comprises a sequence of pnpn-zones. Just as in a diode, the external p-
and n-zones constitute the anode and cathode of the thyristor. In addition, a control
contact (gate) is attached to the internal p-doped layer, via which the thyristor can be
brought into a conducting state. If the thyristor is operated in the reverse direction, both
pn-junctions can take the adjacent voltage. If a voltage is applied in forward direction
and there is no positive voltage at the gate with regard to the cathode, the central pn-
Layer sequence, equivalent circuit diagram, circuit symbol and I/U diagram of
a thyristor (not to scale)
If, with forward voltage applied, a positive voltage is switched between the gate and the
cathode that can supply sufficient current so that it can supply charge carriers to the
central pn-junction, this junction becomes conductive and the thyristor starts conducting.
The thyristor remains in a conductive state even when the gate voltage is turned off,
because the load current, which is now flowing, ensures that the central pn-junction is
flooded with charge carriers. In this way, the thyristor can only be turned off if the load
current flowing through it is less than the required holding current I
. This behaviour is
known as "latch-up" and can be described by the equivalent circuit diagram i
Once the npn-transistor has been turned on by a positive current/voltage pulse, it
provides the base current of the pnp-transistor. This, in turn, provides the base current
for the npn-transistor so that even if there is no external voltage applied to the gate, both
transistors stay turnedon, as long as current is flowing through them.
Another way of firing a thyristor is to apply a forward voltage U
that exceeds the
breakdown voltage of the central pn-junction. The thyristor will fire overhead. This
operating status is critical and should not occur regularly in the application.
Based on the standard thyristor, further high power components have been developed,
including the gate turn-off thyristor (GTO), integrated gate commutated thyristor (IGCT)
and light triggered thyristor (LTT).
The first thyristor was developed byGeneral Electric in theUnitedStates in 1957. Thyristors are also known as
silicon controlled rectifiers (SCRs) or electrically triggered thyristors (ETTs).