Thyristor with pressure contacting

An improved thyristor with pressure contacting in which at least a portion of the control base zone in addition to the cathode zone extends on a single major surface of a wafer-shaped thyristor semiconductor body and the cathode base zone and the control base zone are each contiguous and are provided at least in part at the major surface with completely insulated ohmic electrodes, i.e., cathode and control electrodes. The ohmic cathode and control electrodes are insulated from one another by means of a layer of insulating material which covers those portions of the major surface of the semiconductor body which are not provided with electrodes and the control electrode except for the connecting contact for the control electrode, the planar surface of the layer of insulating material not having to lie at the level of the cathode electrode surface, and the cathode electrode is pressure contacted by means of a contacting wafer which is covered, at least on the surface thereof contacting the cathode electrode, with a highly conductive ductile material and is provided with a recess for the connecting terminal of the control electrode.

BACKGROUND OF THE INVENTION 
The present invention relates to a thyristor with pressure contacting in 
which at least a portion of the control base zone in addition to the 
cathode zone borders on one major surface of a semiconductor wafer-shaped 
thyristor element and the cathode and control base zones each are 
contiguous and are provided at least in part with completely insulated 
ohmic electrodes (cathode and control electrode) at this major surface. 
German Auslegeschrift (Published Patent Application) No. 1,209,207 
discloses a controllable semiconductor rectifier with the above-described 
structural features and an npnp semiconductor body in which part of the 
zone which borders on the cathode zone, here called the emitter zone, and 
constituting the control base zone extends through this emitter zone to 
the surface of the semiconductor body, this part of the control base zone 
being provided with an ohmic electrode at the surface of the semiconductor 
body. The surface of this penetrating portion of the control base zone 
forms a contiguous strip pattern on the surface of the emitter or cathode 
zone which is distributed over the entire surface of the emitter or 
cathode zone so that the area of the strip pattern is smaller than the 
area of the emitter or cathode zone at the surface of the semiconductor 
body and is covered by the ohmic electrode. The strip pattern may be 
formed as a plurality of interconnected fir-tree patterns which radially 
outwardly extend from the center of the major surface or as a plurality of 
strips which intersect at right angles in the shape of a net. 
In the known controllable semiconductor rectifier, the surface of the 
emitter or cathode zone and the ohmic electrode attached thereon are 
situated lower than the strip pattern of the control base zone including 
the above-mentioned ohmic electrode with which the strip pattern of the 
control base zone is provided and the entire surface of the emitter or 
cathode zone and the ohmic electrode applied thereto are contacted with an 
electrically conductive material, for example in the form of a wafer or 
disc. In order to prevent the ohmic electrode of the strip pattern of the 
control base zone, which lies higher than the ohmic electrode of the 
emitter or cathode zone from also being contacted by this wafer or disc, 
it is necessary, however, for this wafer or disc to be provided at its 
contacting surface with a negative pattern with respect to the strip 
pattern of the control base zone which negative pattern can be produced 
only with extreme care and great difficulty. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide perfect and dependable 
pressure contacting for the ohmic electrode of the cathode zone, i.e., for 
the cathode of the above-described thyristor, by means of a contacting 
wafer which is easy to produce and which does not touch the ohmic 
electrode of the strip pattern of the control base zone. 
This is accomplishd according to the present invention in that the control 
and cathode ohmic electrodes are insulated from one another by means of a 
layer of insulating material which covers the portions of the major 
surface of the semiconductor body which are not provided with electrodes 
as well as the entire control electrode except for the connecting terminal 
for the control electrode, the planar surface of this insulating layer 
need not necessarily lie at the level of the cathode electrode surface, 
and the cathode electrode is pressure contacted by means of a contacting 
wafer which is coated, at least on the surface thereof which contacts the 
cathode electrode, with a layer of a highly conductive ductile material 
and is provided with a recess or opening for the connecting terminal of 
the control electrode. 
Regarding the above-mentioned layer of insulating material, the present 
invention is further defined in that during the production of the 
thyristor zone and electrode structure the insulating layer is produced in 
the following manner by known photomasking techniques in an additional 
photoprocess. 
Either a photolacquer which is used in the additional photoprocess and 
which is resistant to the process and remains on the thyristor after 
completion of the process can serve as the insulating layer or the 
insulating layer can be selectively etched out of a layer of quartz or 
nitride which has been sputtered onto the entire area of the cathode and 
the control electrode or finally it may be an insulating lacquer which has 
been applied by means of a screen printing mask. 
According to a further feature of the invention, a hard pressure contacting 
wafer is used which is covered with a soft-melted silver and which is 
centered on the semiconductor body by lacquering. 
The present invention produces dependable pressure contacting for the 
cathode even for thyristors having a complicated cathode and control 
electrode structure. This is possible in that under the contacting 
pressure the ductile layer of the contacting wafer is plastically deformed 
and pressed, for example, around the insulating layer of the control 
electrode which extends beyond the cathode, so that the ductile layer 
comes into intimate contact with the cathode electrode. Advantageous use 
of the present invention is seen for the so-called frequency thyristors in 
which the cathode is provided on the major surface of the semiconductor 
body in sectors which are divided by finger-shaped radially branching 
control electrode sections. Thyristors having such a cathode-control 
electrode structure have the advantage that they can be switched on and 
off faster via the control electrode than thyristors having the 
conventional structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIGS. 1-4 there is shown a pressure contacted thyristor 
with a cathode contact which is disposed above one of the major surfaces 
which is subdivided by finger-shaped radially branching control electrode 
sections, the drawing being a cross-sectional view through one finger of 
the control electrode. This cross-sectional view is not taken along a 
diameter but along a chord 1--1 of the circular major surfaces of the 
disc-shaped semiconductor body. 
As shown in these figures, the thyristor semiconductor body 4, which for 
example is made of silicon, has four layer type zones 5-8 of alternatingly 
opposite conductivity type with an n.sup.+ pnp.sup.+ zone sequence as 
illustrated. The n.sup.+ zones 5 constitute two sections of the cathode 
zone of the device and the p zone 6 constitutes the control base zone 
which extends to the same major surface 13 as the cathode zone 5. The edge 
surface 10 of thyristor semiconductor body 4 is conically tapered in order 
to reduce the edge surface field intensity along the lines of intersection 
of the pn-junction surfaces 11 and 12 with the edge surface 10 whenever a 
high anode voltage is applied to the blocking thyristor. A larger portion 
of the illustrated two sections of the n.sup.+ cathode zone 5 and an 
adjacent smaller portion of the p control base zone 6 are each provided, 
at the smaller major surface 13 of the thyristor device, with a partial 
ohmic electrode 14, 14' of the cathode. Also a small portion of the p 
control base zone 6, which lies between the two sections of cathode zone 5 
and extends to the smaller major surface 13 is provided with a 
finger-shaped portion 15 of the ohmic control electrode. The p.sup.+ anode 
zone 8 is bound to a support wafer 17 over the entire larger major surface 
16 of semiconductor body 4. The support wafer 17, for example, consists of 
molybdenum and constitutes the anode electrode for the device. 
The ohmic cathode and control electrode as a whole may be produced in a 
known manner by means of vapor deposition and subsequent alloying on the 
planar smaller major surface 13 of the silicon thyristor body 4. All of 
the finger-shaped portions 15 of the control electrodes as well as the 
portions of the major surface 13 which lie between the cathode electrode 
sections 14 and 14' are covered with an insulating layer 18, as shown. The 
insulating layer 18 may, for example, be an insulating lacquer which does 
not have too good a plastic deformability which layer is applied for 
example, by means of a screen printing mask. Alternatively the material of 
the insulating layer 18 may be quartz or a nitride, e.g. silicon nitride, 
which for example, was sputtered over the entire major surface 13 and then 
selectively etched to form the desired pattern. 
In the embodiment shown in FIGS. 1-4 the insulating layer 18 protrudes 
beyond the electrode portions or sections 14, 14' for the cathode by, for 
example, 20.mu.. The control electrode is provided with a portion 19 for 
an electrical connection to the control circuit. This connection portion 
19 is shown in FIG. 2 and is not covered by the insulating layer 18. The 
electrode sections 14, 14', as well as the remaining sections of the 
cathode, which are not shown in the drawing, are pressure contacted by 
means of a conductive contacting disc 20 formed of a relatively hard 
metal, which is covered at the contacting surface 20' with a highly 
conductive, ductile layer 21, for example, of silver which has been 
annealed at about 600.degree. C. The contacting disc 20 is provided with 
an opening 22 through which electrical connection to the portion 19 of the 
control electrode may take place. During pressure contacting, the layer 21 
will yield, due to its plastic deformability, with respect to the 
insulating layer 18. This produces impressions 23 in the layer 21 of a 
shape which is completely that of the surface of insulating layer 18 on 
the finger-shaped portions corresponding to part 15 of the control 
electrode, as is shown in FIG. 4 of the drawing. These impressions 23 
together form a negative of the structure pattern of the control electrode 
as a whole. Via the ductile silver layer 21 the contacting disc 20 is in 
intimate contact with the cathode electrode and hence the cathode 5 as a 
whole. The contacting disc 20 is of course centered with respect to the 
thyristor semiconductor body 4 and is pressure contacted. These elements 4 
and 20 are then lacquered together in a known manner with a fixing 
lacquer. 
After application of the insulating layer 18, the cathode electrode 
sections 14, 14' may be reinforced, for example, by galvanic drawing so 
that they become higher than the insulating layer 18 as shown in FIG. 5. 
In this case the contacting surface of the silver layer 21 adapts itself 
during pressure contacting to the protruding cathode electrode, i.e., a 
negative pattern or impression of the cathode electrode structure is 
formed in the silver layer with the same intimate contact of the silver 
layer with the cathode electrode. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.