3-53
OPTOELECTRONIC DEVICES are of two basic types: light producers or light users. The LED is
the most widely used light-producing device. When the LED is forward biased it emits energy in the form
of light. LEDs are used in several configurations as digital equipment readout displays. The
PHOTODIODE, the PHOTOTRANSISTOR, and the PHOTOCELL are all devices that use light to
modify conduction through them. The SOLAR CELL uses light to produce voltage.
The UNIJUNCTION TRANSISTOR (UJT) is a three-terminal, solid-state device with only one
PN junction. The block diagram below shows the difference in construction between normal transistors
and the UJT. The area between base 1 and base 2 of the UJT acts as a variable resister. The emitter of the
UJT acts as the wiper arm. The sequential rise in voltage between the bases is called a voltage gradient.
The UJT conducts when the emitter is more positive than the voltage gradient at the emitter/base contact
point. There are many variations of the UJT which are used in switching circuits, oscillators, and wave-
shaping circuits.
The FIELD-EFFECT TRANSISTOR combines the high input impedance of the vacuum tube with
all the other advantages of the transistor. The elements of the FET are the gate, source, and drain, which
are comparable to the base, emitter, and collector of a standard transistor. The JFET or "junction FET" is
made of a solid bar of either P- or N-semiconductor material, and the gate is made of the opposite type
material, as illustrated below. The FET is called P-channel or N-channel depending upon the type of
material used to make the bar between the source and drain. Voltage applied to the gate controls the width
of the channel and consequently controls the current flow from the source to the drain. The JFET is
normally operated with reverse bias that controls the channel width by increasing or decreasing the
depletion region.
