I have previously asked a question about using thermocouples to maintain a temperature range of 36-39° inside an insulated and dry environment.

The more enlightened engineers advised I look into using either a semiconductor device or an RTD instead.

Parts availability is limited where I'm currently located in the third world and mail order is unreliable at best.

I have a lot of bipolar transistors as well as MOSFETs at my disposal.

I've seen some examples of using 2N3904/3906 as thermal sensors.

The BS270 looks a bit more appealing to me because of the positive temperature coefficient and lower currents required.

My one concern is the inherent D-S resistance warming the device. But if the current is low enough I think the resistance curve with respect to junction temperature looks easier to calibrate than something based on bipolar transistors.

Am I travelling down the wrong path here? Bipolars seem to be more popular for this sort of hack job sensor.

The alternative is for me to manufacture an RTD which is indeed a possibility if that's a better option.

  • \$\begingroup\$ Transistors (bipolar) work because the temperature dependence of base voltage is stable and well defined, allowing a reasonably accurate design. If you want to experiment with you FET, you'll need to be sure that the characteristic you want to use is repeatable and doesn't vary between devices, or you'll end up in the weeds trying to achieve the accuracy you need. \$\endgroup\$
    – user16324
    Commented May 1, 2023 at 11:21
  • 1
    \$\begingroup\$ @user_1818839 So if I were to build a one-off design using some calibration equipment it should work. But it's not necessarily repeatable without matching FETs? \$\endgroup\$ Commented May 1, 2023 at 11:41
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    \$\begingroup\$ How long it'll stay in calibration I couldn't say, but yes, you get the idea. \$\endgroup\$
    – user16324
    Commented May 1, 2023 at 11:54
  • \$\begingroup\$ @user_1818839 Meaning that the parameters of MOSFETs are less stable over time vs BJTs? If the answer is yes, then does this effect apply more to a conventional MOSFET design, a HEXFET design, or multilayer MOSFETs with many paralleled PN junctions? Would a trench design be more or less stable over time than a planar? I am aware of the unavoidable effect of holes appearing in the PN junctions. All these questions are just for theory. You've made the point that MOSFET is not the correct solution to this problem. Thanks for your time. \$\endgroup\$ Commented May 1, 2023 at 13:23

1 Answer 1


Vbe of a diode-connected transistor is a fairly predictable and stable characteristic of a BJT. It is quite linear and yields a large signal (-2mV/K, approximately). It would probably require at least single point calibration in your application, since variability is of the order of +/-10°C without calibration. I expect you will use this because it is very, very simple (resistor + transistor -> ADC) and "good enough" for your application. Now, for general interest--

Better is difference in Vbe of two matched BJT devices at exactly the same temperature (i.e. on the same die) at two currents differing by a large factor such as 10:1. Or Vbe of a single device at two currents, switched fast enough that the temperature does not vary between measurements. For a device with low base spreading resistance such as an BC547 it is almost independent of individual device characteristics. Diode-connected transistors have an ideality factor close to one and varying little from that (perhaps 1.008 or so) This yields a smaller signal (order of 200uV/K) but more than 10x more accurate without calibration (proportional to absolute temperature) - typically better than +/-1°C.

Almost every characteristic of every component has some temperature dependence so one can't say that any given idea is unsuitable, however some are definitely better than others, and I would say the Vbe of a diode-connected transistor or delta-Vbe is very good. The two-current method is the same principle used to measure the die temperature in your computer CPU, for example, and the PTAT voltage is used in band-gap voltage references.

If r is the ratio between currents, the voltage difference is

\$\Delta V_{BE} = \frac{nkT}{q} \ln(r)\$

where n is the ideality factor, k is Boltzman's constant, T is temperature (Kelvin) and q is the charge on an electron.

Note that, except for n (which is very close to 1.00 for a diode-connected transistor, and does not vary much with a given type of transistor), there are only fundamental constants in the equation plus the current ratio (which you control).

(there are a few other sources of error but that's a very good first approximation)

The key thing is that Is (saturation current) in the Ebers-Moll equation is eliminated. Is varies widely and is not constant with temperature.

  • \$\begingroup\$ Thank you for the response and your time sir \$\endgroup\$ Commented May 2, 2023 at 3:55
  • \$\begingroup\$ Like you said, diode connected BJT is incredibly linear across this temperature range. I simulate about 3mV/°C which is plenty to work with. I have a perfect SMD BJT that should yield quick temperature response. Thanks again. \$\endgroup\$ Commented May 2, 2023 at 13:11

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