Self-aligned ZnO thin-film transistors with 860 MHz fT and 2 GHz fmax for large-area applications

High-frequency thin film transistors (TFTs) enable many important thin film circuits used in flexible large-area systems such as large bandwidth instrumentation amplifiers (related to f
T
) and high-frequency oscillators (related to f
max
) [1,2]. In [3] f
max
=10GHz and f
T
=2.9GHz were reported but using Si substrates and pulsed laser deposition for ZnO growth, which are incompatible with low-cost large-area processing on a meter scale. More modest reports of f
max-
=1GHz with sputtered IGZO on glass have relied on a very sensitive alignment process, impractical for fabrication over large substrates [4]. In this work we present a ZnO-channel TFT process fully compatible with flexible large-area substrates. We achieve an f
max
=2GHz and f
T
=860MHz by reducing source/drain (S/D) to gate overlaps (X
ov
) and scaling channel lengths down to 500nm via a self-aligned process.