Effective Charge Carrier Lifetime in Silicon p-i-n Junction Detectors

An estimate of the effective charge carrier lifetime can be obtained from a determination of the collection efficiency as a function of the transit time for charge carriers produced by incident short-range radiation in a p-i-n junction detector. The results of this method have the advantage of being independent of both the charge carrier mobility and the depletion depth of the detector. Effective lifetimes as low as a few microseconds (τe) for electrons produced by natural alpha particles have been observed in several 2 mm thick lithium-compensated silicon detectors operating at room temperature. No significant bias dependent windows which might cause misleading interpretation of the results were observed. Window effects were investigated by measuring the changes in pulse height of detector output signals initiated by natural alpha particles incident on the detectors at angles up to 45 degrees. The electron lifetimes were observed to decrease when the detector was operated at liquid nitrogen temperature. The short effective lifetimes that can occur in lithium-compensated silicon set a lower limit on the field strengths that must be maintained for efficient charge collection in thick p-i-n junction detectors. Effective hole lifetimes can also be obtained using this technique but with the present geometry of lithium-compensated silicon detectors this measurement is more difficult if short-range particles are used. The effects of charge carrier lifetime and amplifier pulse rise and fall times on the observed radiation-induced pulse shapes are discussed.