SPICE models for flicker noise in n-MOSFETs from subthreshold to strong inversion

The two main sources of low-frequency flicker noise are mobility fluctuations and number fluctuations. Our experiments on NMOS noise measurements were done from subthreshold to saturation region of operation for both long-channel (5 μm) and short-channel (as small as 0.6 μm) NMOS transistors. The results suggest that for both types that in the saturation region, the flicker noise is due to the surface state effect and the noise equations, NLEV=2 and 3, in SPICE, HSPICE, and PSPICE are most appropriate. For short-channel devices, due to the effects of velocity saturation and the resulting nonlinear transconductance (g_m) variation with gate bias voltage, the input-referred voltage noise increases as the gate-source voltage increases instead of staying constant as it does for long-channel devices. In the subthreshold region, the input-referred voltage noise decreases drastically as the gate-source voltage increases for both long-channel and short-channel NMOS devices. Simulations have been done using PSPICE and HSPICE, with noise level (NLEV)=3 and device model level 3 and BSIM 3.2 and 3.3. The results from PSPICE version 8.0 level 7 (BSIM 3.3) and SPICE level 3 compare favorably with the measured noise phenomena for the short-channel and long-channel NMOS devices, respectively