GATE TURN-OFF THRYISTOR

Abstract: A gate turn-off thyristor having an array of anode shorts (5) arranged, preferably, in concentric circular formations (11) and a cathode region (4) comprising an array of substan­ tially linear elements (12) parallel to a radius and, divided into a number of sub-arrays each bounded by a sector (AOB) of a circle having a centre (O) co-axial with the anode forma­ tions. Preferably the included angle of a cathode array sector (AOB) is no greater than sixty degrees and the cathode strips (12) subtend an angle with respect to a radius which is no more than thirty degrees. Thus, the dynamic characteristics of the device are rendered, to a great extent at least indepen­ dent of the relative angular orientation of the anode and cathode formations.

Description: Gate Turn-off Thyristor The present invention relates to gate turn-off thyristors, by which is meant a certain type of Silicon Controlled Rectifier in which main-terminal current may be interrupted by application of a reverse bias voltage to the gate, this voltage being the reverse of that required to turn-on the device and is normally a negative gate to cathode voltage. The basic structure of a gate turn-off thyristor involves the same four layer, three junction regenerative two transistor module as a conventional Silicon Controlled Rectifier. However, in order to achieve the gate turn-off capability the high regenerative gain is reduced so that the gate is able to divert sufficient charge carriers away from the cathode upon application of negative gate-to-cathode bias to produce turn-off. In terms of the two transistor module of the thyristor the gain of the pnp part is reduced to as low a level as practicable. A prefered method of achieving this is by selectively shorting parts of the anode emitter junction which also more evenly distributes charge carriers across the device but at the expense of the reverse blocking capability. This is not necessarily a disadvantage since gate turn-off thyristors are not normally used in circuit configurations calling for a reverse block capability. Turn-on the performance is improved when the edge length of the cathode emitter is maximised, and the cathode region, in practice, normally consists of an array of cathode elements. If such a cathode array is incorporated in a thyristor having anode shorts the effect on the thyristor characteristics of their mutual interaction is dependent upon their relative dispositions, particularly relative angular orientation, that the operation characteristics of the device are difficult to predict and reproduce accurately unless the relative alingment can be very closely controlled. One solution to this problem is to employ column or island shorts in the anode which is satisfactory only so long as the inter-short spacing is small compared to the width of a cathode element, and the short cross-sections have correspondingly small diameters. Both these requirements can be difficult to achive in practice. The present invention seeks to avoid these problems by proposing a device configuration which obviates the sensitivity to relative angular alingment of array patterns in the anode and cathode. According to the present invention there is provided a gate turn-off thyristor including incombination an anode shorting array comprising a plurality of concentric polygonal shorting formations and a cathode region comprising an array of substantially linear elements arranged parallel to a radius and bounded bya sector of a circle the centre of which is co-incident with the centre of the anode array. Preferably the concentric shorting formations are circular and each sector of the cathode region has an included angle no greater than sixty degrees. In one form of the invention the elements of each sector array consist of parallel rectilinear strips which subtend an angle with respect to a radius from the common centre no greater than approximately thirty degrees. The invention and how it may be carried out in practice will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig 1 shows a plan view of part of a gate turn-off thyristor including one sector of a cathode array, and Fig 2 shows a view of the device along the section X-X in Fig 1. A gate turn-off thyristor employs the same basic four-layer, three-junction structure of superimposed semi conductor layers of alternating conductivity type as found in conventional Silicon Controlled Rectifiers. These devices also exhibit a similar turn-on capability when a voltage pulse is applied to the gate terminal which forward biases the gate-cathode junction. However, gate turn-off devices differ from conventional thyristors in that they are designed also to turn-off with the application of a reverse bias to the same gate electrode. In order to achieve this capability in practice it is necessary, in terms of the two transistor module of a thyristor to reduce the gain of the pnp component to a suitable low value whilst maintaining the gain of the npn as high as possible. A favoured method achieving this in practice is by the use of anode shorts. The basic structure of such a gate turn-off thyristor is shown in the cross section in Fig 2 which shows the p-type anode region at 1, the n-type base layer at 2, the p-type gate layer at 3 and the array of the emitter region is generally indicated at reference 4. The anode shorting structure, referred to above comprises concentric annular regions 5 of n-type base material which extend from layer 1 to the surface 6 of the anode end layer. Referring now to Fig 1 the thyristor device, indicating generally at 10, is circular in plan about a centre 0, and the anode shorts 5 consist of a set of concentric circlar formations 11 also based on the common centre 0. The array of elements forming the cathode region 4 consists of sets of parallel rectilinear elements, of which 12 is one, which extend generally in radial-like manner within a sector defined by dotted lines AO and BO which subtend an included angle at the centre which is preferably no greater than 60 degrees. Each rectilinear strip 12 in cathode array subtends the same angle to a radius PA or OB which shall be no more than 30 degrees. Therefore the angle at which a cathode element crosses-over the path of an anode short is limited to a narrow range lying 30 degrees either side of a right angle. In the described example the included angle of 60 degrees for each sector of the cathode array gives rise to six identical sectors covering the whole cathode end layer of the device. Thus the relative angular orientation of the anode and cathode patterns is no longer of the importance and the only criticial alingment is that of the pattern centres, which may be easily aligned by reference to a diameter of the basic circular silicon slice. Inspection of the anode shorting arrangement concentric circles is also simplified , e.g. to identify discontinuities, because of the relatively more simple task when compared to, say, locating missing elements of an array of discrete dots or islands in the case when the shorts have a column configuration. In conventional thyristors, i.e. other than gate turn-off types, there is normally a unitary cathode emitter which is turned-on along a limited gated edge length, typically an internal circumference of a centrally located region, although much more complex arrangements have been devised for devices for specific applications. Any emitter short, whether in the anode emitter or cathode emitter has the effect of restricting turn-on in its immediate vicinity. Therefore a linear short which sub-divides an emitter into seperate sub-regions tends strongly to prevent the spread of conduction across that line. In the described embodiment of the drawings the concentric annular shorts are effective to limit the spread of conduction to the annular bands where the corresponding cathode emitter receives a gate turn-on signal. In a conventional thyristor the majority of the device would not satisfactorily switch into conduction. In contrast, in a gate turn-off thyristor of the type described the cathode emitter consists of a multiplicity of a strip-like regions which are operated electrically in parallel and each is gated-on along the whole length of its major edges. Preferably, after the manner of a transistor, the gate signal is maintained for the duration of forward conduction. Therefore, for a device to reach a fully turned-on state a conduction region is required to spread no more than half the width of an emitter strip. In practice, this is almost indistinguishable from the turn-on area arising directly from an applied gate signal, and conduction is never required to spontaneously cross the line of an anode emitter short. In an alternative construction the concentric annular regions 5 of the anode array (see Figs 1 and 2) are discontinuous rings and include at least one and preferably several narrow breaks or gaps in their circumferences. If these gaps are too wide the dynamic characteristics of the device become dependent upon the relative angular orientation of the cathode and anode arrays. To avoid this it is preferred to limit the size of the gaps to no greater than half the width of a cathode emitter strip, i.e. of one of the linear elements in the array comprising the cathode.

Claims: CLAIMS 1. A gate turn-off thyristor including in combination an anode shorting array (1) comprising a plurality of concentric polygonal shorting formations (5) and a cathode region comprising an array (4) of substantially linear elements (12) arranged parallel to a radius and bounded by a sector (AOB) of a circle the centre (0) of which is co-incident with the centre of the anode array (1). 2. A gate turn-off thyristor according to Claim 1 wherein each section of the cathode region (4) has an included angle no greater than sixty degrees. 3. A gate turn-off thyristor according to Claim 1 or Claim 2 wherein the elements of each sector array (4) consist of parallel rectilinear strips (12) which subtend an angle with respect to a radius from the common centre no greater than approximately thirty degrees. 4. A gate turn-off thyristor according to any preceding claim wherein the concentric formations (5) of the anode shorting array (1) are circular. 5. A gate turn-off thyristor according to any preceding claim wherein the concentric polygonal formations (5) of the anode array (1) include at least one discontinuity. 6. A gate turn-off thyristor according to claim 5 wherein said discontinuity is provided by a gap having a width no greater than half the width of an element (12) of the cathode array (4).

Inventors: ANDERSON, THOMAS ALEXANDER
Applicants: WESTINGHOUSE BRAKE AND SIGNAL HOLDINGS LIMITED
Application year: 1985
Main classification: H01L29
IPC codes: H01L29