Page 58 - Power Electronics Handbook

photothyristors 51

maintained for a longer period, during turn-on, to ensure that latching current
is reached.
As expected, it is in the turn-off process that the GTO differs most from
a thyristor. Prior to turn-off all the regions are heavily saturated, and the
excess charge needs to be removed before turn-off can occur, resulting in
storage time and fall time. The excess charge is first removed from the p1
layer, in the vicinity of the gate terminal, and this recovered region then
spreads over the whole junction. Current continues to flow by squeezing
into parts of the junction not yet turned off, until eventually all the region
recovers and the storage period ends. The effect of turn-off is therefore
similar to the turn-on of a thyristor, where the current initially squeezes
into the small turned-on region closest to the gate. Two parameters are
quoted in GTO data sheets, which are not given for thyristors, the gate
turn-off voltage (V,) and the gate turn-off current (Z&. The turn-off
time, turn-on time and turn-off gain are all degraded with temperature
increase.

1.11 Wacs

A four-layer device, such as a thyristor, can conduct in one direction only,
and for operation in an a.c. circuit two thyristors must be co~e~ted in a
back-to-back mode. A triac, or TRIode AC semiconductor switch, is
designed to be able to conduct in both directions, the onset of conduction
being controlled by a gate, as for a thyristor. The triac is a five-layer
device, as shown in Figure 1.29, which can operate in quadrants I or III. It
can be triggered by current into (plus) or out of (minus) the gate terminal,
so that the four operating modes are I(plus), I(minw), III(plus), and
III(minus). Generally the triac is relatively insensitive in mode III(p1us) so
that it is normally operated in I@lus) and III(minus).
For mode I(p1us) terminal is positive with respect to L1, and the gate
is positive to L1, so that the device is in essence a thyristor with layers
plnzpzn3 giving the p-n-p-n arrangement and the gate terminal at p2. For
mode III(minus), p2nglnl are the active layers, and the p2 and n3 regions
are forward biased causing emission of electrons into the main body of the
device and eventual turn-on.
Triac ratings and characteristics are very similar to those of a thyristor,
except that they apply to two quadrants of operation. However, because a
triac conducts in both half cycles of an a.c. waveform it does not have time,
as in the case of a thyristor, to adequately recover its blocking capability,
so its dv/dt characteristic is poorer than that of a thyristor.

1.12 Photothyristors

Photothyristors have the same basic structure as conventional thyristors
and function in similar modes, except that they are triggered by light
instead of gate current. The symbol for the photothyristor, also ded a
light-activated SCR (LASCR) or a light-triggered thyristor (LTT'), is
shown in Figure 1.30(a), and it is seen that the gate terminal is sometimes