SELF-ALIGNED FIELD-EFFECT TRANSISTOR FOR HIGH FREQUENCY APPLICATIONS

Abstract: A metal semiconductor field-effect transistor (MESFET) is disclosed that exhibits reduced source resistance and higher operating frequencies. The MESFET comprises an epitaxial layer of silicon carbide and a gate trench in the epitaxial layer that exposes a silicon carbide gate surface between two respective trench edges. A gate contact is made to the gate surface and with the trench further defines the source and drain regions of the transistor. Respective ohmic metal layers form ohmic contacts on the source and drain regions of the epitaxial layer, and the edges of the metal layers at the trench are specifically aligned with the edges of the epitaxial layer at the trench.

Description: SELF-ALIGNED FIELD-EFFECT TRANSISTOR FOR HIGH FREQUENCY APPLICATIONS Field of the Invention The present invention relates to field-effect transistors ("FETs") in wide bandgap materials, and particularly relates to a self-aligned metal- semiconductor field-effect transistor (MESFET) structure formed in silicon carbide that provides advantages in high frequency operation. Background of the Invention The metal-semiconductor field-effect transistor is one of a family of field-effect devices, the three main types of field-effect transistors being junction field-effect transistors (JFETs) , metal- oxygen-semiconductor field-effect transistors (MOSFETs) , and MESFETs. As known to those familiar with semiconductor devices, JFETs and MESFETs operate identically to one another, the difference being that a MESFET uses a metal-semiconductor rectifying contact for the gate electrode instead of a p-n junction. Each of these structures offer certain processing and performance advantages. For MESFETS, these advantages potentially include low temperature formation of the gate structure (as compared to the generally more sophisticated formation of p-n junctions) , low resistance and low current-resistance ("IR") drop along the channel width, and good heat dissipation for power devices. Alternatively, a MESFET' s metal contact structure is more susceptible to leakage current, particularly at high temperatures, so that JFETs can often operate at higher temperatures than MESFETS. As just noted, the general theory and operations of JFETs and MESFETs are well understood. An illustrative discussion is set forth, for example, in Chapter 6 of Sze, Physics of Semiconductor Devices, 2nd Edition (1981) . Metal-semiconductor field-effect transistors are particularly useful for high frequency applications. Such applications include electrical circuits requiring high power handling capability (greater than 20 watts) while operating a high frequencies such as radio frequencies (500 megahertz) , S-band frequencies (3 gigahertz) , and X-band frequencies (10 gigahertz) , the use of which has become more prevalent in recent years. Because of the increase in high power, high frequency circuits, demand has correspondingly increased for transistors capable of operating reliably at radio and higher frequencies while still handling higher power loads. Previously, bipolar transistors and power MOSFETs have been used for high power applications, but the power handling capability of such devices is generally limited at higher operating frequencies. The MESFET structure is preferred for high frequency applications because only the majority carriers carry current. The MESFET design is also preferred over current MOSFET designs because the reduced gate capacities permit faster switching of the gate input. Therefore, although all field-effect transistors utilize only majority carriers to carry current, a MESFET' s Schottky gate structure makes the MESFET more desirable for high frequency applications. In addition to the device structure, the characteristics of the semiconductor material from which a transistor is formed also affect the operating parameters. Such semiconductor characteristics include electron mobility, saturated electron drift velocity, electric breakdown field, and thermal conductivity. For many years, most high frequency MESFETs have been manufactured of n-type III-V compounds such as gallium arsenide (GaAs) because of their high electron mobilities. Although these devices provided increased operating frequencies and moderately increased power handling capability, their relatively low breakdown voltage and their lower thermal conductivity have limited their usefulness in high power applications. Silicon carbide (SiC) has been known for many years to possess excellent physical and electronic properties which theoretically should provide the basis for electronic devices that can operate at higher temperatures, and higher power at high frequencies than devices produced from silicon or gallium arsenide. Silicon carbide has an electric breakdown field of about 4xl0 6 volts per centimeter, a saturated electron drift velocity of about 2.0xl0 7 centimeters per second, and a high thermal conductivity of about 4.9 watts per centimeter-Kelvin, all of which makes silicon carbide suitable for high frequency, high power applications. Silicon carbide is notoriously difficult to work with, however, particularly because of the high processing temperatures required to grow it, its physical hardness, and its characteristic crystallization in approximately 150 polytypes, most of which are separated by relatively small thermodynamic differences. More recently, however, progress in silicon carbide research has provided the practical success in device manufacture to match silicon carbide's theoretical potential. In particular, U.S. Patent No. 5,270,554 and Canadian Patent No. 1,312,148, the contents of both of which are incorporated entirely herein by reference, disclose successful MESFETs in silicon carbide. Both of these patents represent developments carried out by the assignee of the present invention. The '554 U.S. patent is assigned to the present assignee, and the '148 Canadian patent is exclusively licensed to the present assignee. Canadian Patent No. 1,312,148 also claims priority from U.S. application Serial No. 07/540,488, filed June 19, 1990 (now abandoned) which is also incorporated entirely herein by reference. Additionally, interest is increasing in other wide bandgap semiconductor materials such as Group III nitrides (i.e., Group III of the periodic table) , including binary compounds such as gallium nitride (GaN) , and ternary and quaternary nitrides such as GaAlN and InGaAlN. In seeking improvements in these devices, careful evaluation of their properties has shown that much of the source and drain resistance in MESFETs formed in silicon carbide results from the physical gap between the ohmic contact metal layer and the gate contact. More specifically, recent careful analysis of existing devices has revealed that approximately half of the source resistance of current silicon carbide MESFETs results from this gap. As is known to those familiar with present manufacturing techniques for semiconductor devices, this gap is on the order of about 1 micron. This represents the current smallest tolerance that can be achieved using the masking and etching steps by which semiconductor devices of all types, including those formed in silicon carbide, are typically formed. Thus, although a tolerance as small as 1 micron is generally considered to be excellent in many respects, it has now been discovered that this very small gap contributes significantly to the source resistance of the device. As known to those familiar with semiconductor devices and particularly JFETs and MESFETs, the increased source resistance in turn lowers the potential gain and power efficiency of a MESFET at microwave frequencies. Perhaps most fundamentally, the increased source resistance also lowers the maximum operating frequency, F max . There thus exists the need to reduce the source resistance in JFETs and MESFETs formed in silicon carbide and Group III nitrides in a manner which will increase their gain, their efficiency, and their maximum operating frequencies. Object and Summary o. -he Invention Therefore, it is an object of the present invention to provide a field effect transistor with reduced source resistance and a corresponding increase in gain, power efficiency, and F max . The invention meets these objects with a metal-semiconductor field-effect transistor (MESFET) that comprises an epitaxial layer of silicon carbide having a first conductivity type. A gate trench in the epitaxial layer exposes the silicon carbide gate surface between two respective trench edges, and the trench further defines the source and the dra - regions of the transistor. A rectifying gate contact is on the gate surface, and respective ohmic metal layers form ohmic contacts on the source and drain regions of the epitaxial layer. The edges of these ohmic metal layers at the trench are specifically, aligned with the edges of the trench in the epitaxial layer. In another aspect, the invention comprises a method of forming such a self-aligned metal- semiconductor field-effect transistor in silicon carbide. The foregoing and other objects, advantages and features of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and wherein: Figure 1 is a cross-sectional view of a prior art MESFET; Figure 2 is a cross-sectional view of a MESFET according to the present invention; Figure 3 is a schematic top plan view of a masking pattern according to the method of the invention; Figure 4 is a cross-sectional view of a JFET according to the present invention. Detailed Description Figure 1 is a representation of a typical MESFET formed in silicon carbide and broadly designated at 10. The device 10 includes a silicon carbide substrate 11, and an epitaxial layer of silicon carbide illustrated in Figure 1 in two portions as the channel layer 12 and the cap layer 13. It will be understood, however, that a single layer could suffice. In typical MESFETs, both the channel layer 12 and the cap layer 13 are usually n-type conductivity. The gate contact is illustrated at 14 and is positioned in the trench broadly designated at 15 formed in the cap layer 13. In order to provide ohmic contact to the source and drain regions, a metal suitable for an ohmic contact is deposited on the cap layer and is designated as portions 16 and 17 to distinguish the source portion 16 from the drain portion 17. Typically, an interconnect metal shown as the portions 20 (for the source) and 21 (for the drain) complete the device structure. As is known to those familiar with the manufacture of such devices, a typical method for building the device of Figure 1 is to first grow the epitaxial cap layer 13 on the channel layer 12. The gate trench 15 is next etched, after which the structure is masked and the ohmic metal (nickel or a nickel alloy being typical examples for silicon carbide) is deposited to make the ohmic contacts. The physical limitations of working in such small dimensions are such that it has been difficult to date to align the mask for the metal any closer to the trench than about 1 micron. As a result, a source gap designated at 22 and a drain gap designated at 23 are present on the cap layer 13 between the edge of the respective metal portions 16 and 17 and the edges of the gate trench 15. As foreshadowed earlier, it has now been discovered that although a FET's source resistance is a product of several factors, the largest factor--one which contributes approximately half of the source resistance--is this gap 22 along the cap layer 13 between the ohmic contacts 16 and the trench 15. The invention eliminates this gap 22 and thus eliminates approximately half of the source resistance. Figure 2 illustrates the structure of the invention and shows a MESFET broadly designated at 25. The invention is preferably supported by a silicon carbide substrate 26. Those familiar with wide bandgap materials and devices will recognize, however, that devices such as this have been formed on other substrate materials such as sapphire, silicon, or even diamond, and thus the invention encompasses such substrates. Silicon carbide is preferred, however, because it can provide an exact lattice match with the silicon carbide epitaxial layers to be grown upon it. An epitaxial layer of silicon carbide having a first conductivity type is formed on the epitaxial layer 26. In the preferred embodiment of the invention, and as illustrated in Figure 2, the epitaxial layer is formed of a channel layer 27 and a cap layer 30, both having n-type conductivity. As noted earlier, a single epitaxial layer could suffice. The epitaxial layer has a gate trench broadly designated at 31 that exposes a silicon carbide gate surface between two respective trench edges 32 and 33. The trench thus further defines the source and drain regions of the transistor. A rectifying gate contact 34 is made to the gate surface 35 to define the gate region of the transistor, and respective ohmic metal layers 36 and 37 form ohmic contacts on the source and drain regions of the cap epitaxial layer 30. As illustrated by Figure 2, the edges of the metal layers 36 and 37 are specifically aligned with the edges 32 and 33 of the trench 31 in the epitaxial layer 30. Interconnect metal portions 40 and 41 are also shown in Figure 2 and, providing they are compatible with the ohmic contact metal layers 36 and 37, can be selected in an appropriate manner from a number of high conductivity metals well known in this art, including gold (Au) , silver (Ag) , aluminum (Al) , and copper (Cu) . As for the ohmic contact metal layers 36 and 37, an appropriate contact material is qualified functionally as one that provides ohmic behavior with the semiconductor, and one that doesn't react unfavorably with the etchant. For a FET in silicon carbide, the ohmic contact preferably comprises a nickel (Ni) alloy that is formed by depositing nickel on the SiC surface and then annealing it. As-deposited aluminum (Al) will also form an ohmic contact to SiC. Similarly, the gate contact can be selected from the group consisting of gold, platinum (Pt) , platinum suicide, and titanium, and in a preferred embodiment comprises platinum. In preferred embodiments, a gold overlay covers the platinum gate contact, and successive layers of titanium, platinum, and gold overlay the annealled nickel ohmic contacts . As well known to those familiar with the crystal structure advantages of semiconductor devices, the substrate 26, the channel layer 27, and the cap layer 30 are all preferably formed of the same polytype of silicon carbide, and these are generally selected from the group consisting of the 3C, 2H, 4H, 6H, and 15R polytypes of silicon carbide, with the understanding that bulk single crystal substrates of 3C silicon carbide are generally more difficult to form. In the Group III nitride embodiments of the device, the substrate can be silicon carbide, and if a buffer layer of aluminum nitride (not shown) is included between the substrate and the FET structure, the silicon carbide substrate can be n or p-type, semi- insulating, or even substantially undoped. Sapphire is an appropriate alternative substrate material, and is insulating in character. In order to enhance the operation of the device, the cap layer 30 typically has a higher carrier concentration than the channel layer 27, and both are generally formed of n-type silicon carbide. As further known to those familiar with MESFET operation, the substrate 26 is preferably semi- insulating in character, although it can also have the opposite conductivity type from the epitaxial layers 27 and 30. Thus, when the epitaxial layers are preferably n-type, the substrate will be either semi-insulating or p-type. In yet other embodiments, a p-type, semi- insulating, or even substantially undoped epitaxial layer (not shown) can provide a buffer between the substrate and the FET so that the conductivity of the substrate does not affect the performance of the remainder of the structure; i.e.; can be n-type, p- type, or semi-insulating. As Figure 2 illustrates, because the edges of the ohmic contact layers 36 and 37 are aligned with the edges 32 and 33 of the trench 31, the gaps shown at 22 and 23 in Figure 1 are eliminated, thus immediately eliminating approximately fifty percent of the source resistance in the transistor. Figure 4 is a cross-sectional view of a junction field-effect transistor (JFET) according to the present invention. As in the MESFET embodiment illustrated in Figure 2, the JFET of Figure 4, broadly designated at 45, includes a substrate 46, a channel epitaxial layer 47, and a cap epitaxial layer 50. A trench 51 is formed in the cap layer 50 and includes a gate contact 52 which can be of an appropriate ohmic metal, rather than a Schottky metal. In the embodiment shown in Figure 4, a p-n junction is formed by an implanted well 53 of p-type material in the n-type channel layer 47 prior to adding the gate contact 52. As in the previous embodiments, the ohmic contact metals 54 and 55 respectively are aligned with the edges 56 and 57 of the trench 51. Interconnect metals 60 and 61 complete the structure. In another aspect, the invention comprises a method of forming the self-aligned metal-semiconductor field-effect transistor of the present invention. The first step comprises depositing an ohmic contact metal layer on an epitaxial layer of silicon carbide in a pattern that defines a gate trench location at which the epitaxial layer is exposed between portions of the ohmic contact metal layer. The second step comprises etching a gate trench through the epitaxial layer of silicon carbide with an etchant and process that do not remove the ohmic contact metal layer. As a result, the metal layer effectively masks portions of the silicon carbide epitaxial layer to thereby self-align the edges of the metal and the edges of the trench in the silicon carbide layer to form the source and drain regions . A gate contact is then applied to the epitaxial layer at the bottom of the trench. In sequential fashion, the method can further comprise the step of forming the epitaxial layer of silicon carbide supported by a silicon carbide substrate pricr to the step of depositing the layer of ohmic contact metal. In turn, the step of forming an epitaxial layer can comprise the steps of forming an epitaxial channel layer of silicon carbide on the substrate, and then forming an epitaxial cap layer of silicon carbide on the epitaxial channel layer of silicon carbide. The method can further comprise masking a portion of the ohmic contact layer prior to the step of etching the gate trench to thereby provide portions of the ohmic contact layer that are unaffected by the etch and thus more suitable for providing further interconnecting source and drain contacts to the ohmic contact layer. Figure 3 illustrates such a masking pattern in which the ohmic contact metal layers 36 and 37 identically corresponding to the same reference numerals in Figure 3 are shown as having portions schematically covered by the masks 42 and 43 respectively. Although typical etchants for silicon carbide do not usually remove annealed nickel and some other metals, the etchants will often tend to react to some degree with the metal surface and make it less suitable for further electrical contact. Thus, by masking a portion of the nickel during the etching step, a better surface is preserved for later adding the interconnect metal. As in the structural embodiments, the step of depositing the ohmic contact metal preferably comprise depositing nickel and then annealing it. The step of applying a gate contact comprises applying a gate contact selected from the group consisting of gold, platinum, platinum suicide and titanium, and preferably platinum. Typical interconnect metals are selected for their high conductivity, and include gold, silver, aluminum, and copper. In yet another embodiment of the invention, and using an etchant and process that will etch both silicon carbide and the ohmic contact metal (or a first etchant for the metal and a second etchant for the silicon carbide) , the method can comprise etching a gate trench through the ohmic contact metal layer covering the epitaxial layer of silicon carbide and also through the silicon carbide layer to thereby self- align the edges of the metal and the edges of the trench in the silicon carbide layer. The gate contact is then applied to the epitaxial layer at the bottom of the trench as previously described. The various process steps described herein can be carried out using techniques developed by the assignee of this invention. The growth of silicon carbide epitaxial layers of various types on various substrates are set forth in U.S. Patent Nos. 4,912,063; 4,912,064; and 5,011,549, all of which are incorporated entirely herein by reference. The etching step can be carried out in accordance with the teachings of U.S. Patent Nos. 4,865,685 and 4,981,551, the contents of which are incorporated entirely herein by reference. If desired, surface preparation can be carried out in accordance with U.S. Patent Nos. 4,946,547 and 5,200,022, the contents of both of which are incorporated entirely herein by reference. Finally, if ion implantation is used to form any of the layers or junctions described herein, it can be carried out using the techniques described in U.S. Patent No. 5,087,576, the contents of which are likewise incorporated entirely herein by reference. In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms have been employed, they have been used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims: THAT WHICH IS CLAIMED IS: 1. A field-effect-transistor (FET) that exhibits reduced source resistance and higher operating frequencies, said FET comprising: an epitaxial layer of silicon carbide having a first conductivity type; a gate trench in said epitaxial layer that exposes a silicon carbide gate surface between two respective trench edges, and said trench further defining the source and drain regions of said transistor; a rectifying gate junction to said gate surface; and respective ohmic metal layers forming ohmic contacts on said source and drain regions of said epitaxial layer; characterized in that the edges of said metal layers at said trench are specifically aligned with said edges of said epitaxial layer at said trench. 2. A FET according to Claim 1 wherein said gate junction is a p-n junction or a rectifying Schottky contact . 3. A FET according to Claim 1 wherein said epitaxial layer is supported by a silicon carbide substrate . 4. A FET according to Claim 1 wherein: said silicon carbide epitaxial layer comprises a channel layer and a cap layer; said cap layer has a higher carrier concentration than said channel layer; and wherein said cap layer defines said gate trench therein; and wherein said gate trench exposes the upper surface of said channel layer between two respective trench edges . 5. A FET according to Claim 4 wherein said substrate, said channel layer, and said cap layer are all formed of the same polytype of silicon carbide, and selected from the group consisting of the 3C, 2H, 4H, 6H, and 15R polytypes of silicon carbide. 6. A FET according to Claim 3 wherein said substrate is either semi-insulating or has the opposite conductivity type from said epitaxial layer. 7. A FET according to Claim 3 and further comprising an epitaxial layer between said substrate and said FET epitaxial layer, said epitaxial layer being selected from the group consisting of a p-type epitaxial layer, a semi-insulating epitaxial layer, and a substantially undoped epitaxial layer. 8. A method of forming a self aligned field- effect-transistor (FET) in silicon carbide that exhibits reduced source and drain resistance and higher operating frequencies; the method comprising: depositing an ohmic contact metal layer on an epitaxial layer of silicon carbide in a pattern that defines a gate trench location at which the epitaxial layer is exposed between portions of the ohmic contact metal layer; etching a gate trench through the epitaxial layer of silicon carbide with an etchant and process that do not remove the ohmic contact metal layer so that the metal layer effective masks portions of the silicon carbide epitaxial layer to thereby self-align the edges of the metal and the edges of the silicon carbide layer at the trench; and forming a gate junction to the epitaxial layer at the bottom of the trench. 9. A method according to Claim 8 wherein the step of forming a gate junction comprises applying a rectifying Schottky contact to the epitaxial layer at the bottom of the trench. 10. A method according to Claim 8 wherein the step of forming a gate junction comprises forming a p-type region in an n-type epitaxial layer at the bottom of the trench. 11. A method according to Claim 8 and further comprising the step of forming an epitaxial layer of silicon carbide supported by a silicon carbide substrate prior to the step of depositing the layer of ohmic contact metal. 12. A method according to Claim 11 wherein the step of forming an epitaxial layer comprises the steps of : forming an epitaxial channel layer of silicon carbide; and forming an epitaxial cap layer of silicon carbide on the epitaxial channel layer of silicon carbide. 13. A method according to Claim 8 and further comprising masking a portion of the ohmic contact layer prior to the step of etching the gate trench to thereby provide portions of the ohmic contact layer that are unaffected by the etch and thus more suitable for providing further interconnecting contacts to the ohmic contact layer. 14. A method according to Claim 8 wherein the step of etching the gate trench comprises: etching the ohmic contact metal with a first etchant; and thereafter etching the epitaxial layer with a second etchant. 15. A metal-semiconductor field-effect- transistor (MESFET) that exhibits reduced source resistance and higher operating frequencies, said MESFET comprising: a substrate; an n-type Group III nitride epitaxial layer on said substrate; a gate trench in said epitaxial layer that exposes a gate surface between two respective trench edges, and said trench further defining the source and drain regions of said transistor; a rectifying Schottky gate contact to said gate surface; and respective ohmic metal layers forming ohmic contacts on said source and drain regions of said epitaxial layer, and in which the edges of said metal layer are specifically aligned with said edges of said epitaxial layer at said trench. 16. A MESFET according to Claim 15 wherein said epitaxial layer comprises a channel layer on said substrate and a cap layer on said channel layer, with said cap layer having a higher carrier concentration than said channel layer. 17. A MESFET according to Claim 15 wherein said Group III nitride is selected from the group consisting of binary Group III nitrides, ternary Group III nitrides, and quaternary Group III nitrides. 18. A MESFET according to Claim 15 and further comprising an aluminum nitride buffer layer between said substrate and said FET epitaxial layer. 19. A MESFET according to Claim 15 wherein said substrate is selected from the group consisting of silicon carbide and sapphire.

Inventors: ALLEN SCOTT T, ALLEN, SCOTT, T.
Applicants: ALLEN SCOTT T, CREE RESEARCH INC, ALLEN, SCOTT, T., CREE RESEARCH, INC.
Application year: 1995
Main classification: H01L29
IPC codes: H01L21, H01L29