III-nitride semiconductor device with trench structure

A III-nitride trench device has a vertical conduction region with an interrupted conduction channel when the device is not on, providing an enhancement mode device. The trench structure may be used in a vertical conduction or horizontal conduction device. A gate dielectric provides improved performance for the device by being capable of withstanding higher electric field or manipulating the charge in the conduction channel. A passivation of the III-nitride material decouples the dielectric from the device to permit lower dielectric constant materials to be used in high power applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a trench device realized in a III-nitride material system, and relates more particularly to a class of trench switching devices that are nominally off in a III-nitride material system.

2. Description of Related Art

III-nitride semiconductors are presently known that exhibit a large dielectric breakdown field of greater than 2.2 MV/cm. III-nitride heterojunction structures are also capable of carrying extremely high currents, which makes devices fabricated in the III-nitride material system excellent for power applications.

Development of devices based on III-nitride materials has generally been aimed at high power-high frequency applications such as emitters for cell phone base stations. The devices fabricated for these types of applications are based on general device structures that exhibit high electron mobility and are referred to variously as heterojunction field effect transistors (HFETs), high electron mobility transistors (HEMTs) or modulation doped field effect transistors (MODFETs). These types of devices are typically able to withstand high voltages such as in the range of 100 Volts, while operating at high frequencies, typically in the range of 2-100 GHz. These types of devices may be modified for a number of types of applications, but typically operate through the use of piezoelectric polarization fields to generate a two dimensional electron gas (2DEG) that allows transport of very high current densities with very low resistive losses. The 2DEG is formed at an interface of AlGaN and GaN materials in these conventional III-nitride HEMT devices. Due to the nature of the AlGaN/GaN interface, and the formation of the 2DEG at the interface, devices that are formed in the III-nitride materials system tend to be nominally on, or depletion mode devices. The high electron mobility of the 2DEG at the interface of the AlGaN/GaN layers permits the III-nitride device, such as a HEMT device, to conduct without the application of a gate potential. The nominally on nature of the HEMT devices previously fabricated have limited their applicability to power management. The limitations of nominally on power devices is observed in the need to have a control circuit powered and operational, before power can be safely controlled by a nominally on device. Accordingly, it would be desirable to create a III-nitride heterojunction device that is nominally off to avoid current conduction problems during start-up and other modes.

A drawback of III-nitride devices that permit high current densities with low resistive losses is the limited thickness that can be achieved in the strained AlGaN/GaN system. The difference in the lattice structures of these types of materials produces a strain that can result in dislocation of films grown to produce the different layers. This results in high levels of leakage through a barrier layer, for example. Some previous designs have focused on reducing the in-plane lattice constant of the AlGaN layer to near where the point of relaxation occurs to reduce the dislocation generation and leakage. However, the problem of limited thickness is not addressed by these designs.

Another solution is to add insulation layers to prevent leakage problems. The addition of an insulator layer can reduce the leakage through the barrier, and typical layers used for this purpose are silicon oxide, silicon nitride, saphire, or other insulators, disposed between the AlGaN and metal gate layers. This type of device is often referred to as a MISHFET and has some advantages over the traditional devices that do not have an insulator layer.

While additional insulator layers can permit thicker strained AlGaN/GaN systems to be constructed, the confinement layer produced by the additional insulator results in lower current carrying capacity due to the scattering effect produced on electrons at the GaN/insulator interface. Also, the additional interface between the AlGaN layer and the insulator results in the production of interface trap states that slow the response of the device. The additional thickness of the oxide, plus the additional interfaces between the two layers, also results in the use of larger gate drive voltages to switch the device.

Conventional device designs using nitride material to obtain nominally off devices rely on this additional insulator to act as a confinement layer, and may reduce or eliminate the top AlGaN layer. These devices, however, typically have lower current carrying capacity due to scattering at the GaN/insulator interface.

Accordingly, it would be desirable to produce a heterojunction device or FET that has a low leakage characteristic with fewer interfaces and layers that can still withstand high voltage and produce high current densities with low resistive losses. Presently, planar devices have been fabricated with GaN and AlGaN alloys through a number of techniques, including MOCVD (metal organic chemical vapor deposition) as well as molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).

Materials in the gallium nitride material system may include gallium nitride (GaN) and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and indium aluminum gallium nitride (InAlGaN). These materials are semiconductor compounds that have a relatively wide direct bandgap that permits highly energetic electronic transitions to occur. Gallium nitride materials have been formed on a number of different substrates including silicon carbide (SiC), saphire and silicon. Silicon substrates are readily available and relatively inexpensive, and silicon processing technology has been well developed.

However, forming gallium nitride materials on silicon substrates to produce semiconductor devices presents challenges that arise from differences in the lattice constant, thermal expansion and bandgap between silicon and gallium nitride. The problems attendant with the lattice mismatch between GaN and traditional substrate materials are also prevalent in material layer structures involving GaN and GaN alloys. For example, GaN and AlGaN materials have lattice structures that differ significantly enough to produce interface strain between the layers, contributing to piezoelectric polarization. In many previous devices, the fields generated by the piezoelectric polarization are controlled to improve the characteristics of the devices. Variations in the content of aluminum in the AlGaN/GaN layer structures tends to vary the lattice mismatch between the materials to achieve different device characteristics, such as improved conductivity or isolation barriers.

A number of types of power devices can potentially benefit from a nominally off device with low on resistance. For example, it would be desirable to obtain a power switch, power rectifier, synchronous rectifier, current control device or other power devices that are nominally off when no power is applied. Current control devices can include diodes, pinch resistors, Schottky diodes and the like.

Trench structure semiconductor devices have been available in silicon for a number of years. Often, vertical conduction devices are realized with trench structures that can be formed in silicon as enhancement mode or nominally off devices. The devices, such as a MOSFET switch, typically operate by introducing an electric potential on a gate electrode to form an invertible channel along trench sidewalls, that provides a conductive path for the device. However, the conductive path typically has a given on resistance that is associated with the device voltage rating. For example, the thickness, composition and doping of the device materials contributes to determining device characteristics. These design parameters are manipulated to obtain desired characteristics, but on-resistance and blocking voltage for a given current rated device continue to receive attention for improvements to power semiconductor devices. It would be desirable to obtain a vertical conduction device with reduced on resistance that is capable of blocking large voltages.

One factor that contributes to the breakdown voltage value for a given device is the dielectric breakdown value for a given dielectric in the power semiconductor. For example, in silicon semiconductors, native oxides are available, such as silicon dioxide, that can serve as a suitable gate dielectric. However, no material equivalent to the native oxides for silicon is available for suitable gate dielectrics in the III-nitride material system. In addition, gate dielectric materials that would otherwise be suitable in silicon semiconductors, for example, do not transfer well to III-nitride devices. For example, if silicon dioxide or silicon nitride were to be used for a gate dielectric in a III-nitride device, these conventional dielectrics would rupture or otherwise fail. Typically, the large dielectric breakdown field produced in the III-nitride material system causes large electric fields in the III-nitride semiconductor devices that are greater than can be withstood with conventional dielectric materials.

It would be desirable to obtain a device structure and gate dielectric material suitable for use in a semiconductor device that experiences high electrical fields, without the dielectric material breaking down. It would also be desirable to obtain such a dielectric that is suitable for use in the III-nitride material system.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a nominally off, or enhancement mode FET device, realized in a III-nitride material system. The FET is capable of carrying high currents due to the nature of the III nitride material system, where piezoelectric and spontaneous polarization fields contribute to forming a 2DEG that permits high carrier mobility and large current throughput.

According to one embodiment of the present invention, a nominally off switch is formed with a trench geometry, to permit a 2DEG to be controlled along trench sidewalls. The trench geometry interrupts the 2DEG when the device is inactive to provide an enhancement mode, or nominally of device. Activation of the device, for example through application of an electric potential to a gate contact near the trench sidewalls, causes a 2DEG to form at the trench sidewalls, providing a conduction path for the device.

In accordance with a feature of the present invention, the trench geometry includes angled sidewalls at a particular angle. The angle of the trench sidewalls is controlled to obtain a particular threshold voltage for the device. As the angle becomes shallower so that the sidewalls become more lateral in direction, the threshold voltage decreases. Steeper sidewall angles produce higher threshold voltages. In an exemplary embodiment, the sidewalls are lined with AlGaN and formed at a particular angle to achieve a desired threshold voltage.

In accordance with a feature of the present invention, there is provided a FET device and method for operation that includes III-nitride material alloy layers. Alloys with particular percentages of In, Al or Ga may be used, with the nitrides InN, AlN or GaN and various alloy combinations such as, for example, InAlGaN. The device includes a source, a drain and a gate electrode. The device may be vertical or horizontal in operation for current paths.

According to an embodiment of the present invention, a III-nitride semiconductor device is composed of a multilayer stack of III-nitride materials grown on an appropriate substrate. The stack may consist of alternate layers of N and P type doped III-nitride materials. An example of such a multilayer stack includes N+GaN/P GaN/N−GaN/N+Gan/substrate. Although such a multilayer stack may employ GaN as a material, any of the III-nitride material system compositions may be employed, including AlN, InN, AlGaN, InAlGaN, and any other appropriate alloy. Various types of alloys can be employed in the different layers of the stack to obtain particular characteristics and provide particular advantages from the heterojunction effects.

According to an embodiment of the present invention, a III-nitride semiconductor device is composed of a multilayer stack of III-nitride materials grown on an appropriate substrate. The stack may consist of alternate layers of III-nitride materials having different alloy percentages to produce different bandgaps. An example of such a multilayer stack includes GaN and AlGaN layers alternating with multiple 2DEGs formed at the layer interfaces to provide a high current conduction structure. The stack may consist of a superlattice structure, and may be composed of any of the III-nitride material system compositions, including AlN, InN, AlGaN, InAlGaN, and any other appropriate alloy. Various types of alloys can be employed in the different layers of the stack to obtain particular characteristics and provide particular advantages from the heterojunction effects.

Advantageously, cladding and contact layers may be grown above or below the active region. Other known processes for constructing electrodes, insulation layers and so forth may also be applied to the present invention.

According to a feature of the present invention, a good GaN insulator interface is provided to improve current carrying capacity, rather than additional insulator layers or structures at the active layer. Without added insulation layers, the epitaxial nature of the hetero-interface described here leads to an order of magnitude higher mobility for the electrons in the 2DEG when accumulated.

According to another embodiment of the present invention, a nominally off bi-directional switch realized in a III-nitride material system provides an AlGaN/GaN interface to provide a location for the formation of a 2DEG. A vertical AlGaN layer is provided that lines a trench in which a gate contact is disposed to eliminate the 2DEG locally to obtain an enhancement mode device. Application of an electric potential to the gate contact permits the 2DEG to be established and form a current conduction channel.

According to another feature of the present invention, a method for forming the above described devices is provided where a III-nitride structure is formed on an insulative or highly resistive substrate. Optionally, a buffer layer may be provided between the substrate and a III-nitride body layer, which is preferably composed of N−GaN. The buffer layer may also be composed of a III-nitride multilayer stack with alternating types of III-nitride materials to form a high current carrying region. The buffer layer may include a superlattice structure, for example. A layer of P type III-nitride material, preferably GaN, is deposited over the body layer. A layer of N+III-nitride material, preferably GaN, is then deposited over the P type material. A trench is formed in the III-nitride structure that projects into the body layer. A III-nitride layer with a different alloy percentage and lattice constant or band gap than that of the underlying layers, for example AlGaN, is deposited over the trench structure to line the trench and mesas. The top layer may then be patterned and etched, to remove particular portions of the top III-nitride layer as desired.

Ohmic contacts are then formed atop the III-nitride layer, with appropriate annealing steps to activate the ohmic contacts. An insulation layer is then deposited and patterned, which fills the trench and provides a protective layer over the remaining III-nitride structure surfaces. The insulator deposited in the trench is then etched to form a gate trench, into which is deposited conductive material to form the gate electrode. Additional cladding or contact layers may be formed in a vertical or horizontal relationship with the active regions. For example, known processes for constructing electrodes and insulation layers may be applied in forming the III-nitride device.

According to a feature of the present invention, the layer lining the trenches and the mesas is composed of multiple layers, or stacks. An exemplary embodiment includes a layered liner composed of layers of InGaN and AlGaN. Alternately, the liner is a lattice matched alloy with a larger bandgap.

According to another feature of the present invention, a trench device with a body or base layer composed of P GaN or compensated GaN is provided.

According to another embodiment of the present invention, a method for constructing a III-nitride structure is provided where the structure comprises a semiconductor device on a conductive substrate. The substrate is preferably N+material, overlaid by N+GaN. The N+GaN forms a buffer layer, and may be composed of materials with a gradient composition, multiple layers or multiple buffer layers. The buffer layer is overlaid by an N−GaN layer, which in turn is overlaid by a P GaN layer. The III-nitride structure is then overlaid by an N+GaN layer, and a trench is formed through the N+GaN layer, the P GaN layer and into the N−GaN layer. A layer of AlGaN is then deposited over the trench structure to line the trenches and the mesas. Ohmic contacts may then be formed on the layer of AlGaN. An insulator may then be deposited and patterned atop the III-nitride structure, with the insulator material filling the trench. The insulator in the trench is etched to reform a trench opening so that the trench opening has sidewalls composed of insulator material. A conductive gate material is deposited into the trench opening to form the gate contact for the device. The device may then be finished with cladding and other contact or electrode structures, using known processes. Insulation or protective layers may also be provided to encapsulate the structure to passivate the surfaces or prevent damage to the surfaces of the device.

In accordance with another exemplary embodiment of the present invention, a source region is formed with an intrinsic III-nitride material alloy, for example GaN, and overlaid with a larger bandgap III-nitride material alloy, for example AlGaN. Such a configuration permits the formation of a 2DEG in the source region to obtain high current capacities.

According to another embodiment of the present invention, a method for constructing a FET in a III-nitride material system provides for overlaying a substrate with a compensated N type GaN epi layer, which is overlaid by a lightly doped N type epi layer. The lightly doped N type epi layer is overlaid by a P type epi layer, followed by the deposition of a heavily doped N type epi layer. This initial III-nitride FET structure is used as the basis for forming a FET, or other semiconductor devices according to particular processing steps. A contact deposition is provided on the structure to form contacts on a top and bottom layer of the III-nitride structure. The contact on the top is patterned and etched to form the desired contact structure. After surface passivation of the device following the etch step, a gate dielectric is deposited, followed by the deposition of a gate contact. The resulting device may then be completed according to known steps, including providing cladding or other contact or electrode structures and insulation as desired.

In accordance with another embodiment of the present invention, a method for forming a FET device in a III-nitride material system begins with the formation of a P or N type epi layer over a substrate, followed by the formation of a dielectric portion, which may act as a gate dielectric. Ion implantation follows, to form gate and source semiconductor portions in regions not protected by the dielectric layer. That is, it is possible to use the dielectric layer as a mask for the deposition of the doped source and drain regions. Source and drain contacts are then formed, followed by an anneal step to activate the contacts. The protective dielectric layer may then be removed, the surface of the III-nitride device passivated, and the gate dielectric redeposited. A gate contact may be deposited atop the gate dielectric, to permit the formation of a 2DEG under the gate to form a current conduction channel between the source and the drain when the gate is activated.

According to another embodiment of the present invention, a method for constructing a FET in a III-nitride material system provides for an initial starting wafer that may comprise layers of N+GaN, P GaN, N−GaN, N+GaN and a substrate. The wafer is selectively etched to expose various layers, as well as to provide insulation for devices and channel fabrication. A redeposition step is employed to grow a layer of larger band gap III-nitride material over the wafer. This particular step provides a number of improvements in device performance by increasing the carrier mobility at the interface between the wafer and the larger band gap material. The addition of the larger band gap material is optional, and is not required for operation of the device. N type ohmic contacts are then formed and annealed to provide contacts with low resistivity. A gate structure is formed and may be composed of an insulating material such as SiN, SiO2or other insulating material deposited on the III-nitride larger band gap material. A conductive gate material is then formed over the insulator and the insulator and conductive gate material are patterned to provide a particular structure to achieve desirable characteristics according to the application. Devices formed according to this method can be provided in a number of different geometries, including striped, grid and cellular, including circular, oval, square, polygonal or other cellular geometries.

The large dielectric breakdown field in the III-nitride semiconductor material system permits the construction of nominally off power devices described above with reduced size standoff regions. The material system also permits the production of devices with reduced specific on resistance in comparison with known devices of similar voltage ratings.

According to one embodiment of the present invention, a nominally off FET device with a gate, source and drain area are defined using a two layer AlGaN/GaN material to produce a III-nitride HEMT device. The source and drain regions may be formed according to known methods including ion implantation, etching to remove the barrier region over the source and drain regions and application of a low resistance ohmic contact formation process.

According to another embodiment of the present invention, a vertical trench device is provided with AlGaN material lining the trench. Layers of GaN on the mesa portion of the device produce a 2DEG at the GaN/AlGaN interface, which can be controlled through a gate contact to control vertical conduction. Various configurations of trench geometries, including cellular and striped can be used to realize the device. In addition, the dimensions and doping concentration of the GaN layers produce particular device characteristics. Trenches and mesas may be lined with various III-nitride alloys or layers, including InGaN, AlGaN and GaN, in various combinations. The device exhibits low on resistance with high current carrying capability and high blocking voltage capability.

The device is also characterized by low leakage in the contacts and a high dielectric constant of the gate dielectric. As a result, the device provides a larger dielectric constant compared to conventional insulators such as SiO2and SiN. The high critical fields of the GaN material allow thin layers to withstand large voltages without dielectric breakdown. The dielectric constant of GaN materials is approximately 10, which is a factor of 2.5 times better than SiO2.

In accordance with another aspect of the present invention, a self passivating surface structure for III-nitride materials is provided where the interface between materials is nearly ideal. The self passivation effect permits the gate dielectric to be decoupled from the semiconductor material, so that a wide choice of materials may be used for the gate dielectric. Examples of gate dielectrics that may be used are zirconium oxide, lanthenoid group oxides, diamond, AlN and other piezoelectric and pyroelectric materials.

In accordance with another aspect of the present invention, a III-nitride semiconductor surface may be repassivated according to the self passivation effect. Any number of devices may be fabricated using this effect to provide a good dielectric structure that is capable of resisting high electric fields without breakdown and serve to reduce the electric field on the gate.

In accordance with a method of the present invention, a surface passivation is performed prior to the deposition of a gate dielectric. According to a feature of the present invention, the surface passivation includes a nitrogen plasma anneal. In accordance with another feature of the present invention, the passivation process includes using an encapsulant material containing nitrogen followed by an anneal. According to a further feature of the present invention, the passivation may be performed with a photo-electro-chemical etch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the construction of GaN material devices, a number of factors come into play to impact the functionality and capability of the devices. A large lattice mismatch in III-nitride materials and the strong piezoelectric and polarization effects in these materials significantly impact the electrical properties of III-nitride heterojunction devices. A significant number of reported GaN-based devices to date use strained GaN/AlGaN junctions with alloy compositions that are designed to relieve the strain to avoid dislocations that may be responsible for long term instabilities in the devices. Various devices and systems for building heterojunction devices have been proposed to control the lattice mismatch and the strain of the GaN/AlGaN junctions. These devices are particularly designed to take advantage of piezoelectric and spontaneous polarization effects and to minimize long term instabilities.

GaN/AlGaN devices typically have one or more terminals for controlling electrical power flow in a given device. An electrical potential applied to a terminal controls the flow of current in an electrically conductive channel to which the terminal is coupled. The electrically conductive channel is defined by at least one heterointerface between two different semiconductor materials.

When AlGaN/GaN materials compose the semiconductor materials of a heterojunction device, and AlGaN is used as a barrier layer, polarization charges resulting from the spontaneous polarization properties of AlGaN as well as strain induced characteristics known as piezoelectric polarization fields are present. The control of the formation of these fields in the construction of a III-nitride device leads to different characteristics that make GaN-based devices suitable for a wide variety of applications depending upon how the device is characterized.

Heterojunction devices formed with GaN materials sometimes include a barrier layer of AlGaN that is disposed on the channel layer to induce a 2DEG that produces a high concentration of electrons in the channel to thereby enhance the electrically conductive properties of the channel. Due to the presence of the 2DEG formed at the interface of the AlGaN/GaN layers, fundamentally formed III-nitride devices are nominally on because the presence of the channel permits conduction of current between electrodes, for example.

Referring now toFIG. 1A, an abstract view of a heterojunction trench device according to the present invention is illustrated as device100. Device100includes a body region102composed of a III-nitride material, preferably GaN. A number of trenches104-106are formed in body layer102to provide recesses in body layer102. Body layer102with trenches formed in it is overlaid with another III-nitride material layer108, that has a different alloy percentage, lattice constant and bandgap than body layer102. Preferably, material layer108is composed of AlGaN. Layer108covers mesas and sidewalls as well as bottoms of trenches104-106to form a particular geometry for an interface103between layers102and108. In accordance with this geometry, a 2DEG101is formed at horizontal portions of interface103to provide intermittent high mobility conduction channels at interface103. Note that no 2DEG is provided along sidewalls of trenches104-106. Due to the lack of a 2DEG on sidewalls of trenches104-106, no conduction takes place in a vertical or horizontal direction for device100.

Device100is suitable for receiving a gate electrode in trenches104-106. An electrical potential may be applied to these gate electrodes, which induces a 2DEG along the sidewalls of trenches104-106. The induced 2DEG provides a completed conduction channel for device100along interface103, so that device100is capable of conducting current in a horizontal or vertical direction. Due to the nature of the heterointerface103, device100is nominally off because of the absence of a 2DEG along sidewalls of trenches104-106. Owing to the resistant nature of layer102, device100is capable of withstanding high voltages to permit device100to operate well as a power switching device.

Referring now toFIG. 1B, an embodiment of a completed trench device120according to the present invention is illustrated. Device120includes a body layer122, that may be composed of N−GaN. Prior to the formation of trenches124-126in body layer122, a layer129of P type III-nitride material is provided on layer122, atop which a layer123of highly doped N+III-nitride material is deposited. Trenches124-126are then formed in device120that extend through layers123and129and into layer122. Device120is then overlaid with a layer128of III-nitride material that has a different alloy percentage, lattice constant and bandgap than that of III-nitride material layer122. Preferably, layer128is composed of AlGaN. Gates127are formed in trenches124-126, and capped with an insulator/dielectric121. A contact layer115may then be deposited and patterned over device120to form an ohmic contact with layer128. Various types of passivation and processing techniques may be used in conjunction with the formation of contact layer115to provide a good, low resistance ohmic contact for device120. According to a particular embodiment of the present invention, contact115can operate as a source contact in a FET device.

In operation, device120is nominally off when no electrical potential is applied to gate electrodes127. Application of an electric potential to gate electrode127causes the formation of a 2DEG along sidewalls of trenches124-126. In conjunction with the already formed 2DEG at the tops of the mesas and the bottom of trenches124-126. At that point, device120becomes conductive and provides a high current capacity conduction channel that can be oriented horizontally or vertically. Highly doped N+layer123supplies a number of carriers that can enhance the conduction of the channel formed by the 2DEG in the sidewalls of trenches124-126. P type layer129can be designed to have particular characteristics to control such parameters as on resistance and threshold voltage.

Device120can be constructed with the III-nitride devices having any particular geometry. Accordingly, the trenches may be formed as stripes, in grid patterns, or in cellular formats, such as circular, polygonal, square and so forth. In addition, trenches104-106are shown with vertical sidewalls; i.e. normal to a horizontal plane. It should be noted that the sidewalls of trenches104-106can be at any desired angle other than normal. By varying the angle of the orientation of the sidewalls the threshold voltage of the device can be varied.

Referring now toFIGS. 2A-2G, another embodiment according to the present invention is illustrated as a vertical conduction device130. Initial device130may be preformed as a wafer structure upon which further processing steps may be carried out. Device130is provided as an exemplary embodiment of a III-nitride structure formed on an N+substrate132, that may be composed of any suitable material including silicon and silicon carbide, for example. A highly doped III-nitride layer133is provided on substrate132, preferably composed of N+GaN. A lightly doped body layer composed of a III-nitride material is provided over layer133, where body layer134is preferably composed of N−GaN. Another III-nitride layer135is provided over body layer134, where layer135is a P doped III-nitride material, preferably P GaN. A highly doped III-nitride layer136is provided on layer135, where layer136is also preferably composed of N+GaN.

Referring now toFIG. 2B, a trench140is etched into III-nitride device130to pierce layers136and135, and protrude into layer134. Any type of suitable technique for forming trench140may be used, including etching or other processes suitable for use with the III-nitride material system.

Referring now toFIG. 2C, a III-nitride layer142is deposited over the top of device130, and into trench140, and covers sidewalls of trench140. Layer142is composed of a III-nitride material that has a different alloy percentage, lattice constant or bandgap than layers134,135and136. Preferably, layer142is formed with AlGaN. The AlGaN layer forms a heterointerface with layers134-136, and provides a 2DEG at horizontal interface portions144to form a high mobility conduction channel. The deposition of layer142may be achieved according to a number of techniques, including PECVD and sputtering, for example. Optionally, a thin protective layer may be provided atop device130prior to the deposition of layer142to prevent the outdiffusion of nitrogen from the III-nitride material layers134-136during the annealing of ohmic contacts formed in a layer step.

Referring now toFIG. 2D, ohmic contacts142,143are formed atop layer142of device130. The deposition of the ohmic contact material may be achieved according to any known method, and may include an annealing step to decrease the contact resistance of ohmic contacts142,143. Since device130is a vertical conduction device, ohmic contacts142,143act as sources for current input into device130. Current flows through ohmic contacts142,143, along sidewalls of trench140and through layers132-134when device130is conducting.

Referring now toFIG. 2E, an insulator145is deposited atop device130and into trench140, and will act as a gate dielectric for device130. Insulator145is patterned to expose ohmic contacts143,144, while providing a good insulation distance between subsequent gate contact formation and ohmic contacts143,144. Insulator145may be deposited according to any known technique, and may be composed of traditional materials, such as silicon dioxide or silicon nitride, or dielectric materials that exhibit a much higher dielectric constant and high electric field breakdown characteristics, such as zirconium oxide, lanthanoid group oxides, diamond, AlN and piezoelectric as well as pyroelectric materials. Low temperature dielectric deposition techniques may be used with oxide such as GD oxide or MG oxide. The dielectric used may have a high dielectric constant and may reduce the electric field experienced by the gate electrode.

Referring now toFIG. 2F, device130with insulator145deposited is subjected to a process to form a trench146in insulator145and within trench140to permit the deposition of a gate electrode material. Trench146is formed in insulator145to provide insulator material lining trench140, so that insulator145provides for the isolation of the gate contact from layer142. It should be noted that insulator145may be formed initially with a trench146according to the dielectric material used to form insulator145. For example, insulator145may be deposited using TEOS or other processes that provide a dielectric lining in trench140. The dimensions and composition of insulator145may also be modified to produce particular device characteristics, such as a desired on resistance or threshold voltage. Once trench146is suitably prepared, a conductive gate material147is deposited atop insulator145and into trench146. Any particular known technique for depositing gate electrode147may be used, and gate contact147may be composed of various conductive metals or semiconductor material, depending on desired characteristics. In accordance with a special feature of the present invention, the structure represented by device130may be repeated a number of times on a semiconductor die that is formed on a wafer. Accordingly, devices130may be joined together in parallel, or may be isolated from each other and operated independently with different connections to gate electrode147.

Referring now toFIG. 2G, a device150is illustrated and is formed according to the processes used to form device130, and also includes the formation of a drain contact152. Device150includes two gate electrodes147,149that may be operated simultaneously or independently.

Device150operates as a nominally off, enhancement mode device, because no conduction channel is formed between the source and drain due to the absence of a 2DEG at the sidewalls of trenches146aand146b. A conduction channel, or 2DEG, exists along horizontal portions of the interface between layers142and layers136,134. However, because of the geometry of layer142in trenches146a,146b, the 2DEG is interrupted and vertical portions of the interface between layer142and layers134-136.

The application of an electrical potential to gate contacts147,149, induces an invertible channel to locally invert P type material in layer135to an N type conductor, forming a conduction channel between layers134and136along sidewalls of trenches146a,146b. The conduction channel is formed as a 2DEG that connects with the conduction channels formed under source contacts143,144and148, as well as the conduction channels formed under gates147,148. Through the formation of a channel, device150begins to conduct current between source contacts143,144,148and drain contact152. Because of the completed conduction channel along layer142, device150is also capable of conducting current in a horizontal direction, where current may be propagated through device150with a degree of dispersion or balance, permitting greater current densities and throughput.

Referring now toFIGS. 3A and 3B, cellular geometries for devices155and156, respectively are illustrated. Device155is formed according to polygonal cellular geometries, in this case hexagonal, where each cell is closely coupled to a neighboring cell. It should be noted that the term polygonal is meant to refer to a generally polygonal shape, it is not meant to be limited to a strict polygonal interpretation, but may include polygonal shapes with rounded vertices, which may result from diffusion or other manufacturing processes to form device155. The geometry of device155permits gate contacts157to be joined together easily, so that each cell operates in unison to form a device that has a single overall response, i.e., a single connection for each of a source, a drain and a gate contact.

Referring now toFIG. 3B, cellular device156include source contacts that are arranged in a circular geometry, surrounded by circular insulator material, and hexagonally shaped gate contacts159, which may take the shape of polygons with rounded corners or vertices, as discussed above. The circular shaped source contacts may provide advantages with regard to the generation of an electric field or breakdown voltage, and may derive other benefits from ease of manufacturing. It should be apparent that devices155,156are exemplary embodiments of device geometry for a vertical conduction device, and a number of other geometries are readily available. For example, while devices155,156are illustrated as having hexagonal symmetry to form an array of devices, square, rectangular, linear and other alternate geometries are readily available. In addition, while hexagonal and circular devices are shown, linear, oval and other alternative shapes for individual devices is also contemplated to be within the scope of the present invention.

Referring now toFIGS. 4A-4I, another embodiment of the present invention is illustrated as a horizontal conduction device160. Device160is composed of a resistive or insulating substrate162, overlaid by a III-nitride multilayer stack163, and N−III-nitride body layer164, a P III-nitride165and a highly doped N+III-nitride layer166. Layers164-166are substantially similar to the counterpart vertical conduction devices130-150, and layers164-166are capable of conducting current in a vertical direction. The III-nitride material used to construct layers164-166is preferably GaN.

Multilayer stack163is composed of alternating III-nitride materials, where one III-nitride material is alternated with a III-nitride material that has a different in-plane lattice constant. The different in-plane lattice constant of the two alternating III-nitride materials forms a number of conduction channels at the interface between the alternating materials due to the 2DEG at the heterointerface. Accordingly, stack163is capable of carrying large amounts of current that can be delivered through layers164-166.

Referring now toFIG. 4B, device160is illustrated with a trench170formed through the upper layers of device160. Trench170is formed through layers165,166, and protrudes into layer164, and provides an initial structure for the formation of a gate contact. Referring now toFIG. 4C, layers164-166are etched, or otherwise processed to remove a portion of the layers overlying stack163. This processing step may be carried out according to known techniques for manufacturing semiconductor devices, and in particular III-nitride devices. The same is true of the formation of trench170, where various known processes may be used in the formation of the trench structure. During this step, stack163is exposed at a step161, suitable for the formation of contacts discussed in greater detail below. Stack163, composed of alternating III-nitride materials preferably is constructed with alternating layers of GaN and AlGaN. It should be apparent, however, that other III-nitride materials may be employed to construct stack163, including those discussed above.

Referring now toFIG. 4D, a III-nitride material layer172is formed over device160. Layer172is a III-nitride material that has a smaller in-plane lattice constant than material used to form layers164-166. Preferably, layer172is composed of AlGaN. Layer172is deposited to line trench170so that the bottom and sidewalls of trench170are covered by layer172. Because of the different in-plane lattice constants between layer172and layers164-166, a high mobility 2DEG is formed in horizontal portions of the interface between layer172and layers164-166. Accordingly, a 2DEG is formed below the bottom of trench170, and between horizontal portions of layer172and layer166. Layer172may be deposited according to any known method, including PECVD and sputtering. A protective or insulative layer may be deposited over device160prior to the deposition of layer172to protect layers164-166from outdiffusion during high temperature processing that is typically associated with the formation of ohmic contacts, discussed below.

Referring now toFIG. 4E, device160is processed to remove layer172in the vertical and horizontal portions of step161. This operation may be accomplished through well known masking and etching techniques, for example. After the process for removing the portions of layer172near step161, a portion of stack163is exposed at step161for further processing.

Referring now toFIG. 4F, ohmic contacts174sand174dare formed on device160to provide good low resistive contacts for current conduction. Ohmic contacts174sare formed as source contacts atop layer172, while contact174d, formed as a drain contact, is formed on step161. The formation of ohmic contacts174s,174dmay include a high temperature processing technique to obtain low resistive contacts. Prior to the formation of ohmic contacts174s,174d, a protective layer may be formed over device160that tends to reduce or eliminate the outdiffusion of nitrogen from layers163-166during the high temperature processing technique used to form the ohmic contacts. Ohmic contact174s,174dmay be formed according to any known technique suitable for use in the III-nitride material system. Ohmic contact174sand174dmay be composed of ohmic metals or other conductive materials.

Referring now toFIG. 4G, an insulation material layer176is deposited and patterned on device160according to known techniques. Insulator176is deposited into trench170, and will be the basis for the formation of a gate dielectric in later processing steps. Insulator176is also deposited along the horizontal portion of step161to contribute to protecting and passivating layers164-166of device60. Insulator176is also interposed between ohmic contact174dand layer164, so that conduction in device160is provided through stack163, which is capable of carrying high amounts of current.

Referring now toFIG. 4H, insulator176deposited into trench170is etched or otherwise processed to form a trench178within insulator176in trench170. Trench178is formed in insulator176to permit insulator176to line trench170and act as a gate dielectric for device160. After the formation of trench178, a conductive gate material177is deposited into trench178to provide a gate contact. Material177may be any suitable conductive material for operation of the gate electrode, including polysilicon and conductive metals. Gate contact177is operable to induce a 2DEG along sidewalls of trench170when an electric potential is applied to gate contact177. By inducing a 2DEG on the sidewalls of trench170, a conduction channel is formed between source contacts174sand layer164to provide a path for current to stack163and drain contact174d.

Referring now toFIG. 4I, a completed horizontal conduction III-nitride device180is illustrated with two gate electrodes182,184, in trenches186b,186a, respectively. Device180permits high current conduction from source contacts174sto drain contact174dwhen gates182,184are activated to induce a conduction channel along the sidewalls of trenches186a,186b. It should be apparent that gate electrodes182,184may be operated independently of each other, or may be timed together to a common electrode. Accordingly, gate contacts182,184may be operated independently to form workall conduction channels in only portions of device180. However, when gate electrodes182,184are operated in conjunction with each other, a 2DEG conduction channel is formed over the circuits of layer172, permitting horizontal current flow between layers164-166, which permits the conductive current to be balanced and achieve a higher current density. In operation, current in device1800flows into source electrode174sinto highly doped layer166, along sidewalls of trenches186a,186band into body layer164. The conduction channels along the sidewalls of trenches186a,186bare induced when an electric potential is applied to the gate contacts182,184. The number of source and gate contacts per cell in device180depends upon the conductivity of multilayer stack163, related to current carrying capability. Multilayer stack163includes alternating III-nitride materials to form multiple 2DEG conduction channels for high current capacity. Device180may be completed according to known techniques for cladding, insulation or protective layers, including the formation of source, drain and gate electrodes and contacts.

Referring now toFIGS. 5A-5B, plan views of multi cellular devices185,186, are illustrated with different cellular geometries. Devices185,186are formed with insulating or resistive substrates, and conduct in a lateral or horizontal direction, rather than vertically. Device185has cellular components in the form of polygons, such as hexagons, that are equilateral or symmetrical in shape. Devices185,186include gate electrodes184,187respectively, that may be operated independently, or in conjunction with each other. Device190also includes drain electrodes193that surround the perimeter of each of the cells of device190. Accordingly, drain contacts193may be all connected together. Device190also includes source contacts192that may also be connected together. Accordingly, device190can be formed with a high density cellular structure according to the hexagonal geometry illustrated.

Referring toFIG. 5B, gate electrodes197are provided in a circular geometry, surrounded by insulator material198, also in a circular geometry. Source electrodes199are also circular in nature, while drain contacts195are in the shape of polygons, in particular hexagons. The geometry of the cellular portions of device196can contribute to reduced electric field potential at particular points to improve the device breakdown voltage and other device characteristics. It should be apparent that a number of geometries may be used for the array of cells in the disclosed devices, including square, rectangular, linear and so forth. In addition, the cellular portions of devices190,196may be formed in hexagonal, circular, linear, oval and any other particular geometrical shape depending on the desired device characteristics.

The construction of ohmic contacts, schottky contacts, insulator layers and metallized contacts may be performed according to known techniques. In addition, passivation layers and cladding may be applied to the devices described herein, as well as techniques for forming contacts to current carrying electrodes and gates to provide a finished device.

In addition, the electrodes described herein may be formed with a low resistive ohmic contact process that further improves the operational characteristics of the described devices.

Referring now toFIG. 6, a portion of the cross-section of a vertical conduction device200is shown. Device200is a field effect device, with a gate202, a source204and a drain206. Device200also includes a gate dielectric208that provides insulation from the remainder of the device, while permitting gate202to affect the semiconductor material surrounding it to form an invertible channel when an electric potential is applied to gate202. Device200is a III-nitride device that includes a conductive substrate201, a body layer203, a blocking layer205and a highly doped conducting layer207. Device200is similar in structure and operation130,150,160and180as described above. Accordingly, substrate201is a highly doped N+substrate, while layer203is a lightly doped N−layer. Layer205is a P-type material, while layer207is a highly doped N-type material. In an exemplary embodiment, layer201has a dopant concentration of 5×1018cm−3, layer203has a dopant concentration of 4.7×1016cm−3, layer205has a dopant concentration of 5×1017cm−3and layer207has a dopant concentration of 5×1018cm−3. In this exemplary embodiment, layer201is approximately 10 microns, layer203is approximately 4.5 microns, layer205is 0.4 microns and layer207is 0.5 microns. The gate dielectric layer208separating the gate from the remainder of device206may have an exemplary thickness of from approximately 0.1 to approximately 0.75 microns.

Due to the large dielectric breakdown field of greater than 2.2 MV/cm in III-nitride semiconductors, devices fabricated in the III-nitride material system are extremely useful for power control. However, III-nitride semiconductors do not have a native oxide that is suitable as a gate dielectric material. It is well known that III-nitride semiconductor materials do not behave according to the well developed models provided for Si, GaAs, InP and other thorough investigated materials. However, it is possible to construct III-nitride materials according to particular techniques to obtain a surface that is self passivating, where the surface interface is nearly ideal. The interface properties of III-nitride devices play a critical role in the electrical behavior of devices fabricated in the III-nitride material system. One particular type of device that can take advantage of the characteristics of III-nitride semiconductor materials is a MISFET, where the behavior of the device is determined by the interaction of the semiconductor material with the gate dielectric. Due to the self passivating effect of the III-nitride material, a material used as a gate dielectric is decoupled from the semiconductor material, thereby allowing a wide choice of materials for the gate dielectric. A MISFET device may also take advantage of the large band gap of III-nitride materials to diminish the effects of surface and fixed charges that would otherwise have a significant impact in a corresponding silicon device.

In prior known devices, gate dielectrics composed of silicon dioxide or silicon nitride were commonly used for silicon based devices. However, due to the large dielectric breakdown field of greater than 2.2 MV/cm that III-nitride semiconductors are capable of withstanding, gate dielectric material with greater withstanding properties are indicated. That is, conventional dielectrics are apt to rupture or breakdown when exposed to the high electric fields produced in the III-nitride semiconductor devices contemplated according to the present invention. Accordingly, alternate materials for gate dielectric layer208are considered in accordance with the present invention. Some examples of gate dielectric materials that may be advantageous in accordance with the present invention are zirconium oxide, lanthanoid group oxides, diamond, AlN, piezoelectric and pyroelectric materials, in addition to electret materials that can withstand the higher electric fields generated in the III-nitride semiconductor devices in accordance with the present invention. According to a feature of the present invention, trench surface layers203,205and207may be self passivated to decouple the dielectric material from the III-nitride semiconductor material, to reduce the electric field that the gate dielectric material is exposed to.

According to a feature of the invention, gate dielectric208is composed of a material that is deposited using a low temperature process, such as Gd oxide or Mg oxide, where layers203,205and207are self passivated.

Referring now toFIG. 7, a lateral III-nitride device210is illustrated, with an invertible channel212and a standoff region214. Ohmic contacts215,216provide the conduction points for permitting current to flow into and out of device210. Contacts215,216are disposed atop highly doped N+regions217,218, respectively, which are composed of a III-nitride material, preferably GaN. Standoff region214includes an N-type doping of a III-nitride material, preferably GaN. A gate electrode211resides over channel212, separated from channel212from gate dielectric layer213. As with device200(FIG. 6), device210experiences high electric fields when blocking voltage, that is, when gate electrode211is not activated. In this instance, channel212prevents conduction of current between contacts215,216and a large electric field is generated between contacts215,216and gate electrode211. Under these circumstances, gate dielectric213must be capable of withstanding the high electric field and prevent or substantially reduce any leakage current through gate electrode211. Accordingly, gate dielectric213is composed of a material that is capable of withstanding high electric fields without breakdown, as discussed above, and may be provided over a passivated surface of regions212,214,217and218. By providing a passivated surface in these regions, the electric field experienced by gate dielectric213can be diminished to a level where lower dielectric breakdown materials may be used. This advantageous feature of the present invention permits the use of well known dielectric materials that have a long and well developed history of use in the manufacture of semiconductor devices. The use of these known materials implies that cost can be reduced while a robust device can be produced.

Referring now toFIGS. 8A-8E, the construction of a III-nitride device220in accordance with the present invention is illustrated. Device220is a vertical conduction device with a conductive substrate222that is composed of a highly doped N+semiconductor material, which may include silicon, silicon carbide and so forth. An N-type epi layer223is optionally grown over substrate222, and a lightly doped N-type epi layer224is formed atop layer223. Another P-type epi layer225is formed over layer224, and a heavily doped N-type epi layer is formed over layer225. Layers222-226form the basis of a wafer that can be manufactured according to techniques developed for the processing and manufacturing of III-nitride semiconductor materials. Accordingly,FIG. 8Aillustrates a useful starting point for the manufacture of specific III-nitride devices.

Referring now toFIG. 8B, device220is processed for the formation of contacts221and227, where the contact material is deposited and then annealed to form a low resistance contact. Following the formation of contacts221,227, device220is patterned and etched, or otherwise processed, to form steps228, which can form a portion of a trench if device220is to be constructed as a trench device. The formation of contacts221and227may be brought about by any known method using suitable materials for conduction of large amounts of current. Step228may be formed according to any known method, and may be formed to protrude into layer224, in addition to passing through layers225-227.

Referring now toFIG. 8C, device220undergoes a passivation step in accordance with the present invention to permit layers224-226to be decoupled from a gate dielectric deposited in step228. The passivation process results in exposed surfaces of layers224-226being close to ideal for the formation of an interface with another material, such as a gate dielectric.

Referring now toFIG. 8D, a gate dielectric229is deposited in step228, and optionally patterned or processed to have a specific thickness or density to obtain particular characteristics. For example, gate dielectric229may be formed as a block of dielectric material deposited in the entire area of step228, and then patterned and etched to leave dielectric material229along the walls of layers225-226and the top of layer224in the location of step228. Any particular type of process may be used to form gate dielectric229, and the materials used may be any of those discussed above, or other dielectric material that have a suitably high breakdown value or dielectric constant to permit device220to withstand high voltages without gate dielectric breakdown, in conjunction with passivated surfaces of layers224-226.

Referring now toFIG. 8E, a gate contact is deposited adjoining gate dielectric229to form a field effect device. Gate material230may be formed of conductive material including metals and doped semiconductor material. Any known type of process may be employed to form gate contact230, including masking and deposition according to a number of techniques.

Device220operates with the presence or absence of an electrical potential on gate contact230. When an electrical potential is applied to gate contact230, the resulting electric field induces a conduction channel through layer225to provide a conduction path from contact227to contact221. In the absence of an electric potential on gate contact230, no electric field is generated, and no conduction channel is formed, permitting device220to stand off significant voltage. As the voltage device220stands off increases, a large electric potential is developed between gate contact230and contact227, for example. The electric potential can increase significantly due to the ability of the III-nitride materials in layers222-226to withstand high voltages. However, gate dielectric229must be able to provide a high level of isolation, or exhibit a high breakdown voltage value to obtain the best performance possible with device220. By subjecting device220to a passivation for layers224-226, a newly ideal interface is formed between gate dielectric229and III-nitride layers224-226. Accordingly, gate dielectric229is decoupled from layers224-226, and is not subjected to the high electric fields that may cause dielectric229to break down. The self passivated materials of layers224-226bear the electric field potential, and serve to disperse or balance high electric field to prevent gate dielectric229from being overstressed.

In addition, gate229may be composed of materials that exhibit a high dielectric breakdown, such as those discussed above, to further improve the performance of device220. The selection of dielectric material229relates to the breakdown voltage device220may achieve, as well as the threshold voltage, speed and power rating of device220.

Referring now toFIGS. 9A-9G, the formation of a III-nitride lateral, or horizontal, conduction device240is illustrated. Device240includes a substrate241and an epi layer that is doped according to the type of device desired. For example, as with all the devices discussed in the present disclosure, N or P channel type devices may be constructed in accordance with the structures of the present invention using III-nitride semiconductor materials. Device240is illustrated and this exemplary embodiment as a N channel type device, and that may be enhancement or depletion mode in operation. Referring now toFIG. 9B, a top surface242of device240is first provided with surface protection to prevent damage to layer242during subsequent processing. A surface protection may take the form of the deposition of a thin, nitrogen rich layer that prevents the outdiffusion of nitrogen from layer242, for example.

After surface243is protected, a dielectric material244is deposited and patterned to form a gate dielectric for device240. Referring now toFIG. 9C, N+regions245,246are formed according to known techniques, such as ion implantation or diffusion. During the formation of regions245,246, gate dielectric244may act as a mask to permit the patterning of regions245,246without additional processing.

Referring now toFIG. 9D, contacts247,248are formed over regions245,246, and may be in the form of ohmic contacts that are deposited, patterned and annealed to form a low resistance contact for device240. Contacts247,248may be referred to interchangeably as source and drain contacts, or simply ohmic contacts.

Referring now toFIG. 9E, dielectric material244is removed after a further surface protection step to prevent damage to layer242or regions245,246. Referring now toFIG. 9F, the resulting device with layer242and regions245,246exposed is then processed to passivate device240to provide a near ideal surface243referred to above as self passivated. This nearly ideal surface243permits layer242and regions245,246to be decoupled from a later formed gate dielectric, to permit lower dielectric constant materials to be used as gate dielectrics. After the passivation process, gate dielectric249is deposited over device240, and patterned and processed to provide a dielectric layer between contacts247and248. Gate dielectric249may be composed of any material suitable for withstanding a high electric field and providing a high voltage breakdown, as discussed above. The passivation step applied to surface243results in lower electric fields experienced by gate dielectric249, so that lower dielectric constant materials may be used.

Referring now toFIG. 9G, device240is functionally completed with a gate contact250deposited over gate dielectric249, and isolated from contacts247,248. In operation, device240conducts current or blocks voltage depending upon whether an electric potential is applied to gate contact250. When no electric potential is applied to gate250, no conductive channel is formed between regions245and246, preventing conduction in device240, and permitting device240to withstand high voltages. When an electric potential is applied to gate contact250, a conductive channel is formed in P-type layer242from region245to region246, to permit current flow between contacts247and248. Note that device240is a bidirectional device.

Due to the high voltages that device240withstands when blocking voltage, dielectric249must be capable of withstanding high electric fields without breakdown. During voltage blocking operation, device240develops potentially high electric fields and electric potentials between gate contact250and contacts247,248. These high electric fields can rise to the level of dielectric breakdown with respect to dielectric249due to the nature of the blocking capability of the III-nitride semiconductor materials. However, due to the passivation of surface243prior to the deposition of gate dielectric249, gate dielectric249is decoupled from the III-nitride materials composing regions245,246and layer242. Accordingly, device240is able to obtain better performance using conventional gate dielectric material for a gate dielectric249than is possible with conventional devices where self passivation is not available.