Abstract:
A high electron mobility field effect transistor (HEMT) includes a two dimensional electron gas (2DEG) in the drift region between the gate and the drain that has a non-uniform lateral 2DEG distribution that increases in a direction in the drift region from the gate to the drain.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of U.S. patent application No. 13,478,402, filed on May 23,2012, which is incorporated herein as though set forth in full. This application is related to U.S. patent application Ser. No. 13/478,609 , filed on May 23, 2012 and entitled “HEMT GaN Device with a Non-Uniform Lateral Two-Dimensional Electron Gas Profile and Process for Manufacturing the Same” and to U.S. patent application Ser. No. 13,479,018, filed May 23, 2012 and entitled “Non-Uniform Two-Dimensional Electron Gas Profile in III-Nitride HEMT Devices” 
    
    
     TECHNICAL FIELD 
     This disclosure relates to type III—nitride HEMT devices and in particular to two dimensional electron gas (2DEG) in the drift region. 
     BACKGROUND 
     A high electron mobility transistor (HEMT) is a field effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction). Gallium nitride (GaN) HEMTs have attracted attention due to their high-power performance. In type III-nitride HEMT devices used in power applications there is a design trade-off between the on-state resistance and breakdown voltage (BV). Since the relation between the BV and on resistance is at least quadratic, improvement in the BV for a given drift region length results in a significant improvement in the FOM of the device, defined as BV 2 /Ron. 
     In the prior art type III-nitride HEMT devices have a uniform 2DEG density which results in a peak electric field under or near the gate region. The electric field distribution tends to be closer to a triangular shape than to the desired trapezoidal shape which reduces the breakdown voltage per unit drift region length of the device. The use of field plate and multistep field plates are some of the techniques that are used to improve the electric field distribution. However, field plates typically result in multiple peaks and suffer from less than ideal flat field distribution, and may exhibit a saw tooth profile. Field plates also add to the gate to drain capacitance. In addition, process complexity and cost typically increase with the number of field plate steps. 
     U.S. Pat. No. 7,038,253 to Furukawa describes a GaN based device on silicon (Si) technology which uses a uniform 2DEG profile in the drift region. Because of the absence of any field shaping technique in the Furukawa device, the breakdown voltage and dynamic on resistance from drain to source is limited by a localized increase in the electric field under the gate region thus requiring over design of the device which degrades the figure of merit (FOM) that such a device can achieve. 
     In “High Breakdown Voltage AlGaN/GaN HEMTs Achieved by Multiple Field Plates” by H. Xing et. Al, a field shaping technique that uses multiple field plates is described to improve the electric field distribution. However, multiple field plates do not achieve a uniform electric field, may have a saw tooth type distribution, and introduce gate to drain capacitance. Implementing such a device structure also increases device complexity and cost. 
     What is needed is a significant improvement in the FOM in type III nitride HEMT devices, and in particular an improvement in the breakdown voltage for a given drift region length, so that the FOM of the device, defined as BV 2 /Ron, improves. The embodiments of the present disclosure answer these and other needs. 
     SUMMARY 
     In a first embodiment disclosed herein, a high electron mobility field effect transistor (HEMT) comprises a two dimensional electron gas (2DEG) in the drift region between the gate and the drain that has a non-uniform lateral 2DEG distribution that increases in a direction in the drift region from the gate to the drain. 
     In another embodiment disclosed herein, a high electron mobility field effect transistor (HEMT) comprises lattice damage in a drift region of a carrier supply layer between a gate and a drain, wherein the lattice damage decreases in a direction in the drift region from the gate to the drain. 
     In yet another embodiment disclosed herein, a method of fabricating a high electron mobility field effect transistor (HEMT), the method comprises forming a channel carrier traveling layer on a substrate, forming a carrier supply layer on the channel carrier traveling layer, forming a mask layer on the carrier supply layer, the mask layer configured to be aligned with a drift region from a gate to a drain, and configured to have a lateral variation in a direction from the gate to the drain, and implanting ions through the mask layer into the carrier supply layer. 
     These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the use of a gray scale mask to control ion implantation to be tapered in a drift region in accordance with the present disclosure; 
         FIG. 2  shows a tapered two dimensional electron gas (2DEG) charge density in a type III Nitride device in accordance with the present disclosure; and 
         FIGS. 3A-3C  are flow diagrams for methods of fabricating a HEMT device in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention. 
     Referring now to  FIG. 1 , a field effect transistor (FET) device structure  10  is shown. The FET device structure  10  is composed of a stack of III-V layers, such as GaN layer  14  and AlGaN layer  16 , grown on a substrate  12  that can be any of the suitable substrates that are commonly used to grow type III-nitride materials. Suitable substrates include but are not limited to silicon (Si), Sapphire, silicon carbide (SiC), and bulk single crystal gallium nitride (GaN). 
     The stack of III-V layers may include a buffer layer of GaN or aluminum gallium nitride (AlGaN) grown on the substrate  12 . Then a channel layer also known as a channel carrier travelling layer, such as GaN layer  14 , is grown on the buffer layer. Then a barrier layer also known as a carrier supplying layer, such as AlGaN layer  16 , is grown on top of the GaN layer  14 . An AlN spacer layer may be between the GaN layer  14  and the AlGaN layer  16  to improve device electrical performance. 
     On top of the AlGaN layer  16  a suitable masking layer  50 , which may be Si 3 N 4 , is grown. The masking layer  50  is used as a masking layer to stop the majority of the ions implanted via ion implantation  52  from reaching the AlGaN layer  16 . Only a small fraction of the implanted ions, the tail of the Gaussian distribution, are intended to reach the AlGaN layer  16  to cause damage to the lattice. The small fraction of ions that succeed in reaching the AlGaN layer  16  ideally do not penetrate deep into the AlGaN layer  16 . 
     Further, the masking layer  50  is configured to vary the density of ions implanted along the drift region between a gate and a drain of a field effect transistor (FET). A mask layer  50  may be used, as shown in  FIG. 1 , to form a tapered mask layer  60  in the drift region between points  62  and  64 . The tapered mask layer  60  has a lateral profile and has a height that increases towards the drain. In another embodiment a mask may be used that has with various size openings to vary the density of ions implanted along the drift region. Either type of mask modulates the ion implantation between points  62  and  64 , such that lattice damage due to ion implantation in the AlGaN layer  16  is greater at the point  62 , near the edge of gate region  22 , as shown in  FIG. 2 , and less at point  64  along the drift region towards the drain  20 , as shown in  FIG. 2 . The mask layer  50  may be configured to provide a lattice damage that linearly increases from point  64 , along the drift region from near the drain  20 , to point  62  near the gate  22 . After ion implantation the masking layer is etched and removed. 
     The source contact  18  and drain contact  20  shown in  FIG. 2  may be formed by metal evaporation or metal sputtering. Then a passivation layer  24  may be deposited between the source  18  and the drain  20 . 
     A gate region is then formed by etching through the passivation layer  24  in a gate area between the source  18  and drain  20  and into the AlGaN layer  16 . In another embodiment the etch may extend through the AlGaN layer  16  and partially into the GaN layer  14  to an appropriate depth. A gate dielectric  26  is then deposited over the area between the source  18  and gate  22  and the gate  22  and the drain  20 , and also deposited to line the etched trench that extends into the AlGaN layer  16 . If the etched trench extends into the GaN layer  14 , then the gate dielectric  26  also lines the etched trench that extends into the GaN layer  14 . 
     After deposition of the gate dielectric  26 , gate metal  22  is formed by evaporation or sputtering and fills the etched trench. 
     Various alternating passivation and metallization layers may be formed as a part of back-end processing to improve the parasitic resistance of the device and provide connection to device pads and/or a package. 
     The use of the mask layer  50  with tapered mask layer  60  or in another embodiment a mask with various size openings to control ion implantation and thereby the distribution of lattice damage in the drift region of the AlGaN layer  14  provides a significant improvement of the figure of merit (FOM) in type III Nitride HEMT devices by achieving flat electric field distribution in the drift region between the gate  22  and the drain  20 . By controlling the ion implantation and thereby the lattice damage in the drift region from the gate to the drain, the 2DEG  42  is varied in the drift region to form a non-uniform lateral 2DEG distribution  44 . As shown in the embodiment of  FIG. 2 , the 2DEG increases in the drift region in the direction from the gate  22  towards the drain  20 . A flat electric field distribution results from the non-uniform lateral 2DEG distribution  44  along the drift region which provides the improvement in the figure of merit (FOM). 
     Implementing a non-uniform lateral 2DEG profile  44  along the drift region by causing tapered lattice damage to the carrier supplying layer, such as AlGaN layer  16 , controls the level of damage or stress in that layer. The lateral control of the damage in the carrier supplying layer is achieved by means of ion implantation of a suitable ion specie through a masked layer, such as mask layer  50 , that has a tapered profile, where the vertical height of the masking layer determines the stopping power of the implanted projectiles and hence their projected range. The tapered profile of the masking layer may be produced by gray scale photolithography followed by an etch step. 
     Alternatively the stress in the AlGaN layer  16  may be varied by opening windows in the photo resist with varying size where the size of the opening is a function of the lateral distance from the gate to the drain. The size of the openings may be larger or smaller in the drift region near the gate and decrease or increase, respectively, in the drift region in the direction of the drain. 
     Since the density of charge in the 2DEG region is determined locally by the magnitude of the damage induced by ion implantation, a non-uniform 2DEG distribution  44  is achieved by varying the lattice damage laterally over the drift region. If the lattice damage caused by ion implantation is reduced as a function of a distance from the gate region along the drift region by increasing the height of the mask layer  50  or by reducing the size of openings in the mask, the 2DEG  44  density increases as a function of distance from the gate region along the drift region, as shown in  FIG. 2 . 
       FIGS. 3A-3C  are flow diagrams for methods of fabricating a HEMT a type III Nitride device in accordance with the present disclosure. 
     In step  100  a channel carrier traveling layer  14  is formed on a substrate  12 . Then in step  102  a carrier supply layer  16  is formed on the channel carrier traveling layer  14 . In an embodiment, the layers  14  and  16  are formed by an epi manufacturer. 
     Next in step  104  a mask layer  50  is formed on the carrier supply layer  16 . The mask layer is configured to be aligned with a drift region from a gate to a drain, and configured to be have a lateral variation in a direction from the gate to the drain. Then ions  52  are implanted through the mask layer  50  into the carrier supply layer  16 . 
     In one embodiment the mask layer is formed in step  108  by forming a tapered section on the mask layer that has a thickness that increases in the direction from the gate to the drain by using gray scale photolithography and then in step  110  etching the mask layer to form the tapered section. 
     In another embodiment the mask layer is formed by coating the carrier supply layer with photoresist in step  112  and then in step  114  opening windows in the photoresist of varying size such that the size of the openings decrease in the direction from the gate to the drain. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein. 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . . ”