Optimal current for minimum thermal noise operation of submicrometer MOS transistors

The wide band noise voltage (equivalent thermal noise voltage at the gate) of a submicron MOSFET, working in saturation, exhibits a minimum value at a certain drain current. This is supported by measurements and theoretical analysis based on a suitable thermal noise model. This macroscopic noise model attributes the thermal noise of the drain current to the superposition of two noise sources originating from two separate regions of the transistor´s channel (a gradual channel approximation region and a saturation region). The existence of a minimum of the noise spectral density at an optimum drain current (I_opt), is well proved by measurements and is contradictory to the predictions of the current simulation program with integrated circuit emphasis (SPICE) models. An empirical way for evaluating analytically I_opt is given. The fact of the existence of a noise minimum for a submicron MOSFET, brings a phenomenological equivalence to the bipolar transistor and GaAs MESFET when they are employed at the first stage of an amplifier