Analysis of MOSFET degradation due to hot-electron stress in terms of interface-state and fixed-charge generation

Abstract

An analysis of MOSFET degradation induced by hot-electron stress is carried out by separating the effects of generated fixed-oxide charge ΔQ F, due to trapped electrons in the gate oxide and charge in the generated interface states ΔQ i. Both influences are considered in the subthreshold and above-threshold bias regions using a model for the stressed device which divides its channel into two regions having different threshold voltages. A phyiscal explanation is offered for degradation of transconductance g m by considering the V G dependence of the charge in the interface states ΔQ i and the threshold voltage V T. It is predicted that an increase in Q F alone causes the magnitude of the threshold voltage V T to increase and the device turn-on to become more abrupt (transconductance g m increases), while an increase in Q i alone results in a softer turn-on ( g m decreases) and makes the threshold voltage dependent on V G. The generated interface state density D it and the fixed charge density ΔQ F is extracted by fitting the measurements to the above threshold I D- V G characteristics (small drain voltages) calculated with the model. Furthermore, we find that while both Q F and Q i increase monotonically with stress, the contribution of ΔQ i to the change in V T is significantly larger than that due to the change in Q F. This conclusion is independent of stress time or of the exact division of the channel (the assumed lengths L 1 and L 2). Interface state densities obtained from the measured slopes of subthreshold I D- V G plots (which are affected by states near the middle of the band gap) are appreciably lower than densities deduced from measurements made above threshold (which are sensitive to states near the band edges) using the divided-channel representation. An increased dependence of subthreshold current on V D after stress is attributed to short-channel behavior of the MOSFET in the vicinity of the drain. The effects of hot-carrier stress in p-channel MOSFETS are also interpreted in terms of the two-transistor representation.