In many linear applications of field-effect transistors, the FET is used as a constant-current device for large VDS (i.e., drain-to-source voltage) or as ohmic resistance for small VDS. In the region before pinch-off, the FET is effective as a voltage-variable resistor. When forward-biased, the transfer characteristic (VGS vs. Id) of this device exhibits a linear variation of rDS(on), or drain-to-source on-resistance, with the junction temperature (Tj). This behavior indicates that a FET could be used as a temperaturesensing resistor (TSR) in the range of approximately −40°C to +150°C.
To use a FET as a temperature sensor in a practical application, a metallic cap preferably made with aluminum can be used as a heat sink (Fig. 1). The heat sink senses the temperature bath and provides linear variation of rDS(on) with temperature. Increasing junction temperature causes a linear increase in drain-to-source on-resistance.
In general usage, junction temperature changes with the mode of operation and biasing configuration of the FET. But in this application, the biasing and FET operation are kept constant, varying the junction temperature via the heat sink by heating or cooling above or below room temperature. Compared to N-channel FETS, P-channel FETS produce a larger variation of rDS(on) over temperature.
The plots given are for Motorola’s general-purpose N-channel FET 2N4856. Other families of FETs also exhibit a similar linear response of rDS with respect to junction temperature.
See associated figure