Page 256 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

236 MICROSENSORS

at 300 K), an output on the order of n millivolts per degree can be achieved from a
thermopile. Polysilicon/gold thermocouples have also been made with an output of about
+0.4 mV/K in which the n-type (phosphorous) polysilicon has a lower Seebeck coeffi-
cient of -176 uV/K (for a sheet resistance 8 of 100 fi/sq at 300 K) and the gold has a
standard value of +194 uV/K. However, these are not standard IC process materials and
so polysilicon-based thermocouples are not the preferred fabrication route for low-cost
temperature microsensors.

8.2.3 Thermodiodes and Thermotransistors

The simplest and easiest way to make an integrated temperature sensor is to use a diode
or transistor in a standard IC process. There are five ways in a bipolar process and three
ways in a CMOS process to make a p-n diode (see Table 4.2). The I-V characteristic
of a p-n diode is nonlinear (Figure 4.19) and follows Equation (4.14), which is repeated
here for the sake of convenience:

(8.8)

where I S is the saturation current, typically 1 nA and X is an empirical scaling factor that
takes a value of 0.5 for an ideal diode. Rearranging Equation (8.8) in terms of the diode
voltage gives

V = (8.9)

Therefore, when the diode is operated in a constant current circuit (see Figure 8.9(a)),
I 0
the forward diode voltage V out is directly proportional to the absolute temperature 9 and

Thermodiode Thermotransistor

'BE

(a)

(b)
Figure 8.9 Basic temperature microsensors: (a) a forward-biased p-n diode and (b) an n-p-n
transistor in a common emitter configuration with VCE set to zero

8
The resistance of a square piece of material is independent of its size.
9
Sometimes called a proportional to absolute temperature (PTAT) device.