Some Basic Concepts in Photoconductivity

Let us consider a solid subjected to bulk absorbing radiation producing g0earriers/cm3 sec. The steady state concentration of carriers is given by 

On the other hand the measured current upon irradiation by the same source would be different if the solid has blocking contacts. Perfect blocking contacts are defined as those that permit the photogenerated carriers to drift out of the solid under the influence of the electric field but do not allow any charge to be injected into the solid. The current under these conditions is given by2.  

The tacit assumption here is that the recombination time r between oppositely charged carriers is larger than the transit time Tt of the carriers so that the measured current is equal to the rate at which carriers are being produced (multiplied by e to convert the photon current into electrical current). Any electric field dependence of this current then reflects the electric field dependence of the QE of photogeneration. However, in the presence of substantial carrier recombination (r Tt) the current with blocking contacts is 

The mobility p is the velocity of the charge carriers per unit electric field and can be defined as either the microscopic mobility p0 or the drift mobility pd. The microscopic mobility is the mobility of the carriers in the absence of any traps. For 

---