Simulated operation and properties of source-gated thin-film transistors

Recently, Shannon and Gerstner introduced the source-gated thin-film transistor (SGT) with undoped a-Si:H as the active layer, claiming that it overcomes some fundamental limitations of the field-effect transistor. In the common thin-film transistors, the channel is controlled by the gate and the current saturates when the drain end of the channel becomes depleted due to the drain voltage. For this operation, the channel contacts to source and drain must be ohmic. By contrast, in the SGT, source has a depletion (Schottky) contact to the film and it is situated on the film surface opposite to the channel. This device is rather similar to the top-contact (TOC) organic field-effect transistor (OFET) with Schottky-type contacts which has been investigated by us. We simulated the SGT and found that indeed the operation is essentially the same as that one of the Schottky-contact TOC OFET, and it differs from the verbal description given by Shannon and Gerstner. We found that, apart from the unusual voltage dependencies of the current with an abrupt transition into saturation at lower drain voltage, the main feature of the SGT is a strongly reduced current due to the series resistance of the depleted region between source and channel. The SGT enables indeed higher voltage gain at lower voltages. However, a reduced transconductance and large source-gate overlap capacitance lead to a reduced cut-off frequency. Also, a reduction of the on-current may be limited by requirements on the on-off ratio.