Effect of Forward and Reverse Substrate Biasing on Low-Frequency Noise in Silicon PMOSFETs

The forward body biasing improves the low-frequency noise performance of p-channel metal-oxide semiconductor (PMOS) transistors by about 8 dB/V. Therefore, for analog design, forward body biasing may be preferred if noise is a concern. This is in agreement with the improvement of other MOSFET parameters such as the decrease of the threshold voltage ($V_{T}$) or the increase of unity current-gain frequency ($f_{T}$) on forward substrate- (or body)-source biasing ($V_{BS}$). Also, forward $V_{BS}$ is very attractive for low voltage supply ($V_{DD} < 0.6$ V) and low-power, low-noise circuits. A detailed analysis of the dependence of the noise level on $V_{BS}$ and on the gate-source ($V_{GS}$) biasing showed that the dependence on $V_{BS}$ seems to be smaller in weak inversion, and it increases in strong inversion. The dependence on $V_{GS}$ has a turning point at $V_{GS}\approx 0.8$ V, independent of body bias, which it seems is due to the activation of oxide traps, as the noise waveform showed a random telegraph signal (RTS) component at $V_{GS} > 0.8$ V. Generally, it is confirmed that the spectral density $S_{I}$ of the total low-frequency noise of the drain current $I_{D}$ is proportional to the square of $I_{D}$, i.e., $S_{I}\propto I_{D}^{2}$, but it cannot be clearly ascribed to either number fluctuation or mobility fluctuation models. In addition, both models cannot accurately describe the dependence of the noise level on the body bias.