Page 264 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

244 MICROSENSORS

p-n type (Si or GaAsP) PIN type (Si)

"} hv SiO, S hv
SiO 2

(a) (b)

Schottky type (GaAsP, GaP) Avalanche type (Si)

Au SiO, S hv

Figure 8.16 Some different types of radiation microsensors: (a) p-n diode; (b) PIN diode;
(c) Schottky diode; and (d) Avalanche diode

Then the difference in base-emitter voltages is simply related to I CR, which is proportional
to the illuminance I R, by

A VBE = (V BEi - V BE2) = — In ( ^] oc ln(/ CR) (8.20)
e V /co /
Thus, photodiodes and phototransistors not only have fast response times but can also have
integrated circuitry with, for example, a linearised output voltage or internal temperature
compensation.

8.3.3 Pyroelectric Devices

A pyroelectric material can also be used to make a coulombic radiation sensor and it works
on the principle that the radiation heats up the surface of a pyroelectric crystal (usually
LiTaO 3) and reduces the normal polarisation state of the crystal, thereby inducing the
charge to flow off its surface and creating a voltage. Pyroelectric sensors are not very
sensitive to the wavelength of the radiation and cover a wide dynamic range in the IR
spectrum, as illustrated in Figure 8.15. Their dynamic response depends on the thermal
time constant of the device and the electrical time constant of the associated circuit. The
read-out circuit needs to have a very high input impedance and so is usually CMOS
technology.
Pyroelectric sensors are best-suited for detecting short-term changes in the IR signal,
and the most popular application is to detect the slow movement of a human body in