GB2097580A - Thyristors - Google Patents

Abstract

In an optically controlled thyristor charge carriers generated in a localised region 19 of the base region are directed towards a first amplifying gate 12 by a non-conducting recess 20 surrounding the other three sides of the region 19. The recess 20 may be left open or filled with insulating material. Transfer of triggering current to the cathode is effected via a second amplifying gate 13. A conductive electrode 23 may be interposed between the region 19 and the first amplifying gate 12 to equalize the surface potential across the region so that the current flow spreads out over the whole width of the first gate. <IMAGE>

Description

SPECIFICATION Thyristors This invention relates to thyristors and is particularly, but not exclusively applicable to high voltage thyristors. Although the voltage drop across a thyristor is very low when it is in its conductive state, the voltage which the thyristor is required to withstand when it is non-conductive is high, and can be in excess of a kilovolt. The thyristor is switched between its conductive and non-conductive states by means of a gate signal applied to a gate structure which is located adjacent to the cathode connection.

If the anode connection of the thyristor is connected to a reference potential, the potential of the cathode varies during the operation of the device and consequently the potential of those portions of the gate structure to which the control signal is applied alters also. In very high voltage devices, this can cause serious problems and in order to overcome this difficulty, it has been suggested that the control signal could be applied to the gate structure in the form of a light signal so that the gate structure is electrically isolated from the circuits which generate the gate signal. This would also reduce the problem of spurious triggering of the thyristor when it is operated in an electrically noisy environment in which electrical noise might inadvertently couple to the gate structure thereby triggering the thyristor.It has been proved difficult to produce a satisfactory light activated thyristor which triggers reliably from a light source without causing a serious degredation in the switching characteristics of the thyristor, and the present invention seeks to provide an improved thyristor.

According to this invention a thyristor includes a body of semiconductor material having an amplifying gate located on one major surface of the body adjacent to the main cathode emitter of the thyristor, means for receiving light at a localised area of said surface which is spaced apart from said amplifying gate to generate charge carriers within a defined region of a p-type base layer underlying said localised area, and means for directing said charge carriers under the influence of a potential at said cathode towards an n-type emitter forming part of said amplifying gate so as to control the switching action of the thyristor.

The invention is further described by way of example with reference to the accompanying drawing in which, Figure 1 shows a plan view of a thyristor in accordance with the present invention and Figure 2 is a sectional view of the same thyristor taken on the line X-Y.

The thyristor shown in the drawings consist of a circular slice 1 of silicon having a pair of opposite major faces 2 and 3. The thickness of the slice in relation to its diameter is greatly exaggerated in Figure 2 for the sake of clarity, and in practice the slice is relatively thin. The thyristor is a multi-layer device consisting of a p-type emitter layer 4, an n-type base layer 5, a p-type base layer 6 and an n-type emitter layer 7. The layer 7 is not continuous, but is set into the p-type layer 6 at predetermined locations in the major surface 2.In practice, the thyristor is formed from a homogenous slice of silicon which is initially n-type, and subsequently during processing p-type inpurities are diffused into the body of silicon from both opposite major faces 2 and 3 to form the p-type layers 4 and 6. Subsequent- ly, an n-type impurity is diffused into the layer 6 to form the localised n-type emitters at the required locations. A continuous electrode layer 8 is formed in contact with the p-type layer 4 and this constitutes the anode connection.

A conductive layer is also provided on the upper major surface 2, but this is not in the form of a single continuous layer. Instead the layer is divided up into a number of different regions. The largest regions constitutes a cathode electrode 9 and this is arranged to partly surround an amplifying gate electrode 10, which in turn partly surrounds another amplifying gate electrode 11. The electrode 11 forms part of a first amplifying gate structure 12 and the electrode 10 forms part of a second amplifying gate structure 13. The amplifying gate structures 12 and 13 include regions of the n-type layer 7 and in order that the extent of the n-type later 7 can better be understood it is shown diagonally cross-hatched on Figure 1, although this is not a sectional view.A small rectangular area 14 of the p-type layer lies under part of the electrode 11, and a part annular area 15 of the p-type layer 7 lies under the electrode 10, which is in the shape of a half-circle. The remaining areas of the p-type layer 7 lie under the cathode electrode 9 and constitute the cathode structure of the thyristor. The cathode structure is of the emitter - short kind and these short circuit points are indicated as small dots 16 on Figure 1.

A light guide 17 conducts light in the direction of the arrow 18 from a light source (not shown) to a surface area 19 of the p-type layer 6. This area 19 is almost wholly surrounded by a deep recess 20 formed in the surface 2, and the area 19 is of approximately the same size as the first amplifying gate 12. The recess 20 is of a U-shape groove with the base of the U surrounding the region 19 and the extended arms of the U defining the extent of the n-type emitter region 14. Lateral extensions 21 of this groove bound the extent of the second amplifying gate structure 13. The recess 20 represents a non-conducting region of the p-type layer 6 and it may simply be left as a recess or it may be filled with a suitable non-conductive passivating material.Its purpose is to influence the paths taken by freecarriers which are generated in the region of the p-type layer 6 underlying the surface area 19 when light is projected on to it via the light guide 17.

The thyristor is turned on by means of a light signal received via the light guide 17, as the presence of light of a suitable nature causes the thyristor to conduct. The light must be such as to generate electron-hole pairs at the surface region 19 and typically infra-red light is used. Initially the thyristor is non-conductive and a large voltage difference exists between the cathode electrode 9 and the anode electrode 8 with the cathode being very negative with respect to the anode. Electron hole pairs which are generated at the surface area 19 respond to the potential on the cathode electrode 9.

The electrons flow towards the anode and the holes flow towards the cathode, and because electrical connection is made only to the main cathode electrode, the holes are constrained to flow under the amplifying gates 12 and 13 to reach the cathode.

Charge flow in the p-type region 6 below the emitter 14 causes the first amplifying gate 12 to turn on. This gate drops to anode potential, thereby providing an amplified current flow under the second amplifying gate 13. By a similar mechanism the second amplifying gate 13 turns on, and subsequently the main cathode turns on by the conventional thyristor action. Amplifying gates are in themselves well known and it is not thought necessary to describe in detail the mechanism by which they cause the main emitter region 7 to conduct.

It will be realised that the groove 20 constrains the directions in which the holes generated at the surface area 19 can flow. Initially they flow towards the first amplifying gate 12 from the point at which the infra-red light is incident. A strip electrode 23 is provided on the upper surface of the p-type layer 6, so as to extend from one arm of the U-shaped groove 20 to the other arm, thereby defining the otherwise open boundary of the surface area 19. The electrode 23 acts as a short circuit for the p-type region lying immediately underneath it and prevents a a potential gradient developing along the length of this electrode. This has the effect of spreading the current constituted by the migrating holes in a more uniform manner.By this means, substantially all of the holes generated below the surface area 19 are directed towards the first amplifying gate 12 and the current density at the amplifying gate is fairly predictable. This enables the thyristor to be turned on in a reliable manner from a very low energy light source. It also means that the thyristor can be made insensitive to voltage transients which could in advertentlyturn it on. It is the provision of two amplifying gates arranged as shown which enhances the ability of the thyristor to withstand voltage transients without being adversely affected by them. This parameter is often referred to as dV/dt in connection with the operation of thyristors.

It will be noted that the incident light falls on a surface area 19, which does not possess an emitter layer 7, so that no thyristor action takes place in this region. The holes generated within this region are guided to the first and second amplifying gates where cumulative thyristor action takes place. The holes are prevented from spreading randomly by the non-conducting recess 20, and are forced to flow under both amplifying gates 12 and 13 by the action of the accelerating cathode potential.

Claims (9)

1. A thyristor including a body of semiconductor material having an amplifying gate located on one major surface of the body adjacent to the main cathode emitter of the thyristor, means for receiving light at a localised area of said surface which is spaced apart from said amplifying gate to generate charge carriers within a defined region of a p-type base layer underlying said localised area, and means for directing said charge carriers under the influence of a potential at said cathode towards an n-type emitter forming part of said amplifying gate so as to control the switching action of the thyristor.

2. A thyristor as claimed in claim 1 and wherein said amplifying gate is positioned between said localised area and said main cathode.

3. A thyristor as claimed in claim 1 or 2 and wherein said localised area is bounded by a surface region of low conductivity except for a boundary segment adjacent to said amplifying gate.

4. A thyristor as claimed in claim 3 and wherein said surface region of low conductivity is constituted by a recess in the semiconductor material of which said surface region is formed.

5. A thyristor as claimed in claim 4 and wherein said recess contains non-conductive material.

6. Athyristor as claimed in claim 3 and wherein a conductive layer overlies said boundary segment so as to influence the current density distribution of the charge carriers passing through said boundary segment to said amplifying gate.

7. A thyristor as claimed in any of the claims 3 to 6 and wherein the size of said amplifying gate is at least approximately the same as said localised area.

8. A thyristor as claimed in claim 7 and wherein said amplifying gate as at least partly bounded by extensions of said surface region of low conductivity.

9. A thyristor substantially as illustrated in and described with reference to the accompanying drawing.

9. A thyristor as claimed in any of claims 2 to 8 and wherein a further amplifying gate is positioned between first said amplifying gate and said main cathode.

10. A thyristor as claimed in claim 9 and wherein said further amplifying gate is of at least approximately arcuate shape so as to partly surround first said amplifying gate.

11. A thyristor as claimed in any of the preceding claims and including a light guide arranged to terminate closely adjacent to said localised area so as to direct light substantially only to said localised area.

12. A thyristor as claimed in claim 11 and wherein the end of the light guide remote from said localised area is coupled to a switchable source of infra-red light.

13. Athyristorsubstantially as illustrated in and described with reference to the accompanying drawing.

New claims or amendments to claims filed on 19 April 1982.

Superseded claims 1 to 13.

New or amended claims

1. A thyristor including a body of semiconductor material having a main cathode emitter, and first and second amplifying gates positioned on one major surface of the body between said main cathode emitter and a localised area of said surface which is adapted to receive light; means for generating charge carriers within a defined region of a p-type base layer underlying said localised area in response to light; and means for directing said charge carriers under the influence of a potential at said cathode towards an n-type emitter forming part of said first amplifying gate so as to control the switching action of the thyristor; wherein, said first amplifying gate is positioned adjacent to said localised area, said second amplifying gate is of at least approximately arcuate shape so as to partly surround said first amplifying gate; and wherein said localised area, and said first and second amplifying gates are at least partly bounded by a surface region of low conductivity.

2. A thyristor as claimed in claim 1 and wherein said localised area is bounded by the surface region of low conductivity except for a boundary segment adjacent to said first amplifying gate.

3. A thyristor as claimed in claim 2 and wherein said surface region of low conductivity is constituted by a recess in the semiconductor material of which said surface region is formed.

4. A thyristor as claimed in claim 3 and wherein said recess contains non-conductive material.

5. A thyristor as claimed in claim 4 and wherein a conductive layer overlies said boundary segment so as to influence the current density distribution of the charge carriers passing through said boundary segment to said first amplifying gate.

6. A thyristor as claimed in any of the preceding claims and wherein the size of said first amplifying gate is at least approximately the same as said localised area.

7. A thyristor as claimed in any of the preceding claims and including a light guide arranged to terminate closely adjacent to said localised area so as to direct light substantially only to said localised area.

8. A thyristor as claimed in claim 7 and wherein the end of the light guide remote from said localised area is coupled to a switchable source of infra-red light.