Abstract:
A transistor includes a buffer layer, a channel layer over the buffer layer, a barrier layer over the channel layer, a source electrode electrically connected to the channel layer, a drain electrode electrically connected to the channel layer, a gate electrode on the barrier layer between the source electrode and the drain electrode, a backside metal layer, a substrate between a first portion of the buffer layer and the backside metal layer; and a dielectric between a second portion of the buffer layer and the backside metal layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    None 
       TECHNICAL FIELD 
       [0002]    This disclosure relates to III-nitride transistors and GaN-on-Si transistors, and in particular to breakdown voltages in III-nitride transistors. 
       BACKGROUND 
       [0003]    GaN-on-Si transistors are suitable for making high-efficiency and low-cost power switching module; however, the voltage breakdown performance of GaN-on-Si transistors is limited by the thickness of the III-nitride epilayer stack. It is challenging to grow III-nitride epilayer stack thicker than 5 micrometers, due to the large thermal mismatch between the III-nitride material and the Si substrate. 
         [0004]    It is well known that the Si substrate can limit the breakdown voltage of GaN-on-Si transistors. Bin Lu and Tomas Palacios, “High Breakdown (&gt;1500V) AlGaN/GaN HEMTs by Substrate-Transfer,” IEEE Electron Device Letters, vol. 31, no. 9, pp. 951-953, September 2010 describe the removal of the Si substrate, and the subsequent transfer of the device to a glass substrate. Devices showed higher breakdown voltage after the substrate transfer; however, a disadvantage of this approach is that it is difficult to handle large-size wafers with their substrates completely removed, resulting in low yield and high cost. 
         [0005]    P. Srivastava, J. Das, D. Visalli, M. V. Hove, P. E. Malinowski, D. Marcon, S. Lenci, K. Geens, K. Cheng, M. Leys, S. Decoutere, R. Mertens, and G. Borghs, “Record Breakdown Voltage (2200V) of GaN DHFETs on Si with 2-μm Buffer Thickness by Local Substrate Removal,” IEEE Electron Device Letters, vol. 32, no. 1, pp. 30-32, January 2011 describe an improvement in breakdown voltage after local removal of the Si substrate between the source and the drain. A disadvantage of this approach to improving breakdown voltage is that an unfilled trench in the Si substrate may cause electron trapping problems, which affect the dynamic characteristics of the GaN FETs. In particular the devices with an etched substrate may have poor dynamic on-resistance due to trapping at the substrate/air interface. 
         [0006]    What is needed are III-nitride transistors and a method to fabricate III-nitride transistors with improved breakdown voltage that have good performance characteristics and are cost effective to fabricate. The embodiments of the present disclosure answer these and other needs. 
       SUMMARY 
       [0007]    In a first embodiment disclosed herein, a transistor comprises a buffer layer, a channel layer over the buffer layer, a barrier layer over the channel layer, a source electrode electrically connected to the channel layer, a drain electrode electrically connected to the channel layer, a gate electrode on the barrier layer between the source electrode and the drain electrode, a backside metal layer, a substrate between a first portion of the buffer layer and the backside metal layer, and a dielectric between a second portion of the buffer layer and the backside metal layer. 
         [0008]    In another embodiment disclosed herein, a transistor comprises a plurality of epi layers, a drain electrode electrically connected to a channel layer in the epi layers, a backside metal layer, a substrate between a first portion of the epi layers and the backside metal layer, and a dielectric between a second portion of the epi layers and the backside metal layer, wherein the backside metal layer is on a side of the epi layers opposite the drain electrode. 
         [0009]    In yet another embodiment disclosed herein, a method of fabricating a transistor comprises forming a buffer layer on a substrate, forming a channel layer on the buffer layer, forming a barrier layer on the channel layer, forming a source electrode and a drain electrode on opposite ends of the barrier layer, forming a gate electrode on the barrier layer between the source and drain electrodes, coating the source electrode, the drain electrode, the gate electrode and the barrier layer with a removable material, bonding a carrier wafer to the removable material, etching away a portion of the substrate under the drain, depositing dielectric in the etched away portion of the substrate, depositing backside metal on the substrate and the dielectric, and removing the carrier wafer and the removable material. 
         [0010]    In still another embodiment disclosed herein, a method of fabricating a transistor comprises forming a plurality of epi layers on a substrate, forming a drain electrode electrically connected to a channel layer in the epi layers, coating the drain electrode and a top layer of the epi layers with a removable material, bonding a carrier wafer to the removable material, etching away a portion of the substrate under the drain, depositing dielectric in the etched away portion of the substrate, depositing backside metal on the substrate and the dielectric, and removing the carrier wafer and the removable material. 
         [0011]    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 
         [0012]      FIG. 1  shows an elevation sectional view of a III-nitride transistor in accordance with the present disclosure; and 
           [0013]      FIGS. 2A to 2D  show a method to fabricate a III-nitride transistor in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    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. 
         [0015]      FIG. 1  shows an elevation sectional view of a III-nitride transistor in accordance with the present disclosure. The substrate  12 , which may be silicon (Si), is partially removed in an area underneath the drain  22  and filled with a dielectric  30 , which may be benzo-cyclo-butene (BCB). Backside metal  32  is deposited on the backside of substrate  12  as well as the dielectric  30 . 
         [0016]    For the transistor in  FIG. 1  the drain  22  may be at a high voltage and the backside metal  32  may be a ground. Because a silicon substrate  12  is conductive all of the voltage between the drain  22  and the backside metal  32  must be dropped across the III-nitride layers. By removing at least some or all of the substrate  12  underneath the drain  22 , and filling it in with a dielectric  30 , some of the voltage drop can be across the dielectric  30 , which reduces the amount of voltage drop that would otherwise be across the III-nitride layers, thus improving the breakdown voltage of the device. Note that other substrates may also be conductive such as a type of silicon carbide (SiC) substrate. 
         [0017]    A buffer layer  14  is over the substrate  12  and the dielectric  30 . The buffer layer  14  may be a III-nitride material. A channel layer  16  is over the buffer layer  14 , and may be a III-nitride material. Typically the channel layer is un-intentionally doped GaN layer with the thickness ranging from 5 nanometers (nm) to about 2 micrometers. 
         [0018]    A barrier layer  18  is over the channel layer  16 , and may be a stack of III-nitride materials. The barrier layer  18  has a greater energy bandgap than the channel layer  16 . Typically the barrier layer  18  may be an AlGaN alloy with the Al content of 10˜30% and the thickness of 5-30 nanometers (nm). 
         [0019]    A source electrode  20  and a drain electrode  22  are on opposite ends of the barrier layer  18  and the channel layer  16 , and are typically formed by alloying a stack of Ti/Al/Ni/Au. The source electrode  20  and the drain electrode  22  are in electrical contact with the channel layer  16 . A gate electrode  24  is on top of the barrier layer  18 , and between the source  20  and drain  22  electrodes. 
         [0020]    Transistors with III-nitride on Si, such as GaN-on-Si transistors employing the structure of  FIG. 1  have increased breakdown voltage ratings without the necessity of increasing the thickness of the III-nitride epilayers for the buffer layer  14 , channel layer  16 , and barrier layer  18 . 
         [0021]      FIGS. 2A to 2D  show a method to fabricate a III-nitride transistor in accordance with the present disclosure. 
         [0022]    First, as shown in  FIG. 2A , a III-nitride transistor is formed by growing a buffer layer  14  on a substrate  12 , which may be silicon (Si). The buffer layer  14  may be a stack of III-nitride materials grown on the substrate  12  by chemical vapor deposition or molecular beam epitaxy. 
         [0023]    Then a channel layer  16  is grown on the buffer layer  14 . The channel layer  16  may be a III-nitride material grown on the buffer layer  14  by chemical vapor deposition or molecular beam epitaxy. Typically the channel layer is un-intentionally doped III-nitride layer, such as GaN, with the thickness ranging from 5 nanometers to a few micrometers. 
         [0024]    Next a barrier layer  18  is grown on the channel layer  16 . The barrier layer  18  may be a stack of III-nitride materials grown on the channel layer  16  by chemical vapor deposition or molecular beam epitaxy. The barrier layer  18  is formed to have a greater energy bandgap than the channel layer  16 . Typically the barrier layer  18  may be an III-nitride, such as an AlGaN alloy with the Al content of 10˜30% and the thickness of 5-30 nm. 
         [0025]    Then a source electrode  20  and a drain electrode  22  are formed on opposite ends of the barrier layer  18  and the channel layer  16 , and are typically formed by alloying a stack of Ti/Al/Ni/Au. The source electrode  20  and the drain electrode  22  are formed in electrical contact with the channel layer  16 . Finally, a gate electrode  24  is formed on top of the barrier layer  18 , and between the source  20  and drain  22  electrodes. 
         [0026]    Next, as shown in  FIG. 2B , the structure is mounted onto a carrier wafer  42  after the epi layer and other front side processes, such as forming the electrodes, are complete. The source  20 , drain  22 , gate  24  electrodes and the barrier layer  18  are coated with a removable material  40  such as wax. Then a carrier wafer  42  is bonded to the removable material  40 . 
         [0027]    Once the carrier wafer  42  is attached, the entire structure can be turned over and a portion  44  of the substrate  12  under the drain may be etched away by using plasma etching. 
         [0028]    Then, as shown in  FIG. 2C , dielectric  30  is coated on the backside of the structure in the area  44  of the etched away substrate, and then polished. Next backside metal  32 , such as AuGe, is deposited onto the substrate  12  and the filled dielectric  30 . 
         [0029]    Then, as shown in  FIG. 2D , the carrier wafer  42  and the wax  40  are removed. The result is the structure shown also in  FIG. 1 . 
         [0030]    As described, foregoing method of selective removal of the substrate and deposition of a dielectric in place of the substrate enhances the breakdown voltage rating of III-nitride devices, such as GaN on Si high electron mobility transistors (HEMTs). 
         [0031]    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. 
         [0032]    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 . . . .”