Due to the Fick law, in the junction, the electrons move spontaneously from the semicondtcutor doped n to semiconductor dope p. At the same time, the holes from the semicondcutor doped p to semiconductor doped n. this phenomenon generates the diffusion current. Now, what about the drift current?
The existence of the diffusion current introduces a build-up of charge on either side of the junction. This establishes an electric field, resulting in a current which opposes the diffusion current. As mentioned in another answer, at equilibrium the drift current (from the build-up of charge) and the diffusion current (from the concentration gradient of charge carriers) are equal.
Dear Kevin Joseph May,
That means the drift current is a consequence of diffusion current or? In other word, that would like to say the diffusion and drift current are not created simultaneously? if yes why the total current is nul at equilibrium?
lets see this in following 3stages
1)DIFFUSION CURRENT
after fabrication of PN juction due to difference in dopant impurity on both side there exist a gradient or if we say there will be curve with bend showing slope(in real life you can take it as two water tanks connected with a flow pipe in between then and one tank is completely filled and another tank half field) due to this the transmission of charge will occur from from high doped region to low doped region(water from full filled tank to less field tank) and then after certain time the charge in both the region will be same in terms of dopant number not in terms of charge and then equillibrium or what we say a charge barrier is formed making diffusion current null (as water in both tank at same level hence now no more flow of water in both tanks)
2) Drift current : after diffusion current have become zero the transfer of charge have stopped so now to make charge flow we need a force (in simple language we need high pressure at one end and low pressure at other end),so this force in semiconductor is applied by electric voltage the end connected to +ve end of battery applies the push to charge carries (simple language one at +ve terminal is high pressure area) and resultantly the charge begins to move towards the other end where we have negative end of battery or zero voltage(-ve end take as low pressure zone) so this current is drift current
some note
1)direction of diffusion current may be same and different
2)direction of flow of minority and majority charge carriers differs in drift current
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