Patent GB-1245674-A (GB)
Title: Regenerative gate thyristor construction
IPC: H01L29/08
Filed: 1969-05-14
Abstract: 1,245,674. Thyristors. NATIONAL ELECTRONICS Inc. 14 May, 1969 [22 May, 1968], No. 24629/69. Heading H1K. A thyristor is provided with a regenerative gate structure in which a voltage change at a point adjacent to the triggering gate is transferred to another point at which it initiates a second firing path in the device. As shown, Fig. 2, the cathode emitter region of a thyristor is provided with a lip 20 extending from that part of the emitter region contacted by the cathode electrode 16 towards the gate electrode 18. A contact 30 applied to the lip 20 is connected to an auxiliary gate contact 32 located at the opposite side of the cathode. When the gate electrode 18 is triggered a localized discharge 34 occurs at the nearest point of the lip 20. The current flowing through lip 20 produces a voltage drop at contact 30 and this is applied to electrode 32 to cause a second localized discharge 36. The discharges then spread to provide a rapid turn-on for the device with enhanced di/dt rating. A large number of auxiliary gate contents 48 may be provided each of which is located adjacent to a lip 50 of the emitter region extending from the cathode contact 16 to equalize the loading and to prevent the cathode from shorting out the firing current, Fig. 3. Since each of the lips 50 forms a resistive region across which a voltage drop occurs when the associated gate is triggered, a contact may be provided which is connected to the following gate (as indicated by the dotted line) so that a cascade is formed, the discharge at each emitter lip triggering the following gate. The end of the lip (20, 44) extending towards the main gate contact may be provided with an area at which the breakover voltage is lower than over the remainder of the device. This may be achieved by selective diffusion, by forming a cavity before the emitter diffusion, Fig. 5 (not shown), or before the base diffusion, Fig. 6 (not shown), so that the end of the lip has a downwardly extending projection, or by forming the second base region with a tapered construction, Fig. 7 (not shown). When an overvoltage is applied between the anode and cathode the device breaks over preferentially at the end of the lip and a voltage is developed at the contact in the same way as when the gate is triggered. This results in the whole device being triggered into conduction without a hot spot forming. An interdigitated structure, Fig. 8 (not shown), comprises a cathode electrode (80) having three extending fingers surrounded by extensions of the emitter region, the auxiliary gate contact (90) has four fingers interdigitated with those of the cathode and contacts a long extension of the emitter region at the end of the middle cathode finger. The long extension terminates adjacent to the gate contact (88). A further structure, Fig. 9 (not shown), comprises four circular cathodes connected to a common terminal, three of the cathodes being surrounded by the auxiliary gate contact which is connected to a point on the fourth cathode emitter region located between the main gate contact and the connection to the cathode terminal. The device of Fig. 3 may be modified by omitting the gate contacts 48 and providing an extension of the end of the emitter lip 44 to provide an annular track surrounding but spaced from the ends of lips 50, Fig. 11 (not shown). In a further arrangement, Fig. 12 (not shown), a cathode having a rectangular cut-out at one edge has a radially extending lip of the emitter region crossing the cut-out and terminating adjacent to the gate contact, and two auxiliary emitter lips extending from the sides of the cutout towards the radial lip. The point at which the auxiliary triggering voltage is developed may be brought out to a separate terminal. An isolated emitter area may be provided connected to the main emitter area by a resistor in the form of a track or a component internal or external to the housing. This functions in the same way as the emitter lip (20, 44) to provide the auxiliary triggering voltage. The devices may be produced using diffusion, epitaxial growth, vapour deposition, plating and printing techniques. The thyristors may be triggered by a light source instead of by the gate electrode.