Page 335 - Sensors and Control Systems in Manufacturing

Advanced Sensors in Pr ecision Manufacturing
Photodiodes offer many conveniences and advantages that make 291
them very practical for a wide range of applications:

• They can easily measure from picowatts to milliwatts of opti-
cal power.
• They come in standard packages, or the package can be tooled
to the application.
• Depending on the semiconductor material used, they can
detect wavelengths from 190 to over 2,000 nm.
• They are small and lightweight.
• Almost any photosensitive shape can be fabricated for small
capital investment as low as $3,000 for tooling.
• They have very reproducible sensitivity.
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• Very large areas can be fabricated: > 10 cm .
• They can be very responsive, with rise-times as fast as
10 picoseconds.
If noise presents an obstacle when measuring a few picowatts of
light with a standard photodiode, consider the advantages of an ava-
lanche photodiode (APD) which offers a current gain internal to the
photodiode structure of up to 100 times.
Photodiodes generally require a pre-amplifier to give signal gain
for applications to detect picowatts of light power. But for high optical
power of <10 microwatt levels, a simple load resistor configuration
can give adequate performance and TTL-compatible voltage swings.
Silicon based photodiodes cover the wide range of wavelengths
from 190 to 1100 nm. The lower limit is set by absorption of ultravio-
let light in air. Germanium (Ge) photodiodes overlap the silicon
response spectrum and are usable to about 1600 nm. Semiconductors
that are compounds of gallium, arsenide, indium, antimonide, and
phosphorous can be specially fabricated to cover small sections of the
190-to-2000-nm spectral range. For instance, the fiber optics industry
uses indium-gallium-arsenide “InGaAs” detectors for the 800 to 1800 nm
range. More exotic and expensive photodiodes can sense energy
much further out in the IR spectrum.
Photodiodes are widely used in high-tech societies, in applica-
tions ranging from sensors for door openings, assembly line controls,
load levelers in luxury cars, to personal blood sugar meters for diabetics,
sun-tan exposure meters, smoke detectors, x-ray baggage inspection
systems, and even cranial pressure sensors for head injury patients.

6.6 Phototransistors and Photodarlingtons
Phototransistors are transistors designed to capture light and are
assembled in a transparent package. They are often more convenient