Question

The diffusion current in semiconductors is proportional to
(A) Applied electric field
(B) Square of the applied electric field
(C) Concentration gradient of charge carrier
(D) Potential gradient of applied field

Answer 1

Hint: Diffusion current is a type of current that is generated inside the semiconductors due to diffusion of charge carriers within it. This diffusion current is entirely dependent upon the distribution of different types of charge carriers inside semiconductors.

Complete Solution
Inside any semiconductor due to thermal effect or due to doping there exist many free electrons or lack of electrons (known as holes). Both these electrons and holes are able to move within the semiconductors. Electrons can move themselves due to any potential difference. In case of holes some other electron takes its place by making another hole elsewhere and thus holes move. In general, this distribution of charge carriers doesn't remain uniform throughout the semiconductor. Specifically when two differently doped semiconductors (n-type or p-type) are merged together then the distribution of charge carrier is completely opposite around the merging point and a steep gradient of potential difference is created. This gradient produces a current and that is known as diffusion current. Unlike drift current which is produced by the effect of an external electric field applied across the semiconductor, this diffusion current is produced completely independently of the external field.

Correct answer: The diffusion current in semiconductors is proportional to (c) concentration gradient of charge carrier.

Note: Notice the word ‘diffusion’, it means that something is widely spread. Here, at some places charge carriers remain congested at some places and at other places that charge carriers remain diffused i.e. their concentration remains less there. So a concentration gradient is generated and exactly this concentration gradient produces the diffusion current.