Patent GB-1410726-A (GB)
Title: Thyristor with increased switching on an switching through speed
IPC: H01L29/74
Filed: 1973-01-24
Abstract: 1410726 Thyristors LICENTIA PATENTVERWALTUNGS GmbH 24 Jan 1973 [24 Jan 1972 6 April 1972 10 July 1972 22 Aug 1972] 3633/73 Heading H1K A thyristor (Fig. 1, 2) has a P<SP>+</SP>NPN<SP>+</SP> structure with anode 1 covering the whole area of P<SP>+</SP> region 1 and a cathode 4<SP>1</SP> of annular shape on circular N<SP>+</SP> region 4. A P-type control base region 3 underlies region 4 and overlies a N-type region 2, and highly doped control current paths 5 of P<SP>+</SP> conductivity extend as a finger shaped structure almost to the edge of the wafer, with their centre connected to a control electrode 5<SP>1</SP> passing insulatedly through regions 3, 4 and the central aperture of electrode 4<SP>1</SP>. These paths form a PSN structure with regions 3 and 4, and inject holes into region 3 on application of control voltage between regions 3, 4, while conducting control current into edges of region 3 to produce similar injection there. The value of ignition current depends on the width and number of the control current paths 5, and it is shown that voltage drop is reduced by tapering the path width with spacing from the central control contact. Four control current paths may radiate at #/2 spacings from the control contact terminating in segment shaped ends. In fabrication, a monocrystalline Si wafer predoped to N-conductivity is formed into a PNP regional sequence by diffusion or epitaxial deposition. It is then etched to remove a P-type region, masked and apertured to define the required control current paths, and boron diffused or ion implanted. More P-type material is then deposited to restore thickness, and an upper region of N<SP>+</SP> conductivity is epitaxially formed thereon. In a modification (Figs. 3, 4, not shown) the N<SP>+</SP> emitter region is slotted to expose the regions of the control base overlying the control current paths; and the spacing between the N<SP>+</SP> region and the control current paths is decreased with increased spacing from the central control contact. In a further modification (Fig. 5, not shown) the control current paths in the control base region are of grid shape with a central control contact, and the grid members may decrease in width with increased displacement from the control contact, which may be offset from centre. In a further modification (Figs. 6, 7) the thyristorcomprises a P<SP>+</SP>NPN<SP>+</SP> structure with P<SP>+</SP> region 1 as anode emitter and N<SP>+</SP> region 4 as cathode emitter; an inner P-type control base region 3 and an inner N-type base region 2. The anode 1<SP>1</SP> covers the entire area of region 1 and the cathode 4<SP>1</SP> is annular leaving a central aperture admitting a control electrode 5<SP>1</SP> of control base region 3. Finger shaped P<SP>+</SP> doped current paths 5 radiate from contacts 5<SP>1</SP> almost to the edges of the wafer, adjacent the PN junction between regions 3, 4, and are crosslinked with ring shaped concentric current paths 5<SP>111</SP>. Emitter region 4 is slotted to coincide with and to expose current paths 5, 5<SP>1</SP>, and the slots may decrease in width with increased spacing from the central contact. The latter may be offset from centre at a point of current path intersection, and the ring shaped current paths may be elliptical.