- Two Transistor Model of SCR Definition: The two transistor model of SCR is defined as a representation of an SCR using two interconnected transistors, one pnp and one npn, to explain its operation.
- SCR Structure: An SCR consists of four layers (pnpn) and can be split into two transistors, making its function easier to understand.
- Anode Current Analysis: The anode current in an SCR is analyzed by considering the currents and gains of the two transistors involved.
- Gate Current Impact: Applying a gate current (Ig) increases the cathode current (Ik), affecting the overall anode current.
- Positive Feedback Effect: The positive feedback effect in the two-transistor model causes a rapid increase in anode current, which can be moderated by external resistance.
The two transistor model of SCR explains regenerative turn-on by representing the four-layer device as a cross-coupled PNP-NPN pair.
An SCR is a PNPN thyristor. Splitting the equivalent structure gives a PNP section using junctions J1 and J2, and an NPN section using junctions J2 and J3.
The equivalent transistors are interconnected so that each collector supplies base drive to the other:
For either equivalent transistor, IC is collector current, IE is emitter current, ICBO is collector-base leakage current with the emitter open, and α is common-base forward current gain.
Here IB is base current and β is common-emitter forward current gain.
For equivalent transistor T1, the current relation is:
For equivalent transistor T2, the corresponding relation follows:
Kirchhoff’s current relations connect these branch currents to the anode current:
Combining equations (i) and (ii) gives:
With applied gate current Ig, cathode current equals anode current plus gate current:
Substituting this value of Ik into equation (iii) gives the standard anode-current expression:
The denominator contains 
. As this gain sum approaches unity, the simple model predicts a sharp rise in anode current. The term changes with current and operating conditions; gate current starts the regenerative process rather than directly creating transistor gain. The feedback path in the two transistor model of SCR explains the turn-on sequence.
A positive gate current Ig adds base drive to T2. In the shown current notation, IB2 includes Ig, while the emitter current of T2, IE2, is the cathode current Ik. The common-base relation for α2 is:
The added base drive increases collector current according to:
The collector current IC2 supplies base current IB1 to T1, increasing its collector current:
The IC1 and IB1 relationship increases IC1 as shown by 
, while α1 varies with the higher device current. The new base drive for T2 is represented by 
. It raises emitter current 
, changes α2 and further increases 
.
The relation 
feeds more current back to T1, so α1 and the anode current rise again. This positive feedback drives toward the turn-on condition. Once anode current exceeds latching current, the gate pulse can be removed. The SCR remains on only while current stays above holding current. Actual current is limited by the supply, load, device voltage drop and external resistance; commutation or a current reduction below holding current turns it off.
