Physics and Mitigation of Excess OFF-State Current in InGaAs Quantum-Well MOSFETs

A number of recent reports have noted excess OFF-state leakage current (IOFF) in scaled InGaAs quantum-well nMOSFETs. There is growing evidence that a combination of band-to-band tunneling (BTBT) and a floating-body bipolar gain effect is responsible for this. Unless this issue is effectively addressed, the scaling potential of this transistor structure will be compromised. This paper presents a detailed study of the physics of IOFF and explores IOFF reduction strategies through 2-D device simulations that have been calibrated with experiments. In essence, under OFF conditions at even moderate values of Vds, a BTBT process at the drain-end generates holes in the channel and thereby reduces the source-channel potential barrier. This results in injection of electrons into the channel that contribute to enhanced IOFF while the holes are injected into the source where they recombine. In a nanoscale device, the bipolar effect that is at play here can have a very large current gain and amplify many fold even a small BTBT current. A study of approaches to mitigating this effect is analyzed here. It is concluded that the most effective strategy is to minimize the bipolar current gain rather than BTBT in scaled transistors.