III-V/Ge-based tunneling MOSFET

Tunnel MOSFET (TFET) is one of the most promising steep slope devices, which have been regarded as mandatory for future scaled LSIs for ultra-low power and IoT applications. Fig. 1 shows the band diagram of a typical p-n junction-based TFET. The steep slope property can be realized by the energy filtering effect, where sharp onset of tunneling current is induced by the density-of-state overlap between the valence band of the source and the conduction band of the channel in n-channel TFETs. Thus, any defects in the bandgap such as bulk defects, tail states and interface states at MOS interfaces, generating possible leakage current paths, must be carefully eliminated. Also, the source impurity concentration and the steepness of the source impurity profile have to be sufficiently high, in order to thin the tunneling barrier width and to yield high tunneling current. Fig. 2 summarizes the critical issues and the device engineering of the p-n junction-based TFETs. While most of the requirements are shared with those of advanced CMOS devices, the device engineering inherent to TFET is the formation of the source tunnel junctions, as pointed out above. Here, the optimization of the source doping concentration and the spatial steepness of the dopant concentration profile are important in thinning tunneling barrier. In addition, the choice of the source and channel materials is quite important. Actually, Si-based TFETs have a difficulty in realizing low S.S. and high I
_on
, because of the low tunneling probability due to high and indirect bandgap. Thus, semiconductors such as Ge and III-V are expected as suitable materials with small and/or direct band gap for enhancing I
_on
of TFETs. In addition, type-source/channel junctions composed of II hetero-structures are known to be effective in boosting the TFET performance. From these viewpoints, we are currently working for the following material systems [1-3], (1) InGaAs TFET (2) GaAsSb (source)/InGaAs (channel) TFET (3) Ge TFET (4) Ge (source)/strained Si (channel) TFET.