Charge Collection Efficiency

With irradiation, defects are introduced in the silicon bulk which act as charge traps and recombination centers. This leads to a reduction in the charge collection efficiency. The probability of charge trapping is proportional to the drift time of the carriers. Thus, the charge collection efficiency can be partially restored by applying a higher bias voltage, which results in shorter drift times as discussed in section , p. . While the efficiency curve of non irradiated detectors reaches its plateau at the depletion voltage, irradiated sensors need considerable ``overbiasing'' beyond the depletion voltage. In practice, the efficiency in that case never fully saturates, while the operational voltage is limited by high voltage breakthrough.

Experimental results showing this effect can be found in section , p. .