Current gain of amorphous silicon thin-film transistors above the cutoff frequency

A key challenge for the development of high functionality thin-film large-area electronic systems is the operational frequencies achievable by Thin-Film Transistors (TFTs). These frequencies are typically limited by low transconductances and large (gate and overlap) capacitances. However, we have recently demonstrated energy-harvesting and communication systems, as shown in Fig. 1, utilizing thin-film circuit topologies that allow operation at or above the TFT cutoff frequency (ft) [1,2] by using inductors to 'resonate out' the effect of TFT capacitances. Measurement and modelling of current gain near and above ft is thus critical (as opposed to conventional studies of current gain below ft). In this paper we (1) show above-ft measurements for standard bottom-gate amorphous silicon (a-Si) TFTs and self-aligned bottom-gate a-Si TFTs and (2) illustrate how large TFT gate-drain capacitances lead to a slow current-gain roll-off at frequencies above ft.