Process for the production of a locally high, inverse, current amplification in a planar transistor

A process for the production of a locally high inverse current amplification in a preferably double diffused or implanted inversely operated transistor which includes forming a low doped epitaxial layer of one conductivity type on a high doped semiconductor substrate of the same conductivity type, forming a high doped buried region by ion implantation in the epitaxial layer beneath the zone provided for the collector, forming a low doped second region of the opposite conductivity type, above said first region which covers an area substantially wider than said first region and forming a third high doped region in the surface of the epitaxial layer spaced above said first region and having an area smaller than said first region, the first region forming the emitter, the second region forming the base and the third region forming the collector, the second region extending to the surface surrounding said third region and partially surrounding the first region. A second low doped region may be provided as an injector by diffusing an impurity of the second conductivity type into a fourth region adjacent the surface of the epitaxial layer and laterally spaced from the second region.

BACKGROUND OF THE INVENTION 
The invention relates to a process for the production of a locally high, 
inverse current amplification (upwards current amplification) in a 
preferably double-diffused or implanted, inversely operated planar 
transistor which is arranged in a semiconductor body with an integrated 
circuit. 
Digital circuits are known (Valvo-Reports, Volume XVIII, Edition 1/2 pages 
215 to 226) which employ the so-called integrated injection logic (I.sup.2 
L). The basic gate type of this technique requires only a very small 
crystal surface and the power loss can be kept extremely low. Bipolar, 
double-diffused or implanted transistors are used as switching elements. 
However, in contrast to a transistor of the usual planar technique in an 
inversely operated transistor, the emitter zone does not lie on the 
surface of the semiconductor body or an epitaxial layer deposited on a 
semiconductor substrate, but in the semiconductor body itself, that is to 
say beneath the surface of the epitaxially deposited layer. Thus an 
inversely operated npn-transistor has, for example, a n-conducting 
epitaxial layer, which normally forms the collector of a conventional 
transistor, forms the emitter, whereas the last n.sup.+ diffusion which 
normally forms the emitter of the conventional transistor now serves as 
the collector. This means that the switching element of the I.sup.2 L 
technique is an inversely operated bipolar npn-transistor in the 
conventional planar technique. 
A fundamental advantage of the integrated injection logic consists in the 
high packing density which can be achieved on a semiconductor body for the 
integrated circuit. This is based on the fact that with a corresponding 
circuit concept, no mutual insulation is necessary. On the other hand, 
inversely operated transistors have a relatively small upwards current 
amplification, which here is referred to as upwards current amplification 
(B.sub.up), which corresponds to the inverse current amplification in 
normal operation. The upwards current amplification could in fact be 
improved by a correspondingly high basic doping of the epitaxial layer. 
However, a high doping of this type is not very effective as it reduces 
the efficiency of a lateral pnp-transistor as injector (see Valvo-Reports, 
Vol. 18, Edition 1/2, pages 216 and 217) and at the same time increases 
the emitter-base-capacitance of the I.sup.2 L- transistor. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a novel inversely 
operated transistor which exhibits a high upward current amplification 
even in the case of a low-doped and high-ohmic epitaxial layer or in the 
case of a high-ohmic semiconductor substrate. 
This object is attained in accordance with the invention in that prior to 
the base doping ions are implanted into the semiconductor body beneath the 
zone provided for the collector, and during following temperature 
treatments diffuse out into the adjacent zones. 
The process in accordance with the invention allows the injector to be 
fully effective and at the same time permits a high upwards current 
amplification. As a result of the higher local doping introduced by ion 
implantation merely beneath the "active" base surface, the parasitic 
capacitance between the aforesaid base surface and the aforesaid emitter 
surface is likewise lower than with an increased basic doping of the 
overall epitaxial layer and of the substrate. This results in a 
substantial improvement in the speed-power product, which on the one hand 
is based on the high injector efficiency with a nevertheless adequate 
forwards current amplification and on the other hand on a reduction in the 
parasitic base-emitter-capacitance in relation to an overall increase in 
the doping.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A semiconductor body 1 consists of an n.sup.++ conducting semiconductor 
substrate having therein a buried layer 2. Above the layer 2 is an 
epitaxially deposited layer 3 of n type conductivity. The epitaxially 
deposited layer 3 has, for example, a layer thickness of 5 .mu.m and a 
specific resistance of 0.8 .OMEGA.cm corresponding to a doping of 
approximately 10.sup.16 atoms-cm.sup.-3. In this connection, see the 
horizontal line 31 in FIGS. 3 and 4. The doping course of the 
semiconductor substrate 2 is indicated by a curve 32 in FIGS. 3 and 4. 
Using ion implantation, n.sup.+ conducting zones are introduced into the 
epitaxially deposited layer 3 into those zones which are later covered by 
the collectors. The ions are implanted for example with 100 keV. As a 
result of a post-diffusion, the implanted ions penetrate deeper into the 
epitaxially deposited layer 3, so that the zones 4 which they form assume 
the course shown in broken lines in FIG. 2. FIG. 4 shows in broken lines 
the doping course of the implanted ions following the implantation in the 
form of a broken line curve 41. 
Then, in a known manner, the base is doped with boron by diffusion or ion 
implantation, so that a p-conductive zone 5 with the emitter-base junction 
is formed at the penetration depth x.sub.EB (see FIGS. 3 and 4). At the 
same time the p-conducting zone 5 is being formed, the injector zone 7 may 
be conveniently formed. If desired, the p-conducting zone 7 may be formed 
as the injector by diffusion in the n-conducting layer 3. Finally, in the 
zone 5, an n.sup.+ conducting zone 6 is produced as collector with the 
base-collector junction at the penetration depth x.sub.BC (see FIGS. 3 and 
4) by means of diffusion or implantation. The zone 6 has a penetration 
depth of approximately 1 .mu.m. The doping course of the zone 5 and of the 
zone 6 is referenced 35 and 36, respectively in FIGS. 3 and 4. 
During the base doping, a possible post-diffusion and the doping of the 
collector, the doping which has been introduced by means of ion 
implantation (see Curve 41 in FIG. 4) diffuses out, so that after these 
temperature treatments a doping course is formed as illustrated by the 
curve 42 in FIG. 4. 
FIG. 3 shows the doping course in a section III--III of FIG. 2 without the 
zone 4, and FIG. 4 shows the doping course in a section IV--IV with the 
zone 4. Beneath the collector 6, thus at the point of the actual, 
inversely operated transistor, the epitaxial layer 3 has a higher doping 
which produces a good emitter efficiency and thus a good upwards current 
amplification. 
It will be apparent to those skilled in the art that many modifications and 
variations may be effected without departing from the spirit and scope of 
the novel concepts of the present invention.