Diode Model Of Thyristor

A diode is a two layer, two terminal, p-n semiconductor device. The diode have a junction formed by p-type semi-conductor and n-type semi conductor, the lead joining p-type is known as anode and the other side lead joining the n-type is known as cathode.

A diode is a unidirectional semiconductor device that allows current to flow in one direction only. It is classified as an uncontrolled device because its turn-on and turn-off behavior is determined by the applied voltage and current. Following is the symbol of diode −

Operation Of Diode

Following are the different modes of operation of diode −

• Forward Biased - When the anode terminal of the diode is connected to the positive terminal of the supply and the cathode terminal is connected to the negative terminal, the diode is said to be in forward biased. Under this condition, the diode conducts current and it acts as a ON switch.
• Reverse Biased - When the anode terminal of the diode is connected to the negative terminal of the supply and the cathode terminal is connected to the positive terminal, the diode is said to be in reverse biased. Under this condition, the diode does not conduct current and it acts as a OFF switch.

SCR - Diode Model

A thyristor has three PN junctions J₁, J₂, J₃. Each PN junction is represented by diodes. When thyristor is forward biased and junctions J₁ and J₃ are forward biased and junction J₃ is reverse biased.

Consequently, J₁ and J₃ represented by forward bias diodes DJ₁ and DJ₃ respectively. Similarly, J₂ represented by reverse biased diode DJ₂.

When the gate current I_g flows, p₂ is flooded by electrons from cathode. Subsequently, P₂ looses its own identity. The electrons cross the junction and accelerated by anode to cathode voltage.

The accelerated electrons collide with the covalent bond of silicon and generates more electron-hole pairs or minority carriers in N₁ layers.

Hence, both sides of J₂ flooded by the minority carriers and the device become turn ON.