Amorphous Silicon Avalanche Photodiode Films Using a Functionally Graded Superlattice Structure

The photocurrent multiplication due to impact ionization was observed in an a-Si:H/a-SiC:H staircase photodiode. In the staircase photodiode with one band offset, the photocurrent was doubled and was saturated. It was confirmed that almost all the electrons multiplied after they crossed the band offset. On the staircase photodiode with 3 band offsets, a saturation multiplication gain of about 6 was obtained. The gamma values of the photocurrent characteristics were 1.0, indicating that no excess carriers entered from the electrode and no interband tunneling affected the photoinduced current. These results suggested that the impact-ionization at each conduction band step due to the conduction-band discontinuity could be the dominant mechanism of the photocurrent multiplication.