Hydrogenated amorphous silicon bipolar junction thin film transistors (a-Si:H B-TFTs)

A bipolar junction transistors made of a-Si:H thin films has the potential advantages of a conventional bipolar junction transistors, such as large current driving ability and high switching speed, and low thin film deposition temperatures, which supplies a very thin base layer and may result in the device's low power consumption. In this study, authors fabricated n-p-n bipolar junction transistors from a-Si:H thin films (B-TFT). A stacked structure of n+i/p/i/n+ of a-Si:H thin films was used to form emitter/base/collector layers. All a-Si:H layers and the SiNx passivation film were deposited by PECVD at 250C. The complete transistor was fabricated using five masks. The proper film thicknesses for good junction behaviors were obtained from studying the a-Si:H p-i-n diode characteristics, separately. The B-TFT showed a current gain of 3-6 and a current of 50 A both of which are larger than the literature values. The current noise might be contributed by the high defect densities at junction interfaces, especially the interface between the base and the emitter. The "OFF" current of a few micro-amps and the right shift of the knees in the I-V curves are attributed to the parasitic p-n diodes formed between the base and the collector outside of the emitter active region. The high series resistance of the base layer in the extrinsic region needs to be solved to improve the transistor characteristics.

Hydrogenated amorphous silicon bipolar junction thin film transistors (a-Si:H B-TFTs) Conference Paper