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 J_{1} and J_{2} junctions and another is with J_{2} and J_{3} 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 T_{1} this relation holds

And that for transistor T_{2}

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 T_{2} transistor i.e. I_{B2} = I_{g} and emitter current of the T_{2} transistor I_{E2} = I_{k}. Hence establishment of the emitter current gives rise α_{2} as

Presence of base current will generate collector current as

This I_{C2} is nothing but base current I_{B1} of transistor T_{1}, which will cause the flow of collector current,

I_{C1} and I_{B1} lead to increase I_{C1} as and hence, α_{1} increases. Now, new base current of T_{2} is , which will lead to increase emitter current and as a result α_{2} also increases and this further increases .

As , α_{1} 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.