Understanding of BTI for tunnel FETs

Abstract

We systematically investigated the NBTI degradation of Si-channel p-type tunnel FETs (pTFETs). The NBTI degradation mechanism of pTFETs is almost the same as that of pFETs. It was clarified that the NBTI degradation of pTFETs is only caused by the trap charge and the interface state degradation located in the tunneling region near the n + source/gate edge. Furthermore, in terms of the BTI degradation of n-and p-type TFETs, although the injection sources of carriers inducing PBTI and NBTI are different, applying a drain bias corresponding to the operation conditions has an effect on the BTI lifetime improvement of n-and p-type TFETs. IntroductionTunnel FETs (TFETs) are the most promising steep-slope devices for ultralow-voltage operation [1][2][3][4]. The BTI degradation of TFETs is their most important reliability issue [5][6][7]. The main differences between TFETs and FETs are the operation mechanism and the polarities of the source/drain. TFETs operate by band-to-band tunneling (BTBT) from the source to the channel, and the polarity of the source/drain for TFETs is asymmetric. In n-type TFETs (nTFETs), by a positive bias stress, the threshold voltage (V th ) shifts in the positive direction, while the average subthreshold slope (SS ave ) is not degraded [7]. We have reported that the PBTI lifetime of nTFETs is improved by applying a drain bias, because the PBTI degradation is mainly caused by electron injection from the n + drain [7]. However, the NBTI degradation of pTFETs has been hardly reported and has not been fully understood yet (Fig. 1). In this work, we investigated the NBTI degradation of pTFETs, to systematically understand the BTI of TFETs. For pTFETs, since the tunneling locally occurs near the n + source/gate edge, the impacts of the trap charge and the interface state degradation located in the tunneling or nontunneling region on NBTI were quantitatively examined by simulation. Finally, we investigated the impact of tunneling carriers on the NBTI degradation of pTFETs.