Thyristor Family and other Semiconductor Devices
The P-N-P-N devices with zero, one or two gates constitute the basic thyristor. But today the
thyristor family includes other similar multilayer devices also. The complete list of thyristor
family members include diac (bidirectional diode thyristor), triac (bidirectional triode thyristor),
SCR (silicon controlled rectifier), Shockley diode, SCS (silicon controlled switch), SBS (silicon
bilateral switch), SUS (silicon unilateral switch) also known as complementary SCR or CSCR,
LASCR (light activated SCR),`s, namely, the anode, the cathode and the gate. It is a unilateral
device and conduction takes place from anode to cathode under proper bias conditions (forward
bias).
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Diac, DIode for Alternating Current (bidirectional diode thyristor) is a two-terminal, three
layer device and is commonly used for triggering triacs.
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Triac, TRIode for Alternating Current (bidirectional triode thyristor) is a 3-terminal
semiconductor device and may be considered equivalent to two SCRs connected
in antiparallel.
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SCS (silicon controlled switch) is similar to SCR except that it has two gates and can be turned-
on or off by either gate. It is also a low-powered SCR with two gate terminals. If the anode-
cathode current is less than 4mA, the cathode gate (Gc) is used, and if the current is greater than
4mA, the anode gate (GA) is used. The gate in this device are used to switch the device off. This
kind of device provides both a positive and a negative pulse.
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Shockley diode is a two terminal reverse blocking diode thyristor having no gate it is also called
the four-layer diode, is a unidirectional diac, which is basically an SUS without a gate terminal.
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SBS, Silicon Bilateral Switch is a device consisting of two identical SUS structures arranged
in antiparallel but having only one gate, that is used only for external synchronization or for
proper biasing.
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SUS, Silicon Unilateral Switch also known as complementary SCR or CSCR has gate on the
anode side and can be employed as a programmable unijunction transistor (PUJT).
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LASCR (light activated SCR) is the light-activated SCR which is turned on by photon
bombardment.
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LAS (light activated switch) light activated switch means a switch that becomes either ON or
OFF when the light intensity crosses a set value. You can connect any device like bulb, motor,
alarm etc through this switch.
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LASCS (light activated SCS) it is an SCS that operate in a the influence of a light
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UJT, UniJunction Transistor unlike a bipolar transistor has only one junction, and like other
conventional transistors, it processes the transistor action and operates like a switch. The
characteristics of UJT are similar to those of a SUS. Its construction is, however, different and
it does not belong to thyristor family. Also, it is a solid-state device that has been specifically
designed to provide a sharp pulse when its breakover voltage level is reached and also called a
breakover voltage switch.
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CSCR (complementary SCR) uses a negative gate pulse instead of a positive pulse. This feature
is useful in circuits where the gate must be pulsed by the cathode side of the circuit.
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GTO (gate turn-off) device useful in circuits that use higher frequency up to 100 kHz. It is used
in place of a typical SCR and it provides a greater degree of control at the higher frequencies.
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PUT (Programmable Unijunction Transistor) is similar to the UJT in that it has the ability to
provide a pulse that is used to trigger SCRs and other thyristors.
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SIDAC (Silicon Diode Alternating Diode) A silicon diode for alternating current (SIDAC) is a less
commonly used device, electrically similar to the DIAC, but having, in general, a higher breakover voltage and
greater current handling capacity.
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ASBS (Asymmetrical Bilateral Switch) is similar to the silicon bilateral switch (SBS) except it
has two different firing levels.
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IGBT (Insulated-Gate Bipolar Transistor) is a three-terminal power semiconductor
device primarily used as an electronic switch which, as it was developed, came to combine high
efficiency and fast switching. It consists of four alternating layers (P-N-P-N) that are controlled
by a metal-oxide-semiconductor (MOS) gate structure without regenerative action.
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SAS (Silicon Asymmetrical Switch)
Thyristor modified by adding a semiconductor layer so that the device becomes a bidirectional
switch; used as an alternating current phase control, for synchronous switching and motor speed
control; it is similar to the silicon bilateral switch (SBS) the major difference is that the SAS has
a different firing voltage in the positive quadrant than it does in the negative quadrant
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