Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
2001-11-19
2004-09-07
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S201000, C257S197000, C257S613000, C257S183000, C257S615000, C257S191000, C257S194000, C257S195000
Reexamination Certificate
active
06787822
ABSTRACT:
The invention relates to a heterojunction transistors of the field effect transistor category or of the bipolar transistor category, using III-V semiconductor alloys.
III-V semiconductors, of which the best known is gallium arsenide GaAs, constitute a vast family making use of one or more elements from column III and one or more elements from column V of the periodic table. Thus there exist:
binary compounds such as GaAs or InP;
ternary compounds such as AlGaAs or GaInAs or GaInP; and
quaternary compounds such as GaInAsP or AlGaSbAs.
In ternary or quaternary alloys, the III elements substitute for one another and likewise the V elements substitute for one another so that the III elements as a whole and the V elements as a whole are of equal composition: for example Al
x
Ga
1−x
As or Ga
x
In
1−x
As
y
P
1−y
. To avoid overburdening the text, the simpler notation AlGaAs or GaInAsP is adopted below. Wherever necessary, the concentrations of the various elements are specified. Furthermore, the various elements making up an alloy are given in the standardized order (from the most electropositive to the least electropositive), and not in order of decreasing concentration.
The advantage of such ternary or quaternary alloys or indeed quinary alloys when making electronic components stems from the fact that substituting one III element for another or one V element for another modifies the electronic properties of the alloy, e.g. the effective mass of electrons or holes, or indeed the width of the forbidden band. These modifications are taken advantage of when making heterojunctions, i.e. junctions between two materials of different kinds, e.g. AlGaAs/GaAs or GaInAsP/GaAs, etc.
Semiconductor lasers made of AlGaAs/GaAs or GaInAsP/AlGaInP or InGaAs/GaAs/GaInP are well known examples of using heterojunctions to make lasers that emit at the following wavelengths respectively 870 nanometers (nm), 670 nm, and 980 nm.
So-called high electron mobility field effect transistors (HEMTs) making use of AlGaAs/GaAs or AlGaAs/GaInAs/GaAs heterojunctions are also well known for their performance which is better than that of conventional non-heterojunction GaAs transistors.
Heterojunction bipolar transistors (HBTs) of the AlGaAs/GaAs or GaInAsP/GaAs type are also known for their performance which is better than that of non-heterojunction GaAs bipolar transistors.
In spite of the flexibility with which III-V semiconductor alloys can be combined with one another in the form of a heterojunction, there nevertheless exists a limit on such combination.
Different materials can be associated with each other only if they have the same crystal lattice parameter or have lattice parameters that are very similar, so as to ensure that high mechanical stresses do not appear between the various materials. High stress can lead to dislocations appearing at the interface between the two materials, which dislocations then propagate throughout one of the two materials, thereby degrading the quality of such materials and consequently the quality of the electronic component. When the mismatch between the lattice parameters of the two parameters is small, it is possible to grow a heterojunction by epitaxy without causing dislocations to appear at the interface since the mechanical stress generated is small enough for the material to remain within its bounds for elastic deformation. It is known that elastic deformation is a function both of the difference between the crystal lattice parameters and of the thickness of the stressed layer. Below a certain thickness known as the “critical” thickness, the layer remains in a state of elastic deformation, i.e. free from dislocations, and above the critical thickness the stresses are relaxed by the appearance of dislocations.
Many electronic components make use of this possibility to achieve elastic stress in order to provide heterojunctions having electronic characteristics of interest. For example, AlGaAs/GaInAs heterojunctions are made in which the indium content can be as much as 25%, thereby creating a parameter mismatch approaching 2% and thus imposing a critical thickness of about 10 nm.
This stressed GaInAs material presents advantages over non-stressed GaAs material when making HEMTs: electron mobility is improved; the conduction band discontinuity at the heterojunction interface is advantageously greater. Such HEMTs using the narrow forbidden band material in the elastic stress state are referred as “pseudomorphic” HEMTs. Pseudomorphic HEMTs are in very widespread use for low noise amplification and for power amplification.
HBTs make little use of this pseudomorphic state because the base of an HBT is generally about 100 nm thick, and thus thicker than the critical thickness for lattice mismatch materials that are liable to present electronic characteristics of interest. Thus, no pseudomorphic HBTs are known as industrial products.
Furthermore, because heterojunctions are generally grown epitaxially on a substrate, it is the substrate which determines the lattice parameter. For practical reasons, substrates are binary compounds, and the compound in most widespread use industrially is GaAs. This is followed by InP which is difficult to manufacture and therefore expensive and which is also fragile and brittle. In spite of this handicap, Inp is often used because it makes it possible to implement alloys such as GaInAs or GaInAsP with very high indium contents, up to as much as 60%. Such alloys present electronic properties of interest, such as a narrow forbidden band extending from 0.6 electron volts (eV) to 1 eV.
Thus, in the present state of the art, the HEMTs in most widespread use can be put into two categories. The first category uses a GaAs substrate and arsenides as the narrow forbidden band material: GaAs for non-pseudomorphic HEMTs and GaInAs for pseudomorphic HEMTs having an indium content up to about 25%. The second category uses an InP substrate and arsenides as the narrow forbidden band material: GaInAs with indium at 52% for non-pseudomorphic HEMTs and at up to about 65% for pseudomorphic HEMTs.
In the same manner and in the present state of the art, HBTs can be put into two categories: those made on a GaAs substrate with GaAs as the narrow forbidden band material, and those on an InP substrate with GaInAs (52% indium content) as the narrow forbidden band material. As recalled above, pseudomorphic HBTs have not been developed industrially.
Nevertheless, both in HEMTs and in HBTs, the use of arsenides as the narrow forbidden band material presents limitations.
In HEMTs, the limitations occur as follows: the main characteristic of HEMTs is the association of a material having a broad forbidden band, generally AlGaAs with a material having a narrow forbidden band, GaAs or GaInAs, thus making it possible to obtain a two-dimensional electron gas that accumulates in the narrow forbidden band material when the broad forbidden band material is doped. This transfer of electrons from the broad forbidden band material to the narrow forbidden band material is more effective with increasing conduction band discontinuity &Dgr;Ec. Having a two-dimensional electron gas of high density makes it possible for the drain current of the transistor to be greater, and thus for power amplification to be more efficient. Unfortunately, in the arsenide system with an AlGaAs/GaAs or an AlGaAs/GaInAs combination, the &Dgr;Ec discontinuity is limited. Firstly &Dgr;Ec, which increases with the aluminum content in AlGaAs, cannot exceed a certain value, since above 22% aluminum, troublesome defects known as “DX centers” appear in the AlGaAs, and above 40% the AlGaAs material presents an indirect forbidden band. Secondly, adding indium to GaAs also makes it possible to increase &Dgr;Ec, but as recalled above, in practice it is hardly possible to exceed 25%. As a result, in practice, the AlGaAs/GaInAs system has a &Dgr;Ec maximum of about 400 millielectron volts (meV). In the category of HEMTs on an InP substrate where the broad forbidden band material is AlInAs (53% indium) and the
Flynn Nathan J.
Mandala Jr. Victor A.
Nixon & Vanderhye P.C.
Picogiga International
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