Fundamental limits of the switching abruptness of tunneling transistors

The tunneling field-effect transistor (TFET) is one of the most promising candidates for future low-power electronics because of its potential to achieve a subthreshold swing less than the 60 mV/decade thermal limit at room temperature. It can surpass this limit because the turn-on of tunneling does not sample the Maxwell-Boltzmann distribution of carriers that gives rise to the 60 mV/decade limit in conventional devices. However, theoretical predictions and experimental measurements of TFET device characteristics have differed by a wide margin—experimental subthreshold characteristics have not achieved the switching steepness (i.e., the change in drain current with applied gate voltage) of theoretical simulations. Non-ideal effects, such as non-abrupt band edges, phonon-assisted tunneling, and trap states, are discussed as mechanisms that may degrade theoretical predications. A strained-Si/strained-Ge bilayer TFET is used as a test-bed device to better understand the discrepancy between simulation and experiment. The bilayer TFET studied in this work eliminates channel doping and uses the strained-Si/strained-Ge heterostructure. Band-to-band tunneling occurs perpendicular to the gate, in-line with the gate electric field. Multiple gates are used so that the impact of the directionality of tunneling on switching abruptness can be studied. The band alignment of the strained-Si/strained-Ge heterostructure is extracted from a MOScapacitor structure though an experimental quasistatic CV technique. The extracted effective band gap (related to the tunneling barrier) is shown to be only ~200 meV for the heterostructure, and the valence band offset is shown to be ~100 meV larger than predicted by density-functional theory. New deformation potentials are suggested for the Si-Ge material system based on the experimentally extracted band alignments.