Drift current –

This terms applies to conductors. When an electric field is applied across a metal conductor, the free electrons in conductor drift towards the positive terminal while the holes drift towards the negative terminal and the current flows through the conductor called as drift current. Thus electrons and holes form separate drift current components, therefore total drift current density is given by –

$J(drift)=J_e(drift) + J_h(drift)=eE(n\mu_e=p\mu_h)$

Diffusion Current –

This terms applies to forward biased P-N junction. There is a large concentration of holes & electrons in the P & N regions respectively. When the P-N junction is forward biased, the holes from P-region & the electron from N-region diffuse through the junction thus, constituting the diffusion current. Total diffusion current density is given by –

$J(diff)= J_e(diff)+J_h(diff)=e\cdot D_{e}\dfrac {\partial n}{\partial x}-e\cdot D_{h} \dfrac {\partial p}{\partial x}\\$ Mobility of charge carrier – It is defined as the magnitude of drift velocity acquired by charge carrier in unit field. Thus if $E$ is applied electric field in which the charge carrier acquires a drift velocity $V_d$ then mobility $\mu$ is $\mu=\dfrac {V_d}{E}$.