Current Controlled Field Effect Transistors for Micropower Circuits

At the present time, the micropower circuits used in pocket calculators, digital watches, pagers, etc are based on sub-threshold MOSFETs (Metal Oxide Silicon Field Effect Transistors) operating in the weak inversion regime (e.g. current exponentially dependent on voltage). These devices have found widespread application because small drain current and voltage is required for operation. Critical to the efficacious use of circuits based on sub-threshold MOSFETs is the ability to operate at high speeds, preferably > 1MHz. Unfortunately, despite their utility, weakly inverted MOSFETs are disadvantaged by low operating speed, which is primarily attributable to the methods used to control current. Present techniques use a gate voltage (Vg) to control the current flowing in the weakly inverted channel. However, because drain current (Id) is exponentially dependent on threshold voltage (Vth), small fluctuations in Vth cause large variations in Id. These variations complicate transistor matching and require the usage of excessively long gate lengths (~3mm) which limit the maximum operating frequency of present day micropower circuits to well below 1 MHz.In order to solve these problems Arizona State University researchers have developed alternative micropower circuit technology based upon novel sub-threshold Field Effect Transistors (FETs). Using certain test designs researchers have demonstrated greatly improved transistor matching and higher frequency of operation than is currently achievable with comparable MOSFET devices. It is believed that these improvements will potentially benefit micropower applications in such areas as communications and medical implants, in particular pacemakers.