Photocurrent gain mechanisms in metal-semiconductor-metal photodetectors.

Different gain mechanisms are observed in metal-semiconductor-metal (MSM) photodetectors consisting of coplanar Schottky contacts on a high resistivity GaAs sample. I-V curves taken at different frequencies reveal low-frequency gain due to surface charges. Additionally, the time-resolved photocurrent response indicates the presence of high frequency gain, manifesting itself as a 200 ps long tail in the impulse response. We attribute this effect to injection of electrons by tunneling from the cathode into the semiconductor. The electron injection is caused by holes trapped at defects in the vicinity of the contact. We present a theoretical model for the high-frequency gain, which shows good agreement with the experimental data and explains the observed variation of the tail amplitude with excitation density and contact distance.