Question

(a) What is an extrinsic semiconductor?

(b) What are n-type and p-type semiconductor

Answer 1

(a) Extrinsic semiconductors

A doped semiconductor or a semiconductor with impurity atoms is called an extrinsic semiconductor.

n-Type semiconductor.

The conductivity of intrinsic semiconductor is zero at absolute zero and very small at ordinary temperatures. Germanium has 4.52 x 10²² atoms per cm³. An addition of only one impurity atom per million Ge atom, is sufficient to give desired conductivity to it. The process of deliberately adding suitable impurity atom to the intrinsic semiconductor is called Doping (the impure semiconductor is called a doped semiconductor). If we dope intrinsic Ge. with a controlled amount of pentavalent atoms, say antimony Sb or phosphorus P, which has five valence electrons, the atoms of the impurity element will substitute the Germanium atoms Fig. Four of the five valence electrons is comparatively free to move. The pentavalent atoms are called the donor atoms because they donate electrons to the host crystal extrinsic semi-conductor are called n-type. On giving up their fifth electron, the donor atoms become positively charged. However, the material remains electrically neutral as a whole.

The extra electron of the donor atom orbits around the donor nucleus.

Explanation on the basis of band. In the band language, we would say that this electron, free to move about, has the lowest possible energy in the empty conduction band. Thus, the energy level diagram of a doped n-type semi-conductor is as shown in the Fig. For phosphorus or arsenic in silicon, the lowest donor electron energy level lies below the bottom of the conduction band. This energy is comparable to room temperature. Thermal energy is much smaller than the energy gap E_g = 1.1 eV

Since, it requires less energy to free a donor electron (or ionize the donor atom) than to promote an electron from the valence band to the conduction band, at any non-zero temperature, a sizeable fraction of the donor electrons is in the conduction band. In n-type electron density is greater than hole density n_e > n_h.

p-Type semiconductors

In p-type Ge. If we dope intrinsic Ge, with a controlled amount of trivalent atoms, say indium (In or boron B or aluminium Al) Group III, which has three valence electrons, impurity atom will occupy places of some Ge atoms and there will be one incomplete covalent bond with a neighbouring Ge atom, due to the deficiency of an electron. This is completed by taking an electron from one of the Ge-Ge bonds, thus completing the In-Ge bond. This makes In ionized (negatively charged), and creates a ‘hole’ or an electron deficiency in Ge.

The trivalent atoms are called acceptor atoms and this extrinsic semiconductor is known as p-type.

Explanation on the basis of band theory

In a p-type semiconductor at room temperature, the holes are free to move about in the valence band because the acceptor atoms are nearly all ionized due to thermal energy.

Therefore, the hole density in valence band is equal to the acceptor density Na. In p-type semiconductor the hole density is greater than the electron density,

i.e. Na = n_h >> n_e.