Excess noise characteristics of hydrogenated amorphous silicon avalanche photodiode films using functionally graded structure

An a-Si:H/a-SiC:H staircase photodiode with linearly graded gap multiplication regions is a useful device for photoconversion films. Photocurrent multiplication due to impact ionization was observed in the hydrogenated amorphous silicon (a-Si:H)/hydrogenated amorphous silicon carbide (a-SiC:H) staircase photodiode. In the staircase photodiode with one band offset, the photocurrent was multiplied twice and was saturated. In the staircase photodiode with three band offsets, a saturated multiplication gain of about 6 was obtained. The noise characteristics of the a-Si:H/a-SiC:H staircase photodiode films with one and three linear graded-gap regions were studied. The measured shot noise characteristics of the photodiode film, which was operated in a photocurrent multiplication region, corresponded to the ideal value, which was calculated as free excess noise. It was determined that the shot noise of a-Si:H/a-SiC:H staircase avalanche photodiode film was much less than that for the conventional crystal silicon avalanche photodiode in the photocurrent multiplication region. These results indicate that staircase photodiode films are promising devices for highly sensitive imaging sensors.