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Just as a transistor may be considered as basically a solid-state diode with a third semiconductor layer added to form two back-to-back diode junctions, the SCR
may be
considered as a transistor with an additional semiconductor region. Simple models of the "lower order" devices may be analyzed, therefore, to show the effect of the additional semiconductor regions on the operation of the devices.
An SCR is basically a four layer p-n-p-n unidirectional device designed to provide bistable switching when operated in the forward-bias mode. The device has three
electrodes, referred to as the cathode, the anode, and the gate. The gate is the control electrode for the device. For forward-bias operation, the anode potential must be positive with respect to the cathode. During normal operation, the SCR is turned on by application of a positive voltage to the gate electrode. The SCR then remains on, even though the gate voltage is removed or made negative, until the anode-to-cathode voltage is reduced to a value below that required to sustain regeneration, or
forward current. Faster turn-off can be achieved by a reversal of the forward-current flow.
As shown in Fig. 26, the basic p-n-p-n SCR structure is analogous to a pair of complementary n-p-n and
p-n-p bipolar transistors. Fig. 26(a) shows the schematic symbols for an SCR and equivalent connection of the complementary pair of transistors, and Fig. 26(b) shows the equivalent relationship of the
p-n-p-n SCR structure and the interconnected transistor structures.
The n-p-n and p-n-p transistors in the equivalent model are interconnected so that regenerative action occurs
when a proper gating signal is applied to the base of the n-p-n transistor.
When the model is in the off state, the initial value of principal-current flow is zero. If a positive pulse is then
applied to the base of the n-p-n transistor, the transistor turns on and forces the collector (which is also the base of the n-p-n transistor) to a low potential; as a result, a current Iabegins to flow. Because the
p-n-p transistor Q1 is then in the active state, its collector current flows into the base of the n-p-n transistor (Ic1 =lb2) and sets up the conditions for regeneration. If the external gate drive is removed, the
model remains in the on state as a result of the division of currents associated with the two transistors, provided that sufficient principal current (Ia) is available
When the two-transistor model is connected in a circuit to simulate normal SCR operation, the emitter of the p-n-p transistor is returned to the positive terminal of a dc supply through a limiting resistor R2,
and the emitter of the n-p-n transistor Q2 is returned to the negative terminal of the dc supply to provide a complete electrical path, as shown in Fig. 27.
Theoretically, the model shown in Fig. 27 remains in the on state until the principal current flow is reduced
to zero. Actually, turn-off occurs at some value of current greater than zero. This effect can be explained by observation of the division of currents as the value of the limiting resistor is gradually
increased. As the principal current is gradually reduced to the zero current level, the division of currents within the model can no longer sustain the required regeneration, and the model reverts to the
blocking state.
The two-transistor model illustrates three features of thyristors:
(1) a gate trigger current is required to initiate regeneration, (2) a minimum principal current
(referred to as "latching current") must be available to sustain regeneration, and (3) reduction of principal-current flow results in turn-off at some level of current
flow (referred to as "holding current") that is slightly greater than zero.
 Fig. 28 shows the effects of a resistive termination at the base of the n-p-n transistor on the latching and
holding currents. The collector current through the p-n-p transistor must be increased to supply both the base current for the n-p-n transistor and the shunt current through the terminating resistor.
Because the principal-current flow must be increased to supply this increased collector current, latching and holding current requirements also increase. The use of the two-transistor model
provides a more concise meaning to the mechanics of thyristors. In thyristor fabrication, it is generally good practice to use a low-beta p-n-p unit and to include internal resistance termination for the base
of the n-p-n unit. Termination of the n-p-n provides immunity from "false" (non-gated) turn-on and the use of lower beta p-n-p units permits a wider base region to be used to support the high voltage
encountered in thyristor applications.
On to triacs
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