Semiconductors

INTRODUCTION

On the basis of their electrical conductivity we can classify materials into three categories-conductors, insulators and semiconductors. A conductor conducts electricity and insulator does not conduct electricity. A semiconductor is defined as a material whose conductivity is in between a conductor and an insulator. It is in the range of 10³ to 10^-8 Siemens per centimeter. Silicon, germaniums are the most widely used semiconducting materials.

Materials can also be classified on the basis of their band gap i.e. the energy level difference between the valence band (which contains the outermost orbit electrons) and the conduction band (which consists of the conducting electrons). In case of conductors the two bands overlap each other and thus there is free flow of electrons from valence band into conduction band. In case of semiconductors and insulators there is a band gap, the gap is more in case of insulators as compared to semiconductors. So the valence electron requires less energy to jump from valence band into conduction band as compared to the case of insulators. The electron excitation can be done by applying electric field or thermal excitation.

 

Band structure in semiconductors

Even at conditions such as room temperature the electrons can still overcome the band gap giving conductivity and as temperature rises so too is the number of electrons increases that cross over the conduction band thereby increasing the electrical conductivity. Therefore giving the semiconductor a negative temperature coefficient in terms of resistance.

 

TYPES OF SEMICONDUCTORS:

INTRINSIC SEMICONDUCTOR

A semiconductor material which is highly pure and has no dopant material added to it is called an intrinsic semiconductor. The electrons in valence band acquire sufficient energy even at room temperature to overcome the band gap and form conducting electrons and thus leaving a corresponding vacancy in the valence band. This vacancy can be thought of as a positively charged particle called a hole. As the electrons move from lower levels of valence band to the conduction band the hole move downwards in the valence band. Thus the electrons and holes move in opposite direction and both constitute conductivity.

s=n_e*q*m_e + n_h*q*m_h

Where s is total conductivity

n_e   and n_h are the number of holes, and electrons respectively

q is the charge=1.6*1^-19 C

in an intrinsic semiconductor

n_e = n_h =n_i

so , s=n_i*q*(m_e + m_h)

EXTRINSIC SEMICONDUCTOR:

The energy available at room temperature is not sufficient to excite many electrons to the conduction band; hence the conduction capability is not enough for use in most electronic devices. Hence a small amount (1 maybe 2 atoms of impurity for 10⁶  intrinsic atoms) of doping or addition of impurity elements is done to increase the conductivity. This is done in two ways thereby two types of extrinsic semiconductors are possible:

N-TYPE SEMICONDUCTOR

In this configuration a small percentage of impurities having 5 valence electrons (pentavalent) like phosphorus, arsenic or antimony will then be added with the purity of the semiconductor (germanium or silicon crystal) to create a new N-Type semiconductor.

Silicon atom has four valence electrons and the antimony contains valence electrons of five. There will exist a covalent bonding between the antimony atoms and silicon atoms because of the four surrounding valence electrons and one electron has been left loosely bonded to antimony atom, which upon excitation moves from the valence band on to the conduction band. There is no corresponding hole generation. Thus the number of electrons exceeds the number of holes. Therefore the conductivity in mainly due to electrons and hence they are called majority carriers and holes are called minority carriers.

These donor impurities are called as such because it literally “donates” an extra electron to induce conduction. As it donates one electron for conduction, it becomes a positively charged ion, but it does not take part in any form during conduction because it has been firmly fixated into structure.

s=n_e*q*m_e

Where s is total conductivity

n_e   is the electrons density (number)

q is the charge=1.6*1^-19 C

 

 

P type Semiconductor:

When one wants to create a p-type semiconductor one needs to add a significantly less amount of impurities having 3 valence electrons (trivalent) such as boron or aluminum into the pure semiconductor. The Silicon (Si) atom contains valence electrons of four while boron has only three valence electrons. The three valence electrons which is found in boron form a covalent bond with that of the silicon atom that has four valence electrons resulting in a hole or vacancy from one of the four covalent bonds of silicon. This is also known as acceptor impurity. Hence the number of holes is more than half compared to the amount of electrons in a P type semiconductor. The majority carriers are the “holes” and the minority carriers are the “electrons”.

s= n_h*q*m_h

Where s is total conductivity

n_h   is the number of holes

q is the charge=1.6*1^-19 C

 

 

APPLICATION OF SEMICONDUCTORS:

Semiconductors are immensely used in electronic devices. They are used in

1. diodes
2. transistors
3. rectifiers
4. mosfets
5. jfets

These semiconducting have widespread applications. Semiconductor devices are contained in television, radios, amplifiers, etc. Transistors are used as building blocks of logic gates which are fundamental in design of logic gates which are used to make microcontroller and microprocessor used in computers and various other electronic devices.