Abstract
A High Electron Mobility Transistor structure having: a GaN buffer layer disposed on the substrate; a doped GaN layer disposed on, and in direct contact with, the buffer layer, such doped GaN layer being doped with more than one different dopants; an unintentionally doped (UID) GaN channel layer on, and in direct contact with, the doped GaN layer, such UID GaN channel layer having a 2DEG channel therein; a barrier layer on, and in direct contact with, the UID GaN channel layer. One of the dopants is beryllium and another one of the dopants is carbon.
Claims
1. A High Electron Mobility Transistor structure, comprising: a substrate; a high resistivity GaN buffer layer disposed on the substrate, the high resistivity GaN buffer layer doped with beryllium and having a resistivity greater than 2.2×10.sup.3 ohm*cm; a doped GaN thin film layer disposed on, and in direct contact with, the high resistivity GaN buffer layer, the doped GaN thin film layer having a thickness between 10 and 300 nm and having multiple different dopants where one of the dopants is beryllium and wherein the beryllium doping is 5×10.sup.16 to 3×10.sup.19 atoms/cm.sup.3 and one of the dopants is carbon and wherein the carbon doping is less than the beryllium doping but higher than 1×10.sup.16 atoms/cm.sup.3; and an unintentionally doped (UID) GaN channel layer on, and in direct contact with, the doped GaN thin film layer, the UID GaN channel layer having a 2DEG channel therein and wherein the UID GaN channel layer has a thickness between 50 and 200 nm.
2. The structure recited in claim 1 including a barrier layer on, and in direct contact with, the UID GaN channel layer.
3. The structure recited in claim 1 including a nucleation layer disposed on the substrate.
4. A High Electron Mobility Transistor structure, comprising: a substrate; a high resistivity GaN buffer layer disposed on the substrate, the high resistivity GaN buffer layer having a resistivity greater than 2.2×10.sup.3 ohm*cm; a doped GaN thin film layer disposed on, and in direct contact with, the high resistivity GaN buffer layer, the doped GaN thin film layer being doped with beryllium and carbon and wherein the beryllium doping is 5×10.sup.16 to 3×10.sup.19 atoms/cm.sup.3 and wherein the carbon doping is less than the beryllium doping but higher than 1×10.sup.16 atoms/cm.sup.3; and an unintentionally doped (UID) GaN channel layer on, and in direct contact with, the doped GaN thin film layer, the UID GaN channel layer having a 2DEG channel therein and wherein the UID GaN channel layer has a thickness less than 200 nm.
5. The structure recited in claim 4 wherein the UID GaN channel layer has a thickness between 50 and 200 nm.
6. The structure recited in claim 4 wherein the doped GaN thin film layer has a thickness of 10 to 300 nm.
7. The structure recited in claim 4 including a barrier layer on, and in direct contact with, the UID GaN channel layer.
8. A High Electron Mobility Transistor structure, comprising: a substrate; a high resistivity GaN buffer layer disposed on the substrate, the high resistivity GaN buffer layer doped with beryllium, iron and/or carbon; a doped GaN thin film layer disposed on, and in direct contact with, the high resistivity GaN buffer layer, the doped GaN thin film layer being doped with beryllium and carbon and wherein the beryllium doping is 5×10.sup.16 to 3×10.sup.19 atoms/cm.sup.3 and wherein the carbon doping is less than the beryllium doping but higher than 1×10.sup.16 atoms/cm.sup.3; and an unintentionally doped (UID) GaN channel layer on, and in direct contact with, the doped GaN thin film layer, the UID GaN channel layer having a 2DEG channel therein and wherein the UID GaN channel layer has a thickness less than 200 nm.
9. The structure recited in claim 8 wherein the UID GaN channel layer has a thickness between 50 and 200 nm.
10. The structure recited in claim 8 wherein the doped GaN thin film layer has a thickness of 10 to 300 nm.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a diagrammatical, cross-sectional sketch of a Group III-Nitride HEMT structure according to the PRIOR ART;
(2) FIG. 2 is a diagrammatical, cross-sectional sketch of a Group III-Nitride HEMT structure according to the disclosure;
(3) FIG. 3A is a diagrammatical, cross-sectional sketch of a Group III-Nitride HEMT structure without an additional doped layer between the UID GaN channel and the high resistivity GaN buffer, useful as comparison to the HEMT structure of FIG. 2;
(4) FIG. 3B is a diagrammatical, cross-sectional sketch of a Group III-Nitride HEMT structure utilizing only carbon as a dopant in the doped layer between the UID GaN channel and high resistivity GaN buffer, useful as comparison to the HEMT structure of FIG. 2;
(5) FIG. 3C is a diagrammatical, cross-sectional sketch of a Group III-Nitride HEMT structure utilizing both carbon and beryllium as dopants in the doped layer between the UID GaN channel and high resistivity GaN buffer, one embodiment of the HEMT structure of FIG. 2;
(6) FIG. 4A is a plot of normalized drain leakage and gate leakage current versus drain-to-source voltage for the structure of FIG. 3A, without an additional doped layer, useful for understanding the benefits of the structure of FIG. 2;
(7) FIG. 4B is a plot of normalized drain leakage and gate leakage current versus drain-to-source voltage for the structure of FIG. 3B, utilizing only carbon doping in the doped layer, useful for understanding the benefits of the structure of FIG. 2;
(8) FIG. 4C is a plot of normalized drain leakage and gate leakage current versus drain-to-source voltage for the structure of FIG. 3C, utilizing both carbon and beryllium as dopants in the doped layer in one embodiment of the HEMT structure of FIG. 2;
(9) FIG. 5A is a plot of normalized drain current versus drain-to-source voltage for a quasi-static configuration and a 28V pulsed configuration highlighting the impact of current collapse for the structure of FIG. 3A, without an additional doped layer, useful for understanding the benefits of the structure of FIG. 2;
(10) FIG. 5B is a plot of normalized drain current versus drain-to-source voltage for a quasi-static configuration and a 28V pulsed configuration highlighting the impact of current collapse for the structure of FIG. 3B, utilizing only carbon in the doped layer, useful for understanding the benefits of the structure of FIG. 2;
(11) FIG. 5C is a plot of normalized drain current versus drain-to-source voltage for a quasi-static configuration and a 28V pulsed configuration highlighting the impact of current collapse for the structure of FIG. 3C, utilizing both carbon and beryllium as dopants in the doped layer in one embodiment of the HEMT structure of FIG. 2.
(12) Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
(13) Referring now to FIG. 2, a HEMT structure is shown having: a single crystal substrate, here, for example, Silicon, Sapphire, or Silicon Carbide; a nucleation layer, here, for example, MN or Graphene formed epitaxially on the substrate; a doped high resistivity GaN buffer layer formed epitaxially on the nucleation layer, here such buffer layer being doped with, for example, beryllium doped GaN to have a resistivity of 2.2×10.sup.3 ohm*cm (for 5×10.sup.18 atoms/cm.sup.3 doping), iron doped GaN to have a resistivity of 3×10.sup.5 ohm*cm (see R. P. Vaudo et al., Characteristics of semi-insulating, Fe-doped GaN substrates, Physical Status Solidi 200, 18 (2003)), carbon doped GaN to have a resistivity of 1×10.sup.8 ohm*cm, or combination of dopants that result in high resistivity GaN; a doped GaN layer, having a thickness of 10-300 nm and having more than one dopant, here having: a beryllium dopant having a doping concentration in a range of 5×10.sup.16 to 3×10.sup.19 atoms/cm.sup.3; and, a carbon dopant having a doping concentration less than the doping concentration of the beryllium, but higher than 1×10.sup.16 atoms/cm.sup.3. The doping levels result from the co-deposition of beryllium and carbon impurities during the epitaxial growth of the doped GaN layer. Formed epitaxially on the doped GaN layer is an unintentionally doped (UID) GaN channel layer having a thickness of 50-200 nm and having a two-dimensional electron gas (2DEG) within. Formed epitaxially on the UID GaN layer is a barrier layer, such as AlGaN, InAlGaN, or ScAlN. The optimal doping density, within the ranges provided, is dependent on the thickness of the UID GaN channel layer and the charge density in the channel as controlled by the barrier material composition and thickness.
(14) Referring now to FIGS. 3A, 3B and 3C, three HEMT structures are shown for purposes of comparison. All three structures utilize identical substrate, nucleation layer, and high resistivity GaN buffer layer structures, here SiC, AlN, and carbon and beryllium doped GaN, respectively. The structure shown in FIG. 3A has no doped layer between the UID GaN channel and high resistivity GaN buffer, achieved via a 300 nm UID channel thickness on, and in direct contact with, the high resistivity buffer layer. The structure shown in FIG. 3B has only carbon doping in the doped layer, here with a thickness of 165 nm and doped with a carbon density of 1.2×10.sup.17 atoms/cm.sup.3, between the UID GaN channel, here with a thickness of 110 nm, and high resistivity GaN buffer. The structure shown in FIG. 3C has both carbon and beryllium doping in the doped layer, here with a thickness of 165 nm and carbon doping density of 1.4×10.sup.17 atoms/cm.sup.3 and beryllium doping density of 1.2×10.sup.18 atoms/cm.sup.3, between the UID GaN channel, here with a thickness of 110 nm, and high resistivity GaN buffer. Here, the structures in FIGS. 3A, 3B, and 3C are grown by molecular beam epitaxy (MBE). The doped GaN layers in this particular example are formed under nitrogen-rich conditions with predetermined flux ratios of gallium, nitrogen, and dopants and predetermined growth temperatures, here 660-780° C. as measured by an optical pyrometer, that result in the desired dopant concentrations. While nitrogen-rich conditions are used here, gallium-rich conditions may also be used, although with different predetermined flux ratios and/or growth temperature. Here, beryllium is doped via thermal evaporation from a solid elemental beryllium source and carbon is doped via a CBr.sub.4 gas source.
(15) Referring now to FIGS. 4A, 4B and 4C, normalized drain and gate leakages are shown for three terminal source-gate-drain lateral transistors, with ohmic source and drain contacts and a Schottky gate contact, fabricated from the epitaxial structures shown in FIGS. 3A, 3B and 3C, respectively. More particularly, DC measurements of three terminal source-gate-drain lateral transistors of the drain current and gate current with the gate pinched off at −6V on the gate for varying drain-to-source voltages are shown. The “ideal” is for both the drain and gate leakage current to be as low as possible. Data is normalized to the drain leakage current at 100V for the structure shown in FIG. 3C. The structure shown in FIG. 3A, without a doped layer between the UID GaN and high resistivity buffer, shows high leakage current (˜3-10× the leakage current of the structure shown in FIG. 3C). The structure shown in FIG. 3B with only carbon doping in the doped layer, shows a leakage current that is low and similar to the leakage current of the structure shown in FIG. 3C; the structure in FIG. 3C having both carbon and beryllium dopants in the doped layer.
(16) Referring to FIGS. 5A, 5B and 5C, pulsed IV measurements are shown for the transistors fabricated from the epitaxial structures shown in FIGS. 3A, 3B and 3C, respectively. For the quasi-static curve, the quiescent bias point is V.sub.DQ=V.sub.GQ=0 V. For the 28 V.sub.DQ curve, the quiescent bias point is V.sub.DQ=28 V, I.sub.DQ=110 mA. “Ideal” is to minimize the shaded area, a measure of current collapse (or dispersion), to zero. The data is normalized to the quasi-static drain current at 15 V for the structure shown in FIG. 3C. The structure shown in FIG. 3A, without a doped layer between the UID GaN and high resistivity buffer, shows the current collapse is low (but this is the result of high leakage as shown in FIG. 4A). The structure shown in FIG. 3B, which has only carbon doping in the doped layer, shows relatively high current collapse as compared with the current collapse of the structure shown in FIG. 3C, which has both carbon and beryllium doping in the doped layer, as shown in FIG. 5C. Thus, the structure shown in FIG. 3C, with both carbon and beryllium doping in the doped layer, has both a very low current collapse and low leakage.
(17) A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, one may utilize a stepped or graded doping profile within the doped GaN layer. Also, for example, while doped GaN has been described, it should be understood that other group III-N doped materials may be used such as doped AlGaN. Similarly, the use of different group III-N barrier, channel, or buffer layer materials than GaN and AlGaN may be used, including, for example, composite barriers with more than one material or composition of barrier material (such as AlGaN/InAlN or InAlGaN. Additionally, alternate doping sources such as solid source carbon may be used. Further, the disclosure does not depend on the use of any specific substrate, nucleation layer, or high-resistivity buffer dopant. Accordingly, other embodiments are within the scope of the following claims.
