Thyristors are three-terminal solid-state switches whose bistable state depends on a regenerative feedback mechanism occuring in a p-n-p-n structure.
The silicon-controlled rectifier (SCR) is a device belonging to the class of thyristors with the ability of unidirectional conduction. After defining the switching and holding points of an SCR, the gate turn-on conditions are examined, and it is shown that the V–I coordinates of the switching point are dependent on small-signal current gains of the p-n-p and n-p-n SCR sections. The turn-on occurs when the sum of small-signal alphas is equal to unity.
In the case of a shorted emitter, the turn-on condition is simplified to a product of small-signal current gain and avalanche multiplication factor Mα 1= 1.
At zero potential applied to the center junction the current flowing through the device is I v and at this current level the sum of DC alphas αN and αP is equal to unity.
The holding current I h is greater than I 1 and, therefore, the V–I coordinates of the holding point are dependent on the sum of DC alphas.
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