Bottom-gate-type thin film transistors using ZnO as an active channel layer (ZnO–TFT) have been constructed. The ZnO layers were deposited using pulsed laser deposition at 450 °C at an oxygen pressure of 3 m Torr, and the material that was formed had a background carrier concentration of less than 5×1016 cm−3. A double layer gate insulator consisting of SiO2 and SiNx was effective in suppressing leakage current and enabling the ZnO–TFT to operate successfully. The Ion/Ioff ratio of ZnO–TFTs fabricated on Si wafers was more than 105 and the optical transmittance of ZnO–TFTs fabricated on glass was more than 80%. These results show that it is possible to fabricate a transparent TFT that can even be operated in the presence of visible light.

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.

Recently, high-resolution and high-density solid-state imaging devices have been in high demand due to their high photogain because the smaller pixel size makes the smaller photo-current dependent on the reduced area. Multilayer photodiodes having a-SiC:H/a-Si:H/a-SiN:H structures fabricated on the crystalline silicon (c-Si) substrate were studied for their photo-multiplication effects which marked a gain of 6.6. These multiplication phenomenona arose in the wavelength region longer than 550 nm, while in the shorter-wavelength region, the photogain was less than unity. This is explained by the electron carrier process where the carriers photogenerated in the c-Si substrate are accumulated at the a-SiN:H/c-Si interface and tunneled via localized states in the a-SiN:H layer by the induced high electric field. From these results, it is considerably difficult to support the theory of avalanche effect to explain these photo-multiplication effects.

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