The degradation of MOS transistor operation due to soft breakdown of the gate oxide is studied.Important transistor parameters are monitored under homogeneous stress at different temperature until the soft breakdown occurred.The output and transfer characteristic have small change after soft breakdown as the degradations of drain current and threshold voltage is continuous.However,the increment of gate leakage current increases abruptly after the soft breakdown.The analysis to the increment of gate leakage current after the soft breakdown shows mechanism of similar Fowler Nordheim(FN) tunneling current.
The effect of HALO dose on device parameter degradation of pMOSFET with 2.1nm o xide and 0.135μm channel length at hot carrier stress is analyzed.It is found that the degradation mechanism is not sensitive to HALO dose changing,but the d egradation quantities of linear drain current,saturation drain current,and maxim um transconductance increase with HALO dose enhancing and are larger than those of speculated before.The degradation of device parameters (linear drain current, saturation drain current,and maximum transconductance) is attributed to not onl y the drain series resistance enhancing induced by interface states under spacer oxide and carrier mobility degradation but also the threshold voltage variation and initial threshold voltage increasing with HALO dose enhancing.
An empirical expression for the direct tunneling (DT) current is obtained.This expression can be used to calculate the DT current for nMOSFETs with ultra thin oxide when the oxide thickness is considered as an adjustable parameter.The results have good agreement with the experimental data.And the oxide thickness obtained is less than the value acquired from the capacitance voltage( C V )method.
The effect of neutral trap on tunneling currentin ultrathin MOSFETs is investigated by num erical analy- sis.The barrier variation arisen by neutral trap in oxide layer is described as a rectangular potential well in the con- duction band of Si O2 .The different barrier variation of an ultrathin metal- oxide- sem iconductor(MOS) structure with oxide thickness of4nm is numerically calculated.It is shown that the effect of neutral trap on tunneling cur- rent can not be neglected.The tunneling current is increased when the neutral trap exists in the oxide layer.This simple m odel can be used to understand the occurring mechanism of stress induced leakage current.
According to the definition of interface traps,a new application of relaxation spectral technique to sub-threshold swing shift and sub-threshold gate voltage shift is proposed to extract interface trap density in 1.9nm MOSFET.And thus the energy distribution of interface trap can be determined.According to the two methods,the energy profile of interface traps agrees with those reported in literature.Compared to other methods,this method is simpler and more convenient.
