Similarities and differences between poly-Si and silicon-on-insulator (SOI)

Polysilicon devices are structurally very similar to SOI single crystal devices. They are usually MOS based, the channel is contained within a thin film of material which will often be relatively free of other carriers, and has no substrate connection. The polysilicon of TFTs is often intrinsic in behaviour, whereas with single crystal SOI the film is fully depleted. Although a major advantage of single crystal SOI is the absence of latch-up, a particular problem of bulk CMOS, spurious bipolar effects do occur, and the related instabilities, although less destructive than latch-up, are important. Such effects are also present in polysilicon TFTS. The low value of the effective Gummel number of the region between source and drain, in both structures, makes the current gain of the associated bipolar transistor higher than in the equivalent bulk structures. The low minority carrier lifetimes which are such a disadvantage with TFT devices in respect of leakage current, can inhibit the positive feedback associated with drain multiplication of carriers emitted from the forward biassed source.<
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