Abstract:
An enhancement-mode GaN transistor, the transistor having a substrate, transition layers, a buffer layer comprised of a III Nitride material, a barrier layer comprised of a III Nitride material, drain and source contacts, a gate containing acceptor type dopant elements, and a diffusion barrier comprised of a III Nitride material between the gate and the buffer layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application No. 61/167,817, filed on Apr. 8, 2009, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of enhancement mode gallium nitride (GaN) transistors. More particularly, the invention relates to an enhancement mode GaN transistor with a diffusion barrier. 
       BACKGROUND OF THE INVENTION 
       [0003]    Gallium nitride (GaN) semiconductor devices are increasingly desirable for power semiconductor devices because of their ability to carry large current and support high voltages. Development of these devices has generally been aimed at high power/high frequency applications. 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 (HFET), high electron mobility transistors (HEMT), or modulation doped field effect transistors (MODFET). These types of devices can typically withstand high voltages, e.g., 100 Volts, while operating at high frequencies, e.g., 100 kHz-10 GHz. 
         [0004]    A GaN HEMT device includes a nitride semiconductor with at least two nitride layers. Different materials formed on the semiconductor or on a buffer layer causes the layers to have different band gaps. The different material in the adjacent nitride layers also causes polarization, which contributes to a conductive two dimensional electron gas (2DEG) region near the junction of the two layers, specifically in the layer with the narrower band gap. 
         [0005]    The nitride layers that cause polarization typically include a barrier layer of AlGaN adjacent to a layer of GaN to include the 2DEG, which allows charge to flow through the device. This barrier layer may be doped or undoped. Because the 2DEG region exists under the gate at zero gate bias, most nitride devices are normally on, or depletion mode devices. If the 2DEG region is depleted, i.e. removed, below the gate at zero applied gate bias, the device can be an enhancement mode device. Enhancement mode devices are normally off and are desirable because of the added safety they provide and because they are easier to control with simple, low cost drive circuits. An enhancement mode device requires a positive bias applied at the gate in order to conduct current. 
         [0006]      FIG. 1  illustrates a conventional enhancement mode GaN transistor device  100  without a diffusion barrier. Device  100  includes substrate  101  that can be composed of silicon (Si), silicon carbide (SiC), sapphire, or other material, transition layers  102  typically composed of AlN and AlGaN that is about 0.1 to about 1.0 μm in thickness, buffer material  103  typically composed of GaN that is about 0.5 to about 10 μm in thickness, barrier material  104  typically composed of AlGaN where the Al to Ga ratio is about 0.1 to about 0.5 with thickness from about 0.005 to about 0.03 μm, p-type AlGaN  105 , heavily doped p-type GaN  106 , isolation region  107 , passivation region  108 , ohmic contact metals  109  and  110  for the source and drain, typically composed of Ti and Al with a capping metal such as Ni and Au, and gate metal  111  typically composed of a nickel (Ni) and gold (Au) metal contact over a p-type GaN gate. 
         [0007]    There are several disadvantages of the conventional enhancement mode GaN transistors shown in  FIG. 1 . During growth of the p-type AlGaN  105  (in  FIG. 1 , for example) over the undoped GaN  103  or AlGaN  104 , Mg atoms will diffuse back down the crystal into the active region of the device, leading to unintentional doping of layers  104  and  103 . These Mg atoms act as acceptors, taking electrons, and become negatively charged. The negatively charged Mg repels electrons from the 2-dimensional electron gas. This leads to higher threshold voltage under the gate and lower conductivity in the region between the gate and the ohmic contacts. In addition, the charging and discharging of these Mg atoms can lead to time dependent changes in the threshold and conductivity of the device. A second disadvantage of the conventional GaN transistor is the high gate leakage when the transistor is turned on by applying positive voltage to the gate contact. During growth of layer  106  (in  FIG. 1 , for example), Mg atoms diffuse to the growth surface. When growth is terminated, a heavily doped layer exists at the surface. When positive bias is applied to the gate contact, a large current is generated due to the high doping at the top of this layer. 
         [0008]    It would therefore be desirable to provide a GaN transistor with a diffusion suppression structure which avoids the above-mentioned disadvantages of the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates a cross-sectional view of a conventional enhancement mode GaN transistor device. 
           [0010]      FIG. 2  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a first embodiment of the present invention. 
           [0011]      FIG. 3  is a schematic representation comparing the aluminum content in conventional GaN transistors and the device of  FIG. 2 . 
           [0012]      FIG. 4  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a second embodiment of the present invention. 
           [0013]      FIG. 5  is a schematic representation comparing the aluminum content in conventional GaN transistors and the device of  FIG. 4 . 
           [0014]      FIG. 6  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a third embodiment of the present invention. 
           [0015]      FIG. 7  is a schematic representation comparing the aluminum content in conventional GaN transistors and the device of  FIG. 6 . 
           [0016]      FIG. 8  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a fourth embodiment of the present invention. 
           [0017]      FIG. 9  is a schematic representation comparing the magnesium content in conventional GaN transistors and the device of  FIG. 8 . 
           [0018]      FIG. 10  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a fifth embodiment of the present invention. 
           [0019]      FIG. 11  is a schematic representation comparing the magnesium content in conventional GaN transistors and the device of  FIG. 10 . 
           [0020]      FIG. 12  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a sixth embodiment of the present invention. 
           [0021]      FIG. 13  is a schematic representation comparing the magnesium content in conventional GaN transistors and the device of  FIG. 12 . 
           [0022]      FIG. 14  illustrates a cross-sectional view of an enhancement mode GaN transistor device formed according to a seventh embodiment of the present invention. 
           [0023]      FIGS. 15A-15D  illustrate a method of forming an enhancement mode GaN transistor device according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    In the following detailed description, reference is made to certain embodiments. These embodiments are described with sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be employed and that various structural, logical, and electrical changes may be made. 
         [0025]    Embodiments of the invention described herein relate to an enhancement mode GaN transistor with a diffusion barrier that prevents Mg atoms from diffusing through the crystal into the active regions of the device. The embodiments are based on the addition of a diffusion barrier and/or a graded doping profile to reduce or eliminate the diffusion of dopant atoms (e.g., Mg). In one embodiment of the current invention, a thin AlN, or high Al content AlGaN layer is deposited above the primary channel layer to block the back diffusion of Mg into this region. In another embodiment of the invention, a thin AlN or high Al AlGaN layer is deposited within or above the barrier layer. In another embodiment, the Mg doping profile is controlled to reduce the quantity of Mg diffused into or through the barrier layer by adding an undoped region between the p-GaN layer and barrier layer. In yet another embodiment, a doping modification near the gate contact is used either to facilitate ohmic or Schottky contact formation. 
         [0026]    Referring to  FIG. 2 , a first embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 2  illustrates a cross-sectional view of the device  200 . Device  200  includes substrate  21  composed of Si, SiC, sapphire, or other material, transition layers  22  typically composed of AlN and AlGaN from about 0.1 to about 1.0 μm in thickness, buffer layer  23  typically composed of GaN from about 0.5 to about 10 μm in thickness, channel layer  20  typically composed of GaN or InGaN with a thickness from about 0.01 to about 0.3 μm, barrier layer  27  typically composed of AlGaN where the Al fraction is about 0.1 to about 0.5 with a thickness between about 0.005 and about 0.03 μm, gate structure  26  typically composed of p-type GaN with a refractory metal contact such as Ta, Ti, TiN, W, or WSi 2 . The p-type GaN and refractory metal contact are each between about 0.01 and about 1.0 μm in thickness. Ohmic contact metals  24 ,  25  are composed of Ti and Al with a capping metal such as Ni and Au or Ti and TiN. Diffusion barrier  28  is typically composed of AlGaN, where the Al fraction is between about 0.2 and about 1 with a thickness between about 0.001 and about 0.003 μm. The Al fraction is the content of Al such that Al fraction plus Ga fraction equals 1. Buffer layer  23 , barrier layer  27 , and diffusion barrier  28  are made of a III Nitride material. A III Nitride material can be composed of In x Al y Ga 1-x-y N where x+y≦1. 
         [0027]    In accordance with the above-described embodiment, a double layer of different Al contents is formed. The structure in  FIG. 2  has higher Al content close to the channel layer, and lower Al content near the gate layer. A comparison of Al content between the channel layer and gate layer in a conventional GaN transistor and the structure of  FIG. 2  is shown in  FIG. 3 . In the structure shown in  FIG. 2 , the diffusion barrier layer  28  above the channel layer is high in Al, while the barrier layer  27  is of lower Al content. Although  FIG. 3  shows 2 distinct layers of constant Al content, the combination of layer  28  and  27  into a graded Al content layer can also be employed, such that the Al content is graded from high near the channel layer to low near the gate structure. This grading can be done in many fashions, such as linear, multiple steps down, alternating between high and low Al content while gradually decreasing the average Al content, or alternating between high and low Al content while changing the thickness of the high and low Al layers from thicker high Al near the channel to thinner high Al near the gate. The high Al content material blocks diffusion of Mg and confines it to regions above the channel layer. The high Al content layer also leads to high electron mobility. In the structure shown in  FIG. 2 , however, diffusion still proceeds into the top barrier layer. 
         [0028]    Referring to  FIG. 4 , a second embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 4  illustrates a cross-sectional view of the device  300 .  FIG. 4  is similar to  FIG. 2 , but differs in that diffusion barrier  38  and barrier layer  37  are inverted from their positions in  FIG. 2 , thus providing diffusion barrier  38  next to the gate structure  36 . The dimensions and compositions of the various layers are similar to that of the first embodiment. 
         [0029]    In accordance with the above-described embodiment, a double layer of different Al contents is provided, with similar advantages as the first embodiment. A comparison of Al content between the channel layer and gate layer in a conventional GaN transistor and the structure of  FIG. 4  is shown in  FIG. 5 . In the structure shown in  FIG. 4 , the barrier layer  37  above the channel layer is low in Al, while the diffusion barrier layer  38  is of higher Al content. The high Al content material blocks diffusion of Mg and confines it to regions above the barrier layers. In the structure shown in  FIG. 4 , however, the lower Al content layer does not have the advantage of higher electron mobility possessed by the first embodiment. 
         [0030]    Referring to  FIG. 6 , a third embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 6  illustrates a cross-sectional view of the device  400 . The third embodiment is essentially a combination of the first and second embodiments described above, and includes two diffusion barrier layers  48 ,  49 , one of either side of the barrier layer  47 . The dimensions and compositions of the various layers are similar to that of the first and second embodiments. 
         [0031]    This embodiment has the advantages of both the first and second embodiments described above. The structure of  FIG. 6  has a triple layer of different Al contents with higher Al content close to the gate layer and higher Al content near the channel layer. A comparison of Al content between the buffer layer and gate layer in a conventional GaN transistor and the structure of  FIG. 6  is shown in  FIG. 7 . In the structure shown in  FIG. 6 , the diffusion layer  49  above the channel layer is high in Al, while the barrier layer  47  is of lower Al content, and the other diffusion layer  48  is again of high Al content. The high Al content material of layer  48  blocks diffusion of Mg, and confines it to regions above the barrier layers. The high Al content material of layer  49  leads to higher electron mobility. 
         [0032]    Referring to  FIG. 8 , a fourth embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 8  illustrates a cross-sectional view of the device  500 . This embodiment is similar to the first and second embodiments described above, but has a p-type GaN gate with a Mg doping profile and does not have diffusion barrier layers. The gate layer  57  in this embodiment has lower Mg concentration near the barrier layer  54  and higher Mg concentration near the gate contact  58 . Typical values for Mg concentration in gate layer  57  are about 10 16  atoms per cm 3  near the barrier layer, increasing to about 5×10 19  atoms per cm 3  at the gate contact. 
         [0033]    In accordance with the above-described embodiment, the Mg doping level of the gate layer  57  is low near the barrier layer  54 , and higher near the gate contact  58 . This is shown in  FIG. 9  with comparison to a conventional GaN transistor. The structure in  FIG. 8  has higher Mg content close to the gate layer. The Mg concentration level can begin at zero or a low level, e.g., about 10 16  atoms per cm 3 , and then increase towards the gate contact. The shape of the Mg concentration through the p-type GaN gate layer  57  can vary in a number of ways, some of which are shown in  FIG. 9  (e.g., a linear graded Mg concentration or a spiked Mg concentration near the gate contact). Included in these are versions in which there is a spacer layer above the barrier that does not contain Mg. Associated with this low Mg region is a doping offset thickness. The structure of  FIG. 8  has various advantages. The low Mg concentration near the barrier layer reduces the back diffusion into the barrier layer. Combined with a doping offset, very low unintentional doping of the barrier layers and the buffer layers can be achieved. The high Mg concentration near the gate contact helps create an ohmic contact between the gate contact and p-type GaN that leads to improved device turn on characteristics. 
         [0034]    Referring to  FIG. 10 , a fifth embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 10  illustrates a cross-sectional view of the device  600 . This embodiment is similar to the fourth embodiment, except that the Mg doping profile of the p-type GaN gate layer  67  is different. The gate layer  67  in this embodiment has a lower Mg concentration near the barrier layer  64  and near the gate contact  68 , with an increased concentration in the middle. Typical values for Mg concentration are about 10 16  atoms per cm 3  near the barrier layer, increasing to about 5×10 19  atoms per cm 3  near the center of the p-GaN gate, and decreasing to about 10 16  atoms per cm 3  near the gate contact. 
         [0035]    In accordance with the above-described embodiment, the Mg doping level is low near the barrier layer and higher near the center of the gate. This is shown in  FIG. 11  with comparison to a conventional GaN transistor. The shape of the Mg concentration through the p-type GaN layer can vary in a number of ways, some of which are shown in  FIG. 11  (e.g., a peaked Mg concentration or a flat topped Mg profile). The structure of  FIG. 10  has higher Mg content in the center of the gate layer. The low Mg concentration near the barrier layer reduces the back diffusion into the barrier layer. Combined with a doping offset, very low unintentional doping of barrier, channel, and buffer layers can be achieved. The low Mg concentration near the gate contact allows formation of a Schottky contact between the gate contact and p-type GaN that leads to improved device gate leakage. 
         [0036]    A sixth embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 12  illustrates a cross-sectional view of the device  700 . This embodiment is similar to the fifth embodiment, except that n-type doping is provided through addition of Si in gate layer  77  near the gate contact. Typical values for Mg concentration are similar to the fifth embodiment. Si concentration near the gate contact can rage from about 10 15  to about 10 19  atoms per cm 3 . 
         [0037]    In accordance with the above-described embodiment, the Mg doping level is low near the barrier layer and higher near the center of the gate. Si atoms are added near the gate contact. This is shown in  FIG. 13  with comparison to a conventional GaN transistor. The low Mg concentration near the barrier layer reduces the back diffusion into the barrier layer. Combined with a doping offset, very low unintentional doping of barrier, channel, and buffer layers can be achieved. The low Mg concentration near the gate contact results in a low hole density. The hole density is further reduced by the addition of Si atoms. Part A of  FIG. 13  illustrates the addition of Si atoms to reduce the density of holes. The density of Si atoms is less than or equal to the density of Mg atoms. This very low hole density improves the formation of a Schottky contact. Further increasing the Si content beyond the level of Mg, results in a p-n junction. Part B of  FIG. 13  illustrates the addition of Si atoms far beyond the density of Mg atoms near the gate contact. This results in a p-n junction within the gate structure and can lead to further reduction in gate leakage. 
         [0038]    A seventh embodiment is now described with reference to the formation of an enhancement mode GaN transistor.  FIG. 14  illustrates a cross-sectional view of the device  800 . This embodiment is similar to the fifth and sixth embodiments, except that region  89 , composed of a portion of the spacer layer, remains above the barrier layer in the region outside the gate region. Typical values of layer  89  thickness are about 0% to about 80% of the spacer layer thickness. 
         [0039]    An additional advantage of the low doped or undoped layer is a reduction in damage from manufacturing, and an improvement in manufacturing tolerances. Referring to  FIGS. 15A-15D , the steps in fabrication consist of: (a) deposition of AlN and AlGaN transition layers  82  on substrate  81 , GaN buffer layer  83 , channel layer  80 , barrier layer  84 , p-GaN layer  87 , and gate contact material  88 ; (b) etching of the gate contact and most of the p-GaN layer  87  leaving a small amount of material  89 ; (c) passivation of the surface through deposition of an insulating material such as SiN  90 ; and (d) etching open contact area and depositing ohmic contact material to form source  86  and drain  85 . The advantage is achieved in step (b). During the etch of p-GaN, the etching is stopped before reaching the barrier layer. This is done to avoid causing damage to this sensitive material that can result in high resistivity in the channel layer, and trapping of charge at the SiN interface. Without use of the low doped spacer layer, layer  89  is composed of p-GaN. This leads to negative charge in layer  89  that repels electrons from the channel layer and increases resistance to current flow when the device is on. The use of an undoped spacer layer allows the etching of step (b) to terminate above the barrier layer, thus avoiding damage, without leaving highly doped material that is detrimental to resistance of the channel layer. The spacer layer may be grown at a very high temperature (around 1000° C. to around 1100° C.), grown at around 900° C. with high ammonia, and/or grown very slowly. 
         [0040]    The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modifications and substitutions to specific process conditions can be made. Accordingly, the embodiments of the invention are not considered as being limited by the foregoing description and drawings.