An IMPATT diode having Si-SiGe heterostructure grown on a Si substrate with the SiGe layer being disposed in the generation zone of the IMPATT diode. The SiGe layer may be replaced by a Si/SiGe superlattice.

BACKGROUND OF THE INVENTION 
The present invention relates to an IMPATT (Impact Avalanche Transit Time) 
diode composed of a monocrystalline silicon substrate to which a 
heterostructure-semiconductor layer sequence has been applied in an 
alternating arrangement of at least two different semiconductor layers 
forming at least one heterojunction. 
IMPATT diodes are powerful millimeter wavelength components. They are 
utilized, in particular, to generate oscillations. 
Avalanche multiplication by impact ionization and movement of the charge 
carriers through a drift space together may produce a negative resistance. 
A structure suitable to generate and combine these mechanisms is, for 
example, a pn-junction. 
By using a heterojunction instead of a pn-junction, the voltage required 
for the avalanche breakdown of the IMPATT diode can be reduced. The 
heterostructure of the IMPATT diode must be such that the semiconductor 
material of the generation zone (avalanche breakdown zone) has a smaller 
band gap than the material of the drift zone. This reduces the ionization 
threshold energy and thus increases the efficiency of the IMPATT diode. 
However, prior art solutions have the drawback that the semiconductor 
materials employed place the efficiency of the IMPATT diode, for example 
for a frequency of 80 GHz, at below 10%, and limit the wavelength range to 
the millimeter range. Moreover, the use of economical Si substrates is 
possible only within limits. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide an IMPATT diode 
having a heterostructure which can be produced economically, has high 
efficiency and whose wavelength range includes the millimeter and 
submillimeter ranges. 
The above object is achieved according to the present invention in that in 
an IMPATT diode composed of a monocrystalline Si substrate to which a 
heterostructure semiconductor sequence has been applied in an alternating 
arrangement of at least two different semiconductor layers forming at 
least one heterojunction, the generation zone of the IMPATT diode includes 
at least one SiGe layer. 
According to one embodiment of the invention the substrate is formed of 
p.sup.+ -Si, and the heterostructure semiconductor layer sequence includes 
an undoped SiGe layer, an n-doped Si layer and an n.sup.+ -doped Si layer 
grown on the substrate. 
According to another embodiment of the invention, the heterostructure 
semiconductor layer sequence grown on the p.sup.+ -Si substrate is 
composed of a p-doped Si layer, an undoped SiGe layer, an n-doped Si 
layer, and an n.sup.+ -doped Si layer. 
According to further embodiments of the invention, the generation zone of 
the IMPATT diode is configured such that the undoped SiGe layer of each of 
the two embodiments mentioned above is replaced with an undoped Si/SiGe 
superlattice. 
According to a feature of the invention, a predeterminable stress 
distribution is produced in the heterostructure semiconductor layer 
sequence to result in a reduction of the band gap of the semiconductor 
materials forming the generation zone. 
The invention has the advantage that the IMPATT diode is manufactured on a 
cost efficient Si substrate and its efficiency is improved by a suitable 
Si/SiGe heterostructure. 
The invention will be described in greater detail below for embodiments 
thereof and with reference to the schematic drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown an IMPATT diode according to the 
invention including a p.sup.+ -doped Si substrate 1 having a charge 
carrier concentration of more than 10.sup.19 cm.sup.-3, on which there is 
applied a semiconductor layer sequence composed of: 
an undoped, i.e., intrinsic, SiGe layer 2 having a layer thickness of about 
0.02 .mu.m; 
an n-doped Si layer 3 having a charge carrier concentration of about 
1.4.10.sup.17 cm.sup.-3 and a layer thickness of about 0.3 .mu.m; and 
an n.sup.+ -doped Si layer having a charge carrier concentration of about 
5.10.sup.19 cm.sup.-3 and a layer thickness of about 0.1 .mu.m. 
The symbol "+" indicates heavy doping. Suitable doping materials for 
silicon are, for example, Sb, P, and As for n-type doping, and B, Ga, A1 
and In for p-type doping. 
In operation, a reverse voltage of such magnitude that the space charge 
layer extends over semiconductor layers 2 and 3 is applied to the 
structure of FIG. 1. At the boundary between the p.sup.+ -doped Si 
substrate 1 and the SiGe layer 2, the maximum field intensity is so great 
that impact ionization begins. 
The semiconductor layer structure of an IMPATT diode according to FIG. 1 
differs from that of an IMPATT diode based on Si and provided with a 
pn-junction in that a SiGe layer 2 has been grown at the location where 
the field intensity necessary for impact ionization, and thus avalanche 
multiplication, occurs. Advantageously, SiGe has a smaller band gap 
E.sub.g than Si (E.sub.g =1.12 eV) and thus a lower ionization threshold 
energy than Si. For an Si.sub.0.6 Ge.sub.0.4 layer of a thickness of 0.02 
.mu.m, the band gap E.sub.g =0.76 eV. 
Additionally, the different lattice constants of Si and SiGe produce a 
lateral mechanical tension in the semiconductor layers. If, for example, 
an Si.sub.0.6 Ge.sub.0.4 layer 2 having a subcritical layer thickness of 
about 0.02 .mu.m is precipitated onto an Si substrate 1, a lateral 
pressure stress develops in the SiGe layer since the natural lattice 
constant of monocrystalline SiGe is greater than that of monocrystalline 
Si. A layer thickness is considered subcritical if the layers composed of 
different semiconductor materials have the same lattice constant in the 
lateral direction. This then indicates pseudomorphous growth. The pressure 
stress causes the SiGe layer to be compacted by up to 4%. The mechanical 
stress created by the pseudomorphous growth results in a reduction of the 
band gap of SiGe and results in an additional reduction of the ionization 
threshold energy of SiGe. 
The embodiment according to FIG. 2 shows an IMPATT diode having a double 
heterostructure. The double heterostructure of FIG. 2 differs, for 
example, from the single heterostructure of FIG. 1 by a further p-doped Si 
layer 5 grown between the p.sup.+ -doped substrate 1 and the undoped SiGe 
layer 2. Si layer 5 has a layer thickness of about 0.25 .mu.m and a 
positive charge carrier concentration of about 1.6.10.sup.17 cm.sup.-3. 
With respect to layer thickness and layer composition, substrate 1 and 
semiconductor layers 2, 3, 4 are selected analogously to the corresponding 
layer of the structure shown in FIG. 1. The mechanical stress in the SiGe 
layer 2 caused by the different lattice constants of Si and SiGe 
advantageously results in a reduction of the band gap in the SiGe layer 
and a reduction in the ionization threshold energy. 
The greater ionization rate of SiGe has the advantage that the spatial 
region in which power loss occurs is reduced and the efficiency of the 
IMPATT diode is augmented. The efficiency of heterostructure IMPATT diodes 
according to the present invention, for a frequency of 100 GHz, lies 
between 12 and 15%. 
To increase the efficiency of the IMPATT diode of this type, it is 
additionally of advantage to configure the generation zone as an undoped 
Si/SiGe superlattice. In a Si/SiGe superlattice having a suitable period 
duration, the Ge percentage x in the Si.sub.1-x Ge.sub.x layers of the 
superlattice can be selected to be greater, if the layer thickness is the 
same as for an individual SiGe layer, than in a corresponding individual 
Si.sub.1-x Ge.sub.x layer. By increasing the Ge percentage x of the 
Si.sub.1-x Ge.sub.x layers and optimally configuring the period duration 
of the Si/SiGe superlattice, it is possible, as shown in FIG. 3, to 
produce a miniband 6 in the energy band scheme of the Si/SiGe superlattice 
whose band gap toward the conduction band E.sub.c is smaller than the band 
gap between the valence and conduction bands of an individual SiGe layer 
having the same layer thickness. For example, for an individual Si.sub.1-x 
Ge.sub.x layer having a layer thickness of 0.02 .mu.m, the maximum Ge 
percentage x=0.4, and the corresponding band gap E.sub.g =0.76 eV. For a 
symmetrical Si/Si.sub.1-x Ge.sub.x superlattice having a layer thickness 
of 0.02 .mu.m and being composed of five periods, only three of which are 
shown in FIG. 3, and with each period having a period duration L=0.004 
.mu.m, the maximum Ge percentage x=0.8 and the corresponding band gap 
E.sub.g =0.6 eV. In a symmetrical superlattice, all individual layers of 
the superlattice have the same layer thickness. 
The critical layer thickness of the Si/Si.sub.1-x Ge.sub.x superlattice is 
greater, with the identical Ge percentage x, than in an individual 
Si.sub.1-x Ge.sub.x layer. Thus, the layer thickness of the generation 
zone can be selected to be greater if a Si/SiGe superlattice is employed 
than for an individual SiGe layer. This has the advantage that the 
superlattice IMPATT diode can be used over a broader frequency range than 
the corresponding heterostructure IMPATT diode which includes only a 
single SiGe layer. 
The wavelength range of the IMPATT diodes according to the invention covers 
the millimeter and sub-millimeter ranges. 
The heterostructure semiconductor layer sequences of the IMPATT diodes 
according to the invention are produced, in a known manner, by means of a 
molecular beam epitaxy process. 
The present invention is not limited to the stated p.sup.+ inn.sup.+ or 
p.sup.+ pinn.sup.+ structure of the IMPATT diodes, but can also be used 
for inverse structures which are produced in that, in the embodiments, the 
conductivity types of the semiconductor layers are reversed (n-type 
conductivity is replaced by p-type conductivity and vice versa). Moreover, 
the present invention is not only suitable for IMPATT diodes with 
quasi-Read structures as described but, moreover, can also be applied for 
Misawa diodes. 
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.