Page: Basic Structure and Working of SCR
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The thyristor or SCR is a three terminal, three junction semiconductor device made of four alternate diffused layers of p and n-type silicon. Figure 1 shows the cross sectional, diagrammatical representation. and circuit symbol. The outer p-layer is the anode, the outer n- layer is the cathode, and the inner p-layer is called the gate.

When the anode is positive with respect to the cathode, junctions $J_{1}$ and $J_{3}$ are forward biased, while junction $J_{2}$ is reverse biased. Due to the reverse biasing of junction $J_{2}$ only a small leakage current flows through the device and the thyristor is said to be in the forward blocking state or in the OFF-state. When this forward voltage is increased to a sufficiently large value, the reverse biased junction $J_{2}$ is broken down by avalanche multiplication; this voltage is called the forward break-over voltage $V_{F B O}$ . Since the other two junctions are already in the forward biased condition, there will be free movement of charge carriers across all three junctions; thus a large forward current flows and the device is then said to be in a conducting state or in the ON-state. Approximately 1V to 2V is dropped across the device in the ON-state, which is the ohmic drop in all four layers. In this state, the anode current is only limited by the external impedance or resistance.

The ON-state anode current must be more than the rated minimum current value known as the laching current $I_{L}$ , in order to maintain the required number of charge carriers flowing across the junction $J_{2} ;$ otherwise, the device will be reverted back to the forward blocking OFF-state as the anode-to-cathode voltage is reduced. Thus, the latching current is the minimum anode current required to maintain the device in the ON-state just after the thyristor has been turned on and removes the gate signal.

Due to the free movement of large charge carriers there is no depletion layer on junction $J_{2},$ therefore, the device behaves like a conducting diode and there is no control over the device. However, if the forward anode current is reduced below a level known as the holding current $I_{H},$ a depletion layer will develop around the reverse biased junction $J_{2}$ due to a reduced number of free carriers and the thyristor will go into the forward OFF-state. Thus, the holding current is the minimum anode current required to maintain the thyristor in the ON-state and its value is less than the latching current value which is of the order of milli-amperes.

When the cathode is made positive with respect to the anode, junction $J_{2}$ is forward biased and the $J_{1}, J_{3}$ junctions are reverse biased. Then the thyristor is in the reverse blocking state and a very small leakage current flows, known as the reverse current $I_{R} .$ In most cases the reverse breakdown voltage is slightly higher than the forward breakover voltage.

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