Silicon Controlled Rectifier SCR | Two Transistor Model | Operating Principle

What is Thyristor or SCR?

It is a silicon based semiconductor device, which is used in electrical circuits for switching operation. SCR, whose full form is silicon controlled rectifier, is also a well known member of thyristor family. Although there are many different members are available in thyristor family, but silicon controlled rectifiers are so widely used that as if thyristor and SCR become synonymous.
The characteristic of thyristor consists of the characteristic of thyratron tube and characteristic of transistor.

In other words, it can be said, that the characteristic of thyristor is combination of characteristics of thyratron tube and transistor. That is why the name of thyristor consists of first four letters of thyratron tube and last five letters of transistor. [THYRItron + transISTOR].

The device has ideal states, i.e. On and OFF. Generally an SCR consists of two PN junctions, but sometimes it may also consist of more than two P-N junctions.

If we see from the constructional and operational point of view, it is a four layer (PNPN) three terminals (Anode, Cathode, Gate) semi controlled device. This device has tow states i.e. on and OFF. We can turn it ON by sending a gate current signal between second P layer and cathode. But we cannot turn it OFF by control signal. That means we have control upon its turn ON, once it goes to conduction mode, we lose control over it. It can block both forward and reverse voltage but can conduct only in one direction. In very high power application, like AC –DC converter, AC – AC converters, engineer’s first choice is always thyristor or silicon controlled rectifier due to its very low conduction loss.

Two Transistor Model of SCR

Basic operating principle of SCR, can be easily understood by the two transistor model of SCR or analogy of silicon controlled rectifier, as it is also a combination of P and N layers, shown in figure – 1.

schematic diagram of thyristor

Figure-1. Schematic Diagram of Thyristor

This is a pnpn thyristor. If we bisect it through the dotted line then we will get two transistors i.e. one pnp transistor with J1 and J2 junctions and another is with J2 and J3 junctions as shown in figure – 2.
schematic diagram of two transistor model

Figure-2. Schematic Diagram of Two Transistor Model

When the transistors are in off state, the relation between the collector current and emitter current is
two transistor model

Two Transistor Model

Here, IC is collector current, IE is emitter current, ICBO is forward leakage current, α is common base forward current gain and relationship between IC and IB is

Where, IB is base current and β is common emitter forward current gain.
Let’s for transistor T1 this relation holds

And that for transistor T2

Now, by the analysis of two transistors model we can get anode current,

From equation (i) and (ii), we get,

If applied gate current is Ig then cathode current will be the summation of anode current and gate current i.e.

By substituting this valyue of Ik in (iii) we get,

From this relation we can assure that with increasing the value of (α1 + α2) towards unity, corresponding anode current will increase. Now the question is how (α1 + α2) increasing? Here is the explanation using two transistor model of SCR.

At the first stage when we apply a gate current Ig, it acts as base current of T,2 transistor i.e. IB2 = Ig and emitter current i.e. Ik = Ig of the T,2 transistor. Hence establishment of the emitter current gives rise α2 as

Presence of base current will generate collector current as

This IC2 is nothing but base current IB1 of transistor T,1, which will cause the flow of collector current,

IC1 and IB1 lead to increase IC1 as Ia = IC1 + IB1 and hence, α1 increases. Now, new base current of T2 is Ig + IC1 = (1 + β1β2)Ig, which will lead to increase emitter current Ik = Ig + IC1 and as a result α2 also increases and this further increases IC2 = β2(1 + β1β2)Ig.

As IB1 = IC2, α1 again increases. This continuous positive feedback effect increases (α1 + α2) towards unity and anode current tends to flow at a very large value. The value current then can only be controlled by external resistance of the circuit.