Drift and Diffusion current

Drift and Diffusion current

The flow of current through a semiconductor material is normally referred to as one of the two types.

Drift current
• If an electron is subjected to an electric field in free space it will accelerate in a straight line form the –ve terminal to the + ve terminal of the applied voltage.

• However in the case of conductor or semiconductor at room temperature, a free electrons under the influence of electric field will move towards the +ve terminal of the applied voltage but will continuously collide with atoms all the ways as shown in figure 1.9.

 

Each time, when the electron strikes an atom, it rebounds in a random direction but the presence of electric field doesnot stop the collisions and random motion. As a result the electrons drift in a direction of the applied electric field.

•  The current produced in this way is called as Drift current and it is the usual kind of current flow that occurs in a conductor.

Diffusion current

• The directional movement of charge carriers due to their concentration gradient produces a component of current known as Diffusion current.

• The mechanism of transport of charges in a semiconductor when no electric field is applied called diffusion. It is encountered only in semiconductors and is normally absent in conductors.

With no applied voltage if the number of charge carriers (either holes or electrons) in one region of a semiconductor is less compared to the rest of the region then there exist a concentration gradient.

• Since the charge carriers are either all electrons or all holes they sine polarity of charge and thus there is a force of repulsion between them.

• As a result, the carriers tend to move gradually or diffuse from the region of higher concentration to the region of lower concentration. This process is called diffusion and electric current produced due to this process is called diffusion current.

• This process continues until all the carriers are evenly distributed through the material. Hence when there is no applied voltage, the net diffusion current will be zero.

Fermi-level

Fermi level indicates the level of energy in the forbidden gap.

1. Fermi-level for an Intrinsic semiconductor

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• We know that the Intrinsic semiconductor acts as an insulator at absolute zero temperature because there are free electrons and holes available but as the temperature increases electron hole pairs are generated and hence number of electrons will be equal to number of holes.
• Therefore, the possibility of obtaining an electron in the conduction band will be equal to the probability of obtaining a hole in the valence band.

• If Ec is the lowest energy level of Conduction band and Ev is the highest energy level of the valence band then the fermi level Ef is exactly at the center of these two levels as shown above.

2. Fermi-level in a semiconductors having impurities (Extrinsic)
a) Fermi-level for n-type Semiconductor
• Let a donar impurity be added to an Intrinsic semiconductor then the donar energy level (ED)  shown by the dotted lines is very close to conduction band energy level (Ec).
• Therefore the unbonded valence electrons of the impurity atoms can very easily jump into the conduction band and become free electros thus, at room temperature almost all the extra electrons of pentavalent impurity will jump to the conduction band.

• The donar energy level (ED) is just below conduction band level (Ec) as shown in figure1.10(a). Due to a large number of free electrons, the probability of electrons occupying the energy level towards the conduction band will be more hence, fermi level shifts towards the conduction band.

b) Fermi-level for P-type semiconductor

• Let an acceptor impurity be added to an Intrinsic semiconductor then the acceptor energy level (Ea) shown by dotted lines is very close to the valence band shown by dotted lines is very close to the valence band energy level (Ev).

• Therefore the valence band electrons of the impurity atom can very easily jump into the valence band thereby creating holes in the valence band.

 

• The acceptor energy level (EA) is just above the valence band level as shown in figure 1.11 (b).
•  Due to large number of holes the probability of holes occupying the energy level towards the valence band will be more and hence, the fermi level gets shifted towards the valence band.