Two transistor model is used to explain the principle of operation of the thyristor in a very simple way. Since a thyristor is a four-layered PNPN device, it can be considered as a combination of two transistors, one transistor as PNP and the other transistor as NPN. A two-transistor model is obtained by separating the two middle layers of the thyristor into two parts as shown in the below figure.

In this model, base current I_b1 of transistor Q₁ is equal to the collector current I_c2 of transistor Q₂ and vice-versa. The equivalent circuit of the two transistor analogy is shown below.

From the above equivalent circuit, the base and collector currents of the transistors Q₁ and Q₂ can be written as,

I_b1 = I_c2 and I_c1 = I_b2 …(1)

Also, from the above figure, we have, cathode current is the sum of anode current and gate current.

I_k = I_a + I_g …(2)

We know that, collector current I_c is related to emitter current I_e as,

I_c = αI_e + I_cbo

Where,

• α = Common-base current gain ≃ I_c/I_e
• I_cbo = Leakage current of collector-base.

Hence, in the given equivalent circuit,

I_c1 = α₁ I_e1 + I_cbo1 and

I_c2 = α₂ I_e2 + I_cbo2 …(3)

But, I_e1 of Q₁ = I_a and I_e2 of Q₂ = I_k. Subsituting the values of I_e1, I_e2 in equation 3, we get,

I_c1 = α₁ I_a + I_cbo1 and

I_c2 = α₂ I_k + I_cbo2 …(4)

The sum of two collector currents is equal to an anode current. Therefore,

I_a = I_c1 + I_c2 …(5)

Substituting equation 4 in equation 5, we get,

I_a = α₁ I_a + I_cbo1 + α₂ I_k + I_cbo2 …(6)

From transistor analysis, we have,

I_e1 = I_b1 + I_c1

I_b1 = I_e1 – I_c1 …(7)

Substituting equation 2 in equation 6, we get,

If the leakage currents I_cbo1 and I_cbo2 of transistors Q₁ and Q₂ are negligibly small, then,

From the above equation, it can be analyzed that, if α₁ + α₁ = 1, then anode current I_a becomes infinity and hence thyristor enters into conduction state from non-conduction state. In other words, if I_g reaches a value such that α₁ + α₂ = 1, then SCR is turned ON and this action is called the Regenerative action.

The regenerative action i.e., the condition α₁ + α₁ = 1 can be obtained in the following ways,

• The increase in temperature increases the leakage current by which the required above condition can be obtained to turn on the SCR.
• The SCR can be also turned ON by increasing α₁ and α₂. Since α₂ depends on (I_a + I_g) and α₁ on I_a, the current gain of the transistor Q₂ can be increased without affecting voltage V_a and current I_a by injecting the current I_g through the gate in the same direction of I_a. Thus the above condition can be obtained.