# Two Transistor Model of SCR or Thyristor

Published on 24/2/2012 & updated on 5/12/2018Basic 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.**Please Rate this Article**