1. Field of the Invention
The invention relates to heterojunction field-effect transistors and to methods for making and operating heterojunction field-effect transistors.
2. Discussion of the Related Art
The thin portions 14′ of the AlxGa(1-x)As layer 14 are infiltrated by metal that, was diffused out of the metallic source and drain electrodes 16, 18. The diffused metal, which is schematically indicated by large dots D, makes the lower boundaries of the source and drain electrodes 16, 18 less well-defined. In particular, the diffused metal, D, extends, at least, to the interface 22 between the GaAs and AlxGa(1-x)As layers 12, 14. For that reason, the diffused metal, D, improves the electrical contact between the source and drain electrodes 16, 18 and the AlxGa(1-x)As/GaAs heterojunction interface 22.
Between the source and drain electrodes 16, 18, the AlxGa(1-x)As layer 14 forms a mesa-like structure, M, that rises vertically above the source and drain electrodes 16, 18. The mesa-like structure, M, supports the metallic gate electrode 20 above the source and drain electrodes 16, 18. That is, the mesa-like structure, M, electrically isolates the metallic gate electrode 20 for the source and drain electrodes 16, 18. The mesa-like structure, M, also electrically insulates the metallic gate electrode 20 from the AlxGa(1-x)As/GaAs heterostructure interface 22.
In the FET 10, a two-dimensional electron gas (2DEG) may be trapped at the AlxGa(1-x)As/GaAs heterojunction interface 22. The 2DEG can carry a substantial portion or all of the source-drain current in the FET 10. The 2DEG can have a high mobility in the FET 10. The voltage on the metallic, gate electrode 20 controls the density of electrons in the 2DEG.
In the field-effect transistors (FETs) as shown, e.g., in
One embodiment features an apparatus that includes an FET. The FET includes a region of first semiconductor and a layer of second semiconductor that is located on the region of the first semiconductor. The layer and region form a semiconductor heterostructure. The FET also includes source and drain electrodes that are located on one of the region and the layer and a gate electrode located to control a conductivity of a channel, portion of the semiconductor heterostructure. The channel portion is located between the source and drain electrodes. The gate electrode is located vertically over the channel portion and portions of the source and drain electrodes.
In some embodiments, the heterojunction is in a GaAs/AlxGa(1-x)As heterostructure, wherein 0<x<1.
In some embodiments, the FET further includes a dielectric layer located between the channel portion and the gate electrode and between the gate electrode and the portions of the source and drain electrodes.
Another embodiment features a method. The method includes providing a semiconductor heterostructure and forming source and drain electrodes on the semiconductor heterostructure. The method includes depositing a dielectric, layer over parts of the source and drain electrodes and the part of semiconductor heterostructure located between the source and drain electrodes. The method includes forming a gate electrode vertically over the part of the semiconductor heterostructure between the source and drain electrodes and vertically over parts of the source and drain electrodes.
In some embodiments, the heterostructure is a GaAs/AlxGa(1-x)As heterostructure, wherein 0<x<1.
In the Figures and text like reference numerals indicate element functions.
In the Figures, the relative dimensions of some features may be exaggerated to more clearly illustrate one or more of the structures therein.
Herein, various embodiments are described more fully by the Figures and the Detailed Description of Illustrative Embodiments. Nevertheless, the inventions may be embodied in various forms and are not limited to the embodiments described in the Figures and Detailed Description of Illustrative Embodiments.
Herein, a semiconductor heterostructure refers to a structure in which a layer of a first semiconductor is located on a region of a second semiconductor, wherein the first and second semiconductors are crystalline and are formed of different alloys. The portion of the semiconductor heterojunction that is near the interface between the layer and region is referred to as a semiconductor heterojunction. The layer of first semiconductor may be epitaxially grown, e.g., on the region of second semiconductor.
In various embodiments, the source and drain electrodes 36, 38 have a conducting contact with the semiconductor heterojunction. For example, the conducting material of the source and drain electrodes 36, 38 may be vertically diffused into the underlying layer 34 of second semiconductor to form a highly conducting connection with the semiconductor heterojunction. For that reason, the bottom boundary of the source and drain electrodes 36, 38 may be rough.
The FET 30 is configured such that a 2DGCC may be trapped at the interface 44 between the layer 34 of second semiconductor and the region 32 of first semiconductor. In particular, the alloy compositions of the first and second semiconductors are selected to enable application of a voltage to the gate electrode 42 to cause such trapping. In the presence of the trapped 2DGCC, the semiconductor heterojunction around the interface 44 functions as an active channel portion of the semiconductor heterostructure. The active channel portion can carry a current between the source and gate electrodes 36, 38.
The GaAs layer 32 has an atomically flat top surface 44 and is typically undoped. The GaAs layer 32 may function as a mechanical support substrate or may rest on a surface of a separate support substrate 46, e.g., a [100] of a GaAs substrate.
The crystalline AlxGa(1-x)As layer 34 is located on the crystalline GaAs layer 32 and forms a smooth AlxGa(1-x)As/GaAs interface 44 with the crystalline GaAs layer 32. The crystalline AlxGa(1-x)As layer 34 may have a variety of semiconductor compositions and is also typically undoped. The parameter “x” defining the compound semiconductor alloy may be in the range [0.05, 0.5] and may be in the range [0.1, 0.25]. An exemplary crystalline AlxGa(1-x)As layers 34 has a thicknesses of about 100 nanometers (nm) and may or may not be capped with a thin GaAs layer (not shown), e.g., about 5 nm of GaAs.
The source and drain electrode 36, 38 are located on the surface of the crystalline AlxGa(1-x)As layer 34. The source and drain electrodes 36, 38 may be formed, e.g., of metal layers or multi-metal layers. One exemplary conducting multi-metal layer has a bottom-to-top layer structure of about 4 nm of nickel (Ni), about 100 nm of germanium (Ge), about 200 nm of gold (Au), and about 80 nm of Ni. The lower boundaries of the source and drain electrodes 36, 38 may smooth or unsmooth. The metal/material of the source and drain electrodes 36, 38 is typically vertically diffused into the AlxGa(1-x)As layer 34, e.g., as illustrated schematically by black dots, D, in
The dielectric layer 40 vertically overlies the part of the AlxGa(1-x)As layer 36, e.g., the part located between the source and drain electrodes 36, 38, and vertically overlays portions of the source and drain electrodes 36, 38. The dielectric layer 40 may be formed of an inorganic dielectric or an organic dielectric, wherein the selected dielectric is conventionally used to in the fabrication of FETs in the microelectronics industry. An exemplary dielectric layer 40 is an amorphous Si3Ni4 or SiO2 layer, e.g., with a thickness of about 120 nm. Another exemplary dielectric layer 40 is a layer of an organic dielectric such as polyimide. The dielectric layer 40 may also include a sequence of dielectric layers.
The gate electrode 42 is located on the dielectric layer 40 and vertically overlays both portions of the source and drain electrodes 36, 38 and the pan of the AlxGa(1-x)As layer 34 located between the source and drain electrodes 36, 38. Thus, edges of the gate electrode 42 are not aligned with the edges of the source and drain electrodes 36, 38. The underlying dielectric layer 40 electrically insulates the gate electrode 42 from the AlxGa(1-x)As layer 34 and the source and drain electrodes 36, 38. Exemplary gate electrodes 42 may be formed of a conductor conventionally used in microelectronics fabrication. For example, the gate electrode may be ah aluminum layer with a thickness of about 30 nm or more.
The FET 30A is configured enable trapping of a 2DEG at or around the interface 44 between the GaAs and AlxGa(1-x)As layers 32, 34. Indeed, the semiconductor heterojunction located between the source and drain electrodes 36, 38 can function as an active channel for the FET 30A. Around the interface 44, the density of electrons in the trapped 2DEG is determined by the voltage applied to the gate electrode 42. The 2DEG can have a very high mobility in the FET 30A.
In the FET 30A the presence of a trapped 2DEG may be observed, e.g. by the observation of a quantum Hall effect. Indeed, conventional measurements may show the presence of several states of the fractional quantum Hall effect.
In some embodiments of the FET 30A, electron densities may be in the range of 0.5×1011/cm3 to 2.2×1011/cm2 in the trapped 2DEG, and mobilities of the trapped electrons may be larger than 10×106 cm2/V-sec.
In the FET 30B, the crystalline substrate 46 may be, e.g., a crystalline GaAs substrate with a smooth [100] top surface.
In the FET 30B, the crystalline GaAs layer 32 may be about 200 nm of GaAs that is epitaxially grown on the top surface of the crystalline substrate 46.
In the FET 30B, the crystalline AlxGa(1-x)As layer 34 may be an epitaxially grown AlxGa(1-x)As layer, and the alloy parameter may, e.g., satisfy x≅0.24. The AlxGa(1-x)As alloy layer may have a thickness of about 200 nm. The AlxGa(1-x)As layer 34 may or may not be covered by a thin epitaxially grown capping layer, e.g., about 5 nm of GaAs (not shown).
In the FET 30B, metal of the source and drain electrodes 36, 38 may be diffused through the AlxGa(1-x)As layer 34 to the AlxGa(1-x)As/GaAs interface 44 as illustrated schematically by black dots, D. Such diffused metal can function as a highly conductive electrical connection between the source and drain electrodes 36, 38 and the semiconductor heterojunction and can also function as a p-type dopant for the AlxGa(1-x)As/GaAs heterostructure. Exemplary constructions for the source and drain electrodes 36, 38 include Au and beryllium (Be). One exemplary construction provides a bottom AuBe layer with a thickness of about 250 nm and a top Au layer with a thickness of about 60 nm. Another exemplary construction provides a bottom AuBe layer with a thickness of about 80 nm, a middle titanium (Ti) layer with a thickness of about 50 nm, and a top Au layer with a thickness of about 200 nm. In both constructions, the bottom AuBe layer may have, e.g., an alloy composition in which about 98-99 weight percent (wt %) of the alloy is Au and about 1-2 wt % of the alloy is Be.
In the FET 30B, the dielectric layer 40 may be; e.g., silicon nitride or silicon dioxide. For example, the dielectric layer 40 may be formed of about 120 nm of amorphous silicon nitride.
In the FET SOB, the edges of the gate electrode 42 are again not aligned with the edges of the source and drain electrodes 36, 38. Instead, the gate electrode 42 vertically overlays portions of the source and drain electrodes 36, 38 and vertically overlays the portion of the AlxGa(1-x)As/GaAs heterostructure that is located between the source and drain electrodes 36, 38. An exemplary gate electrode 42 may be about 30 nm or more of vapor-deposited Al.
In some embodiments, the FET 30 may include a thin dielectric layer (not shown) interposed between the auxiliary electrodes 49 and the second crystalline semiconductor layer. For example, the third dielectric layer may be about 50 nm of silicon nitride. Such a thin dielectric layer would electrically insulate the auxiliary gates 49 from the underlying second crystalline semiconductor layer 34.
During operation, the auxiliary gates 49 can be biased to either laterally limit or more generally deplete the active semiconductor channel of charge carriers in the relevant 2DGCC. For example, the auxiliary gates 49 can be unbiased or biased oppositely than the overlying gate electrode 42. In the later case, the auxiliary gates 49 would typically deplete underlying portions of the semiconductor heterojunction of charge carriers of the 2DGCC, e.g., thereby limiting the lateral extent or conductivity of the active channel that connects the source and drain electrodes 36, 38.
In some embodiments, the FETs 30, 30A, 30B, and 30G of FIGS. 2 and 2A-2D, the semiconductor heterostructure may have mesa structure. Then, portions of the source and drain electrodes 36, 38, the dielectric layer 40, and the gate electrode 42 can, overlap the edges of the mesa structure. Thus, portions of these structures will be located on the semiconductor heterostructure and portions of these structures will be located off the semiconductor heterostructure.
Referring again to
The method 50 includes providing a region of a first crystalline semiconductor with an atomically smooth top surface (step 52). The region of first material may be a GaAs layer, e.g., GaAs layer 32 of
The method 50 includes forming on the top surface of the region of first semiconductor a layer of a second crystalline semiconductor thereby producing a semiconductor heterojunction (step 54). The layer of second crystalline semiconductor may be an AlxGa(1-x)As layer, e.g., the AlxGa(1-x)As layer 34 of
In some embodiments, the method 50 may include doing a mask-controlled etch to produce a mesa structure from the semiconductor heterostructure produced at the forming step 54. For the AlxGa(1-x)As/GaAs, one suitable wet etchant is a solution of 100 volume parts H2O, 10 volume parts phosphoric acid, and 2 volume parts of 30% H2O2. For a starting the AlxGa(1-x)As/GaAs heterostructure structure that includes about 100 nm of the AlxGa(1-x)As on about 200 nm GaAs, the wet etch may produce a mesa with a height of about 200 nm.
Next, the method 50 includes forming source and drain electrodes, e.g., the source and drain electrodes 36, 38, on the semiconductor heterostructure that was produced at the step 54 (step 56). For the above described AlxGa(1-x)As/GaAs heterostructure, the source and drain electrodes are formed on the AlxGa(1-x)As layer or GaAs cap layer as appropriate. The forming step 56 may include performing one or more conventional metal vapor-depositions under the control of a mask, e.g., a mask produced by a conventional lithographic process. The source and drain electrodes may be formed of a metal layer or a metal multi-layer and may have various thicknesses and lateral dimensions.
For an AlxGa(1-x)As/GaAs heterostructure that provides electron-mediated conduction, the forming step 56 may involve performing a series of depositions that forms a metal multilayer for the source and drain electrodes. From bottom-to-top, the metal multilayer may include, e.g., 4 nm of Ni, 200 nm of Au, 100 nm of Ge, and 80 nm of Ni.
For an AlxGa(1-x)As/GaAs heterostructure that provides hole-mediated conduction, the forming step 56 may also involve performing a series of depositions that forms a metal multilayer. From bottom-to-top, one metal multilayer may include, e.g., an AuBe layer with a thickness of about 250 nm and an Au layer with a thickness of about 60 nm. The bottom AuBe layer may have an exemplary alloy composition in which about 98 wt % to about 99 wt % is Au and about 1 wt % to about 2 wt % is Re.
In embodiments that include forming an AlxGa(1-x)As/GaAs mesa structure, the method 50 may position the source and drain electrodes to overlap edges of the mesa.
The method 50 may include then, performing an anneal to cause metal and/or conductive material to diffuse vertically from the deposited source and drain electrodes to into the underlying semiconductor heterostructure (step 58). Such an anneal causes a substantial amount of the metal or conductive material of the source and drain electrodes to diffuse to the heterojunction, e.g., to the AlxGa(1-x)As/GaAs interface for embodiments based on above AlxGa(1-x)As/GaAs heterostructure. For the exemplary AlxGa(1-x)As/GaAs heterostructure of
In some embodiments, the method 50 may also include performing a mask-controlled metal deposition to form the auxiliary gate electrodes on or over the semiconductor heterostructure, e.g., to form the electrodes 49 of
The method 50 includes depositing a dielectric layer over that portion of the semiconductor heterostructure located between the source and drain electrodes and depositing the dielectric layer over adjacent parts of the source and drain electrodes themselves (step 60). The dielectric layer may be, e.g., Si3N4 or SiO2 and may be deposited by any conventional process. For the above-described AlxGa(1-x)As/GaAs heterostructure, a conventional plasma enhanced chemical vapor deposition (PECVD) of amorphous Si3N4 can, e.g., produce the dielectric layer, the dielectric layer 40 of
The method 50 also includes forming a gate electrode on the dielectric layer (step 62). The gate electrode is formed over the region between the source and drain electrodes and over adjacent portions of the source and drain electrodes themselves. That is, the forming step 62 produces a gate electrode whose edges are not aligned vertically over the edges of the source and drain electrodes. One exemplary process for forming the gate electrode includes forming a patterned mask on the dielectric layer, doing ah evaporation-deposition of metal to form the gate electrode, and then removing the mask by a conventional process. For example, the deposition may involve thermally evaporating Al to deposit Al at a rate of about 0.5 nm/sec for 10 seconds and continuing the deposition at a rate of about 1.5 nm/sec until a layer of about 30 nm or more of Al has been deposited.
In some embodiments, the method 50 also includes dispersing a covering of large non-sticking grains, e.g., indium lumps, on the source and drain electrodes prior to performing the dielectric, deposition of step 60. The grains are selected such that a mild mechanical processes can remove the grains thereby exposing metallic contact areas on the underlying source and drain electrodes. The exposed metallic contact areas can then, be used to make electrical connections to the source and drain electrodes of the final FET.
From the above disclosure, the figures, and the claims, other embodiments will be apparent to those of skill in the art.
Number | Date | Country | |
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Parent | 11526968 | Sep 2006 | US |
Child | 12813991 | US |