GB2033661A - Thyristors - Google Patents

Abstract

A rapid switching thyristor in which the charge carrier lifetime is set by recombination centres so as to be low to reduce the circuit commutated turn-off time comprises an emitter region (11) penetrated by shorted channels (113) extending from the control base region (13), the charge carrier lifetime being arranged to be homogeneous and low in accordance with a desired ignition resistance in the event of a rise in the voltage under the emitter region which recurs forwardly biased across the thyristor element each time the thyristor turns off, and the charge carrier lifetime being set to be still lower in at least one sharply delimited small area (b) arranged along the edge (111) of the emitter region (11) or forming a part thereof facing the control electrode (G) and intersected by this edge. Where an auxiliary ignition emitter is provided between the main emitter and the control electrode, the auxiliary emitter has a higher density of shorted channels at its edge facing the control electrode and the small low lifetime areas are formed at the edge of the main emitter area facing the auxiliary emitter. <IMAGE>

Description

SPECIFICATION Improvements in thyristor elements This invention relates to a thyristor element for rapidly switching thyristors in which the life of the charge carriers is see with the aid of recombination centres so as to be low in order to cut down the circuit commutated turn-off time, having a structure comprising an emitter region and a control base region, the emitter region being penetrated by shorting channels starting from the control base region. The invention also relates to a method of setting a low charge carrier life in a thyristor element whereby the thyristor element which is ready for connection and provided with metallic contacts is subjected at one main surface to electron radiation directed orthogonally thereof and having energy greater than 1 MeV (megaelectronvolts).

A Asemiconductor component, for example a so- called thyristor disc, having improved frequency characteristics and structural features stated above but without any shorting channels in the emitter region is known from German Patent Specification No. 1 489087 as is a method of manufacturing this type of semiconductor component.

In order to achieve the improved frequency characteristics the circuit commutator turn-off time is also reduced in the known element by reducing the service life of the carriers by arranging inside its volume through which current passes separate areas or an area which is organized in raster shape having recombination centres in a concentration which is greater than that of the bordering volume of the emitter region and of the control base region.

The said separate area or areas are constructed by diffusing-in atoms of gold for example or radiating on to the thyristor disc through a mask arranged on a main surface of the element.

The areas of reduced carrier service life are designed to be effective only at a small depth below the n emitter region and are designed expressly so as not to affect the two operationally effective pnjunctions between the p-base and n-base region or the n-base and p-emitter region, i.e. they are designed at this depth not to effect any local reduction in the carrier life.

Furthermore, it is known from German Offenlegungsschrift No. 2402 205 to cut down the switch-off time of a thyristor having the structural features stated at the outset and in turn without shorting channels in the emitter region. According to this known arrangement, the operational thyristor element which is constructed so as to be ready for connection is subjected at one main surface, without masking, to an electron radiation directed orthogonally thereto and having an energy greater than 1 MeV. As a result, in the volume of the thyristor element the service life of a charge carrier becomes almost homogeneous in terms of a space and not as described above ata lower level and distributed locally.When using a radiation dose of the order of magnitude of 1014 eiectrons/cm2 a clear reduction in the switch-off time is effected which however goes hand in hand with a large increase in the drop in forward voltage.

The present invention aims to achieve a clear reduction in theturn-offtime by means of lowering the life of the charge carriers distributed locally in a thyristor element, without increasing the drop in forward voltage.

This is difficult to achieve because of the further aim of sufficiently reducing the turn-off time without at the same time, increasing the firing current or the switch-on losses, whereby a thyristor element having the structural features stated at the outset and the emitter region penetrated by shorting channels is taken as the starting point.

In accordance with the invention, there is provided a thyristor element for rapidly switching thyristors, in which element the life of the charge carriers is set with the aid of recombination centres so as to be low in order to cut down the circuit commutated turn-off time, wherein the element comprises an emitter region and a control base region, the emitter region being penetrated by shorting channels starting from the control base region, wherein the life of the charge carriers in the thyristor element is arranged to be homogeneous and low in accordance with a desired ignition resistance in the event of a rise in the voltage under the emitter region which recurrs forwardly biased across the thyristor element each time the thyristor turns off and wherein the said life of the charge carriers is set to be still lower in at least one sharply delimited small area arranged along The edge of the emitter region or forming a part thereof facing the control electrode and intersected by the said edge of the emitter region.

Thyristor elements having a centrally arranged control electrode for example and having shorting channels in the emitter region are known on the market but do not have the structural features in accordance with the invention whereby these shorting channels are arranged in an area of the emitter region bordering the edge which faces the control electrode in a manner which is closertogetherthan inside said area of the emitter region. Thus a higher firing resistance is achieved as compared to the rise speed of the voltage recurring forwardly biased across the thyristor element for each given charge carrier life. Narrow or dense edge shorting on the other hand causes a further increase in the control or firing current, as already mentioned, since the emitter shorting resistance becomes less.Moreover, this has a negative effect on the permitted steepness of the rise in current of the thyristor and causes higher switch-on losses.

Advantages of using the structure comprising one or more small areas in accordance with the invention and stated above may be seen in that, with thyristor elements having emitter shorting channels, the increase in the control or firing current is prevented but the increase in the firing resistance during the rise in the recurring voltage is not made more nar row in consequence of the narrow edge shorting.

The increase in the control or firing current may be prevented in the region structure with only a limited number of small areas.

The object stated above is solved in a thyristor element having a structure. comprising an additional auxiliary emitter region penetrated by shorting channels starting from the control base region, wherein the shorting channels of the auxiliary emitter region are arranged in an area bordering the edge of the emitter regions or a part thereof facing the control electrode and are closer together than in the inner area thereof and are closer than in the main emitter region, the or each small area being arranged along the edge of the main emitter region facing the auxiliary emitter region. In this case, a plurality of small areas are preferably provided.

The or each small area preferably extends for no more than 1 mm into the emitter region and no more than 0.5 mm from the edge of the emitter region into the control base region. The so-called narrow or dense edge shorting of the emitter region can thus be shifted into the auxiliary emitter region. Thus the increased firing resistance of the thyristor element is maintained and an increase in the firing and control current may be prevented unrestrictedly. During each firing in the first instance the auxiliary thyristor system is fired via the auxiliary emitter with normal control current i.e. without an increased demand for control current.Thus the anode current of the socalled auxiliary thyristor is not derived from a control generator provided in order for the actual thyristor to derive the control current but is derived only from the current source supplying the anode current of this thyristor.

In orderto achieve this in the auxiliary emitter region, in accordance with an embodiment of the invention the control electrode has a shape which concentrates the control current on to a part of the edge of the auxiliary emitter region facing the control electrode or is surrounded by a diaphragm concentrating the control current. The control electrode may be circular for example and have a projection-like extension. Moreover, it may also be circular and be partially connected to the control base region which is p-conducting by an n-conducting region which extends beyond the edge thereof and has the shape of an open circular ring.

The invention also extends to a method of setting a a low carrier service life in a thyristor element having the features above described, in which method the thyristor element which is ready for connection and is provided with metallic contact layers is subjected on one main surface to electron radiation directed orthogonally with respect thereto and having an energy greater than 1 MeV, wherein the electron radiation is effected by means of a number of spatially individual sharply delimited beams of radiation which are directed on to the parts of the main surfaces via the small areas which are closed off and by a radiation dose of 5.1013to 5.1014 electrons/cm2; and wherein diffusion with gold or platinum through a main surface into the thyristor element not provided with electrodes precedes radiation with electrons having sharply delimited beams of radiation.

A local reduction may be provided additionally even in sharply delimited small areas close to the control electrode. An increase in the proportion of the surfaces of the smaller areas which are under consideration is not provided overall so that the measures in accordance with the invention of setting the service life of the charge carriers so as to be low in any case do not affect any increase in the forward voltage drop. The method of setting a low service lifefor the charge carriers in sharply delimited and sealed off small areas of a thyristor element considerably control butes to the success of the invention.

The invention will now be further described, by way of example, with reference to the drawings, in which:~ Fig. 1 shows a cross-section taken on the diameter of a thyristor element having a central control electrode and an annular emitter region; Fig. 2 shows a cross-section taken on the diameter of a thyristor element having a central control electrode and an annular emitter region as well as an annular auxiliary emitter region; Fig. 3 is a plan view of the thyristor element shown in Fig. 2 on the cathode side of said element; and Fig. 4 is a diagram showing the path of the anode current 1A and the anode voltage U in a thyristor element according to the invention.

A short representation of the physical processes which occur during the turn-off phase in a thyristor element precedes any explanation of the invention.

The turn-off phase is subdivided into four time portions I to IV which are shown in Fig. 4 and are designated as follows: Time portion I. The commutation phase to... to II. Extraction-recombination phase .... . t2 Ill. Pure recombination phase .... . t3 IV. The phase of the recurring voltage having a positive dU/dt t3 . . . t4 During the forward phase i.e. before the point in timet0,the base regions 13 and 14 of a thyristor in accordance with Figs. 1 and 2 are swamped with charge carriers of both types. At the point in time to, the commutation phase I begins in which phase the forward current of the thyristor is driven downwardly at a certain negative dl/dt to zero.The time between to and t1 depends on the conditions of the outer circuit but not on the thyristor element.

At the point in time t1 (which corresponds to the zero axis crossing of the current) the charge carrier density in regions 13 and 14 is still very high and the current continues to flow through the pn-junctions 1 and 6 as an excessively high blocking current. Initially a blocking voltage is not able to build up across these pn-junctions. At the point in time t2the carrier concentration near the junctions 1l and 13 is built up to such an extent thatthe transitions are polarized in a blocking or reverse direction sothattheexces sively high blocking current begins to fall off. The time t1 and t2 need not be taken into consideration because of its shortness (1 to 2 ,tlS) but it can be regarded as a purely extraction phase and as the first part of the extraction-recombination phase II which ends at t'2.

Since at the point in time t3 the junctions Ii and 3 are polarized in a blocking direction, a further unhindered flowing away of the carriers present in high concentration in the two base regions 13 and 14 is prevented so that the excessively high blocking current decays. During the part of the extractionrecombination phase II which follows, the charge carrier density is reduced. It decays exponentially with a decay constant ra by way of a first approximation.

applies to the reciprocal of m where Text, is the extraction-time constant and shl the service life of the carriers in the case of a high carrier density.

Approximately at the point in time t'2 the phase III begins from which time onwards a further extraction of electrons is prevented by the existence of a p-base potential barrier. The charge carrier density in the base regions 13 and 14 is reduced in practice only by recombination now. At the point in time t3 phase IV begins and the carrier concentration builds up in the base regions 13 and 14 until the mean pn-junction 13 is ableto absorb and blockvoltageintheforward direction with a certain voltage rise dU/dt. The turnoff process is thus terminated. The carrier concentration still present in the regions 13 and 14 is designated Nz in the following.

The turn-offtime t5 of the thyristor is determined largely by the length of the time portions 11 and Ill of t1 tot3: to = ti +time Fort" and till relationships having NI, (carrier concentration in the time portion II) apply and Nl,, (carrier concentration in the time portion Ill).

N112 til = ra. 1 n N1113 N111 till rhl . 1n Nz From this it is possible to see that there are basically two possibilities of reducing to namely on the one hand by reducing Thi whereby t11 is reduced only slightly while till is reduced significantly or by ensuring that the permitted carrier concentration Nz is able to rise again in the regions 13 and 14 in the case of voltage recurring forwardly biased with the increase dU/dt, so that t", is reduced.Of these possibilities until now the shl reduction has been practised and results in an increase in the forward voltage drop UT.

In order that the permitted residual concentration Nz of excess charge carriers may be large in order to utilize the second possibility, the thyristor must be able to carry a correspondingly large "forward current pulse" (without igniting), which pulse appears as an increased blocking current during the increase in the recurring voltage dU/dt. This may be achieved with a highly effective emitter short-circuit.

The effectiveness of this emitter short-circuit is capable of being shown by the reciprocal of the "specific emitter short-circuit resistance" in the following relationship:

in which Is carries away the current density of a homogeneousforward-blocking current which is intended to be distributed and is homogeneous not via the p-base-n±emitter junction but by means of the emitter short-circuits. Because of the lateral currents in the p-base region 13 and increase in potential occurs the maximum value of which is designated Urn in a region which is evenly coated with emitter short-circuits in the stated relationship. In the case of a non-uniform coating the maximum value is to be taken too.

An unintentional ignition of a thyristor occurs in each case when a blocking current or a dU/dt current pulse or approximately at the point in time t3 a positive so-called recovery current pulse produces a potential Urn which is higher than 0.6 to 0.7 Vat 250C or higher than 0.35 to 0.4 Vat125"C.

If the specific emitter short-circuit resistance is designed to be small so that a high charge carrier residual concentration Nz is permissible as described above, then the shorting channels 113 in the emitter region 11, particularly in their edge region 114 which faces the control electrode G, must be arranged very closely adjacent each other (see Fig. 1). In applications of a athyristor for switching in the kilohertz region this type of "dense shorting" is not advantageous because of the small firing spread speed caused thereby. A "dense shorting" has disadvantages particularly in the surroundings of the control electrode.

If a forward voltage is applied to a structure comprising regions of a thyristor element 1 in the blocking condition then parallel to the pn-junction 13 a space charge region is formed in which a capacitative displacement current flows as a hole current to the cathode emitter region 11 and flows to the anode region 15 as a corresponding electron current. The size of the displacement current lc depends on the following relationship: lc = C dU/dt and is thus dependent on the speed at which the voltage applied to the pn-junction 13 rises.

If the density of the shorting channels 113 of the emitter region 11 and thus the resistance in the shorting channels is selected so that all of the hole currents flowing in the control base region 13 can flow away through the shorting channels as a capacitative displacement current lc to the n-emitter region 11 then the voltage required for this or the potential Urn remains below 0.6 to 0.7 at 250C or 0.35 to 0.4 V at 125"C that there is no injection of elec trons out of the n±emitter region into the p-base control region 13.

In Fig 1. below the surrounding region of the con trol electrode G a hole current flowing to the inner edge 111 of the annular emitter region appears dur ing build-up of the space charge region and thus in turn a higher hole concentration as compared to the inner emitter region appears which causes thorough ignition of the thyristor when there is a sufficiently high residual concentration Nz in the case of the recurring voltage having the given dU/dt. In order to prevent injection of electrons at the inner edge of the n±emitter region 11 the above-mentioned residual concentration Nz must be reduced i.e. the time portion till of the recombination phase must be extended or in a region along the inner edge 111 of the emitter regions the density of the shorting channels 113 must be increased again.

An increase in the density of this shorting of the inner edge of the emitters causes considerable disadvantages for the thyristor since a) if in the region along the edge of the emitter regions the density of the shorting channels is increased until all of the holes flowing to the edge region as a capacitative displacement current may flow away through the shorting channels this takes place in reverse too when an ignition current is impressed.

The required ignition current is then undesirably large.

b) if a relatively large proportion of the surfaces of the inner edge area 114 of the emitters is coated with shorting channels, ignition may take place only at certain points and the firing spread speed is smaller than with only a small number of shorting channels.

High switching-on losses are produced as a result of this because in the first ,as during switching-on the current-carrying surface is small and the current density is correspondingly large with a given increase in current dl/dt. For thyristors switching in the kilohertz range this is particularly disadvantageous in the higher frequency range.

The disadvantages due to a high density of the shorting channels in the region along the edge 111 of the emitter regions are avoided and a high carrier density N, in the p-base region may be permitted and thus a small turn-off time may be achieved if, in the region B lying between control electrode G and the emitter region 11, atomic recombination centres are present, for example atoms of gold, and radiation defect centres are located in the sharply delimited small area b which is evenly arranged along the edge 111 of the emitter regions in a region 0.3 to 1 mm wide and if moreover in the region b the charge carrier service lifer is arranged so as to be considerably less than in the adjacent regions 13 and 14.In the regionb the recombination rate is high and during the recombination phase III the carrier density is reduced more rapidly than in the remaining volume of the thyristor. The recombination rate is reduced at the point in time t3, when the permitted residual charge N, is still relatively large in the region remote from G and in the region about the control electrodes, until the capacitative displacement current concentrating at the edge 111 of the emitters is no longer able to carry out complete ignition of the thyristor element. By means of this measure, the disadvantages indicated above of a large density of the shorting channels in the region of the edge of the emitter regions are largely avoided.Owing to this measure of the so-called local T-reduction in the reg ion b, the forward voltage drop UT does not rise in these regions and in any case is not larger than without this measure since the surface parth of the region b is small overall in comparison to the surface of the emitter region and in comparison to the cathode surface area K and therefore practically negligible.

The local r-reduction described above may be achieved by means of electron radiation of the thyristor element 1 at the cathode side main surface H which may be implemented having beams E of rays directed orthogonally on to the surface parth, these beams being sharply delimited. In the area B of the base regions 13 and 14 on the other hand, T may be adjusted to a desired value with the aid of a large-area Au diffusion which precedes electron radiation. As an alternative, platinum may be diffused instead of gold.

As indicated above it is possible by using the measure of local low setting to reduce the turn-off time to a desirable extent without increasing the forward voltage drop and this aim may be achieved with an arrangement and density of the shorting channels 113 which is predetermined as desired.

Limits are set however on the measure of local low setting of T in the region b of the thyristor element if as is usually required the firing sensitivity of the component is to be large. The firing condition of a thyristor is provided as is known according to the relationship Ico+A1 I,, IA 1-(A1 + A2) where 1A = anode current of the forward-conducting thyristor; ICC = the outwardly flowing electron current in the space charge region at the pn-junction 13.

A1 = the current amplification factor of the n+ pn transistor part of the thyristor; A3 = the current amplification factor of the pnp transistor part of the thyristor; I, = the current required for firing the necessary con trol current.

With the reduction of T in the region B and in the small region b, the factors A1 and A3 are smaller. ICC andlor Is must be greater then so that the thyristor may ignite again whereby the anode current 1A is greater than the so-called blocking current of the thyristor. The increase in charge carrier current ICC however requires a fairly large voltage at the pnjunction 12 so that the anode-cathode voltage required to ignite the thyristor is greater. This is not desirable in many applications.

If the firing capacity of the thyristor which has been hindered because of the local low setting of 7 in the region b is to be improved only by means of a corresponding increase in control or firing current then it has been shown that firing currents of 1 to 2 A are required as long as the carrier service lifer in the edge region 114 of the emitter region 11 is adjusted to be so low that no ignition of the thyristor is possible during the turn-off process. Thyristors of low turn-off time in which the firing current requirement is large may be less acceptable for some thyristor applications as thyristors having a small turn-off time may be dispensed with.

One solution developed from the measures of local low setting of T in thyristor elements 1 according to Fig. 1 may be used in a so-called amplifying gate thyristor 1 as shown in Figs. 2 and 3. As Fig. 2 shows, atomic recombination centres, for example gold, are present in a region B' located between an auxiliary emitter region 12 and the emitter region 11 and radiation defect centres are located in turn in sharply delimited sealed-off small areas b' which are arranged evenly distributed along the edge 111 of the emitter region 11 which faces the region 12.

Furthermore, shorting channels 123 in an area 124 of the edge 121 of this region are arranged in the case of auxiliary emitter region 12 more closely adjacent each other than in the inner region, said region being related to the control electrode G. In contrast, the emitter region 11 is penetrated by a small number of evenly spaced shorting channels 113 so that the potential Urn which is explained above is smaller than the voltage required in orderto inject electrons out of the n±emitter region 11 in the case of the given voltage rise dU/dt. In the area B' (between the regions 12 and 11)the carrier lifer is set so as to be just as low as in the rest of the volume for example by diffusing atoms of gold into it through the main surface H of the element.In the small area b' on the other hand T is adjusted with the aid of electron radiation so as to be lower comprising individual beams e E of radiation directed orthogonally onto the main surface parts h'. In the auxiliary emitter region 12 the shorting channels 123 are arranged so closely adjacent each other in the edge area 124 that the thyristor is ignited during a rise in the voltage dU/dt of the recurring forward voltage in a not unintentional manner. If the shorting channels 123 in the edge region 124 of the region 12 are arranged very closely adjacent each other in accordance with Fig. 2, the control or firing current required for ignition may be undesirably large for the thyristor element 1. However, this may be avoided by a particular shaping of the control electrode auxiliary emitter system having the auxiliary cathode K'. One example of this is shown in Fig. 3. The circular control electrode G has an extension G' which causes the control current flowing from G to the region 12 to be concentrated on a preferred part of the edge 121 of the regions whereby the current required for firing is reduced again. This measure causes the system (G, 12) to be fired only at certain points; the control current is thus rather small (some 100 mA) and only flows shortterm so that there is no thermal overloading of importance and thus no danger of destruction.

The control electrode auxiliary emitter system (G, 12) may be more easily fired as a result of this shap ing without a high control current having to be impressed.

Claims (10)

1. A thyristor element for rapidly switching thyristors, in which element the life of the charge carriers is set with the aid of recombination centres so as to be low in order to cut down the circuitcom- mutated turn-off time, wherein the element comprises an emitter region and a control base region, the emitter region being penetrated by shorting channels starting from the control base region, wherein the life of the charge carriers in the thyristor element is arranged to be homogeneous and low in accordance with a desired ignition resistance in the event of a rise in the voltage under the emitter region which recurrs forwardly biased across the thyristor element each time the thyristor turns off and wherein the said life of the charge carriers is set to be still lower in at least one sharply delimited small area arranged along the edge of the emitter region or forming a part thereof facing the control electrode and intersected by the said edge of the emitter region.

2. A thyristor element according to claim 1 and comprising an additional auxiliary emitter region penetrated by shorting channels starting from the control base region, wherein the shorting channels of the auxiliary emitter region are arranged in an area bordering the edge of the emitter regions or a part thereof facing the control electrode and are closer together than in the inner area thereof and are closer than in the main emitter region and wherein the or each said small area is arranged along the edge of the main emitter region facing the auxiliary emitter region.

3. A thyristor element according to claim 2, wherein a plurality of small areas are provided.

4. A thyristor element according to Claim 2 or Claim 3, wherein the control electrode has a shape which concentrates the control current onto a part of the edge of the auxiliary emitter region facing the control electrode or is surrounded by a diaphragm concentrating the control current.

5. A thyristor element according to Claim 4, wherein the control electrode is circular and has a projection-like extension.

6. Athyristor element of a thyristor according to Claim 4, wherein the control electrode is circular and is partially connected to the control base region which is p-conducting by an n-conducting region which extends beyond the edge thereof and has the shape of an open circular ring.

7. A thyristor element according to any preceding claim, wherein the or each small area extends for no more than 1 mm into the emitter region and no more than 0.5 mm from the edge of the emitter region into the control base region.

8. A thyristor element substantially as described herein with reference to the drawings.

9. A method of setting a low service life for the carriers in a thyristor element as claimed in any one of the preceding claims, in which the thyristor ele ment which is ready for connection and is provided with metallic contact layers is subjected on one main surface to electron radiation directed orthogonally with respect thereto and having an energy greater than 1 MeV, wherein the electron radiation is effected by means of a number of spatially individual and sharply delimited beams of radiation which are directed onto the parts of the main surfaces via the small areas which are closed off and by a radiation dose of 5. 1013 to 5. 1014 electrons/cm2; ; and wherein diffusion with gold or platinum through a main surface into the thyristor element not provided with electrodes precedes radiation with the electrons having sharply delimited beams of radiation.

10. A method according to claim 9 and substantially as herein before described.