Impatt Diode

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.

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.

FIG. 1 is a schematic illustration of the structure of a single heterostructure IMPATT diode according to the invention.

FIG. 2 is a schematic illustration of the structure of a double heterostructure IMPATT diode according to the invention.

FIG. 3 shows the energy band scheme for an Si/Si.sub.0.2 Ge.sub.0.8 supperlattice for an IMPATT diode according to the invention.

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.