OPERATING PRINCIPLE OF A GATE TURNOFF THYRISTOR GTO
GTO being a monolithic p-n-p-n structure just like a thyristor its basic operating principle can be explained in a manner similar to that of a thyristor. In particular, the p-n-p-n structure of a GTO can be thought of as consisting of one p-n-p and one n-p-n transistor connected in the regenerative configuration as shown in Figure.
With applied forward voltage VAK less than the forward break-over voltage both ICBO1 and ICBO2 are small. Further, if IG is zero IA is only slightly higher than (ICBO1 + ICBO2). Under this condition both ∝n and ∝p are small and (∝p + ∝n) <<1. The device is said to be in forward-blocking mode.
TURNING ON THE PROCESS OF GTO
To turn the device on either the anode voltage can be raised until ICBO1 and ICBO2 increases by avalanche multiplication process or by injecting a gate current. The current gain ∝ of silicon transistors rises rapidly as the emitter current increases. Therefore, any mechanism which causes a momentary increase in the emitter current can be used to turn on the device. Normally, this is done by injecting current into the p-base region via the external gate contract. As ∝n + ∝p approaches unity the anode current tends to infinity. Physically as ∝n + ∝p near unity, the device starts to regenerate and each transistor drives its companion into saturation. Once in saturation, all junctions assume a forward bias and the total potential drop across the device becomes approximately equal to that of a single p-n diode. The anode current is restricted only by the external circuit. Once the device has been turned on in this manner, the external gate current is no longer required to maintain conduction, since the regeneration process is self-sustaining. Reversion to the blocking mode occurs only when the anode current is brought below the “holding current” level.
TURN-OFF PROCESS OF GTO
To turn off a conducting GTO the gate terminal is biased negative with respect to the cathode. The holes injected from the anode are, therefore, extracted from the p base through the gate metallization into the gate terminal (Figure b). The resultant voltage drop in the p base above the n emitter starts to reverse biasing the junction J3 and electron injection stops here. The process originates at the periphery of the p base and the n emitter segments and the area still injecting electron shrinks. The anode current is crowded into higher and higher density filaments in most remote areas from the gate contact. This is the most critical phase in the GTO turn-off process since highly localized high-temperature regions can cause device failure unless these current filaments are quickly extinguished. When the last filament disappears, electron injection stops completely and the depletion layer starts to grow on both J2 and J3. At this point, the device once again starts blocking forward voltage. However, although the cathode current has ceased the anode-to-gate current continues to flow (Figure b) as the n base excess carriers diffuse towards J1. This “tail current” then decays exponentially as the n base excess carriers reduce by recombination. Once the tail current has completely disappeared does the device regain its steady state blocking characteristics? “Anode Shorts” (or transparent emitter) helps reduce the tail current faster by providing an alternate path to the n base electrons to reach the anode contact without causing appreciable hole injection from the anode.
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