Thyristors
by Chris Woodford. Last updated: May 29, 2021.
Transistors are the tiny electronic components
that changed the world: you'll find them in
everything from calculators and
computers to
telephones, radios, and
hearing aids. They're amazingly versatile, but that doesn't mean they can do everything. Although we can use them to switch tiny electrical currents on and
off (that's the basic principle behind computer memory), and
transform small currents into somewhat larger ones (that's how an
amplifier works), they're not very useful when it comes to handling
much bigger currents. Another drawback is that they turn off
altogether as soon as the switching current is removed, which means
they're not so useful in devices such as alarms where you want a
circuit to trigger and stay on indefinitely. For those sorts of jobs,
we can turn to a somewhat similar electronic component called a
thyristor, which has things in common with
diodes, resistors,
and transistors. Thryristors are reasonably easy to understand,
though most of the explanations you'll find online are unnecessarily
complex and often confusing beyond belief. So that's our starting
point: let's see if we can take a clear and simple look at what
thyristors are, how they work, and what kinds of
things we can use them for!
Artwork: A typical thyristor looks a bit like a transistor—and works in a
closely related way.
What are thyristors?
First, let's nail some terminology. Some people
use the term silicon-controlled rectifier (SCR)
interchangeably with "thyristor." In fact, silicon-controlled
rectifier is a brand name that General Electric introduced to
describe one particular kind of thyristor that it made. There are
various other kinds of thyristors too (including ones called
diacs and triacs, which
are designed to work with alternating current), so the terms aren't completely
synonymous. Nevertheless, this article is about keeping things
simple, so we'll just talk about thyristors in the most general
terms and assume SCRs are exactly the same thing. We'll refer to them as thyristors throughout.
Photo: Thyristors are widely used in electronic power control circuits like this one.
Three connections
So what is a thyristor? It's an electronic
component with three leads called the anode (positive terminal),
cathode (negative terminal), and gate. These are somewhat analogous
to the three leads on a transistor, which you'll remember are called
the emitter, collector, and base (for a conventional transistor) or
the source, drain, and gate (in a field-effect transistor, or FET).
In a conventional transistor, one of the three leads (the base) acts
as a control that regulates how much current flows between the other
two leads. The same is true of a thyristor: the gate controls the
current that flows between the anode and the cathode.
(It's worth noting that you can get thryistors
with two or four leads, as well as three-lead ones. But we're keeping
things simple here, so we'll just talk about the most common variety.)
Transistors versus thyristors
If a transistor and a thyristor do the same job,
what's the difference between them? With a transistor, when a small
current flows into the base, it makes a larger current flow between
the emitter and the collector. In other words, it acts as both a
switch and an amplifier at the same time:
How a transistor works: A small current flowing into the base makes a larger current flow between the emitter and the collector. This is an n-p-n transistor with red indicating n-type silicon, blue indicating p-type, black dots representing electrons and white dots representing holes.
A similar thing happens inside a FET, except that we apply a small voltage to the gate to produce an
electric field that helps a current flow from the source to the
drain. If we remove the small current at the base (or gate), the large current
immediately stops flowing from the emitter to the collector (or from the source to the drain in a FET).
Now often that's not what we want to happen. In
something like an intruder alarm circuit (where maybe an intruder
steps on a pressure pad and the bells start ringing), we want the
small current (activated by the pressure pad) to trip the larger
current (the ringing bells) and for the larger current to keep on flowing
even when the smaller current stops (so the bells still ring even if
our hapless intruder realizes his mistake and steps back off the pad). In a thyristor, that's
exactly what happens. A small current at the gate triggers a much
larger current between the anode and the cathode. But even if we then
remove the gate current, the larger current keeps on flowing from the
anode to the cathode. In other words, the thyristor stays ("latches") on
and remains in that state until the circuit is reset.
Where a transistor generally deals with tiny electronic
currents (milliamps), a thyristor can handle real (electric)
power currents (several hundred volts and 5–10 amps is typical).
That's why we can use them in such things as factory power switches,
speed controls for electric motors,
household dimmer switches, car ignition switches,
surge protectors, and
thermostats. Switching time
is practically instantaneous (measured in microseconds), and that useful feature,
coupled with a lack of moving parts and high reliability, is why thyristors are often used
as electronic (solid-state) versions of relays
(electromagnetic switches).
How does a thyristor work?
Thyristors are a logical extension of diodes and
transistors, so let's briefly recap on those components. If
you're unfamiliar with solid-state electronics, we have longer and
clearer explanations of how diodes work and
and how transistors work,
which you might like to read first.
A thyristor is like two diodes
Recall that a diode is two layers of semiconductor
(p-type and n-type) sandwiched together to produce a junction
where interesting things happen. According to how you wire up a
diode, current will either flow through it or not, making it the
electronic equivalent of a one-way street. With a positive connection
to the p-type (blue) and a negative connection to the n-type (red), a diode is
forward biased, so electrons (black dots) and holes (white dots) move
happily across the junction and a normal current flows:
A forward-biased diode: current flows across the junction between the p-type (blue) and n-type (red), carried by electrons (black dots) and holes (white dots).
In the opposite configuration, with a positive connection to the n-type and a
negative to the p-type, a diode is reverse biased: the
junction becomes a huge chasm that electrons and holes can't cross
and no current flows:
A reverse-biased diode: with the battery connected the other way, the "depletion zone" at the junction gets wider, so no current flows.
In a transistor, we have three layers of semiconductor arranged alternately (either p-n-p or n-p-n), giving
two junctions where interesting things can happen. (A FET is slightly
different, with extra layers of metal and oxide, but still
essentially an n-p-n or p-n-p sandwich.). A thyristor is simply the next step in the
sequence: four layers of semiconductor, again arranged alternately to
give us p-n-p-n (or n-p-n-p if you swap it around) with three
junctions in between them. The anode connects to the outer p layer,
the cathode to the outer n layer, and the gate to the internal p
layer, like this:
A thyristor is like two junction diodes connected together, but with an extra connection to one of the inner layers—the "gate."
You can see that this resembles two junction diodes connected in series, but with the extra gate connection at the bottom.
Just like a diode, a thyristor is a rectifier: it conducts in only one direction. You can't make a thyristor simply by wiring two diodes in series: the extra gate connection means there's more to it than that. If you're quite familiar with electronics, you'll note the resemblance between a thyristor and a Shockley diode (a kind of double diode with
four alternating semiconductor layers, invented by Transistor pioneer William Shockley
in 1956). Thyristors evolved from Shockley's transistor and diode work,
which was further developed by Jewell James Ebers,
who developed the two-transistor model we're going to cover next.
Artwork: General Electric introduced the first commercially successful thyristor (then called a silicon-controlled rectifier) in July 1957, thanks to the efforts of Robert Hall, Nick Holonyak, F. W. "Bill" Gutzwiller,
and others. This is a basic illustration of a thyristor from one of Bill Gutzwiller's patents. Artwork from
US Patent 3,040,270: Silicon controlled rectifier circuit including a variable frequency oscillator courtesy of US Patent and Trademark Office.
A thyristor is like two transistors
What's less obvious is that the four layers work like two
transistors (an n-p-n and a p-n-p) that are connected together so the
output from one forms the input to the other. The gate serves
as a kind of "starter motor" to activate them.
A thyristor is also like two transistors connected together, so the output from each one serves as the input to the other one.
The three states of a thyristor
So how does this thing work? We can put it into three possible states, in all three of which it's either completely off or completely on, which means it's essentially a binary, digital device. To understand how these states work, it helps to keep diodes and transistors in mind:
Forward blocking
Normally, with no current flowing into the gate, a thyristor is switched off: no current can flow from the
anode to the cathode. Why? Think of the thyristor as two diodes joined
together. The upper diode and the lower diode are both forward biased.
However, that means the junction in the center is reverse biased, so there's no way for current to
get all the way from the top to the bottom. This state is called forward
blocking. Although it's similar to forward-bias in a conventional diode, no current flows.
Reverse blocking
Suppose we reverse the anode/cathode connections. Now you can probably see that both the
upper and lower diodes are reverse biased, so still no current flows through the thyristor. This is
called reverse blocking (and it's analogous to reverse bias in a simple diode).
Forward conducting
The third state is the really interesting one. We need the anode to be
positive and the cathode negative. Then, when a current flows into the gate, it
switches on the lower transistor, which switches on the upper one,
which switches on the lower one, and so on. Each transistor
activates the other. We can think of this as a kind of internal, positive feedback in which the two transistors keep feeding current to each other
until both of them are fully activated, at which point current can flow through them
both from the anode to the cathode. This state is called forward conducting and it's how a
thyristor "latches" (stays permanently) on. Once a thyristor is latched
on like this, you can't turn it off simply by removing the current to the
gate: at this point, the gate current is irrelevant—and you have to
interrupt the main current flowing through from the anode to the
cathode, often by switching off power to the entire circuit. Don't follow that?
Check out the animation in the box below, which I hope will make it clear.
Types of thyristors
Somewhat simplified, that's the crux of how a
thyristor works. There are numerous variations, including
gate-turn off (GTO) devices
(that can be turned on or off by the action of the gate), AGT (anode gate thyristor)
devices that have a gate going to the internal n-type layer near the anode (instead of the p-type layer near the cathode),
photoelectric thyristors in which the base is activated by light, and all kinds of others. But they all work in broadly similar ways,
with a gate tripping one transistor, which then trips the other.
