SEMICONDUCTOR

                        Semiconductor

Types of semiconductor

(1). Intrinsic semiconductor

        The pure semiconductor is called intrinsic semiconductor. 

For ex. Germanium (Ge) and silicon (So) etc. 

In it electrons and holes are the charge carriers which is produces by the thermal excitation. 

In it the no. Of density of electrons (ne) is equal to the no. Of density of holes(nh). 

It shows very low conductivity due to having equal no. Of electrons and holes. 

(2). Extrinsic semiconductor

        When we mix a certain amount of impurities atom. In the pure semiconductor it is called extrinsic semiconductor. 

Doping

The process of mixing the impurities atom in the pure semiconductor is called doping and the impurity atom called dopent. 

Need of doping

To increase the conductivity of pure semiconductor. 

Types of impurities atom(dopent) 

(1). Pentavalent impurities

They have valency of 5 like phosphorus, antimony, bismith etc. 

(2). Trivalent impurities 

They have valency of 3 like boron and aluminum. 

  Types of extrinsic semiconductor

There are 2 types of extrinsic semiconductor 

According to the impurities atom

(1). n - type semiconductor

        When the pentavalent impuritie atom mixed with pure semiconductor it produces n - type semiconductor. 

In it impuritie atom donates extra electrons to the pure semiconductor for the conduction. 

In it electrons are majority charge carriers and holes are minority charge carriers. 

ne is greater than nh. 

(2). P - type semiconductor

        When the trivalent impuritie atom mixed with pure semiconductor it produces p - type semiconductor. 

It called an trivalent impuritie atom called acceptor atom because it creates a whole which can accept the electron from the neighbouring bond. 

In it holes are majority charge carriers and electrons are minority charge carriers. 

nh is greater than ne. 

Energy bands in solids

An isolated atom have fixed orbit having fixed energies but when  large no. Of atoms combined together then the orbit overlaps on each other and forms energy bands. 

The highest energy band Which is filled with balance electron is called valence band. 

The lowest unfilled aloud energy band next  to valence band is called conduction band. 

The gap  between valence band and conduction band is known as energy band gap Or energy gap. 

Classification of conductors, insulators and semiconductors on the basis of band theory

If Ev, Ec and Eg be the energies of conduction band, valence band and energy band gap. Then it is found that

Eg = Ev - Ec

Conductors

In the conductors valence bands and conduction bands are overlaps on each other and there is no energy gap between valence band and conduction band. 

On applying the energy of 0.1 - 0.7eV the electrons easily  jumps from valence to conduction band. 

Insulators

In it the valence band is fully filled while the conduction band is empty and there is a energy gap of eg. = 6eV( is greater than 3eV) 

Semiconductor

In it there is the energy gap of less than (3eV) present between valence band and conduction band. 

For ex.    Eg. For si = 1.7eV

                Eg. For Ge = 0.8eV

Mobility of a charged carrier

The drift velocity acquire by a charged carrier  in a unit electric fields 

Or

It is define by the ratio of drift velocity to the applied E.F denoted by "u"

            u = vd/E

The mobility of an electron in the conduction band in a conductor is greater than the mobility of a hole in the valence band.