Such absorption in the presence in the forbidden zone of the semiconductor of local levels of impurity can cause transitions of electrons between levels of impurity and zones. The photoconductivity caused by such transitions is called as impurity photoconductivity. For realization of such transitions smaller energy of quantum, than for realization of transitions from a valent zone to a conductivity zone is necessary. Therefore impurity absorption takes place with big lengths of waves of an incident light.
This type of absorption takes place in that case when optical excitement of electrons comes from a valent zone in a conductivity zone. For the semiconductor with direct valleys upon vertical transitions energy of a photon of h has to be not less than width of the forbidden zone, that is
The main shortcoming to which usually point, - lack of strengthening. But it is rather conditional. Almost in each optoelectronic device the photodetector works for this or that coordinating electronic scheme. And introduction of the intensifying cascade to it is much simpler and more expedient, than giving to a photodetector of strengthening functions unusual for it.
There are 3 physical factors influencing a lag effect: time of diffusion or drift of nonequilibrium carriers through the base; flight time through p-n transition of i; time of a recharge of barrier capacity of p-n of transition which is characterized by Rsbar time constant.
Fundamental difference of optrons as elements of communication is in use for transfer of information electrically of neutral photons that causes a number of advantages of optrons which are inherent also in all other optoelectronic devices in general. Though at optrons is, certainly, and the shortcomings.
The photo diode can possess big speed, but its coefficient of strengthening of photocurrent does not exceed unit. Thanks to existence of optical communication optoelectronic integrated chips possess a number of essential advantages. Almost ideal galvanic outcome of the operating chains at preservation between them strong functional communication.
At absorption of quanta of light in p-n transition or in the areas adjoining to it new carriers of a charge are formed. The nonbasic carriers of a charge which arose in the areas adjacent to p-n to transition at distance which is not exceeding diffusive length diffundirut transition to p-n and pass through it under the influence of electric field. That is the return current when lighting increases. Absorption of quanta directly in p-n transition leads to similar results. The size at which the return current increases, is called as photocurrent.
As a result of such absorption amplitude of fluctuations of knots of a lattice increases. In this case neither concentration of carriers of a charge, nor their mobility does not change. Therefore absorption of light a crystal lattice is not photoactive.
Absorption of light free carriers of a charge and a crystal lattice cannot directly cause change of concentration of carriers of a charge. However increase of concentration of carriers of a charge in these cases can result from secondary effects when absorption of light considerably increases kinetic energy of free carriers of a charge or increases concentration of phonons which then give the energy on excitement of carriers of a charge.