Semiconductors are those materials, which behaves like a conductor or insulator depending upon the condition and scenarios.
We will have a look into these scenarios further in this article. There are many types of semiconductors such as intrinsic, extrinsic, p-type or n-type and even p-n types semiconductor.
Hearing all these names leads to so much confusion that it gets difficult to understand semiconductor. So in this article, we will be discussing about the types of semiconductors and how they work.
Semiconductor is of two types intrinsic and extrinsic. Extrinsic semiconductors are further categorized into p-type and n-type semiconductors on the basis of impurity added to make a semiconductor.
So this was just the short definition of all the things, so let’s uncover each of these topics in detail for a better understanding of types of semiconductors.
What is semiconductor ?
A semiconductor is a material that behaves like a conductor or insulator depending upon the situation. The conductivity of semiconductor material lies between a conductor and an insulator as the name suggests.
It has less conductivity than of conductors such as metal (Copper, aluminium, etc) and more conductivity than of insulators such as glass.
Now, let’s talk about different types of semiconductors which I had mentioned before.
Types of semiconductor:
So semiconductors are of two types Intrinsic Semiconductors and Extrinsic Semiconductors depending upon how they function and work.
We will discuss all these semiconductors here and their key feature of how they work. When will one show conductivity and when will they behave like an insulator.
Germanium and Silicon are the common elements we use for intrinsic semiconductor. You can also call them as Ge or Si by there symbol.
In an intrinsic semiconductor, an atom of element forms 4 four covalent bonds with its neighbour atoms. Here is an image of an ideal element of Ge or Si where no covalent bond is broken between the atoms:
|An ideal Ge or Si atom. Image source: NCERT|
In this ideal state, this will behave like an insulator but as temperature rises, some bonding electrons get thermal energy due to which they may break-away and become a free electron.
This leads to a hole or an electron vacancy where the electron was bonded before getting ionized. Here is the image which shows the vacant space and free electron after the rise of temperature.
|A Ge or Si element after an increase in temperature. Image source: NCERT|
In this condition, Ge or Si will act as a conductor because now a charge carrier or a free electron is available inside the element.
So this was an intrinsic semiconductor that behaves like an insulator when the temperature (T) is equal to 0K (T=0K).
When temperatures rise, some electron gets thermal energy and they act as a free electron leaving a vacant space and in this condition, the element will act as a conductor because it now has a charge career (T>0K).
There is one problem with intrinsic semiconductor, they don’t provide you with the total control of conductivity or insulation which makes it hard to use in the practical world and therefore we use extrinsic semiconductor.
So let’s understand about extrinsic semiconductors.
As I told earlier, the conductivity of intrinsic semiconductors depend on temperature and on room temperature, the conductivity of an intrinsic semiconductor is very low which makes it impossible to make any useful electronic devices using these conductors.
To overcome this problem, a small amount of impurity is added into an intrinsic semiconductor to increase there conductivity at room temperature.
Such semiconductors made from adding impurity into an intrinsic semiconductor to increase conductivity at room temperature are called extrinsic semiconductors.
This process of adding desirable impurity is called doping and the impurity atom which is to be doped is called dopant. You cannot use any element as impurity or dopant to increase the conductivity of intrinsic semiconductors.
The dopant needs to be such that it does not change the shape of original lattice of intrinsic semiconductor and the size of dopant and atom of semiconductor should nearly be the same.
Now to fulfil these two conditions, we can only use some specific elements as a dopant and these elements are categorized into two parts on the basis of their valency: 1. Pentavalent elements 2. Trivalent elements.
Some of the pentavalent elements are 1. Arsenic 2. Antimony 3. Phosphorus etc and some of the tetravalent elements are 1. Indium 2. Boron 3. Aluminium etc..
Now, let’s discuss how doping changes the number of charge careers and conductivity in a semiconductor and doping of these pentavalent and tetravalent elements give entirely different types of semiconductors (n-type and p-type).
So let’s discuss each of them.
Now, when we doped a pentavalent impurity into an intrinsic semiconductor, we get n-type semiconductors where “n” stands for a negative charge. Why? Because in n-type semiconductors, electrons are the majority charge careers and the reason behind increased conductivity. Don’t worry I will explain it to you.
So when a Ge or Si is doped with a pentavalent impurity such as arsenic or antimony, they replace some of the Ge or Si atoms.
Now, pentavalent atoms have 5 valence electron in their outermost shell, 4 of them creates a covalent bond with their neighbour Ge or Si atoms and are left with one free electron which helps in conductivity of semiconductor even at room temperature.
Here is an image which shows a pentavalent impurity being doped in Gi or Se element, leaving behind a free electron in the material for conductivity and because pentavalent atom has formed 4 bonds and has one free electron, therefore, impurity is acting as donor core because it has donated an electron.
|Intrinsic semiconductor after being doped with a pentavalent impurity|
Now, after doping even at room temperature, it has one extra electron to carry charge and therefore conductivity is increased in the semiconductor.
Now, when a tetravalent impurity is being doped into an intrinsic semiconductor, we get p-type semiconductors where “p” stands for positive charge, because in p-types semiconductors, a positive charge is the majority charge careers and they increase the conductivity of the semiconductor.
Fun Fact: Electric current cannot just be created by electron but it can also be created by proton. Just because proton is heavier compared to electron it required high energy to move and produce electric current.
Now, tetravalent atoms have only three electrons in their outermost shell and all three of them create a covalent bond with their neighbour Ge or Si atoms, but as Ge or Si requires four covalent bonds, one vacant space is left for an electron and a hole is created.
This hole now acts as a charge career in p-type semiconductors.
Here is the image which shows a tetravalent impurity being doped in Gi or Se element, leaving behind a free space or hole in the material for conductivity and because tetravalent atom has formed 3 covalent bonds and is left with one free space, impurity is acting as acceptor core because it needs an electron to fulfil all the bonds.
|Intrinsic semiconductor after being doped with a tetravalent impurity|
Difference between intrinsic and extrinsic:
|Difference between intrinsic and extrinsic semiconductor|
|Pure semiconductor||Impure semiconductor|
|Poor conductivity||Better conductivity then intrinsic|
|Conduction depends on the temperature||Conduction depends on impurity level and temperature|
|An equal amount of electrons and holes are present||The majority of holes or electrons depends on types of extrinsic semiconductor|
|It’s not further classified||It’s further classified as an n-type and p-type semiconductor|
|Example: Ge and Si||Examples: GeAs and GaP|