Carrier - diffusion drift velocity

It is difficult to estimate the driving force dEp /qdx for the movement of the majority carriers, since it is necessary to account for diffusion and migration. However, for minority carriers, it can be assumed that the driving force dEpp/qdx is entirely due to the electrical field in the depletion layer. Since the mean electric field is h /dsc, the drift velocity of a hole is of the order of Hp (j>sc/dsc- The transit time of the minority carrier through the depletion layer, r dsc), can be expressed as equivalent to an RC time constant using Eq. 6-7 ... 

The charge carrier mobility (p) of a material is defined as the ratio between the drift velocity of the charge carrier (v) induced by the electric field and the amplitude of the applied electric field (F) (p = v/F). In general, carrier mobility is dictated by the diffusion coefficient (D) since the charge transport follows a diffusive mechanism. Carrier mobility is related to the diffusion coefficient via the Nemst-Ejnstein equation ... 

Hirao et al. (1995, 1995a) and Hirao and Nishizawa (1996) measured hole mobilities and diffusion coefficients of 4-dibenzylamino-2-methylbenzaldehyde-1,1-diphenylhydrazone (BMH) and DEH doped PC. The experimental technique involved the fitting of a theoretical expression to the photocurrent transients. The theoretical equation was derived by assuming that a carrier packet is spread by diffusion and that the carriers drift with a constant velocity. The photocurrent is given by the expression... 

---