Threshold voltage extraction circuit for low voltage CMOS design using basic long-channel MOSFET

The threshold voltage (Vth) is a key parameter in MOSFET design and modeling. There are many definitions and extraction methods, each one given with a focus on different aspects. This work presents a simple circuit that extracts the threshold voltage under low voltage conditions and using a feedback loop in order to reach supply independence. The circuit has been simulated using the BSIM3v3 model for a 0.18µm CMOS process. The extracted value of VTHN is very close to the model nominal value (VTHO) used from the model parameters being the variation of +0.327% and −0.294% from VDD=0.6V to VDD=3V. The bias current I is from 810.7nA to 872nA for the same supply voltage variation.

I. Introduction

The threshold voltage is a fundamental parameter for MOSFET modeling and characterization, which represents the start of significant drain current flow. It has been given several definitions [1], but it may be essentially understood as the gate voltage value at which the transition between weak and strong inversion takes place in the channel of the inversion-type MOSFET [2].