Abstract:
A HEMT device has a substrate; a buffer layer disposed above the substrate; a carrier supplying layer disposed above the buffer layer; a gate element penetrating the carrier supplying layer; and a drain element disposed on the carrier supplying layer. The carrier supplying layer has a non-uniform thickness between the gate element and the drain element, the carrier supplying layer having a relatively greater thickness adjacent the drain element and a relatively thinner thickness adjacent the gate element. A non-uniform two-dimensional electron gas conduction channel is formed in the carrier supplying layer, the two-dimensional electron gas conduction channel having a non-uniform profile between the gate and drain elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. patent application Ser. No. ______ filed on the same date as this application and entitled “Controlling Lateral Two-Dimensional Electron Gas HEMT in III-Nitride Devices Using Ion Implantation Through Gray Scale Mask” (attorney docket no. 626621-4) and to U.S. patent application Ser. No. ______ filed on the same date as this application and entitled “HEMT GaN Device with on Uniform Lateral Two-Dimensional Electron Gas Profile and Process for Manufacturing the Same” (attorney docket no. 626622-2). 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to III-Nitride HEMT devices having a non-uniform Two-Dimensional Electron Gas (2DEG) profile in the carrier supplying layer thereof. 
       BACKGROUND 
       [0003]    III-Nitride High Electron Mobility Transistor (HEMT) devices are often used in power applications and/or high temperature applications in RF circuits and in other applications, including in power supplies for electrically powered motor vehicles. 
         [0004]    A design trade-off between the on-state resistance (R on ) and breakdown voltage (BV) of a HEMT can be improved significantly by following the teachings contained herein. Since the relation between the BV and R on  is at least quadratic, improvement in the BV for a given drift 
         [0005]    region length results in a significant improvement in the figure of merit (FOM) of the device, defined as BV 2 /R on . 
         [0006]    HEMTs utilize two semiconductor materials with different band-gaps, forming an electron potential well at a heterointerface between the two semiconductor materials, which materials might be, for example, AlGaN and GaN. The potential well confines electrons and defines a two-dimensional electron gas (2DEG) conduction channel. Due to the two-dimensional nature of the electrons in the conduction channel, the carrier mobility is enhanced. 
         [0007]    Prior art III-Nitride HEMTs utilize 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 a more desirable trapezoidal shape which reduces the BV per unit drift region length of the device. The use of a field plate and/or multi step field plates are some of the techniques that are used in the prior art to improve the electric field distribution but these techniques typically result in multiple peaks and suffer from less than ideal flat field distribution (they can exhibit a saw tooth type profile) which also adds to the gate to drain capacitance. In addition, process complexity and cost typically increase with the number of field plate steps (levels) utilized. 
         [0008]    The prior art includes: 
         [0009]    Furukawa, U.S. Pat. No. 7,038,253 issued on May 2, 2006 discloses a GaN based device that represents state of the art GaN on Si technology which uses a uniform 2DEG profile in the drift region. In the absence of any field shaping technique it is expected that the breakdown and dynamic Rdson performance of the device of this patent will be 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 can be achieved by such a structure. 
         [0010]    H. Xing et al. have proposed a device structure that was published in a paper entitled “High Breakdown Voltage AlGaN/GaN HEMTs achieved by Multiple Field Plates”, (see H. Xing, Y. Dora, A. Chini, S. Hikman, S. Keller and U. K. Mishra, “High Breakdown Voltage AlGaN—GaN HEMTs Achieved by Multiple Field Plates,” IEEE Electron Device Letters, IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 4, pp. 161-163, April 2004), which utilizes a field shaping technique that used multiple field plates to improve the electric field distribution, however, this technique is less favorable than the technology disclosed herein since multiple field plates will not achieve a uniform electric field (will have a saw tooth type distribution) and will increase the gate to drain capacitance. In implementing such structure increases device complexity and cost. 
         [0011]    C. M. Waits, R. Ghodssi, and M. Dubey, “Gray-Scale Lithography for MEMS Applications”, University of Maryland, Department of Electrical and Computer Engineering, Institute for Advanced Computer Studies, College Park, Md., USA, 2006. 
         [0012]    W. Henke, W. Hoppe, H. J. Quenzer, P. Staudt-Fischbach and B. Wagner, “Simulation and experimental study of gray-tone lithography for the fabrication of arbitrarily shaped surfaces,” Proc. IEEE Micro Electro Mechanical Syst. MEMS 1994, Oiso, Japan, pp. 205-210. 
         [0013]    C. M. Waits, R. Ghodssi, M. H. Ervin, M. Dubey, “MEMS-based Gray-Scale Lithography,” International Semiconductor Device Research Symposium (ISDRS), Dec. 5-7, 2001, Washington D.C. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0014]    The invention is concerned with a device structure and a method of implementing a non-uniform two dimensional electron gas profile between the gate and drain electrodes. By implementing a tapered AlGaN layer (charge supplying layer) from the gate to the drain, one can obtain a monotonically increasing 2DEG profile that results in a uniform electric field distribution hence maximizing the FOM of the device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1   a  is a schematic presentation of charge density before tapering the carrier supplying layer (preferably AlGaN), hence the uniform 2DEG density (depth is not drawn to scale) of a partially formed HMET device; 
           [0016]      FIG. 1   b  is a schematic presentation of charge density after tapering the carrier supplying layer (preferably AlGaN) hence the non-uniform 2DEG density (depth is not drawn to scale) of the partially formed HMET device of  FIG. 1   a  after the AlGaN layer has been tapered; 
           [0017]      FIG. 1   c  is similar to  FIG. 1   b , but depicts the partially formed HMET device of  FIG. 1   b  in a perspective view as opposed to an elevational view thereof; 
           [0018]      FIG. 2  is a schematic presentation of charge density (depth is not drawn to scale) of the preferred embodiment of a completed HMET device of  FIG. 1   a  after the carrier supplying layer (preferably AlGaN) has been tapered in the region between the gate and the drain; 
           [0019]      FIG. 3  depicts are alternative method of creating a taper in the carrier supplying layer (preferably AlGaN); 
           [0020]      FIG. 4  depicts both the electric field distribution (which is flat) and the tapered 2DEG density for the device of  FIG. 2 ; and 
           [0021]      FIG. 5  depicts both the electric field distribution (with a triangular profile) and a flat 2DEG density for prior art HEMT devices having zero, one and two field plates. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    As depicted by  FIGS. 1   a ,  1   b ,  1   c  and  2 , and in order to improve the electric field distribution along the drift region  26  (the region between the gate  30  and the drain  34 ) and hence improve the breakdown voltage capability per unit drift region length of a HMET device  8 , a non-uniform lateral 2DEG profile  12  along the drift region in layer  18  is provided, in one preferred embodiment, by monotonically tapering the profile (thickness or z-direction) of a carrier supplying layer  16  alone the x-direction between the gate  30  and drain  34 . The taper is identified by numeral  14  in  FIGS. 1   b  and  1   c . The carrier supplying layer  16  may be, for example, AlGaN (but not limited to) in an AlGaN/GaN HEMT. Monotonically tapering the profile (thickness) of the carrier supplying layer  16  effectively creates a non-uniform profile of two dimensional electron gas (2DEG)  12  where the 2 DEG density increases with the increasing thickness of carrier supplying layer  16 . 
         [0023]      FIG. 1   a  shows HMET device  8  in the process of being fabricated. The gate, source and drain electrodes of the device  8  have not yet been formed. The device  8  in the process of being fabricated is comprised in this figure of a stack of III-V layers, preferably grown on a substrate  10 . Substrate  10  may be any of the suitable substrates that are commonly used to grow III-Nitride materials, for example Si, Sapphire, SiC, bulk single crystal GaN, and others. As can be seen in  FIG. 1   a , substrate  10  provides a supporting surface for a layer  18  of GaN material in one embodiment (but other materials such as AlGaN may be used instead for layer  18 ) or a superlattice formed of alternating layers (e.g. alternating AlGaN/GaN or alternating AlN/GaN) may be used instead for layer  18  or a combination of C-doped GaN buffer and AlGaN back barrier may be used instead for layer  18  or any combination of the aforementioned in still other embodiments may prove to be suitable for layer  18 , which layer functions as a buffer layer in HMET device  8 . 
         [0024]    A carrier supplying layer  16  is preferably formed of AlGaN material and preferably with a suitable Al mole fraction that typically ranges between 20 to 30%, and is grown or otherwise formed on buffer layer  18 .  FIG. 1   a  also shows a layer of photoresist  22  disposed on the carrier supplying layer  16 , which layer  22  has been photolithographically processed, preferably by gray scale lithography, to allow a triangularly shaped wedge (when viewed in cross section) portion  24  to be etched away from the photoresist layer  22 . The layer of photoresist  22  and its triangularly shaped wedge portion  24  is then removed during a subsequent RIE etch process which transfers the wedge pattern  24  from the photoresist  22  and into the carrier supplying layer  16  to thereby define taper  14  therein (see  FIGS. 1   b  and/or  1   c ). The process is preferably optimized so that photoresist  22  remains in the area where the carrier supplying layer  16  layer is preferably kept intact outside the taper area  14  so that its thickness is preferably not decreased outside of the taper or wedge region  14  by the aforementioned RIE etch. Any remaining photoresist  22  may thereafter be removed with a suitable chemical etchant. 
         [0025]    The thicker the carrier supplying layer  16  in a given position in the x-direction along the drift region  26  (see  FIG. 2 ) where x is the horizontal direction from the edge of gate  30  (facing the drain, where x=0) towards the drain  34  (where x=LD at the edge of drain  34  facing the gate  30 ). The taper  14  in carrier supplying layer  16  (see  FIGS. 1   b  and/or  1   c ) is preferably produced by gray scale photolithography of the photoresist layer  22  (to remove the photoresist wedge portion  24  therefrom) as mentioned above. This process is followed by the aforementioned controlled RIE where initially the remaining photoresist  22  is removed in the area of the wedge pattern  24  and eventually is completely removed either as the RIE process progresses or by the chemical etch mentioned above. The carrier supplying layer  16  under the thinner part of the wedge pattern  24  experiences a longer RIE etch time than carrier supplying layer  16  under a thicker part of the wedge pattern  24 , resulting in a profile transfer from the photoresist wedge pattern  24  to the carrier supplying layer  16 . The carrier supplying layer&#39;s thickness is preferably uniform in a lateral direction along the lateral extents of the gate or drain regions (along the y-direction of  FIG. 1   c ). 
         [0026]    An alternative method for forming such a tapered pattern or wedge  14  in the carrier supplying layer  16  is shown in  FIG. 3 . This alternative method involves opening windows  24 ′ in the photoresist layer  22  with varying sizes where the size of the opening is a function of the lateral distance from where the gate will be formed to where the drain will be formed (the pattern is similar to that typically used in Gray scale lithography). Since the photoresist  22  is completely removed in the open windows  24 ′, the loading effect of the RIE etch will result in a faster etch rate in larger photoresist window openings than in smaller window openings hence implementing a taper  14  in the carrier supplying layer  16  as depicted by  FIGS. 1   b  and  1   c . The mask pattern in this alternative method is similar to a conventional gray scale mask, but rather than relying on the intensity of light for making different openings to create a tapered profile  24  in the photoresist  22 , this alternative method relies on the loading effect of the RIE etch process to etch more of the carrier supplying layer  16  in the wider open windows. 
         [0027]    Irrespective of which method is used to form the taper  14 , the taper  14  ends at a step  20  (see  FIG. 1   b  or  1   c ) back to the normal height of layer  16  near where the gate  30  will be formed. The taper  14  smoothly ends where the carrier supplying layer  16  returns to its otherwise normal height at the other end  38  of the wedge or taper  14  near (and preferably immediately adjacent) where the drain  34  will be formed. The gate  30 , drain  34  and a passivation layer  28  with eventually occupy this region as shown in  FIG. 2 .  FIG. 1   b  illustrates that the taper  14  varies in the x-direction.  FIG. 1   c  illustrates that the taper  14  preferably does not vary in the y-direction. 
         [0028]    After the tapering of the AlGaN layer  16  is completed preferably using the techniques discussed above with respect to either  FIGS. 1   a  and  1   b  or  FIG. 3 , ohmic contacts  36  and  34  to the source and drain 2DEG regions are formed preferably using a stack of metal lift off followed by an Rapid Thermal Anneal (RTA) treatment. Thereafter a dielectric  28  is deposited at the exposed surfaces for passivation and it is subsequently patterned in the source and drain contact  36  and  34  areas to open the ohmic contacts  36  and  34  followed by the formation of a gate  30  stack. First a gate foot is preferably etched in the passivation dielectric  28  using either a dry etching or wet etching or a combination of dry/wet etching. The techniques disclosed herein are suitable for use with either an enhancement mode HMET device or a depletion mode HMET device. In a preferred embodiment, as in an enhancement mode device, a fluorine treatment or a combination of fluorine treatment and gate recess (with a further dry etch) can be performed to deplete the channel under the gate  30  of its 2DEG. Thereafter a suitable gate dielectric  32  is deposited. In a preferred embodiment the gate dielectric  32  could by of Al 2 O 3  oxide deposited by atomic layer deposition (ALD), however, gate dielectric materials other than Al 2 O 3  may be utilized and may be deposited using methods other than ALD including but not limited to PECVD, LPCVD, in-situ grown in MOCVD reactor, etc. 
         [0029]    A gate  30  metal stack is then deposited and patterned. Further steps to implement multi-step field plates can then be used where the cumulative effects of field shaping techniques using both non-uniform 2DEG density profile and multi-step field plate techniques can be combined. Additional inter-metallic dielectric and metal layers may be used to reduce the interconnect resistance particularly if the resulting HMET is a large power device. 
         [0030]    Since the density of charge in the 2DEG region  12  is determined locally by the thickness of the carrier supplying layer  16  at any given position, a non uniform 2DEG distribution is achieved by controlling the height (thickness) of the carrier supplying layer  16  which should increase as a function of distance laterally away from the gate  30  along the drift region toward the drain  34 . See  FIG. 2  where the right hand side of the gate  30  structure is preferably positioned where the step  20  formerly occurred and the left hand side of the drain  34  is preferably positioned at the other end  38  of the wedge or taper  14 . The dependence of the 2DEG density on the thickness of the carrier supplying layer  16  is illustrated in the following paper: Smorchkova, I. P. et al., “Polarization-induced charge and electron mobility in AlGaN/GaN heterostructures grown by plasma-assisted molecular epitaxy,” Journal of Applied Physics, Volume 86, Issue 8, pp. 4520-4526, October 1999, and in particular  FIG. 5   c.    
         [0031]    From the band energy diagram of  FIG. 5 , which is from the paper by Smorchkova, I. P. et al identified above, it can be seen that when a thin AlGaN layer is utilized as the carrier supplying layer  16 , the Fermi level is above the donor surface states level which results in the donors atoms being filled and not supplying the 2DEG electrons to the well at the GaN/AlGaN interface. As the AlGaN thickness increases the Fermi level moves downwards and eventually overlaps with the donor state energy level resulting in some of the donors are being empty with their originally compensating electrons being transferred to the 2DEG well. The further the overlap is the higher the 2DEG density is. 
         [0032]    A flat electric field distribution is achieved using the non-uniform 2DEG density concept as shown in FIG. 5 of the paper by Smorchkova, I. P. et al identified above which maximizes the lateral breakdown voltage per unit length of the drift region and improves immunity to dynamic R on  degradation. In contrast, as shown in  FIGS. 5   a - 5   c  for various configurations, if a uniform 2DEG density profile is used, then the electric field profile is either triangular (no field plate) or has multiple peaks as shown in  FIGS. 5   b  and  5   c  (multiple field plates). The uniform 2DEG and zero or more field plates results in a reduced breakdown voltage BV and less immunity to dynamic R on  degradation compared tapered 2DEG distribution shown in  FIG. 4 . 
         [0033]    If one adds one or more field plates to the embodiment of  FIG. 2  as mentioned above, that will increase the complexity of the process since additional process steps are then required which will increase the cost of making the device. However, adding one or more field plates to the embodiment of  FIG. 2  may yield additional performance benefits. Whether the improvement in performance by adding one or more field plates to the embodiment of  FIG. 2  is justified by the increase in cost of manufacture is a matter of design choice. 
         [0034]    Not shown in the drawings, but as is well known, a spacer layer of AlN, for example, may be inserted between layers  16  and  18  to improve device electrical performance. 
         [0035]    This concludes the detailed description including preferred embodiments of the present invention. The foregoing description including preferred embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible within the scope of the foregoing teachings. Additional variations of the present invention may be devised without departing from the inventive concepts as set forth in the following claims.