Carrier mean drift velocity

The simplest model of charge transport in delocalized bands is the Drude model, which assumes the carriers are free to move under the inhuence of an applied electric held, but subject to collisional damping forces. Note that the scattering centers are not the nuclei of the background material, but rather phonons (lattice vibrahons) or impurities. A statistical equahon for estimahng the mean drift velocity of the carriers in the direction of the electric held may be written as... 

A statistical equation for estimating the mean drift velocity of the carriers in the direction of the electric field Fx can be written as... 

The derivation of the current equation for a PC detector is a more involved than for a PV device - but we will find that the result will be similar. The current / in a conductor is the product of the carrier concentration n, the mean drift velocity v, the charge e... 

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 ... 

Dsually V is lO to lOO times greater than S. Carriers interact with acoustic phonons by means of a deformation potential O, and a particular carrier eiqperiences all values and directions of kinetic velocity. In ID this is both forward and backward velocity along the chain. The drift velocity V is then a small bias on this kinetic motion due to the applied field. 

The drift mobility ( x) of a photoinjected carrier is its mean velocity per unit field ( ) and may itself be field dependent. The transit time (t J and mobility are related to specimen thickness (L) according to equation 1 ... 

From the decay curves shown in fig. 4 it is cle u that a t " decay Is not found in the time regime 10 nsecs - 1 ijuaec thou after 1 ftaec there is evidence that such a t decay is asynp-totically approached. Consider carriers created at random positions hy the li t pulse which subsequently drift freely in ID with a velocity v to randomly positioned traps of mean separation s. It can be shown in these circumstamces the current I(t) falls as carriers trap with a decay of the form... 

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