RESISTANCE

TECHNICAL FIELD

[0001] This disclosure relates generally to gaiiiumide (GaN) semiconductor devices arid more particularly to gallium nitride (GaN) semiconductor devices having low ohrnic contact resistance.

BACKGROUND

[0002] As is known in the art, second generation GaN HEMT devices (30-300 GHz) must nave higher sheet charge in the channel, thinner and higher A! mole fraction AlGaR

InAlN, or InGaAlN Schottky contact layer thicknesses, and lower parasitic ohnrie contact resistance (<0.2 ohm mm) than present the present devices,

[0003] One method used to form ohmic contacts for the first generation devices includes forming a Ti/ ^' Al/Bamer/Au witli rapid thermal annealing at §50-900 C which generally result in devices having high ohmic contact resistance (>0.2 ohm aim), and lower yields for source/drai contact spacing of <-2 am.

[0004] One metliod suggested to produce devices having lower ohmic contact resistance is shown in FIGS. 1A-1C. Here, a substrate of, for example, silicon carbide (SiC) or silicon Si, has a gallium nitride (GaN) layer epitaxial formed on the substrate. A semiconductor layer (i.e., a Schottky contact layer) having a higher band gap that the GaN, (e.g., a layer of AlGaN, InAIN, or InGaAlN) is formed on the GaN layer resulting in two dimensional electron gas (2D EG) layer being produced at the interface between the Ga layer and the higher hand, gap Schottky contact layer. Next, a mask is formed on the Schottky contact layer and the exposed portions of the Schottky and GaN are etched in the source and drain contact regions as shown using any suitable dry etch. The resulting structure is a mesa shaped structure as shown in FIG. 1 B, An ohmic contact layer of n+ doped GaN is deposited over the etched structure as shown in FIG. 1C. it is noted that the ends (i.e., edges) of the two dimensional electron gas (2DEG) layer now are in direct contact with the ohrnic contact layer of n+ doped GaN, This method suffers from two issues: Firstly, etching and exposure of the two dimensional electron gas (2DEG) on the edges might i compromise the carrier concentration and mobility of the electrons in fee vicinity of fee exposed etched surface. Secondly, electro injection at the source and collection of electrons at the drain is only through a thin (-50 Angstrom) contact between the two dimensional electron gas and fee ohraic contact layer of n-¾- doped GaN.

SUMMARY

[0005] In accordance with the present disclosure, a semiconductor structure is provided having: a substrate and a mesa structure disposed on the substrate. The mesa structure includes: a lower semiconductor layer; an upper semiconductor layer having a different band gap from., and in direct contact with, the lower semiconductor layer to form a two- dimension electron gas region between the upper semi conductor layer, the two-dimension electron gas region having outer edges terminating at sidewalls of the mesa; an electron donor layer disposed on side wall portions of the mesa structure and on the region of the two-dimension electron gas region terminating at sidewalk of the mesa; and an ohmic contact material disposed on the electron donor layer.

[0006] In one embodiment, a semiconductor structure is provided having: a substrate; and a mesa structure disposed on the substrate, The mesa structure includes: a lower semiconductor layer; an upper semiconductor layer having a higher band gap than, and in direct contact wife, fee lower semiconductor layer to form a two-dimension electron gas region between the upper semiconductor layer, the two-dimension electron gas region having outer edges terminating at sidewalls of the mesa; aa additional semiconductor layer having a higher band gap than the lower semiconductor layer disposed on sidewall portions of the mes structure, on the region of the two-dimension electro gas region terminating at sidewalls of the mesa and on and m direct contact with the lower semiconductor layer to form a two-dimension electron gas region between the additional, layer and the lower semiconductor layer; and a ohmic contact material disposed on the electron donor layer,

[0007] In one embodiment, a semiconductor structure is provided having: a substrate; and a mesa structure disposed on fee substrate. The mesa structure includes: a lower semiconductor layer; an upper semiconductor layer having a different band gap from, and in direct contact wife, fee lower semiconductor layer; an electron donor layer disposed, on sidewall portions of the mesa structure and on the region of the two-dimension electron gas region terminating at sidewalls of the mesa; and an oliniic contact material disposed on the electron donor layer.

[0008] In one embodiment, a semiconductor structure is provided having: a substrate and a mesa structure disposed on the substrate. The mesa s ructure comprising.' a lower semiconductor layer; an upper semiconductor layer having a higher band gap from, and in direct contact with, the lower semiconductor layer to form a two-dimension electron gas region between the upper semiconductor layer and the lower semiconductor layer, the two- dimension electron gas region having outer edges tenninating at sidewalls of the mesa; and, an electron, donor layer di sposed on sidewall portions of the mesa structure and on the region of the two-dimension electron gas region tenninating at sidewalls of the mesa; and an ohmic contact materi l disposed on the electron donor layer.

[0009] Wit such an arrangement, the electron donor layer is disposed between and in direct contact with the outer edges of the two-dimension electron gas region and the ohmic contact material, forming, in effect, what might he considered as a high electron mobility transistor (HEMT). The width of the regions for electron injection from the source ohmic contact material and collection of these injected electrons into, and then through, the two- dimension electron gas region, now is significantly increased resulting in lower contact resistance between the electron donor layer and two dimensional electron gas layer, In effect, the formation of this sideways HEMT increases the concentration of electrons all along the contact between the lower semiconductor layer and the electron donor layer results in lower ohmic contact resistance.

[0010] In one embodiment, ohmic contact material terminates at a top portion of the mesa structure.

In one embodiment, the lower semiconductor layer is Ga .

I one embodiment, the upper semiconductor layer includes A .

In one embodiment, the electron donor layer is n-type doped AlGaN.

In one embodiment, the ohmic contact materia! is n~type doped GaN,

In one embodiment, a gate electrode in Sehottky contact with the upper

semiconductor layer.

[0011] In one embodiment, an ohmic contact is in contact with the ohmic contact material. The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. DESCRIPTION OF DRAWINGS

[0012] FIGS. 1 A-1C are diagrammatical sketches showing the fabrication of a semiconductor structure at various stages IK the fabrication thereof according to the PRIOR ART; and

[0013] FIGS. 2A-2C are diagrammatical sketches showing the fabrication of a semiconductor structure at various stages in the fabrication thereof according to the disclosure.

[0014] Like reference symbols In the various drawings indicate like elements.

DETAILED DESCRIPTION

[0015] Referring now to FIG. 2A, a semiconductor structure 10 is shows having: a substrate 12 _} here a semiconductor substrate of silicon carbide (SIC) or silicon (Si), for example. A lo wer layer 14 of gallium arsenide (GaN) is formed, by any conventional manner on the substrate 12. An upper layer 16 of aluminum gallium nitride (AlGaN), or indium aluminum nitride (MAIN) or indium gallium aluminum arsenide (InGaAlN) ½ formed on the lower layer of gallium arsenide (GaN), as shown. Because the upper layer has a higher band gap than the lower layer, a two dimensional electron gas (2DEG) region 18 is formed between the upper and lower layers 14, 16.

[0016] Next the structure 10 shown in FIG. 2B is masked and dr etched to form a mesa structure 20 shown in FIG. 2B, Here, the structure 10 shown in FIG, 2A is etched to a depth from 300 Angstrom to 1000 Angstrom (preferably 600 Angstrom).

[0017] Next, as shown in FIG, 2C, an additional electron donor N+ layer 22 of AlGaN ^' with Ai concentration in the 5-30% (preferably 15-20%) is grown using, for example, molecular beam epitaxy (MBE) with a thickness of 30-200 Angstrom (preferably 50-100 Angstrom). Here, additional electron donor N+ layer 22 of AlGa has a doping concentration of 10 ^i¾ -5x 10 ¹⁹ electrons per cm ³ . TMs additional electron donor N+ layer 22 of AIGaN having a higher band gap the band gap of the lower GaN layer 14 creates a two dimensional electron gas (2DEG) region 24 GaN on the sidewal ls of the mesa structure 20. It is first noted that the additional electron donor N+ layer 22 of AIGaN forms a two- dimension electron gas region 24 between the lower semiconductor layer 14, the two- dimension electron gas region 16 having outer edges terminating at sidewalls of the mesa 20 and that the additional electron donor layer 22 Is disposed on sldewall portions of the mesa stractore 20 and on the region of the two-dimension electron gas region 22 terminating at sidewalls of the mesa 20. In effect a HEMT structure may be considered as being formed from a combination of doping (similar to GaAs pHEMTs) and spontaneous polarization all along the sidewa!l where it contacts the lower GaN layer 14. This is followed by growth of the additional GaN N+ ohmic contact layer 30 to a thickness of 200- 1000 Angstrom (preferably 600 Angstrom).

[0018] It is noted that:

1 , The additional electron donor N+ layer 22 of AIGaN has larger band gap than GaN layer 14;

2, In effect a HEMT is formed at the interface between the higher band gap material of the additional electron donor N+ layer 22 of AIGa and the GaN layer 14 and therefore electrons reside inside the GaN layer 14 at the Interface; and

3, The additional electron donor N+ layer 22 of AIGaN has both a higher band gap than the GaN layer 22 and also has N+ doping.

[001 ] The, source and drain contact regions 30, 31 here N-f GaN, are formed in the etched regions, as shown in FIG. 2C and terminate at the upper surface of the upper layer 16. Next, ohmic source and drain contacts 32, 34 are formed with the source and drain contact regions 30 and a gate contact 36 is formed within Schotiky contact with, the surface of the upper semiconductor layer 6 using any conventional processing,

[0020] The width of the regions for electron injection from the source contact regions 30 and collection of them at the drai contact region 31 is now significantly increased resulting in lower contact resistance between the re-grown additional layer of N layer 22 of AIGaN and two dimensional electron gas region 24 formed thereby. Thus, in the structure shown in FIG, 2C, there is direct contact between the source and drain contact regions 30, 31 through the two dimensional electron gas region 24 on the sideways of the mesa; i.e., as a result of the through etch and re-grown additional N+ layer 22 of AlGaN. Thus, a GaN HEMT is in effect formed on the sidewalk of the etched source drain GaN regions 30, 31 through growth of the N÷ AlGaN layer first before growing GaN N+ regions 30, 31, Formation of this sideway GaN HEMT increases the concentration of electrons all along the contact between the GaN layer 16 and AlGaN layer 18 and results in lower contact resistance,

[0021] It should now be appreciated a semiconductor structure according to the present disclosure includes a substrate; a mesa structure disposed on the substrate, the mesa structure comprising: a lower semiconductor layer; an upper semiconductor layer having a different band gap from, and in direct contact with, the lower semiconductor layer to form a two-dimension electron gas region between the ^" up er semiconductor layer, the two- dimension electron gas regio having outer edges terminating at sidewall s of the mesa; an electron donor layer disposed on sidewall portions of the mesa structure and on. the region of the two-dim en sion electron gas region terminating at sidewa!ls of the mesa; an ohmic contact material disposed, on the electron donor layer,

[0022] The semiconductor structure also includes one or more of the following features; wherein fee ohmic contact material terminates at a top portion of the mesa structure;

wherein the lower semiconductor layer is GaN; wherein the upper semiconductor layer includes AIM; wherein the electron donor layer is n-type doped AlGaN; wherein the ohmic contact, material is n-type doped GaN; a gate electrode in Sehottky contact with the upper semiconductor layer, and an ohmic contact in contact with the ohmic contact material. [0023] A semiconductor structure according to the present disclosure may include a substrate; a mesa structure disposed on the substrate, the mesa structure comprising: a lower semiconductor layer; an upper semiconductor layer having a different hand gap from, and. in direct contact with, the lower semiconductor layer' to form a two-dimension electron gas region between the upper semiconductor layer, the two-dimension electron gas region, having enter edges terminating at sidewalk of the mesa; an electron donor layer disposed on sidewall portions of the mesa, structure and on the region of the two- dimension electron gas region terminating at sidewalls of the mesa; an ohmic contact material disposed, on the electron donor layer. [0024] A semiconductor structure according to the present ^' disclosure may include a substrate; a. mesa structure disposed on the substrate, the mesa structure comprising: a lower semiconductor layer; an upper semiconductor layer having a higher band gap than, and in direct contact with, the lower semiconductor layer to form a two-dimension electron gas region between the upper semiconductor layer, the two-dimension electron gas region having outer edges terminating at sidewalls of the mesa; an additional semiconductor layer having a higher band gap than the lower semiconductor layer disposed on sidewall portions of the mesa structure, on the region of the two-dimension electron gas region terminating at sidewalls of the mesa and on and in direct contact with the lower semiconductor layer to form a two-dimension electron gas region between the additional layer and the lower semiconductor layer; and an ohmic contact material disposed, on the electron donor layer,

[0025] A semiconductor structure according to the present disclosure may include a substrate; a mesa structure disposed on the substrate, the mesa structure comprising: a lower semiconductor layer; an upper semiconductor layer having a different band gap from, and in direct contact with, the lower semiconductor layer; an electron donor layer disposed on sidewall portions of the mesa structure and on the region of the two- dimension electron gas region terminating at sidewalls of the mesa; an ohmic contact material disposed on the electron donor layer,

[0026] A semiconductor structure according to the present disclosure may include a substrate; a mesa structure disposed on. the substrate, me mesa structure comprising: a lower semiconductor layer; an upper semiconductor layer having a higher band gap than, and in direct contact with, the lower semiconductor; an additional semiconductor layer having a higher band gap than the lower semiconductor layer disposed on sidewall portions of the mesa structure on, and in direct contact with, the lower semiconductor layer; and an ohmic contact material disposed on the electron donor layer. Furthermore, the additional layer may be an electron donor layer.

[0027] 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. Accordingly, oilier embodiments are within the scope the following claims.