SOLUTION

SCRs have three operating regions such as

1. Forward conduction state
2. Forward blocking state
3. Reverse blocking state

Forward Blocking Region

The forward blocking region is the widest operating region. In this region, the anode terminal is positive with respect to the cathode. Junctions J₁and J₃ are forward biased (FB), but junction J₂ is reverse biased (RB). The anode current (I_A) and the voltage across anode to cathode (V_AK) vary in a wide range with the amplitude of gate current.

The width of the forward-biased junction depletion layer (J₁ and J₃) and the reverse-biased junction depletion layer (J₂) depend on the dropping density and biasing voltage. The forward-biased junction depletion layer width is thin while the reverse-biased junction depletion layer width is thick. When a voltage is applied, as junction J₁and J₃ are forward biased, the applied voltage should appear across junction J₂ as depicted in Fig.

Forward Conduction Region

When the SCR is forward biased and the gate current is applied through the gate terminal (G), the junction J₂ breakdown. Then all the layers are filled with charge carriers. The SCR provides a very low impedance between A and K and current flows through anode-to-cathode and amplitude of the current is limited by external circuit parameters. The value of gate voltage V_G varies within a few volts.

Reverse Blocking Region

During reverse biased, the anode terminal is negative with respect to the cathode, junctions J₁ and J₃ are reverse biased and junction J₂ is forward biased as depicted in Fig. As the N₁ layer is lightly doped and the wide reverse-biased depletion layer at J₂ is large. The depletion layer width of junctions J₂ and J₃ are thin. A reverse leakage current flows from the cathode (K) to anode (A). When the applied voltage increases to breakdown voltage V_BD, the junction breaks.