Enhanced capacitance ratio and low minimum capacitance of varactor devices based on depletion-mode Ga-doped ZnO TFTs with a drain-offset structure

Depletion-mode Ga-doped ZnO thin-film transistors (TFTs) with a drain-offset configuration were fabricated employing a plasma-enhanced chemical vapour deposited Si3N4 gate insulator and a metal organic chemical vapour deposition grown Ga-doped ZnO channel layer. For comparison, conventional TFTs with drain-to-gate overlap were also fabricated. Capacitance–voltage characteristics of gate-to-drain capacitors and current–voltage characteristics of TFTs were measured. Although drain-offset TFTs exhibit poor device characteristics compared with conventional TFTs, these TFTs show better C max/C min ratio and C min values. The C max/C min ratio is as large as 71.83 and C min is as small as 0.3 fF µm−1 normalized for channel width, demonstrating the potential of these devices as varactors for circuit applications. Their better varactor performance is ascribed to the presence of a very small capacitance in the drain-offset region. The effect of drain-offset length variation on TFT and gate-to-drain capacitor performance is also reported.