1. Field of the Invention
This invention relates to a power semiconductor component having at least three layers, including first and second layers extending up to a first surface which has sectionalized contact planes resulting in a step-like structure for contacting the first and second layers, and to a process for its manufacture.
2. Description of the Prior Art
Such a component as noted above is known, for example, from German Offenlegungsschrift No. 2,719,219. The semiconductor device described therein has four layers in a pnpn arrangement. The n-doped cathode layer 8 is subdivided into several sections, in order to allow contacting in two planes (FIG. 1). It has a surface doping of 5.times.10.sup.20 atoms cm.sup.3 and is 10 .mu.m thick. The p-base layer 6, located underneath, has a surface doping of 2.times.10.sup.18 atoms/cm.sup.3 and is 53 .mu.m thick. The width of the cathode sections is at least 320 .mu.m. The maximum reverse voltage of the pn junction between the cathode layer 8 and the p base 6 is 10 volt. The semiconductor device described is designed for a total current capacity of 400 A.
A process for manufacturing the abovementioned semiconductor device is also indicated (FIG. 6). For this purpose, boron or gallium is first diffused into both sides of an n-doped silicon substrate 42. The doping of the n substrate is about 10.sup.13 -10.sup.16 atoms/cm.sup.3. The p layers which have been diffused in, the p base layer 46 and the anode 44 have a surface doping of about 2.times.10.sup.18 atoms/cm.sup.3. To form the cathode layer 48, phosphorus is diffused into the p base layer 46. This n layer 48 is photolithographically subdivided into several sections. Aluminium layers are then applied to the anode layer 44, to the cathode sections 48 and to the subdivided p base layers 46.
The known semiconductor device can be switched off by applying a negative current pulse to the p base layer 6 (gate). The power required for this purpose and supplied from the outside is considerable. The load current is constricted towards the middle of a cathode section by this current pulse and a part of the load current is thus drawn away to the gate. The current density of the remaining load current then becomes very high. If the switching-off step proceeds too slowly--for example in the case of unduly high switching-off amplification--or not homogeneously over the entire area of the semiconductor device, the latter is locally overloaded thermally and hence destroyed. The switching-off current via the gate produces a lateral voltage drop which should be smaller than the breakdown voltage of the pn junction between the cathode and the p base layer. However, the maximum reverse voltage of this pn junction is so low that an avalanche breakdown of this junction can occur during the switching-off step. The manufacturing process described is admittedly adequate for the structure having the dimensions of the known semiconductor device, but finer structures can hardly be produced satisfactorily this way.
German Offenlegungsschrift No. 2,855,546 describes a field-controlled thyristor which has characteristics similar to those of a p-i-n diode. This thyristor consists of a thin, heavily p-doped anode zone 15, a thick, lightly n-doped semiconductor substrate 11, several deep p-doped grid zones 12 and thin, heavily n-doped cathode zones 14 arranged in between (FIG. 3). The grid zones 12 consist of channels having a width of about 12 .mu.m and a depth of 15 to 40 .mu.m. To produce these grid zones 12, channels are etched into a &lt;110&gt;-oriented silicon substrate with a mixture of potassium hydroxide and isopropanol in a ratio of about 3:1 (FIG. 1). For this purpose, a mask 13 of silicon dioxide is first formed. The channels are then epitactically refilled with heavily p-doped silicon. A thin layer of poly-silicon then additionally forms on the silicon oxide layer which is removed by etching. A mask of silicon oxide is then formed for the cathode zones 14 which are produced by diffusion of phosphorus. Finally, the grid zones 12 and the cathode zones 14 are provided with metallizations, silicon dioxide being left in between for electrical insulation.
The silicon with &lt;110&gt;-orientation, required for the process described, cannot be drawn without dislocations. For this reason, the silicon wafers can be obtained only in an elliptical form and must be sand-blasted to give a round form for further processing. The base material for the component described is therefore much more expensive than that for conventional components.