Types of semiconductors - class-XII

p and n type materials are NOT positively and negatively charged. An n-type material by itself has mainly negative charge carriers (electrons) which are able to move freely, but it is still neutral because the fixed donor atoms, having donated electrons, are positive

In N-type semi-conductor majority carriers are electrons. So, when the voltage is applied across the semi-conductor electrons started to drift from negative to positive terminal as result current starts to flow in opposite direction of electrons.

Fermi energy is determined as the energy point where the probability of occupancy by an electron is exactly $50\ %$ or $0.5$, i.e., $\dfrac{1}{2}$. For the intrinsic semiconductor, since electrons and holes are always created in pairs, $n = p = ni$. Hence, there are equal number of holes and electrons in valence band and conduction band respectively. Therefore, the Fermi energy level lies in the middle of the forbidden gap, i.e., energy band gap.

The acceptor valence band is close to the valence band of host crystal

The donor valence band lies little below the conduction band of the host crystal.

An intrinsic semiconductor at 0 K, has electrons only in the valence band. Forbidden gap - 3 eV, as a result, electrons do not have sufficient energy to excite to the conduction band. Since there are no electrons in the conduction band, so it behaves as an insulator at 0 K.

A pure semiconductor or semiconductor with no impurities is called intrinsic semiconductor. When a pure semiconductor is doped with an impurity, the process is called doping, due to increased conductivity of an extrinsic semiconductor. Such a semiconductor is called extrinsic semiconductor.

As we know Cu is a conductor,so increase in temperature will lead to increase in resistance. 

Doping is the process of adding impurities to intrinsic semiconductors to alter their properties.
Doping of intrinsic semiconductor is done to increase the concentration of majority charge carrier so that it can be use as p-type or n-type semiconductor in diode.

In an extrinsic semiconductor, doped with an impurity. For example, an n-type semiconductor which is formed by doping n-type impurity so that number of electrons are more than the number of holes. So, the number density of holes is not equal to the number density of electrons. Because in an extrinsic semiconductor, one of the two carriers are in abundance and the other one is in majority.

In an intinsic semiconductor number density of electrons and holes are always equal. As the number of electrons generated in the conduction band is equal to the number of holes generated in the valance band.

A pure semiconductor is called an intrinsic semiconductor. It has thermally generated current carriers. Germanium and silicon are frequently used as semiconductors. The intrinsic semiconductor has no impurity added to them.

The conductivity of an extrinsic semiconductor is more than that of an intrinsic semiconductor. This is because, in an intrinsic (pure) semiconductor, the carriers are only thermally generated carriers. But in extrinsic semiconductors, which is doped with a p-type or n-type impurity to form p-type and n-type semiconductor so as to increase the number of holes or number of electrons in a semiconductor respectively. More the charge carriers, more is the conductivity.

Donor impurities are the ones that donate electrons to the semiconductor atom. For each donor atom added, there is one electron donated to the semiconductor atom. As a result, the electrons in such type of semiconductors are majority charge carriers. Therefore, this extrinsic semiconductor formed by adding donor impurities are called n-type semiconductors.

A transistor is a device which consists of a thin layer of one type of extrinsic semiconductor sandwiched between two thick layers of other types of extrinsic semiconductor. There are two types of transistors:

1. pnp transistor and,
2. npn transistor
   So, a transistor is made of extrinsic semiconductor.

A semiconductor diode consists of p-n junction having metallic contacts at both the ends. A single piece of a semiconductor material (either Si or Ge) with one portion doped with a pentavalent impurity and other portion doped with trivalent impurity behaves as a p-n junction. Means, extrinsic semiconductor or semiconductor doped with impurities of both p and n are used in p-n junction diode.

p type semiconductor has more number of holes than electrons.
P-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities like boron, aluminium and other group IIIA elements