Two Transistor Model of SCR or Thyristor

Basic operating principle of SCR, can easily be understood by the two transistor model of SCR, as it is a combination of p and n layers.

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 below.

The relation between the collector current and emitter current is shown below

Here, I_C is collector current, I_E is emitter current, I_CBO is forward leakage current, α is common base forward current gain and relationship between I_C and I_B is

Where, I_B 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 I_g then cathode current will be the summation of anode current and gate current i.e.

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

From this relation we can assure that with increasing the value of towards unity, corresponding anode current will increase. Now the question is how increasing? Here is the explanation using two transistor model of SCR.
At the first stage when we apply a gate current I_g, it acts as base current of T₂ transistor i.e. I_B2 = I_g and emitter current of the T₂ transistor I_E2 = I_k. Hence establishment of the emitter current gives rise α₂ as

Presence of base current will generate collector current as

This I_C2 is nothing but base current I_B1 of transistor T₁, which will cause the flow of collector current,

I_C1 and I_B1 lead to increase I_C1 as and hence, α₁ increases. Now, new base current of T₂ is , which will lead to increase emitter current and as a result α₂ also increases and this further increases .
As , α₁ again increases. This continuous positive feedback effect increases 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.