GB1592877A - Thyristor fired by collapsing voltage - Google Patents

Description

(54) THYRISTOR FIRED BY COLLAPSING VOLTAGE (71) We, WESTINGHOUSE ELECTRIC CORPORATION, of Westinghouse Building, Gateway Center, Pittsburgh, Pennsylvania, United States of America, a company organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to semiconductor switching devices, and more particularly to two terminal silicon thyristors.

In addition to the well known gating of thyristors by means of third terminal, it is known in the art that thyristors may be switched from the forward blocking mode to the forward conducting mode by rapidly increasing the voltage across the thyristor.

Thyristors which are designed to be fired by such positive dv/dt are sometimes referred to in the art as reverse switching rectifiers.

The thyristor device of the present invention differs from a reverse switching rectifier in that a rapidly decreasing voltage (i.e., negative dv/dt) is used to initiate firing.

It is the principal object of the invention to provide a semiconductor device which is fired by a collapsing voltage. The collapsing voltage generates a displacement current which is used to effect firing of the thyristor.

The word "firing" is used in this specification to describe the dynamic process in which a thyristor switches from the forward blocking mode the forward conducting mode. The expression "turnon" is also used in the art to describe this switching phenomenon.

The invention resides broadly in a semiconductor switching device having a conducting state and a non-conducting state, the device having a plurality of semiconductor regions of alternating conductivity types; and a structure for initiating switching of a forward voltage across the device by initiating a change of state of the device from said nonconducting to said conducting state, said structure for initiating switching being disposed in one of said regions and being adapted for the initiation of the switching from said non-conducting to said conducting state by providing a current for the initiation of switching of the device during an externally initiated collapse of the forward voltage across said device.

A preferred embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which: Figure 1 is a plan view of a thyristor embodiment of the present invention; Figure 2 is a cross-sectional view of the embodiment of Figure 1; Figure 3 is a plan view of a first practical embodiment of the present invention; Figure 4 is a cross-sectional view of the embodiment of Figure 3; Figure 5 is a plan view of a second practical embodiment of the present invention Figure 6 is a cross-sectional view of the embodiment of Figure 5; Figures 7a and 7b are cross-sectional views of the embodiment of Figure 1, schematically illustrating the firing process in time sequence; Figure 8 is a graph illustrating the time sequence of events described in Figures 7a and 7b; Figure 9 is a simplified circuit model of the thyristor of the present invention; and, Figures 10, 11 and 12 are practical examples of circuit applications of the thyristor of the present invention.

Figures 1 and 2 illustrate a basic thyristor embodiment 10 of the present invention comprising a body of semiconductor material 12, which is preferably silcion. The body 12 has four layers of alternate conductivity types which, by way of example, are designated with the letters "N" and "P".

Those skilled in the art will recognize that a complementary device may be produced by interchanging the "N" and "P" conductivity types in description which follows. Starting from top surface 13 of body 12, a first layer consists of two N-type emitter regions 14 and 16, which form PN junctions 15 and 17 with a second P-type layer or base region 18 as shown. A third layer, 20 of N-type conductivity lies beneath base region 18, PN junction 19 being formed therebetween. A fourth layer or anode emitter region 22 of P-type conductivity lies beneath layer 20, PN junction 21 being formed therebetween. Electrodes 24 and 26 make ohmic contact with regions 14 and 16 respectively at top major surface 13.

Electrode 28 makes ohmic contact with region 22 at bottom major surface 29 as shown.

Base region 18 extends past emitter regions 14 and 16 to surface 13 at at least three points. A first portion 30 of base region 18 extends to surface 13 as shown, thereby separating regions 14 and 16. A second portion 32 of base region 18 extends to surface 13 on a side of region 14 separated from porton 30 as shown. A third portion 34 of base region 18 extends to surface 13 on a side of region 16 separated from portion 30 as shown. Electrodes 24 and 26 make ohmic contact with portions 32 and 34 respectively at surface 13.

Disposed in portion 30 of region 18 is a barrier or means for concentrating or channelling lateral flowing current in region 18. What is meant by lateral flowing current is, for example, a current which flows from the portion of base region 18 under emitter region 14 from right to left in the view of Figure 2, and which continues to flow through portion 30 of region 18 into the portion of region 18 under emitter region 16. A presently preferred current concentrating means is an etched moat 36.

A suitable alternative to a moat is a N-type conductivity region which may be located in approximately the same position as moat 36.

An isthmus 38 lies in portion 30 of region 18 between portions of the moat 36 as shown in Figure 1, the isthmus 38 carrying a concentrated lateral flowing current near surface 13 in portion 30 of region 18. It is preferred that region 18 be produced by diffusion so that the resistivity in region 18 decreases as the distance from surface 13 decreases. Thus the isthmus 38 presents a relatively low resistance path to lateral flowing current.

While the embodiment of Figures 1 and 2 is symmetrical on either side of moat 36, region 14 functions as a main cathode emitter and region 16 functions as an auxiliary emitter. Connections to an external circuit are made at cathode electrode 24 and anode electrode 28 as schematically illustrated by the letters "K" and "A".

Now referring to Figures 3 and 4, a presently preferred embodiment 110 is illustrated, which is particularly suited to applications in the hundreds of amperes range. Thyristor 110 is circular in general configuration; however, similar numerals designate parts similar to those described above in conjunction with basic thyristor device 10.

In particular, semiconductor body or wafer 112 has four layers of alternate conductivtiy, wherein the top layer comprises two N-type emitter regions 114 and 116. Auxiliary emitter region 116 is ringshaped and surrounds circular-shaped main emitter region 114. Regions 114 and 116 form PN junctions 115 and 117 respectively with P-type base region 118 disposed therebelow. N-type region 120 and P-type region 122 lie beneath region 118, PN junctions 119 and 121 interfacing the regions as shown. Electrodes 124, 126 and 128 make ohmic contact to regions 114, 116 and 122 respectively. Base portions 130, 132 and 134 terminate at major surface 113.

Portion 130 separates region 114 from region 116.

Disposed in portion 130 of base region 118 between regions 114 and 116 is a barrier or moat 136 similar in form and function to the barrier 36 described above. Moat 136 circumscribes main emitter region 114 almost entirely, a narrow isthmus 138 providing a path near surface 113 for current flowing laterally through portion 130 of base region 118. As means of reducing surface recombination, an insulating layer 140, such as silicon dioxide, is disposed on surface 113 covering at least the portion of junctions 115 and 117 in the vicinity of isthmus 138. The insulating layer 140 minimizes recombination of electrons injected by emitter regions 114 and 116 into base region 118 under layer 140.

Portions 132 are known in the art as shorts or shunts since they provide direct contact between cathode electrode 124 and base region 118. Shunt portions 132 are disposed throughout emitter region 114 in a regular pattern known in the thyristor art, a preferred spacing between adjacent shunts being 25 to 40 mils and a preferred shunt diameter being 4 to 12 mils.

Semiconductor wafter 112 has a bevelled edge 141, for which the angle of inclination and method of forming are known in the art.

Disposed on bevelled edge 141 is an insulating and protective coating 142, a high temperature curing silicone varnish being one of several suitable coating materials.

A second preferred embodiment 210 is illustrated in Figures 5 and 6. The device 210 is fully analagous to device 110, similar parts being designated by similar numerals.

However, device 210 differs from device 110 in that main emitter region 214 and associated electrode 224 surround an auxiliary emitter region 216 and associated electrode 226.

For purposes of comparison, the generic structure of Figure 2 is located in a portion of the cross-sections of devices 110 and 210 designated by numeral 10'. The operation of the inventive devices 10, 110 and 210 is functionally similar apart from geometrical differences.

For ease of illustration, the operation of device 10 will now be described diagrammatically with the aid of Figures 7a, 7b and 8.

At t=0 a forward blocking voltage VAK=VO exists across device 10 with the polarity shown at terminals "A" and "K". At some later time t=t1, the voltage is caused to decrease sharply by means of external control, causing a displacement current id to flow from cathode electrode 24 to anode electrode 28 along the solid line paths shown in Figure 7a. The current id is proportional to the capacitance of forward blocking junction 19 and the rate of decrease in voltage. Some of the current id is channelled through isthmus 38 and then through a portion of base region 18 under auxiliary emitter region 16 as shown. A voltage drop "idr" therefore exists in base region 18 under auxiliary emitter region 16, wherein "r" is the resistance along the current path, which is proportional to the resistivity of the base region 18. When a voltage drop of greater than about 0.7 volts exists under auxiliary emitter region 16, electrons are emitted from region 16 as shown. These electrons are replenished at the interface between metal electrode 26 and base region 18 where a positive current of holes flows from auxiliary emitter electrode 26 into base region 18 as illustrated by the dashed line.

At t=t2 the voltage VAX has dropped from an initial value of V0 to about 0.25 V0 as shown in Figure 8. By this time the electron emission from the edge of region 16 has caused localized lowering of junction resistance in PN junction 19, which allows a localized forward firing current if to flow.

The current i, flows through isthmus 38 and then through base region 18 under main emitter region 14 to cathode electrode 24, thus producing a voltage drop ("i,r") having the polarity shown in Figure 7b. When ifr exceeds about 0.7 volts, electrons are emitted from the edge of emitter region 14 as shown. Those skilled in the art will recognize that such electron emission will fire the thyristor.

Device embodiments 110 and 210 function similarly to device embodiment 10 as just described. Both devices 110 and 210 are fired by a collapsing voltage which produces a displacement current and resulting emission from an auxiliary emitter.

In the case of the thyristor device 110, which is illustrated in Figures 3 and 4, the aforementioned displacement current flows from the cathode electrode 124 through the shunts 132 into base region 118. A portion of the displacement current then flows laterally (i.e., radially outward) until it encounters moat 136, which causes a concentrated current to flow through isthmus 138. The concentrated current continues laterally outward similarly as described above in conjunction with the discussion of device 10 in Figure 7a. A voltage drop occurs under auxiliary emitter region 116 of device 110 in a portion of base region 118 in the vicinity of isthmus 138.

Electron emission then occurs from auxiliary emitter region 116 through a portion of PN junction 117 nearest isthmus 138. The junction resistance of a portion of PN junction 119 under the isthmus 138 is lowered as a result of the electron emission, which causes a localized forward firing current to flow from anode electrode 128 through the low resistance portion of PN junction 119 and then through the shunts 132 to cathode electrode 124. The firing current produces a voltage drop in base region under main emitter region 114, causing electron emission from-region 114 which fires the thyristor 110.

The positioning of shunts 132 is important. The nearest shunt 132 to isthmus 138 must be spaced at a distance far enough from isthmus 138 so that the voltage drop under emitter region 114, as measured from the nearest shunt 132 to the isthmus 138, is sufficiently high to cause electron emission from the edge of emitter region 114 nearest the isthmus 138.

Again referring to Figures 5 and 6, thyristor device 210 functions in an analogous manner to device 110 discussed above. Briefly, when the voltage VAX collapses, a displacement current flows from cathode electrode 224 to anode electrode 228. Some of the displacement current flows laterally through isthmus 238 causing electron emission from auxiliary emitter region 216. The electron emission from region 216 lowers the junction resistance of PN junction 219 in the vicinity of the emission, which permits a forward firing current to flow from anode electrode 228 to cathode electrode 224 causing electron emission from emitter region 214 and a resulting firing of the device 210. The shunts 232 of device 210 serve the same purpose as shunts 132 of device 110 discussed above.

As a practical example of the invention, a working embodiment has been made having the basic geometry of thyristor device 110 shown in Figures 3 and 4. The working embodiment had the following dimensions: overall diameter equals 33 mm, diameter of cathode electrode 124 equals 22 mm, inside diameter of auxiliary emitter electrode 126 equals 28 mm, outside diameter of auxiliary emitter electrode 126 equals 32 mm, width of auxiliary emitter region equals 2 mm, width of moat 136 equals I mm, spacing between moat edge and adjacent emitter regions 114 and 116 equals 0.5 mm, width of isthmus 138 measured from the ends of moat equals 2 mm, peripheral overlap of electrode 126 onto base region 118 equals 0.25 mm. In addition, the working embodiment employed a hexagonal arrangement of shunts 132, each shunt having approximately a 0.2 mm diameter and a 0.9 mm spacing between nearest neighboring shunts. However, the shunts 132 were arranged so that the distance from the edge of emitter region 114 to the nearest shunt 132 (i.e., distance "X" in Figure 4) was approximately 4.5 mm.

A better understanding of the dynamics of thyristor firing has lead to the abovedescribed structure and presently preferred dimensions. As discussed in more detail above, it is necessary to produce a critical voltage of greater than about 0.7 volts under an emitter region in order to cause the desired electron emission from the emitter.

The distance "X" must therefore be long enough to produce a voltage drop in excess of 0.7 volts, and most preferably greater than about 1.0 volts. The working embodiment described above had a sheet resistivity of about 600 ohms/cm2 in base region 118. The distance "X" was therefore selected to be 4.5 mm so that the forward firing current passing laterally through base region 118 would produce the desired voltage drop of greater than about 1.0 volts.

Similar considerations affected the choice of the width of the auxiliary emitter region 116. The displacement current, being concentrated as it passes through isthmus 138, has a magnitude sufficient to produce the critical voltage drop in a distance of 2 mm given a sheet resistivity of 600 ohms/cm2.

Now referring to Figure 9, a simplified circuit model 310 of the present thyristor device is shown, which by way of analogy further describes its operation. When the voltage VAX collapses, a displacement current flows from capacitor C which fires thyristor Tl, which in turn fires the main thyristor T2.

Figure 10 shows a general circuit application for the present thyristor device 310. Initially no current flows through the circuit, the switch 350 being open-circuited.

When the switch 350 closes, the thyristor 310 is turned-on by the collapsing voltage, provided that the conduction state impedance of the switch 350 is greater than that of the thyristor 310. The switch may take any form, such as mechanical, electromechanical or solid-state electronic.

Figure 11 shows an application having parallel operation of thyristors 310 and a conventional three-terminal thyristor 360.

By applying a signal to the gate 362 in the usual manner known in the thyristor art, the three-terminal thyrsitor 360 is fired, which causes all thyristors 310 to fire.

Series parallel operation is also contemplated, as illustrated in Figure 12.

Parallel banks of thyristors 310 are connected in series as shown. Each bank has a control device 370 in parallel, the device 370 being capable of firing its entire bank.

For example, a light activated switch may be used as a control device 370, which would enable simultaneous firing of all parallel banks. A light activated switch has the advantage of enabling "gating" of each bank without regard to the voltage level at each bank.

These and other advantages of the thyristor device of the present invention will be appreciated by those skilled in the art.

For example, it may be advantageous in certain applications to have conventional three-terminal thyristors constructed with the turn-on capability described herein. If gate failure then occurs in such a device, firing will still take place under collapsing voltage conditions.

WHAT WE CLAIM IS: 1. A semiconductor switching device having a conducting state and a nonconducting state, the device having a plurality of semiconductor regions of alternating conductivity types; and a structure for initiating switching of a forward voltage across the device by initiating a change of state of the device from said non-conducting to said conducting state, said structure for initiating switching being disposed in one of said regions and being adapted for the initiation of the switching from said non-conducting to said conducting state by providing a current for the initiation of switching of the device during an externally initiated collapse of the forward voltage across said device.

2. A device according to Claim 1 including an ohmic connector connecting said one region and the switching initiating structure.

3. A device according to Claim I or 2 wherein a second of said regions is disposed in said one region and spaced apart from the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. As a practical example of the invention, a working embodiment has been made having the basic geometry of thyristor device 110 shown in Figures 3 and 4. The working embodiment had the following dimensions: overall diameter equals 33 mm, diameter of cathode electrode 124 equals 22 mm, inside diameter of auxiliary emitter electrode 126 equals 28 mm, outside diameter of auxiliary emitter electrode 126 equals 32 mm, width of auxiliary emitter region equals 2 mm, width of moat 136 equals I mm, spacing between moat edge and adjacent emitter regions 114 and 116 equals 0.5 mm, width of isthmus 138 measured from the ends of moat equals 2 mm, peripheral overlap of electrode 126 onto base region 118 equals 0.25 mm. In addition, the working embodiment employed a hexagonal arrangement of shunts 132, each shunt having approximately a 0.2 mm diameter and a 0.9 mm spacing between nearest neighboring shunts. However, the shunts 132 were arranged so that the distance from the edge of emitter region 114 to the nearest shunt 132 (i.e., distance "X" in Figure 4) was approximately 4.5 mm. A better understanding of the dynamics of thyristor firing has lead to the abovedescribed structure and presently preferred dimensions. As discussed in more detail above, it is necessary to produce a critical voltage of greater than about 0.7 volts under an emitter region in order to cause the desired electron emission from the emitter. The distance "X" must therefore be long enough to produce a voltage drop in excess of 0.7 volts, and most preferably greater than about 1.0 volts. The working embodiment described above had a sheet resistivity of about 600 ohms/cm2 in base region 118. The distance "X" was therefore selected to be 4.5 mm so that the forward firing current passing laterally through base region 118 would produce the desired voltage drop of greater than about 1.0 volts. Similar considerations affected the choice of the width of the auxiliary emitter region 116. The displacement current, being concentrated as it passes through isthmus 138, has a magnitude sufficient to produce the critical voltage drop in a distance of 2 mm given a sheet resistivity of 600 ohms/cm2. Now referring to Figure 9, a simplified circuit model 310 of the present thyristor device is shown, which by way of analogy further describes its operation. When the voltage VAX collapses, a displacement current flows from capacitor C which fires thyristor Tl, which in turn fires the main thyristor T2. Figure 10 shows a general circuit application for the present thyristor device 310. Initially no current flows through the circuit, the switch 350 being open-circuited. When the switch 350 closes, the thyristor 310 is turned-on by the collapsing voltage, provided that the conduction state impedance of the switch 350 is greater than that of the thyristor 310. The switch may take any form, such as mechanical, electromechanical or solid-state electronic. Figure 11 shows an application having parallel operation of thyristors 310 and a conventional three-terminal thyristor 360. By applying a signal to the gate 362 in the usual manner known in the thyristor art, the three-terminal thyrsitor 360 is fired, which causes all thyristors 310 to fire. Series parallel operation is also contemplated, as illustrated in Figure 12. Parallel banks of thyristors 310 are connected in series as shown. Each bank has a control device 370 in parallel, the device 370 being capable of firing its entire bank. For example, a light activated switch may be used as a control device 370, which would enable simultaneous firing of all parallel banks. A light activated switch has the advantage of enabling "gating" of each bank without regard to the voltage level at each bank. These and other advantages of the thyristor device of the present invention will be appreciated by those skilled in the art. For example, it may be advantageous in certain applications to have conventional three-terminal thyristors constructed with the turn-on capability described herein. If gate failure then occurs in such a device, firing will still take place under collapsing voltage conditions. WHAT WE CLAIM IS:

1. A semiconductor switching device having a conducting state and a nonconducting state, the device having a plurality of semiconductor regions of alternating conductivity types; and a structure for initiating switching of a forward voltage across the device by initiating a change of state of the device from said non-conducting to said conducting state, said structure for initiating switching being disposed in one of said regions and being adapted for the initiation of the switching from said non-conducting to said conducting state by providing a current for the initiation of switching of the device during an externally initiated collapse of the forward voltage across said device.

2. A device according to Claim 1 including an ohmic connector connecting said one region and the switching initiating structure.

3. A device according to Claim I or 2 wherein a second of said regions is disposed in said one region and spaced apart from the

switching initiating structure, said second region is so dimensioned and disposed in said one region that the forming of said current to initiate switching of the device is assisted.

4. A device according to Claim 1, 2 or 3 including a second structure associated with said one region being disposed to concentrate said current flowing through said one region for being directed in relation to said switching initiating structure.

5. A device according to Claim 4 wherein said second structure comprises a moat disposed in said one region between said second region and said switching initiating structure, said moat forming an isthmus for concentrating current.

6. A device according to any of the -preceding claims wherein a portion of said one region adjacent said switching initiating structure is constructed such that initial switching is assisted.

7. A device according to any of the preceding claims including a switch connected in parallel with the device said switch having a higher conductivity state impedance than said device and being adapted to collapse a voltage across said device.

8. A semiconductor switching device substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.