introduction

Now a days technology is very vast and modern but in 20th century first integrated circuit (IC) was invented by Jack Kilby while working at Texas Instruments in 1958. Intel is one of the most popular brand in manufacturing IC’s it’s latest IC is Intel Core i7 which contain 731 million transistors which are packed  in 1.67 square inch chip. Most of the devices manufactured in 20th century now there principles are used in vast fields.

semiconductor materials

semiconductors are special class of elements having a conductivity between that of a good conductor and that of an insulator.

There are two classes of semiconductors 1)single-crystal 2)compound

Single-crystal semiconductors are Germanium (Ge) and silicon (Si) because they have repetitive crystal structure.

while compound semiconductors are gallium arsenide (GaAs) gallium Nitride (GaN) they are formed by two or more semiconductors with different atomic numbers.

Most commonly used semiconductors in electronic devices are Ge , Si , GaAs.

covalent bonding and intrinsic materials

Atom is the smallest particle of any material. There are three fundamental components of atom

1)electrons (-ve charge)

2)protons (+ve charge)

3)neutrons (neutral)

In lattice structure protons and neutrons form nucleus and electrons revolve around nucleus in fix orbit.

silicon have three orbits and total 14 electrons and 4 valence electrons while germanium have 32 electrons and 4 valence electrons. Atoms have 4 valence electrons called tetravalent.

gallium have 31 electrons and three valence electrons. Atoms have 3 valence electrons are called trivalent. Arsenic have 33 electrons and 5 valence electrons. Atoms have 5 valence electrons called pentavalent.

above figure showing covalent bonding between gallium and arsenide.

mutual sharing of valence electrons is called covalent bonding

Every element wants to make it’s outermost shell stable so gallium and arsenide form mutual bond to complete there outermost shell.

energy band theory

Electrons in the outermost shell of an atom are called valence electrons and orbit occupying these electrons called valence band. It might be completely filled or partially filled but can’t be empty. The band above the valence band is conduction band and they are completely filled electrons occupying this band are known as free electrons. These bands do not play any role in conduction process.

Insulators

are those materials which did not conduct electric current because there valence shells are completely filled

1)an empty conduction band

2)a full valence band

3)a large energy gap

 

conductors

Those materials which have plenty of free electrons for electrical conduction. In which valence band and conduction band overlaps with each other.

Semiconductors

Semiconductors are those which at room temperature have

1)partially filled conduction band

2)partially filled valence band

3)very thin forbidden gap

a very common term used 0 K which means there are no electrons in conduction band and valence band is completely filled. However with increase in temperature some electrons posses sufficient energy to jump from low energy band(valence) to high energy band(conduction). This transfers some free electrons in the conduction band and creates some vacancies of electrons in valence band. The vacancy of electrons is called a hole. It behaves like a positive charge thus Si and Ge at room temperature are semiconductor.

relative mobility

the ability of free carriers to move throughout the material

Intrinsic and extrinsic semiconductor

A semiconductor in its extremely pure form is called intrinsic semiconductor. Purity in semiconductors matters for charge flow. The process of adding some impurity in pure semiconductor lattice is called doping which contain small number of atoms of some other suitable elements are added as impurity in the ratio of 1 to 10^6. The doped semiconductor materials are called extrinsic semiconductor

Silicon and Germanium are intrinsic semiconductor. In solid crystalline form the atoms of these elements arrange themselves in such a pattern that each atom have a equidistant neighbors.

When a silicon crystal doped with a pentavalent element e.g arsenic,antimony or phosphorous etc four valence electrons of the impurity atom from covalent bond with the four neighboring Si atoms, while the fifth valence electron provides a free electron in the crystal. Such a doped semiconductor is called n-type semiconductor. The phosphorous atom is called a donor atom because it readily donates a free electron.

On the other hand when a silicon crystal doped with a trivalent element e.g aluminium,boron,gallium three valence electrons form covalent bond with 3 neighboring Si atoms while the one missing electron in the covalent bond with the fourth neighboring Si atom is called a hole which in fact is vacancy where an electron is accommodated such a semiconductor is called p-type semiconductor. In case of gallium and silicon gallium is called acceptor atom because it is easy for gallium ion to accept a valence electron form a nearby silicon atom, thus creating a hole in valence bond.

majority and minority carriers

In an n-type material the electron is called the majority carrier and the hole the minority carrier. In a p-type material the hole is the majority carrier and the electron is the minority carrier.

Semiconductor Diode

Now both n-type and p-type materials are available we can construct our first electronic device The Semiconductor Diode it is created by just joining n-type and p-type material together just the joining of one material with a majority carrier of electrons to one with a majority carrier of holes.

Diodes are represented by symbol

no applied bias (v = 0v)

After the formation of junction(p-n) the free electrons in the n-region, because of there random motion diffuse into p-region. As a result of this diffusion a region is formed around the junction in which charge carriers are not present this region is called depletion region.

the small circles show the holes whereas the circles with + and – sign show the +ve and -ve ions which constitute the depletion region. Due to the charge on these ions a potential difference develops across the junction its value is 0.7V in case of silicon and 0.3V in case of germanium this potential difference creates potential barrier stops further diffusion of electrons into p-region. In the absence of an applied bias across a semiconductor diode, the net flow of charge in one direction is zero.

Reverse bias (v < 0v)

When potential applied across the p-n junction while +ve terminal is connected with n-type and -ve terminal is connected with p-type

The positive ions in the depletion region of n-type will increase due large number of free electrons drawn to the +ve potential of applied voltage same in p-type material. This cause the extension of depletion region and current stops flowing

forward bias (v > 0v)

In this case p-type is connected with +ve terminal and n-type is connected with -ve terminal electrons in n-type material and holes in p-type material to recombine with the ions near the boundary and reduce the width of depletion region.

Next page:

-Applications of Diode

https://www.electricalengineering4u.com/electronic-devices-and-circuit/applications-of-diode/