Page 263 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

RADIATION SENSORS 243

UV Vis Near IR Thermal IR
Wavelength (Um)
I 1 I T i r 1
0.2 0.4 0.6 0.8 1 8 10 20
Si

Ge
PbS
PbSe
InAs (77 K)
InSb(77K)
MCT (77 K)
Pyroelectric detectors

Figure 8.15 Some common semiconducting materials used in radiation microsensors and their
dynamic range within the UV-to-IR spectrum
The most obvious advantage of a photovoltaic cell over a photoconductive cell is that it
is compatible with a bipolar process (e.g. Si or GaAs). However, the reduced size and
integrated electronics lead to a higher sensitivity, faster response time (us instead of ms),
and better stability.
Photosensitive diodes, known as photodiodes, can be made in a standard vertical bipolar
process (see Section 4.3.4) such as a p-n diode, or variations on this process, such as
a /7-Si/insulator/n-Si PIN diode, Schottky-type diode, and silicon avalanche diode (see
Figure 8.16).
The basic principle of all these photodiodes is that the photon creates an electron-hole
pair in the space-charge region of the junction. These charges are then separated by the
local field to the different doped regions, and they modify the diode voltage V d. The diode
voltage is the open circuit voltage V oc and can be measured by reverse-biasing the diode
and finding the voltage dropped across a high external load resistor R L. Then the output
voltage is given by

In — I and so V oc a In(I R) (8.19)

where I R is the photocurrent and is proportional to the intensity of incident radiation
and, as usual, I s is the reverse saturation current. Indeed, this equation is the same one
that applied to a thermodiode (see Section 8.2.1 and Equation (8.9)), and so the device
will normally be temperature-sensitive. Thus, the performance of a photodiode can be
improved by either running it at a constant voltage - as low as possible to reduce junction
noise - or it must be temperature-compensated.
An alternative approach is to fabricate a phototransistor rather than a diode. Like
a PTAT device, an IC can be built using two identical transistors; the first produces a
photoinduced collector current I CR and the other produces a reference collector current I C0.