TECHNICAL FIELD

The present invention relates to a thyristor with a thyristor structure produced in a semiconductor body and with a controllable semiconductor member, integrated with the thyristor, for controllable shorting of at least one of the emitter junctions of the thyristor.

BACKGROUND ART

From Swedish patent specification 7507080-5 (publ. No. 392 783) a thyristor of this kind is known. Field effect thyristors for controllable shorting (by-pass) of one of the emitter junctions of the thyristor are produced in the same semiconductor body as the thyristor itself. The controllable shorting of the emitter junction obtained with the aid of the field effect transistors may be used, for example, for turn-off of the thyristor or for blocking of the turn-on of the thyristor. However, a thyristor of this kind has certain drawbacks.

The doping concentrations of the different regions of the field effect transistors must be adapted to the doping concentrations of the thyristor. This makes it difficult simultaneously to obtain an optimum function of the transistors and of the thyristor. Also the type of doping of the transistors is determined by the embodiment of the thyristor, which imposes further limitations when designing the transistors.

In the prior art thyristor described above it is difficult to avoid charge carrier injection from the thyristor structure into the shorting transistors during the conduction interval of the thyristor. It has proved that such an injection may give rise to disturbance of the function of the device, such as a partial shorting of the

emitter junction of the thyristor during the conduction interval and an ensuing deterioration of the conduction properties of the thyristor. Further, in a thyristor of this known kind a parasitic thyristor effect may arise and disturb the function of the component .

In a thyristor of the kind described above the shorting transistors must be arranged on the surface of the semiconductor body beside the emitter part or parts which are to be shorted by the transistors. It has proved that the transistors will then take up a considerable part of the surface of the semiconductor body, which considerably reduces that part of the area which is available for the emitter layer of the thyristor. This means a considerable reduction of the current handling capacity of the thyristor.

SUMMARY OF THE INVENTION

The invention aims to provide a thyristor of the kind mentioned in the introduction, in which

a) the charge carrier injection from the thyristor structure into the shorting controllable semiconductor members is completely eliminated,

b) both the thyristor structure and the shorting controllable semiconductor members can be optimized independently of each other both as regards the doping concentration and the doping type,

c) a considerable increase of the emitter area of the thyristor and hence of its current handling capacity can be obtained,

d) the shorting controllable semiconductor members can be made in a semiconductor material other than that of the thyristor structure, which makes possible an improved shorting function and greater freedom in the

choice of the type of the controllable semiconductor members,

e) in the manufacture of thyristors with controllable shortings both on the anode side and on the cathode side, an improved manufacturing result in the manufacture of the controllable semiconductor members can be obtained,

f) great freedom is provided when choosing design and type of the controllable shorting semiconductor member, and

g) the area available for the shorting members can be made large, which makes possible a low efficient on- state resistance of these members and hence an effective shorting effect.

What characterizes a thyristor according to the invention will become clear from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following with reference to the accompanying Figures 1-7. Figure la shows a section through part of a thyristor according to the invention. Figure lb shows the same thyristor seen from the cathode side. Figure 2 shows an embodiment of the invention in which the controllable semiconductor members consist of field effect transistors of MOS type. Figure 3 shows a different embodiment in which the controllable semiconductor members consist of bipolar transistors. Figure 4 shows an additional variant in which the controllable semiconductors consist of field effect transistors with pn-type control electrodes (JFET transistors) . Figure 5 shows a thyristor according to the invention in which the controllable semiconductor members consist of MOS transistors and the thyristor is provided with double-sided control means, i.e.

provided with members for controllable shorting of the emitter junctions on both the cathode and anode sides, electrically insulating layers according to the invention being arranged only on the cathode side of the thyristor. Figure 6 shows an alternative embodiment, in which the controllable semiconductor members which short the junction between an emitter layer and a base layer are connected to the latter layer with the aid of a conducting material arranged in channels in the emitter layer. Figure 7a shows a perspective sketch of an embodiment of an MOS transistor which is suitable for controllable shorting. Figure 7b shows the section marked A in Figure 7a; Figure 7c shows the section marked B in Figure 7a, and Figure 7d shows the section marked C in Figure 7a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figur la shows a section through a thyristor according to the invention. In a semiconductor body 1 in the form of a silicon wafer, a thyristor structure is produced. The thyristor has a plurality of cathode emitter layers 5a, 5b, 5c, 5d, distributed over its surface, an anode emitter layer 4, and the base layes 2 and 3. The anode emitter layer is provided with a metal contact 11. On the cathode side of the thyristor a number of schematically shown controllable semiconductor members 6a, 6b, 6c, 6d are arranged. On the cathode surface of the semiconductor body a number of insulating layers 12a, 12b, 12c, 12d are arranged. These layers suitably consist of silicon dioxide and may have a thickness of, for example, 1 μm. Possibly, one single continuous silicon dioxide layer may be arranged on the cathode surface and be provided with suitably arranged openings for contacting the base layer 2 of the thyristor and its cathode emitter layers 5a, 5b, etc. On top of this insulating layer or these insulating layers those layers of semiconductor material are arranged in which the controllable semiconductor members 6a, 6b etc., are produced. The latter layers may, for example, have a

thickness within the interval of 0.3-5 μm, for example 3-4 μm. The material in these layers may be the same as in the semiconductor body 1, i.e. in this case silicon. The base layer 2 of the thyristor is contacted with the aid of metal layers 7a, 7b, 7c, with the aid of which the controllable semiconductor members are connected to the base layer. The base layer 2 is provided on its surface with heavily p-doped regions 51a, 51b, 51c to attain low-ohmic resistive contact with the metal layers 7a, 7b, 7c. The emitter layers 5a, 5b etc. are contacted with the aid of metal layers 8b, 8c, etc., with the aid of which the requisite connection between the emitter layers and the controllable semiconductor layers is obtained. On top of the controllable semiconductor members, layers 9a, 9b, 9c of a suitable electrically insulating material are arranged, for example so-called PSG glass (phosphorus silicate glass) . On top of the parts now enumerated, the cathode contact 10 of the thyristor is arranged in the form of a metal layer which covers the entire cathode surface of the thyristor and makes contact with the emitters 5a, 5b, etc., by way of the metal layers 8b, 8c, etc.

Figure lb schematically shows the thyristor according to Figure la seen from the cathode side. The continuous lines in the figure show the extent of the square emitter layers 5a, 5b, 5c, 5d. Figure la shows the section marked by A-A in Figure lb. The emitter parts may, for example, have the side length 5-50 μm. The extent of the controllable semiconductor members 6a, 6b, 6c, 6d is shown in dashed lines in the figure. Other embodiments of the emitter layers than that shown in Figure lb are known and possible. For example, the emitter parts may be formed as elongated strips instead of as squares. Alternatively, the thyristor may be provided with one single, continuous emitter layer with a plurality of openings for contacting the base layer 2 of the thyristor.

The semiconductor layers, in which the controllable semiconductor members are produced, may be produced on top of the silicon dioxide layers 12a, 12b, etc., in several different ways known per se. For example, a polycrystalline silicon layer may be deposited on the silicon dioxide layer and by suitable heat treatment be converted into a monocrystalline structure. Alternatively, the semiconductor layers 6a, 6b, etc., may, from the beginning, constitute part of the semiconductor body 1. By ion implantation of oxygen to a suitable depth, a thin layer of the semiconductor body, located below the surface, is then transformed into silicon dioxide, which gives the oxide layers 12a, 12b, etc. According to a third alternative, the semiconductor body 1 may first be provided with a silicon dioxide layer of a suitable thickness. A second semiconductor body with the same size and shape as the body 1 is also provided with a silicon dioxide body on one of its surfaces. The two bodies are then brought together with the silicon dioxide layers facing each other, and the bodies are bonded together by heat treatment. Finally, such a large portion of the second semiconductor body is removed, for example by etching or grinding, that only the layers 6a, 6b, etc., remain of this body.

In a further method for the manufacture of a thyristor according to the invention, the thyristor structure and the controllable shorting semiconductor members are produced separately. The latter members may then have the shape of thin plates or films of semiconducting material, which are applied on the thyristor structure. The controllable members may, for example, be produced on electrically insulating substrates, which are applied on the thyristor surface by bonding, glueing or in some other way. Alternatively, the surface of the thyristor structure may be provided with layers of electrically insulating material, on which the controllable members are applied. In this method of manufacturing the thyristor structure is manufactured separately and the controllable shorting components

separately, and therefore the manufacturing processes may be optimized for each component separately.

Figure 2 shows an embodiment of the invention in which the controllable semiconductor members consist of MOS transistors. The figure shows the righthand part of the emitter layer 5b and the semiconductor member 6b in Figure la. On top of the silicon dioxide layer 12b a silicon layer is arranged, in which the collector and emitter regions 21, 23 of the transistor, as well as its channel region 22, are produced. A highly n-doped layer 24 nearest the contact 7b gives a low-oh ic resistive connection to this contact. The control electrode 26 of the transistor consists of a layer of doped polycrystalline silicon and is separated from the transistor itself by a thin silicon dioxide layer 27. The transistor is of n-channel type and of enhancement type. It is normally non-conducting but at a positive voltage on the control electrode, an n-conducting channel 25 is induced between the collector and emitter of the transistor. The transistor then forms a low resistance conducting connection between the emitter 5b of the thyristor and its base layer 2, i.e. the emitter junction is shorted.

Figure 2 shows an extra metal layer 8bl arranged between the metal layer 8b and the cathode contact 10. This layer only has the function of building up the metal to a suitable thickness prior to the application on to the cathode contact.

The field effect transistor shown in the figure is, as mentioned above, of n-channel type but may alternatively be made as a transistor of p-channel type. Instead of the transistor of enhancement type shown above, a transistor of depletion type may alternatively be used.

Figure 3 shows an alternative embodiment of the invention, in which the controllable semiconductor members consist of bipolar transistors of npn-type. The figure shows a section

through such a transistor and adjoining parts of the thyristor. The transistor has an emitter layer 33 and a collector layer 31, both being n-doped. Between these layers the p-doped base layer 32 of the transistor is arranged and provided with a metal contact 36 for control of the transistor. A silicon dioxide layer 35 covers the greater part of the surface of the transistor and separates the transistor from the metal layer 8b, except on that region where the metal layer makes contact with the emitter layer 33 of the transistor. As in Figure 2, a heavily n- doped layer 34 is arranged nearest the metal contact 7b to give a low-ohmic resistive contact. With the aid of a suitable control signal on the contact 36, the transistor may be brought to alternatively adopt non-conducting or conducting states, whereby, in the latter state, the transistor forms a low resistance current path between the metal layers 8b and 7b, thus effectively bridging the junction between the emitter layer 5b and the adjoining base layer 2.

Alternatively, the bipolar transistors may be of pnp-type.

Figure 4 shows an additional embodiment of a thyristor according to the invention, in which the controllable semiconductor members consist of field effect transistors with pn-type control electrodes (JFET transistors) . The transistor of this type ^" shown in Figure 4 has an n-doped main part 41, the opposite parts of which form the collector and emitter regions of the transistor. Heavily n-doped layers 34a, 34b are arranged nearest the metal layers 7b and 10 to provide low-ohmic and resistive contacts. The transistor has a p-doped control region 42 connected to a control contact 45 in the form of a metal layer. On top of the transistor a silicon dioxide layer 44 is arranged and provided with an opening for contacting the control region. If no control voltage, or a positive control voltage, is supplied to the control region of the thyristor via the contact 45, the transistor is conducting and current may

flow with a low resistance through the transistor from the metal contact 7b to the cathode contact 10, the emitter junction of the thyristor thus being shorted. If a negative control voltage is applied to the control contact of the transistor, a barrier layer will extend from the pn-junction of the transistor. An example of the extent of such a barrier layer is shown by the dashed line 43 in the figure. The greater the negative voltage being applied to the control contact 45, the greater will be the extent of the barrier layer, and at a sufficiently high negative voltage the connection through the transistor is completely throttled and the shorting of the emitter junction is thus cancelled.

Figure 5 shows a thyristor with double-sided control means according to the invention, i.e. a thyristor whose emitter junctions can be shorted both on the cathode side and on the anode side. On the cathode side a MOSFET structure is arranged for shorting of the emitter junction and separated from the semiconductor body 1 by a silicon dioxide layer 12b. The transistor has collector and emitter regions 53 and 55 as well as a channel region 52. An n-doped region 54 separates the channel region from the metal contact 7a. The transistor has a control electrode 57 of high-conductivity polycrystalline silicon, which is separated from the transistor by a silicon dioxide layer 56. The base layer 2 is provided on its surface with a heavily p-doped region 51 for obtaining a low-ohmic resistive contact with the metal layers 7a and 7b. The construction and function of the transistor correspond, in principle, to what has been described above with reference to Figure 2.

The thyristor has the weakly n-doped base layer 3 and, between it and the anode-side surface of the semiconductor body, a further n-doped layer 3b. The thyristor has a plurality of p-doped anode-emitter regions, each one with a heavily p-doped central part - 4al, 4bl - surrounded by a more weakly p-doped region - 4a2, 4b2. In each one of the

anode-emitter regions, heavily n-doped regions 58a, 58b are arranged. The regions 58a, 58b and the region 3b form collector and emitter parts, respectively, of field effect transistor structures with the channel regions 4a and 4b. Control electrodes 59a, 59b, 59c of polycrystalline silicon are arranged on the surface of the semiconductor body and separated therefrom by thin silicon dioxide layers 60a, 60b, 60c. The field effect transistors are non-conducting in the absence of voltage on the control electrodes but at a positive voltage on the control electrodes, n-conducting channels are produced between the layers 58a, 58b on the one hand and the region 3b on the other hand. This results in shorting of the anode-side emitter junctions of the thyristor.

In the manufacture of thyristors with double-sided control means, the invention provides considerable advantages. First, the shorting structures on the anode side can be produced, and only thereafter is that semiconductor layer on the cathode side deposited where the shorting structures on that side are produced. In this way, the shorting structures of the cathode side can be produced in a new and undamaged surface, which is important in the manufacture of MOS components.

If desired, as an alternative to the embodiment shown in Figure 5, also the transistors arranged on the anode side for shorting the emitter junctions can be produced in semiconductor layers which are separated from the semiconductor body 1 with the aid of insulating layers in the same way as on the cathode side.

In the embodiment of a thyristor according to the invention shown in Figure 6, the thyristor has the cathode emitter layer 5a, 5b, 5c. On top of the silicon dioxide layer 61, MOS transistor structures are produced, which constitute n- channel transistors of enhancement type. Each transistor has collector and emitter regions 63, 66 and channel regions

6 . Each transistor has a control electrode 68 which is surrounded by a thin silicon dioxide layer 67. A heavily n- doped region 62 is arranged for effective contacting of the transistor.

The transistors are connected to the emitter layer with the aid of the metal contact 10. To obtain a connection between the transistors and the p-base layer 2, the emitter layer 5 and that semiconductor layer in which the transistors are produced are provided with channels 69a, 69b. These may have the shape of elongated grooves or of a plurality of discrete holes or openings and may be produced by deep etchings. The walls of the channels are provided with silicon dioxide layers 70a, 70b, applied, for example, by CVD deposition. The channels are filled with an electrically conducting material 71, which also extends up to the surface of the transistors and makes contact with the layers 62. This material may, for example, consist of doped polycrystalline silicon and forms a low resistance connection between the p-base 2 and the field effect transistors.

In the embodiment shown in Figure 6, practically the whole area of the semiconductor body may be utilized for the cathode emitter layer 5, which provides the greatest possible current handling capacity for a certain area of the body. Further, practically the whole area of the semiconductor body may be utilized for the controllable semiconductor members, which provides the greatest possible freedom when designing these members and a possibility of giving these members a low on-state resistance and hence of achieving very effective shortings of the emitter junctions of the thyristors. In this embodiment, the cathode emitter layer need not be given the finely-divided structure which is usually used in this kind of thyristors. This is an advantage, since such a finely-divided pattern of the emitter layer runs the risk of being damaged in the manufacture of the semiconductor layer or layers in which

the controllable semiconductor members are to be produced. In the embodiment shown in Figure 6, the cathode emitter layer 5 may be made as one single continuous layer, whereupon the oxide layer 12 is generated and that semiconductor layer is deposited in which the controllable semiconductor members are to be produced. After that, the channels 69 for communication between the field effect transistors and the p-base layer are produced.

In a thyristor with controllable shortings, it is of great importance that the on-state resistance of the controllable semiconductor members which are used for the shortings is as low as possible. Figure 7 shows an embodiment of an MOS field effect transistor which may be given a very low on- state resistance and is therefore especially well suited for use in a thyristor of the kind referred to here. Figure 7a shows a perspective sketch of the transistor, which is arranged on a silicon dioxide layer 81, which in turn is applied on the surface of that body in which the thyristor is produced. The n+-conducting collector and emitter layers 82 and 83, respectively, of the transistor are arranged, respectively, nearest to and furthest away from the observer in the figure. A number of parallel slots, running from the collector to the emitter region, are etched down through the semiconductor layer in which the transistor is produced, all the way down to the silicon dioxide layer 81. The control electrode 85 of the transistor consists of polycrystalline silicon. It is arranged on top of the transistor and also extends down into the etched slots to the layer 81. The control electrode 85 is separated from the transistor itself by means of a thin insulating layer 86 of silicon dioxide. Those parts of the control electrode which extend down into the slots are designed 85b, 85c, 85d, 85e in the figure. In this way, the transistor receives a plurality of separate channel regions, which are separated by the slots and those parts of the control electrode which are located in these slots, and of which the region 84e is shown in Figure 7a.

Figure 7b shows a section through the transistor along the dashed line designated A in Figure 7a. Figure 7c shows a section along the dashed line designated B in Figure 7a. Figure 7, finally, shows a section along the dashed line designated C in Figure 7a.

At a positive voltage on the control electrode, channel regions 87a, 87b, 87c, 87d, 87e, 87f are produced. These channel regions extend not only along the upper surface of the semiconductor layer but also along the walls of the etched slots. The current through the transistor flows perpendicularly to the paper in the section shown in Figure 7d, and the width of the channel region, i.e. its extension perpendicular to the current direction, will be large in this embodiment, which results in the transistor having a low on-state resistance and providing an effective shorting of the emitter junctions of the thyristor.

In the embodiments of the invention described above, silicon dioxide has been used as insulator between the thyristor structure and the controllable semiconductor members. Also other insulating materials are possible, for example silicon nitride.

In the foregoing, thyristors have been described in which both the thyristor structure itself and the controllable semiconductor members are produced in silicon bodies. However, also other semiconductor materials are feasible within the scope of the invention, such as germanium or gallium arsenide. A considerable advantage which may be attained in a thyristor according to the invention is that the controllable semiconductor members may be manufactured in a layer of a semiconductor material with smaller band gap than the semiconductor material of the thyristor body. This makes it possible to attain an exceedingly low on-state voltage drop of the controllable semiconductor members and hence a very effective shorting effect. Thus, for example, the controllable semiconductor members could be made of

silicon and the thyristor of gallium arsenide; alternatively, in the case of a silicon thyristor, the controllable semiconductor members could be made of germanium. In this embodiment the lower band gap of the material of the controllable semiconductor members makes possible a great freedom when designing the controllable semiconductor members. For example, this design makes possible a use of Darlington transistors, IGBT (Insulated Gate Bipolar Transistors) or thyristors for the controllable shorting of the emitter junctions of the thyristor.

In the embodiments described above, field effect transistors of MOS type, JFET (field effect transistors with pn-control electrodes) and bipolar transistors have been used as controllable shorting members. Also other components may be used as the controllable semiconductor members. In a thyristor according to the invention, in which the controllable semiconductor members are separated from the thyristor structure by means of insulating layers, the controllable semiconductor members may be chosen largely freely, and the only requirement that need be placed is that the controllable semiconductor members have a sufficiently low on-state voltage drop. Other feasible components are, for example, MESFET (field effect transistors with a metal control electrode arranged directly on the semiconductor material) , MISFET (a general designation for field effect transistors with insulated control electrodes) , and IGBT (Insulated Gate Bipolar Transistors) . In those cases where field effect transistors are used as controllable semiconductor members, they may be made as so-called DMOS transistors (Double Diffusion MOS Transistors) . In this method, the control electrode of a transistor is used as mask in the diffusion steps used for the manufacture of the different regions of the transistor.

There has been described above how the controllable shorting semiconductor members are produced in one single layer of semiconductor material, which is separated from the

thyristor body by an electrically insulating layer. Alternatively, additional layers of semiconductor layers may be arranged on top of the above-mentioned layer of semiconductor material and separated therefrom and from each other by electrically insulating layers. All of these layers can then be utilized for producing the controllable shorting semiconductor layers, whereby a considerably larger area is made available to these members and their effective on-state resistance can be reduced.

There has further been described above a thyristor with a finely-divided emitter structure and where the controllable shorting semiconductor members are arranged distributed and in immediate proximity to the emitter junctions of the thyristor. Alternatively, one or more controllable semiconductor members may be arranged beside the thyristor structure itself, on the same semiconductor body as this and separated from this by electrically insulating layers. Suitable connections are then arranged, for example in the form of metal layers, between the semiconductor members and the thyristor structure.