Investigation of a novel tunneling transistor by MEDICI simulation

The shrinking MOSFET has an increasing subthreshold leakage current caused by various mechanisms. For example, the band-to-band tunneling (BTBT) current from drain to channel results in a large subthreshold leakage current in a 50nm MOSFET [1]. Recently, the double-gate (DG) fully depleted (FD) MOS was investigated. In the DG-MOS, the channel region is intrinsic or lightly doped silicon so that the threshold voltage fluctuation problem can be avoided. The reduced leakage current has been proven [2]. In this paper, a DG-NMOS structure shown in Fig. 1 is simulated using MEDICI device simulator. The transfer characteristics of the DG-NMOS with the different gate material work functions φm are shown in Fig. 2. It can be seen that the threshold voltage can be adjusted by the work function of the gate material. However, the band-to-band tunneling leakage still exists in this DG-NMOS, when the transistor is turned off. As shown in Fig. 1, the band-to-band tunneling is still on, when Vgs =0V. Fig. 3 shows the band diagram of this off-state DG-NMOS. The bending of bands enables the band-to-band tunneling at the junction of channel-drain. It seems that with the scaling dimension of MOSFET, the increasing leakage current is always a problem because of the n-p-n or p-n-p structure of MOSFET.