Back in 1999, Bob Pease touched on the operation of CMOS transistors in subthreshold mode. In his article, he pointed out that analog designers can use CMOS ICs such as the CD4007, a dual matched pair of transistors with another pair arranged as an inverter. Pease noted:
“Everybody knows that FETs are characterized by the threshold which causes the FET to put out ID = 1 µA. So currents below 1 µA are sort of imaginary, right? Wrong. Everybody knows that FETs have a square-law characteristic. As the current gets smaller and smaller, the gm [transconductance] per microampere keeps rising to very high levels. Wrong again. The gm per milliampere rises, but tapers up to levels such as 90 or 120 mV per decade. These transistors behave exponentially at low levels, just like bipolars.”
Pease had characterized the subthreshold performance of the CD4007 and included one of his famous hand-drawn figures (Fig. 1). What a digital designer might dismiss as leakage current is a predictable and useful operation mode of CMOS transistors. Pease knew this in 1999, but many analog engineers appreciated this fact. Robert Chao, of Advanced Linear Devices, worked at Supertex (now part of Microchip Technology). While there, he helped develop ICs used in smoke detectors. Long battery life is essential in a smoke detector.