Tubes, Discrete Solid State Devices, and Integrated Circuits 325false-triggers the SCR and starts its conducting without
gate control current. Apart from this time-related
parameter and its gate characteristics, SCR ratings are
similar to those for power diodes.
SCRs can be used to control dc by using commu-
tating circuits to shut them off. These are not needed on
ac since the anode supply voltage reverses every half
cycle. SCRs can be used in pairs or sets of pairs to
generate ac from dc in inverters. They are also used as
protective devices to protect against excessive voltage
by acting as a short-circuit switch. These are commonly
used in power supply crowbar overvoltage protection
circuits. SCRs are also used to provide switched
power-amplification, as in solid-state relays.
Triacs. The triac in Fig. 12-18 is a three-terminal semi-
conductor that behaves like two SCRs connected back
to front in parallel so that they conduct power in both
directions under control of a single gate-control circuit.
Triacs are widely used to control ac power by phase
shifting or delaying the gate-control signal for some
fraction of the half cycle during which the power diode
could be conducting. Light dimmers found in homes
and offices and variable-speed drills are good examples
of triac applications.
Light-Activated Silicon-Controlled Rectifiers. When
sufficient light falls on the exposed gate junction, the
SCR is turned on just as if the gate-control current were
flowing. The gate terminal is also provided for optional
use in some circuits. These devices are used in projector
controls, positioning controls, photo relays, slave
flashes, and security protection systems.
Diacs. The diac is shown in Fig. 12-19. It acts as two
zener (or avalanche) diodes connected in series, back to
back. When the voltage across the diac in either direc-
tion gets large enough, one of the zeners breaks down.
The action drops the voltage to a lower level, causing a
current increase in the associated circuit. This device is
Opto-Coupled Silicon-Controlled Rectifiers. An opto-
coupled SCR is a combination of a light-emitting diode
(LED) and a photo silicon-controlled rectifier
(photo-SCR). When sufficient current is forced through
the LED, it emits an infrared radiation that triggers the
gate of the photo-SCR. A small control current can
regulate a large load current, and the device provides
insulation and isolation between the control circuit (the
LED) and the load circuit (the SCR). Opto-coupled
transistors and Darlington transistors that operate on the
same principle will be discussed later.12.2.4 TransistorsThere are many different types of transistors,^1 and they
are named by the way they are grown, or made. Fig.
12-20A shows the construction of a grown-junction
transistor. An alloy-junction transistor is shown in Fig.
12-20B. During the manufacture of the material for a
grown junction, the impurity content of the semicon-
ductor is altered to provide npn or pnp regions. The
grown material is cut into small sections, and contacts
are attached to the regions. In the alloy-junction type,
small dots of n- or p-type impurity elements are
attached to either side of a thin wafer of p- or n-type
semiconductor material to form regions for the emitter
and collector junctions. The base connection is made to
the original semiconductor material.
Drift-field transistors, Fig. 12-20C, employ a modi-
fied alloy junction in which the impurity concentration
in the wafer is diffused or graded. The drift field speeds
up the current flow and extends the frequency response
of the alloy-junction transistor. A variation of the
drift-field transistor is the microalloy diffused tran-
sistor, as shown in Fig. 12-20D. Very narrow base
dimensions are achieved by etching techniques,
resulting in a shortened current path to the collector.
Mesa transistors shown in Fig. 12-20E use the orig-
inal semiconductor material as the collector, with theFigure 12-18. Schematic of a triac.
Anode (2)/cathode (1)GateAnode (1)/cathode (2)Figure 12-19. Schematic of a diac.CurrentVoltage