Explain Diffusion.
Diffusion: Though, the mobility of the carriers in a semiconductor is greater than the electrons in metals, the conductivity in the former is much less than which in the latter due to the too little current carriers. The conductivity is much less that the random movement of the carriers because of unequal carrier densities plays a greater part in conduction than the drift because of the applied fields. Diffusion arises fundamentally from density difference and resulting current is termed as diffusion currents.
The defining equation for diffusion currents in one direction are given below:
Jn = eDn (∂n/∂x) for electron
JP = eDP (∂P/∂x) for holes
Here Jn = diffusion current density of electrons
JP = diffusion current density of holes
Dn =diffusion constant of electron
DP = diffusion constant of holes
∂n/∂x = gradient of electron density
∂P/∂x =gradient of hole density.
Thus, the diffusion current because of the random motion of carriers from the dense to the less dense regions is proportional to the gradient or rate of raises of carrier density along with distance.
The coefficient of proportionality is termed as the diffusion constant and is denoted with D.