ARCHITECTURES: MOSFET touts zero-V turn-on threshold of 0 V

San Francisco — After years of quietly serving analog customers with precision op amps and comparators, Bob Chao has built the device that could make him famous. The company he founded, Advanced Linear Devices Inc., has produced a MOSFET with an effective zero-volt turn-on threshold. The device supports ultralow-power applications like sensor arrays and medical implants.

The zero-threshold MOSFET is based on a floating-gate charge technology. By programming the charge level on the MOSFET gate, Advanced Linear Devices (ALD) can control the turn-on/turn-off thresholds for the device. The Sunnyvale, Calif., company has been using this technology for years to control the input offset voltages of precision op amps and, more recently, the trip levels of comparators.

Caltech professor Carver Mead and his student Chris Diorio (now at the University of Washington), the founders of Impinj in Seattle, initially used floating-gate technology to trim current-weighing bits on high-resolution A/D converters. Impinj's focus is now on programming RFID tags.

With ALD now producing the N-channel MOSFETs (packaged as duals and quads), designers can build practical electronic systems operating way below 1 volt. The current state of the art in ultralow-power systems is 0.7 V, give or take 500 millivolts, said Chao, the president and CEO of ALD. With a zero-threshold device, small amplifiers can be built to operate at supply voltages of only 0.2 V and with power dissipation of less than 1 microwatt. Zero-threshold MOSFETs can alter current analog circuit design, Chao said.

Using this new device like a matched-pair current source, a simple circuit can be constructed to operate with as little as a 20-mV supply. More complicated circuits, using several stages stacked on top of each other, could operate on as little as 0.2 or 0.45, Chao believes.

Applications for sub-1-V amplifiers and switches include implantable medical devices, microelectromechanical-system sensor arrays, automotive and aviation fail-safe circuits, as well as cell phones.

These quad/dual N-channel matched-pair MOSFETs are level-matched at the factory using a floating-gate charge controller ALD calls EPAD, for electrically programmable analog device. The floating gate is polysilicon embedded in the gate oxide. The charge level on this gate controls the sensitivity of the MOSFET switch, and can be set with the EPAD programming tool.

With CMOS, EPAD programming can set thresholds from 0.01 V to - 0.01 V with an offset voltage of just 2 mV. Thus, the zero-threshold MOSFET can be classified as either an enhancement-mode device (when operated above the threshold voltage) or a depletion-mode device (when operated at or below threshold voltage).

The quad and dual MOSFET arrays have part numbers ALD110800 and ALD110900. Their gate threshold voltage is 0.0 V 0.01 V at 1 microamp. Pricing starts at 91 cents each in quantities of 1,000 pieces.