As its name suggests, a photodiode responds to light (‘photo’ = light). It is operated in reverse bias. When light of varying intensity is incident on it, a current is produced proportionally. Because of the reverse bias, the current is minimum when it is dark and the current is maximum when it is bright.
Photodiodes can be used anywhere light intensity can trigger a circuit, such as in burglar alarms, counting devices, automatic doors on buildings or optical fibre signal receivers for telecommunications.
anode cathode FIG. 33.29b A photodiode circuit symbol Transistors
Transistors are one of the most common electrical devices in the world. They allow for extremely fast switching and amplifying of signals based on a simple three-layer arrangement of semiconductors. The transistor replaced old vacuum tubes used in early electronic equipment.
The two main types of transistor used are NPN and PNP. The letters refer to the semiconductor make-up of the material used.
We will look at the NPN transistor, which is the most common. The NPN bipolar transistor is made up of three main parts (Fig. 33.30):
• base: a thin layer of doped p-type semiconductor • collector: a thick layer of doped n-type semiconductor • emitter: a thick layer of doped n-type semiconductor.
NPN base collector NOTE
Chapter 25 explained the positive (p)/negative (n) doping required for semiconductor layers.
FIG. 33.29a A sunshine detector based on a photodiode
A photodiode is a semiconductor that responds proportionally to light intensity.
NOTE
Light-dependent resistors (LDRs) vary resistance with light intensity. Photodiodes work by a depletion layer change from incident light. Because of this, photodiodes tend to be used when more sensitivity is required in measurement and response.
emitter FIG. 33.30a An NPN transistor FIG. 33.30b Transistors arranged on a circuit board
