Abstract:
A FET includes a first and second set of finger arrays that each include a source, gate and drain. A first source pad is electrically coupled to source electrodes in the first set of finger arrays. A second source pad is electrically coupled to the source electrodes in the second set of finger arrays. A common drain pad is electrically coupled to drain electrodes in the first and second set of finger arrays. A first gate pad is electrically coupled to gate electrodes in the first set of finger arrays. A second gate pad is electrically coupled to gate electrodes in the second set of finger arrays. A substrate is also provided on which are disposed the first and second set of finger arrays, the first and second source pads, the common drain pad, and the first and second gate pads.

Description:
STATEMENT OF RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/655,979, filed Oct. 16, 2012, entitled “LAYOUT DESIGN FOR A HIGH POWER, GaN-BASED FET”, which is a continuation of U.S. Patent Application Serial No. 12/821,492, filed Jun. 23, 2010, entitled “LAYOUT DESIGN FOR A HIGH POWER, GaN-BASED FET”, now U.S. Pat. No. 8,319,256, issued Nov. 27, 2012, which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a high voltage field effect transistor (FET), and more particularly to the layout of a FET which uses a gallium nitride-based compound semiconductor. 
       BACKGROUND OF THE INVENTION 
       [0003]    A field-effect transistor (also referred to as a “FET”) is a conventional semiconductor device employable in switch-mode power supplies for use in data processing and telecommunication systems as well as for other applications that use conditioned power for operation of sensitive electronic circuits. The field-effect transistors have almost universally replaced bipolar transistors previously used for inverters (a type or portion of a power supply) as well as the p-n and Schottky diodes used for rectification. The proliferation of the field-effect transistors has, at least in part, been driven by the need to provide highly efficient power supplies with low dc output voltages such as five volts or less at higher current levels. The broad acceptance of the field-effect transistors for the aforementioned applications is a consequence of the low forward voltage drop and fast switching speed as well as the low power consumption employed to enable or disable conduction thereof. As a result, the use of the field-effect transistors has contributed to compact and efficient power supplies that can be produced at low cost. 
         [0004]    As the loads for the power supplies are generally designed with integrated circuits employing shrinking feature sizes for the circuit elements, a need is continually evolving for new designs with lower output voltages (e.g., one volt or less) at higher current levels (e.g., 50 to 100 amperes or more). Present switch-mode power supplies providing input-output circuit isolation (via a transformer) and employing silicon-based field-effect transistors as synchronous rectifiers therein and designed with best current practice are usually operable with switching frequencies only up to several hundred kilohertz (“kHz”) due, at least in part, to the slower switching speeds of the silicon-based field-effect transistors. To accommodate continuing developments in integrated circuit technology, however, power supply switching frequencies above one megahertz (“MHz”) are desirable to reduce the size of magnetic devices and the capacitive filtering elements of the power supply without compromising the power conversion efficiency. In accordance therewith, field-effect transistors with previously unrealizable characteristics are not only being requested, but are necessary to satisfy the conditions. 
         [0005]    To provide increased power handling capabilities, transistors with a larger effective area have been developed. However, as the area of a transistor increases, the transistor may become less suitable for high frequency operations that, typically, require a small source to drain distance so that the carrier transit times are limited. One technique for increasing the area of a transistor while still providing for high frequency operations is to use a plurality of transistor cells that are connected in parallel. Such a configuration includes a plurality of elongated gate “fingers” which control the flow of current through each of the plurality of unit cells. Thus, the source to drain distance of each cell may be kept relatively small while still providing a transistor with increased power handling capability. 
         [0006]    A material of choice for high performance field-effect transistors is a semiconductor with high electron mobility, and wide band gap for high breakdown voltage that is capable of being processed with conventional equipment and methods not substantially different from those already developed for silicon and present generations of compound semiconductors. A particularly desirable material is the compound semiconductor known as gallium nitride (“GaN”), which has been used for integrated circuits operable at frequencies well above one gigahertz, and which has been used to produce power field-effect transistors with high performance characteristics. The GaN-based FET device is capable of maximizing electron mobility by forming a quantum well at the heterojunction interface between a AlGaN barrier layer, which has a large band gap, and the GaN layer, which has a narrower band gap. As a result, electrons are trapped in the quantum well. The trapped electrons are represented by a two-dimensional electron gas in the undoped GaN layer. The amount of current is controlled by applying voltage to the gate electrode, which is in Schottky contact with the semiconductors so that electrons flow along the channel between the source electrode and the drain electrode. 
         [0007]    As the market for GaN-based FETs continues to grow, many improvements remain desirable to enhance various operating characteristics such as the breakdown voltage V br , on-resistance and the cut-off frequency. 
       SUMMARY 
       [0008]    In accordance with the present invention, a FET is provided which includes a substrate, a buffer layer disposed on the substrate, a channel layer disposed over the buffer layer and a barrier layer disposed over the channel layer. Source, gate and drain electrodes are located over the barrier layer and extend in a longitudinal direction thereon. A portion of the channel and barrier layers define a mesa extending in the longitudinal direction and the source and drain electrodes extend beyond an edge of the mesa. The gate electrodes extend along an edge sidewall of the mesa. A conductive source interconnect is disposed over the buffer layer and have a first end electrically connected to the source electrode. A first dielectric layer is disposed over the buffer layer and over the source interconnect. A gate via is formed in the first dielectric layer. A conductive gate node extends along the buffer layer and electrically connects the portion of the gate electrode extending along the sidewall of the mesa. A gate pad is disposed on the first dielectric layer adjacent the mesa. A conductive gate connect strip is located over the gate node and is in contact therewith. The gate strip is in electrical contact with the gate pad. A source via is formed in the first dielectric layer and a source pad is formed in the source via. The conductive source interconnect has a second end in electrical contact with the source pad. 
         [0009]    In accordance with another aspect of the invention, a FET is provided which includes a first and second set of finger arrays that each include a source, gate and drain. A first source pad is electrically coupled to source electrodes in the first set of finger arrays. A second source pad is electrically coupled to the source electrodes in the second set of finger arrays. A common drain pad is electrically coupled to drain electrodes in the first and second set of finger arrays. A first gate pad is electrically coupled to gate electrodes in the first set of finger arrays. A second gate pad is electrically coupled to gate electrodes in the second set of finger arrays. A substrate is also provided on which are disposed the first and second set of finger arrays, the first and second source pads, the common drain pad, and the first and second gate pads. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a cross-section through two GaN-based FET cells constructed in accordance with the present invention. 
           [0011]      FIG. 2  is plan view of the GaN-based FET shown in  FIG. 1 . 
           [0012]      FIG. 3  is a cross-sectional view taken along line A-A in  FIG. 2 . 
           [0013]      FIG. 4  is an expanded plan view of the GaN-based FET showing multiple fingers. 
           [0014]      FIG. 5  is an even more greatly expanded plan view of the GaN-based FET shown in  FIG. 4 . 
           [0015]      FIG. 6  is cross-sectional view of the FET taken along line C-C in  FIG. 4 . 
           [0016]      FIG. 7  is cross-sectional view of the FET taken along line D-D in  FIG. 4   
           [0017]      FIG. 8  is expanded plan view of a GaN-based FET in which the gate and source pads overlap. 
           [0018]      FIG. 9  is detail plan view of the FET shown in  FIG. 8 . 
           [0019]      FIG. 10  is cross-section view of a FET through line E-E in  FIG. 9 . 
           [0020]      FIG. 11  is cross-sectional view of a FET through line F-F in  FIG. 9 . 
           [0021]      FIG. 12(   a ) shows the dependence of the specific on-resistance on the gate-to-drain spacing L gd  for two different gate lengths L g . 
           [0022]      FIG. 12(   b ) shows the dependence of the breakdown voltage on the gate-to-drain spacing L gd . 
           [0023]      FIG. 13  shows the dependence of the cut-off frequency of the device on the gate-to-drain spacing L gd  for two different gate lengths L g . 
           [0024]      FIG. 14  is a plan view of an alternative embodiment of a FET layout in which there are two sets of finger arrays. 
           [0025]      FIG. 15  shows yet another embodiment similar to  FIG. 14  but in which the gate pads do overlap with the source pad. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  is a cross-section through two GaN-based FET cells constructed in accordance with the present invention. As shown, a heterojunction structure is formed on a semi-insulating substrate  10  such as a sapphire substrate. The heterojunction structure includes a buffer layer  20  of GaN, for example, formed on the substrate  10 . A mesa  90  is formed on the buffer layer  20 . The mesa  90  includes an undoped GaN layer  30  and an undoped barrier layer  40  (e.g., AlGaN), which is generally much thinner than the undoped GaN layer  30 . The undoped GaN layer  30  serves as the channel layer. Source electrodes  50  and drain electrodes  70  are arranged on the undoped barrier layer  40 . Gate electrodes  60  are also formed onto the undoped AlGaN layer  40  and are situated between the source electrodes  50  and the drain electrodes  70 . 
         [0027]      FIG. 2  is plan view of the GaN-based FET in which two gate “fingers” are shown. It should be noted that in  FIGS. 1 and 2 , as well as the figures that follow, like elements are denoted by like reference numerals. The source electrodes  50 , gate electrodes  60  and drain electrodes  70  are interdigitated. The electrodes (fingers) extend in the longitudinal direction with a width W and length L. The cross-sectional view of  FIG. 1  is taken through the center of the mesa  90  along line B-B in  FIG. 2 . As  FIG. 2  shows, the gate electrodes  60  extend beyond the edge of the mesa  90  in the longitudinal direction. In addition, the channel  30 , barrier layer  40  and source and drain electrodes  50  and  70  also extend beyond the edge of the mesa  90  in the longitudinal direction. This is most clearly shown in  FIG. 3 , which is a cross-sectional view taken along line A-A in  FIG. 2 . In some embodiments of the invention the source and drain electrodes  50  and  70  extend about 2-3 microns beyond the edge of the mesa. The gate electrodes  60  extend from the upper surface of the mesa  90  and down along the mesa sidewall  92  where they contact the buffer layer  20 , as shown in the combination of  FIG. 1  and  FIG. 4 . 
         [0028]      FIG. 4  is an expanded plan view of the GaN-based FET showing multiple fingers along with the gate pad  120  and source pad  150  located on the buffer layer  20 .  FIG. 5  is an even more greatly expanded plan view of the GaN-based FET showing two gate pads  120 , source pad  150  and drain pad  162 .  FIG. 4  is a detail from the portion of  FIG. 5  represented within circle  15 . As best seen in  FIG. 4 , the gate electrodes  60  along the mesa sidewall  92  are each connected to a conductive gate interconnect  130 . As shown, adjacent conductive gate interconnects  130  meet at conductive gate nodes  145 . The conductive gate nodes  145  are each connected to a conductive gate connect strip  110 , which in turn is connected to the gate pad  120 . In this way the gate electrodes  60  are in electrical communication with the gate pad  120 . Similarly, the source electrodes  50  are connected to conductive source interconnects  140 , which in turn are connected to source pad  150 , thereby providing electrical communication between the source electrodes  50  and the source pad  150 . As shown in  FIG. 5 , the drain pad  162  is configured similarly. One advantage of the interconnect layout arrangement shown in  FIGS. 4 and 5  is that it avoids the use of an air-bridge, which is often used to connect the source and drain contact while isolating the gate contacts. This is advantageous because air-bridges generally require complex processing steps and can reduce the overall reliability of the device. 
         [0029]      FIG. 6  is cross-sectional view taken along line C-C in  FIG. 4 . Source interconnects  140  are located over the buffer layer  20 . Optionally, a dielectric layer  152  (e.g., silicon nitride, silicon dioxide) may be provided on the buffer layer  20 . In this case the source interconnects  140  (and source pad  150 ), as well as the gate interconnects  130  (and gate pad  120 ), are formed on the dielectric layer  152 . The dielectric layer  152  serves to reduce current conduction among the gate source and drain pads, which can be especially problematic when the buffer layer  20  is not highly resistive. A dielectric layer  155  is formed over the source interconnects  140 . Gate vias  156  are formed over the gate nodes  145  and filled with conductive materials to form gate interconnects  130 . Finally, as further shown in  FIG. 6 , gate connect strip  110  and gate pad  120  are formed over the gate interconnects  130 . 
         [0030]      FIG. 7  is cross-sectional view taken along line D-D in  FIG. 4 . A source pad via  158  is formed in the dielectric layer  155 . The source pad  150  is formed in and above the source pad via  158  and electrically contacts the source interconnects  140  shown in  FIG. 4 . 
         [0031]    In the embodiments of the invention shown in  FIGS. 4-7  the gate connect strip  110  that connects all the gate interconnects  130  through the gate vias  156  does not overlap the source pad  150 . The gate connect strip  110  does, however, partially overlap the source interconnects  140 . One advantage of these embodiments of the invention is that the gate pad  120  and source pad  150  do not overlap, thereby contributing to a reduction in the overall capacitance of the device. 
         [0032]    In other embodiments of the invention the gate pad  120  and the source pad  150  overlap. Such an arrangement is shown in the plan view of  FIG. 8  (compare to  FIG. 5 ) and in the detail shown in  FIG. 9 , which depicts the region shown within the circle  18  of  FIG. 8 .  FIG. 10  is cross-section view through line E-E in  FIG. 9  and  FIG. 11  is cross-sectional view through line F-F in  FIG. 9 . One advantage of the embodiments of the invention shown in  FIGS. 8-11  is that a relatively low spreading resistance can be achieved between the electrodes and the pads. However, the area of overlap between the gate and source pads needs to be carefully arranged so that excessive capacitance can be avoided. 
         [0033]    The dimensions of the various features shown in the figures and discussed above may be selected by those of ordinary skill in the art in accordance with well-known principles that involve tradeoffs in various operating parameters. However, for purposes of illustration and not as a limitation on the invention the following dimensions have been found to be suitable for a number of different device applications. For instance, in one embodiment of the invention the gate electrode length is about 3 microns or less, which ensures that the Rds-on is relatively low while still achieving a relatively fast switching speed. If the gate length were much smaller, the gate resistance would increase during switching due to charging and discharging of the gate. In some embodiments of the invention the source and drain electrode lengths are about 10 microns and each finger is about 1000 microns long. In other embodiments each finger is between about 250 microns and 1 mm in length. If the length of the source and gain electrodes were significantly reduced, good current spreading might not be achieved and thus the ohmic contact would be affected. On the other hand, if the electrodes are too big the device will occupy excessive space and manufacturing costs would increase. The length of the gate connect strip  110  may be selected to be about 40 microns to achieve a reasonable gate resistance and to allow for adequate fabrication tolerances. The gate to drain distance is generally about 15 microns or less, with a typical value of about 10 microns. Various other dimensions are denoted on the figures in units of microns. The gate to source spacing will generally be determined by the limitations of the process that is employed, but in some cases will advantageously be submicron. 
         [0034]      FIG. 12(   a ) shows the dependence of the specific on-resistance on the gate-to-drain spacing L gd  for two different gate lengths L g . Likewise,  FIG. 12(   b ) shows the dependence of the breakdown voltage on the gate-to-drain spacing L gd . These results, based on simulations, show that a low on-resistance and a high breakdown voltage can be achieved by reducing the gate length and increasing the gate-to-drain spacing L gd . 
         [0035]      FIG. 13  shows the dependence of the cut-off frequency of the device on the gate-to-drain spacing L gd  for two different gate lengths L g . These simulated results show that the cut-off frequency is degraded as the gate-to-drain spacing L gd  increases. Consistent with  FIGS. 12(   a ) and  12 ( b ), they also show that a longer gate length does not enhance device performance. 
         [0036]      FIG. 14  is a plan view of an alternative embodiment of a FET layout in which there are two sets of fingers  145  instead of the single set of fingers shown in previous embodiments (compare to  FIG. 5 ). That is, in  FIG. 14  the total number of fingers of doubled while their widths are halved (e.g., there are 400 fingers, 200 in each set with each finger having a length of 500 microns). The device includes two source pads  150 , one drain pad  162  and four gate pad  120 s. The drain pad is located between each set of fingers. In  FIG. 14  the gate pads  120  do not overlap with the source pad.  FIG. 15  shows another embodiment similar to  FIG. 14  but in which the gate pads  120  do overlap with the source pad. One advantage of the arrangement shown in  FIGS. 14 and 15  is that the source and drain pads are now closer to one another, thus reducing the spreading resistance of the metal interconnects. 
         [0037]    Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, while the depletion mode FET has been described as a GaN-based device, the invention more generally encompasses a depletion mode FET that is formed from any Group III nitride compound semiconductor in which the group III element may be gallium (Ga), aluminum (Al), boron (B) or indium (In).