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The anode
connects to the P-type, cathode to the N-type and the gate to the P-type as shown below.
Figure: 1.5. Structure of thyristor
In an SCR, the intrinsic semiconductor is silicon to which the required dopants are infused. However,
doping a PNPN junction is dependent on the SCR application.
Modes of Operation in SCR
OFF state (forward blocking mode)
− Here the anode is assigned a positive voltage,
the gate is
assigned a zero voltage (disconnected) and the cathode is assigned a negative voltage. As a
result, Junctions J1 and J3 are in forward bias while J2 is in reverse bias. J2 reaches its
breakdown avalanche value and starts to conduct. Below this value, the resistance of J1 is
significantly high and is thus said to be in the off state.
ON state (conducting mode)
− An SCR is brought to this state either by increasing the potential
difference between the anode and cathode above the avalanche voltage or by applying a positive
signal at the gate. Immediately
the SCR starts to conduct, gate voltage is no longer needed to
maintain the
ON state and is, therefore, switched off by −
o
Decreasing the current flow through it to the lowest value
called holding current
o
Using a transistor placed across the junction.
Reverse blocking
− This
compensates the drop in forward voltage. This is due to the fact that a
low doped region in P1 is needed. It is important to note that the voltage ratings of forward and
reverse blocking are equal.
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Characteristics of Thyristor
A thyristor is a four layer 3 junction p-n-p-n semiconductor device consisting of at least three p-n
junctions, functioning as an electrical switch for high power operations.
It has three basic terminals,
namely the anode, cathode and the gate mounted on the semiconductor layers of the device. The symbolic
diagram and the basic circuit diagram for determining the characteristics of thyristor is shown in the
figure below,
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