Introduction:
The two-transistor model of a thyristor is a simplified representation of a thyristor using two transistors. It is a commonly used model to understand the basic working principle of a thyristor. In this model, one transistor is of n-p-n type and the other is of p-n-p type.

Explanation:
The two-transistor model of a thyristor consists of the following:

N-P-N Transistor:
The n-p-n transistor in the model represents the n-p-n structure present in a thyristor. It consists of three layers, namely, the n-type layer (emitter), the p-type layer (base), and the n-type layer (collector). The thyristor operates in the forward-active mode, where the base-emitter junction is forward biased, and the base-collector junction is reverse biased.

P-N-P Transistor:
The p-n-p transistor in the model represents the p-n-p structure present in a thyristor. It also consists of three layers, namely, the p-type layer (emitter), the n-type layer (base), and the p-type layer (collector). The p-n-p transistor operates in the reverse-active mode, where the base-emitter junction is reverse biased, and the base-collector junction is forward biased.

Working:
The two-transistor model of a thyristor represents the four layers of a thyristor and their respective junctions. It helps in understanding the flow of current and the triggering mechanism of a thyristor.

When a positive voltage is applied to the anode terminal of the thyristor, the n-p-n transistor is forward biased, allowing the flow of current from the emitter to the collector. This represents the forward conduction state of the thyristor.

When a positive trigger voltage is applied to the gate terminal, it forward biases the base-emitter junction of the p-n-p transistor. This causes the p-n-p transistor to conduct, resulting in a positive feedback loop. The conduction of the p-n-p transistor further enhances the conduction of the n-p-n transistor, leading to a rapid increase in the overall current flowing through the thyristor. This represents the on-state or latching state of the thyristor.

Conclusion:
The two-transistor model of a thyristor consisting of one n-p-n transistor and one p-n-p transistor provides a simplified representation of the internal structure and operation of a thyristor. It helps in understanding the conduction and triggering mechanism of a thyristor.