Thyristor With Starting and Generating Cathode Base Contacts for Use in Rectifier Circuits

1. Field of the Invention

The invention relates to an output thyristor with at least two electrodes placed on the base area of the cathode side, wherein one of the two electrodes serves as control electrode (ignition or starting gate) and the second electrode serves as the discharge of a current (the generator gate). This invention further relates to use of this kind of thyristor in rectifier circuits in which several thyristors are connected in parallel.

2. Description of the Prior Art

In large plants in the power electronics industry frequently several thyristors must be connected in parallel. It is possible that in unfavorable usage conditions, individual thyristor elements might not ignite or start or might become non-conductive because of low current. Then, with a subsequent increase in current the thyristors connected to the non-conductive thyristor could be overloaded and destroyed. To avoid such accidents, presently relatively expensive measures must be taken in the connections (cf. e.g. DE-OS 26 12 549).

From DE-OS 21 46 178 there is now known a thyristor on whose cathode side base area are provided several electrodes. An amplifying gate (auxiliary thyristor) directs currents free of the boundary layer over several ignition electrodes distributed over the upper surface of the thyristor to ensure a symmetrical ignition procedure. One of the amplifying gates of the ignited chief thyristor's output current will not suffice to start a corresponding thyristor connected in parallel.

Accordingly, one object of this invention is to provide a novel thyristor whose ignition and generator gates are dimensioned in such a fashion that the generator current emitted from a started thyristor is sufficient to start a corresponding unconnected thyristor. In this the anode current of the ignited thyristor, J.sub.o, should be smaller than its impulse current or surge limit value, but preferably smaller than three times the continuous current limit.

This and other objects are achieved by providing a novel output thyristor with at least two electrodes located on the cathode side base zone, whereby one of the two electrodes is used as a control electrode (starting gate) and the second electrode is used for the output of current (generator gate), characterized by the fact that the existing lateral resistance R.sub.GK ' between the generator gate and the cathode zone and/or the edge length of the generator gate is selected such that at the generator gate of the started thyristor--with an anode current which is smaller than the surge limit of the thyristor in question--the removable current I.sub.G is sufficient to start a corresponding thyristor in parallel over its starting gate, and that the lateral resistance between the starting gate and the cathode zone--R.sub.GK --is selected as small as possible (R.sub.GK <R.sub.GK ') so that the starting voltage on the starting gate can act directly on the cathode side emitter crossing, J.sub.1.

The output thyristor may be provided with emitter short circuits between the lateral resistances R.sub.GK ' or R.sub.GK which are located between the generator gate or starting gate and cathode zone so that both the following relations hold: ##EQU1##

wherein I.sub.G ' indicates the minimum gate current with which the thyristor will still start.

In a preferred embodiment, the starting gate and the generator gate are located near one another, and the contact of the gates takes place over a thin edge area oriented toward the cathode, under which the edge concentration of the cathode base zone is greater than in the area between the gate and the cathode zone. The thyristor further includes between the remaining part of the gate and the base zone a 500 to 2000 A thick layer of insulation. Parallel to the part of the gate which is insulated from the cathode base zone there is a lamellar emitter short circuit.

Proceeding from a specified thyristor structure, the desired starting sensitivity of the thyristor starting gate is obtained through lessening of the existing lateral resistance R.sub.GK between the gate and the cathode zone, and the required generator current is obtained through increasing the corresponding lateral resistance R.sub.GK ' and/or through lengthening of the edge of the generator gate. Furthermore, the lateral resistance R.sub.GK ' between the generator gate and the cathode zone is increased by etching in this area of the cathode base zone a groove of some 30 to 40 .mu.m depth.

Several of the above-described thyristors are used in a rectifier circuit, in which the thyristors are connected in parallel, whereby the generator gate of one is connected directly with the starting gate of the next thyristor.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view of a known thyristor;

FIG. 2 is an equivalent circuit diagram of a thyristor of FIG. 1 in the started condition;

FIG. 3 is a top view of a thyristor of the model of the invention (without metalizing of the cathode);

FIG. 4 is a cross-sectional view of the thyristor of the type shown in FIG. 3 taken along profile line S-S;

FIG. 5 is a schematic circuit illustration of several thyristors according to the invention, connected in parallel.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, a thyristor is presented whose cathode-, P-base-, N-base-, and anode zone are indicated by 1,2,3 and 4. The existing semiconductor junctions are indicated with J.sub.1, J.sub.2, and J.sub.3. The cathode- and anode zones are provided with the contacts K and A, and the P-base zone with the gate G. Under the gate G the P-basis zone 2 exhibits a higher (P+) doping than under the cathode zone 1.

If the gate is to work as an ignition or starting gate, then the lateral resistance between the gate and cathode zone 1 must be as small as possible, because then the decaying voltage over this resistance is small and the voltage existing at the gate works directly on the junction J.sub.1. Preferably the decaying voltage drop across the resistance R.sub.GK with the minimal starting current I.sub.G ' should amount to less than 0.2 V. The thyristor then ignites with a small anode voltage. In order to make the resistance R.sub.GK as small as possible, the distance between gates, or the gateside edge of the P+-zone 5 and the gateside edge of the cathode zone 1 is made as small as possible. A further possibility of raising the sensitivity lies in selecting the cathode side gateside edge as short as possible.

Otherwise, as in the case of the starting gate, one should note with the dimensioning of the generator gate that the existing lateral resistance between the gate, or the P+-zone, and the cathode zone 1--R.sub.GK '--should be selected as large as possible. At the least R.sub.GK ' should be three times greater than R.sub.GK. This should be explained in the equivalent circuit diagram of a started thyristor presented in FIG. 2. Accordingly the started thyristor between cathode contact K and anode contact A consists of two characterizing resistances, R.sub.1 and R.sub.2, one a diode polarized in the direction of flux through the crossing J.sub.1, and two, the non-reactive voltage drop in the flooded area (P- and N-base zones 2 and 3). On the other hand, between gate G and anode A is a pure transistor structure. This transistor T is controlled through the lateral electron current in the N-base 3 through a corresponding resistance R.sub.N. As soon as the collector-emitter voltage in the thyristor exceeds the saturation voltage U.sub.sat, a constant collector current I.sub.c flows. The current I.sub.G removable from gate G is then: ##EQU2## Depending on the size of the resistance R.sub.GK ' in the P-base zone 2, a further current superimposes itself on the collector current I.sub.c. A raising of the generator current I.sub.G, obtained by selection of a longer gate edge, is also possible in principle, by selecting a sufficiently large resistance and/or collector current I.sub.c. One obtains a large value for I.sub.c especially when the current J.sub.o flowing through the thyristor is selected correspondingly large, since I.sub.c .about..sqroot.j.sub.o ; where j.sub.o signifies the current density of the started thyristor underneath the cathode. For large resistance values R.sub.GK ' the gate in an ideal case can deliver the current I.sub.c with a voltage of U.sub.AK -U.sub.sat.

From the explanations above it follows that it would be hardly reasonable with thyristors connected in parallel to use only a single gate at any one time and to connect these to one another because the requirements of large generator currents or high starting sensitivities lead either to opposing solutions (greater or lesser resistances R.sub.GK ; long or short edge length of the gates) or to require currents J.sub.o which can amount to a multiple of the rated current. The latter is allowed only rarely to be realized, because the danger exists that the started thyristor will be destroyed. This happened by way of example to a thyristor obtained commercially with a central gate and an edge length of 2.4 cm., with a required starting current of 415 mA. The generator current I.sub.G on the other hand, with a current J.sub.o of 1500 A (j.sub.o .apprxeq.100 A/cm), itself amounted only to 240 mA and was not sufficient to start a corresponding thyristor connected in parallel.

In FIGS. 3 and 4 this thyristor is presented with an altered gate structure. Neither the emitter short circuits designated by the numeral 6 nor the potential or measurements of zones 3 and 4 were altered. On the other hand, the central gate was replaced by the two gates next to one another, the starting gate ZG and the generator gate GG. The edge length of the starting gate ZG amounts to 1 cm and the length of the generator gate GG is just about double that. The resistance between starting gate ZG and cathode zone 1 was reduced from the original 300 .mu.m to 50 to 100 .mu.m. For starting, there then suffices a gate voltage of 1 V with a starting current of only 175 mA.

The distance between the gate and cathode, measured from the cathode side edge of the P+ -zone 5" up to the gate side edge of the cathode zone 1, is 100 to 200 .mu.m. To raise the corresponding resistance R.sub.GK ' is required an additional etching of groove 7 in the P-base 2. A depth of 30 to 40 .mu.m. is sufficient for the groove. This etching offers the advantage that the resistance R.sub.GK ' can be raised dramatically without increasing the distance of the gates from the cathode zone.

For contact, gates 8, 8' of metallized plane surfaces lying next to one another are sufficient. For the thyristor these areas are fully passive. To insulate along the P-base 2, the metallizing on a thin layer of oxide (1500 to 2000 A) 9 is used. The current surge which results from a voltage climb in this region can be directed over the neighboring short circuit 10 to the cathode.

As can be seen from FIG. 5, for linking several thyristors in parallel--Th.sub.1, Th.sub.2, Th.sub.3 --the generator gate GG of one can be connected directly with the starting gate ZG of the next thyristor.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.