Carrier Transport in Semiconductor

CARRIER TRANSPORT IN SEMICONDUCTOR

Carrier Transport

In semiconductor electrons and holes are called carriers, which moves from one position to another and this movement of charge carriers is called carrier transport.

In the absence of field [RANDOM MOTION]

In the absence of external field, the charge carrier moves in random direction due to its thermal energy. The path of the charge carrier changes due to scattering by the vibration of the lattice points and by the coulomb force of ionised donar and acceptor atoms in the semiconductor.

In the presence of field [DRIFT]

Now when the charge carriers are subjected to external fields, then they move with a velocity called drift velocity, and reaches a steady state.

At steady state:

Thus during steady state a steady current flows due to the carrier transport in semiconductors.

Let us discuss the carrier transport in 'n' and 'p' type semiconductor.

(i) Carrier transport in n-type semiconductor

We know in 'n' type semiconductor the electrons are majority charge carriers and holes are minority charge carriers. Apart from this, there will be equal number of immobile positive ions (grey shaded) in n-type semiconductor.

Let us consider an n-type semiconductor placed between a pair of electrodes, for which a voltage is applied as shown in Fig. 2.23.

Due to field applied the electrons in the semiconductor move towards the positive terminal and they disappear. At the same time equal number of electrons are generated at the negative terminal. These electrons are attracted by the immobile positive ions present in the semiconductor and therefore a continuous flow of electrons from one terminal to the other terminal takes place through the semiconductor.

The net current flow in the semiconductor depends on the biasing voltage.

Note: Here, as the current contribution due to hole is very less, it is neglected.

(ii) Carrier transport in 'p' type semiconductor

We know, in 'p' type semiconductor the holes are majority charge carriers and electrons are minority charge carriers. Apart from this, there will be equal number of immobile negative ions (grey shaded) in 'p'-type semiconductor.

Let us consider a 'p' type semiconductor placed between a pair of electrodes, for which a voltage is applied as shown in Fig. 2.24.

Here, due to field applied, the holes move towards the negative terminal and they combine with the electrons coming out from the negative terminal and disappears. At the same time equal number of holes are generated near the positive terminal. These holes attract the immobile negative ions present in the semiconductor towards the positive terminal and thus causes current to flow.

During this process an electron is lost by the acceptor atom and therefore it try to get back an electron from the adjacent atom to fill that hole in the semiconductor. This process continues and hole current (As discussed in earlier chapter) occurs inside the semiconductor.

Note: Here, the current inside the semiconductor is due to holes and outside the semiconductor is due to electrons.