P-poly and n-poly gate ultra-fine film SIMOX transistors

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Abstract
Ultrathin-film silicon-on-insulator transistors using both n-polysilicon and p-polysilicon gates are discussed. The channel implants used for the n- and p-channel transistors and the corresponding threshold voltages, gains and mobilities are tabulated. For comparable threshold voltages, the use of p-polysilicon gates increased the n-channel mobility by 40% but decreased the p-channel mobility by a similar factor. The output characteristics of 1- mu m gate length n-channel transistors with similar threshold voltages are shown. The use of p-polysilicon gates increased the breakdown voltage by almost 0.5 V and increased the drive current by 5%. The p-polysilicon gate transistor suffers from punchthrough at low gate voltages. This can be improved by substrate bias and/or modifications to channel doping profiles. The punchthrough characteristics of a p-poly gate transistor which received a channel implant of 2*10/sup 12/ cm/sup -2/ boron at 100 keV are shown. For p-channel transistors with similar thresholds, the use of p-polysilicon gates reduced the drive current by 10. It also considerably improved the punchthrough characteristics. The tradeoffs between n- and p-polysilicon gates are shown in a table. The use of p-polysilicon gates on the n-channel devices and n-polysilicon gates on the p-channel transistors would offer optimum performance, especially for very thin SOI films. Numerical simulations of the devices are shown and the methods available to control punchthrough are discussed.

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