Noise optimization of the source follower of a CMOS pixel using BSIM3 noise model

TL;DR

This paper analyzes and optimizes the noise performance of the CMOS pixel source follower, focusing on 1/f and thermal noise, deriving optimal dimensions and capacitance ratios to minimize total input noise for scientific imaging.

Contribution

It provides a detailed analytical study of noise sources in CMOS source followers using the BSIM3 model, deriving optimal gate dimensions and capacitance ratios for noise minimization.

Findings

Optimal gate dimensions for minimum 1/f noise derived and validated.

Total input noise depends on capacitor ratio and bias current.

Numerical simulations confirm the analytical results.

Abstract

CMOS imagers are becoming increasingly popular in astronomy. A very low noise level is required to observe extremely faint targets and to get high-precision flux measurements. Although CMOS technology offers many advantages over CCDs, a major bottleneck is still the read noise. To move from an industrial CMOS sensor to one suitable for scientific applications, an improved design that optimizes the noise level is essential. Here, we study the 1/f and thermal noise performance of the source follower (SF) of a CMOS pixel in detail. We identify the relevant design parameters, and analytically study their impact on the noise level using the BSIM3v3 noise model with an enhanced model of gate capacitance. Our detailed analysis shows that the dependence of the 1/f noise on the geometrical size of the source follower is not limited to minimum channel length, compared to the classical approach to achieve the minimum 1/f noise. We derive the optimal gate dimensions (the width and the length) of the source follower that minimize the 1/f noise, and validate our results using numerical simulations. By considering the thermal noise or white noise along with 1/f noise, the total input noise of the source follower depends on the capacitor ratio CG/CFD and the drain current (Id). Here, CG is the total gate capacitance of the source follower and CFD is the total floating diffusion capacitor at the input of the source follower. We demonstrate that the optimum gate capacitance (CG) depends on the chosen bias current but ranges from CFD/3 to CFD to achieve the minimum total noise of the source follower. Numerical calculation and circuit simulation with 180nm CMOS technology are performed to validate our results.