states. Thus an increase s observed apparently in the bandgap.

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About Soumali Bhattacharya

I am currently invested in the field of Electronics and communication.
My articles are focused towards the major areas of core electronics in a very simple yet informative approach.
I am a vivid learner and try to keep myself updated with all latest technologies in the field of Electronics domains.

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