New Observations of FN Stress Induced Performance Degradation of RF MOSFETs

1. Introduction Continued shrinking of RF MOSFET gate length leads to thinner silicon oxide film and higher electrical field and thus can results in reliability issues [1]. Previous works to evaluate the RF circuit degradation mainly focused on hot carrier instability (HCI). HCI stress will induce interface states and oxide traps near the drain junction and thus causes the degradation of f t and f max of MOSFETs and circuit failure [2-4]. However, the reliability issue of RF MOSFETs under FN stress which often occurs in some applications, such as RF switcher and passive mixer, has rarely been reported. In these circuits, the drain of the MOSFETs is biased at low voltage and the effect of HCI stress is considerably reduced, and meanwhile MOSFETs mainly suffer a uniform stress in the oxide film, which means FN stress becomes the dominant factor for the degradation of these circuits. The degradation of RF MOSFETs under FN stress is expected to exhibit different characteristics from that under HCI stress due to different physical mechanisms. In this paper, the impacts of FN stress on the RF performance of 130nm MOSFETs are investigated. It is first reported that f t and f max shows different degradation tendency under FN stress. In addition, the gate bias exerts converse influence on the trapped charge generation during stress and the trapped charge impact on the performance degradation induced by FN stress. The optimized gate bias can be obtained when considering the tradeoff between the dependence of RF performance on the gate bias and the generation rate of trapped charges during stress, which can give valuable design guidelines for RF CMOS circuits.