Abstract
In order to improve the performance of the a-Si : H phototransistor (PT) and the a-Si : H/a-SiC : H heterojunction phototransistor (HPT), the a-Si : H/a-SiC : H superlattice (SL) was used for the collector regions of these two devices individually. The resulting superlattice phototransistor (SLPT) and superlattice heterojunction phototransistor (SHPT) were successfully fabricated. Due to the avalanche multiplication in the interpolated a-Si : H/a-SiC : H SL region, the optoelectronic characteristics of the SLPT and the SHPT are considerably better than those of PT and HPT. The SLPT has high photocurrent response and an optical gain as high as 287, when V CE = 14 V, I C = 0.74 mA, and P in (incident power) = 5 μW emitted from HeNe laser. This is the highest optical gain obtainable for various amorphous phototransistors yet reported. The typical shapes of the photo I− V curves for SHPT are much like those of the crystalline-Si (c-Si) phototransistor. This indicates the SHPT is suitable for analog circuits in optoelectronic applications. The SHPT has a maximum optical gain of 78, when V CE = 30 V, I C = 0.208 mA, and P in = 5 μW. The response time for SHPT and SLPT are respectively 4.1 ± 0.1 μs and 6.2 ± 0.1 μs at a load resistance R L = 1 kΩ .
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