Detection of deep level generation-recombination centers in silicon transistor by laser irradiation

A new method for non-destructive identification of deep level recombination centers in processed silicon wafers, by laser irradiation of the base-collector junction of a phototransistor, is proposed. By using a laser with photon energies equal to half the silicon band gap, electrons trapped in the deep levels of reversed biased C-B junctions are raised in the conduction band, while new electrons from the valence band are optically excited to the mid-gap deep level, leaving behind holes. These photo-generated holes, separated by the electric field of the C-B junction, reach the base and decrease the potential energy barrier of the emitter-base junction and thus an increased number of electrons from the emitter overcome the barrier to the collector junction. Therefore, due to the transistor effect, by measuring the I_CEO current, we obtain the photogenerated current (assisted by deep levels) amplified by the current gain of the transistor. A high resolution signal processing method for photogenerated current evaluation is also proposed.