Abstract:
A field effect transistor (FET) includes source and drain electrodes, a channel layer, a barrier layer over the channel layer, a passivation layer covering the barrier layer for passivating the barrier layer, a gate electrode extending through the barrier layer and the passivation layer, and a gate dielectric surrounding a portion of the gate electrode that extends through the barrier layer and the passivation layer, wherein the passivation layer is a first material and the gate dielectric is a second material, and the first material is different than the second material.

Description:
TECHNICAL FIELD 
     This disclosure relates to heterostructure field effect transistors (HFET), which are also known as high electron mobility transistors (HEMTs), and in particular to normally-off type HFET transistors. 
     BACKGROUND 
     GaN-based transistors are typically of the normally-on type due to the spontaneous formation of a polarization-doped two dimensional electron gas (2DEG) at the AlGaN/GaN interface. However, normally-off type devices are desirable in a number of applications and particularly in high voltage power-switching applications, where the normally-off functionality reduces power consumption and improves safety. High-voltage power-switching devices also require a high breakdown voltage in addition to a low on-resistance. Methods of making AlGaN/GaN transistors normally-off include gate recess etching, fluorine plasma exposure, the use of thin or low Al-composition AlGaN barrier layers, p-type depletion layers, etc. Any method for fabrication of a normally-off device should ideally not compromise the breakdown voltage of the device and should maintain a low on-resistance. Another issue is charge trapping at the drain side of the gate, which can result in a phenomenon known as “current collapse” under high-voltage operation. To avoid current collapse, the surface of the device must be passivated by a dielectric material that has a high-quality interface with GaN (typically SiN). 
     The prior art includes flourine-treated normally-off type GaN devices, as described by K. S. Boutros, S. Burnham, D. Wong, K. Shinohara, B. Hughes, D. Zehnder, and C. Mcguire, “Normally-off 5 A/1100V GaN-on-Silicon Device for high Voltage applications”, International Electron Devices Meeting 2009; and hybrid MOS-HFET devices which utilize a single dielectric layer as a gate insulator and surface passivation layer as described by H. Kambayashi, Y. Satoh, S. Ootomo, T. Kokawa, T. Nomura, S. Kato, and T. P. Chow, “Over 100 A normally-off AlGaN/GaN hybrid MOS-HFET on Si substrate with high-breakdown voltage”, Solid State Elec., vol. 54 issue 6 pp. 660-664 (2010), and T. Oka and T. Nozawa, “AlGaN/GaN recessed MIS-Gate HFET with high threshold voltage normally-off operation for power electronics applications”, IEEE Elec Dev. Lett. vol. 29 no. 7 (2008). 
     The disadvantages of Flourine-treated devices include poor threshold voltage uniformity and reliability. The disadvantages of prior art MOS-HFET devices, which use a thick SiO 2  or SiN layer as both a gate dielectric and a passivation layer, include poor channel mobility and on-resistance due to a poor quality, thick, low k dielectric under the gate, as well as poor surface passivation by SiO 2 , and threshold voltage hysteresis due to a poor quality interface between the gate dielectric and underlying epitaxial material. 
     These types of “hybrid” MOS- or MIS-HFET devices are known to result in a normally-off device with a high breakdown voltage. However, these hybrid MOS-HFET devices have the disadvantage of low electron mobility in the active region under the gate due to a poor quality interface between the gate dielectric and the underlying GaN, resulting in increased on-resistance compared to a traditional GaN HFET. 
     What is needed is a device with a normally-off operation with low gate current, high breakdown voltage, and low on-resistance, as well as low threshold voltage hysteresis and current collapse. The embodiments of the present disclosure answer these and other needs. 
     SUMMARY 
     In a first embodiment disclosed herein, a field effect transistor (FET) comprises a source electrode, a drain electrode, a channel layer, a barrier layer over the channel layer and coupled to the source and drain electrodes, a passivation layer over the barrier layer for passivating the barrier layer between the gate electrode and the source electrode and between the gate electrode and the drain electrode, a gate electrode extending through the barrier layer and the passivation layer, and a gate dielectric surrounding the portion of the gate electrode that extends through the barrier layer and the passivation layer, wherein the passivation layer is a first material and the gate dielectric is a second material, and wherein the first material is different than the second material. 
     In another embodiment disclosed herein, a method of fabricating a field effect transistor comprises forming a channel layer, forming a barrier layer over the channel layer, forming a passivation layer over the barrier layer, etching away a first area of the passivation layer for a source electrode and a second area of the passivation layer for a drain electrode, forming a source electrode and a drain electrode on the barrier layer, etching away a third area of the passivation layer and a fourth area extending through the barrier layer for a gate electrode, forming a gate dielectric on the surface of the third area and the fourth area, and forming a gate electrode in the third area and in the fourth area, wherein the passivation layer is a first material and the gate dielectric is a second material, and wherein the first material is different than the second material. 
     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 an elevation sectional view of a hybrid MOS-HFET with a layer used as both a gate dielectric and a passivation layer in accordance with the prior art; 
         FIG. 2  shows an elevation sectional view of a hybrid MOS-HFET in accordance with the present disclosure; 
         FIG. 3  shows transfer curves comparing characteristics with and without a post deposition anneal (PDA) of the Al 2 O 3  gate dielectric for a hybrid MOS-HFET in accordance with the present disclosure; 
         FIG. 4  shows a 200 ns pulsed common-source current voltage measurement of an annealed Al 2 O 3  hybrid MOS-HFET in accordance with the present disclosure; 
         FIG. 5  shows a common-source DC current voltage and zero-bias breakdown measurement of an annealed Al 2 O 3  hybrid MOS-HFET in accordance with the present disclosure; 
         FIG. 6  shows a comparison of a prior art normally-off GaN power device with a hybrid MOS-HFET in accordance with the present disclosure; 
         FIG. 7  shows a common-source DC current voltage measurement of an annealed hybrid Al 2 O 3  MOS-HFET with a gate periphery of 20 mm with the gate biased from +3V in −0.5V steps in accordance with the present disclosure; and 
         FIG. 8  is a flow diagram of a method of fabricating a hybrid MOS-HFET 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. 
       FIG. 1  shows an elevation sectional view of a hybrid MOS-HFET  10  with a layer  12  used for both surface passivation of the AlGaN layer  14  between the gate  16  and source  20  and drain  22  electrodes and as a gate dielectric beneath gate  16  in accordance with the prior art. In the prior art for hybrid AlGaN/GaN MOS- or MIS-HFETs, the layer  12  may be a plasma enhanced chemical vapor deposition (PECVD) SiN or SiO 2  layer  12  and be greater than 20 nm thick. The result the use of the layer  12  as both a surface passivation layer and a gate dielectric is a low-mobility channel with high on-resistance and low g m . In addition, prior art MIS-HFETs can suffer from threshold voltage hysteresis related to a poor quality interface between the layer  12  and the GaN epi layer  18 . 
     The prior art method of fabricating a normally-off low on-resistance GaN HFET, involves etching completely through the AlGaN barrier layer  14  in the gate  16  region of the device and depositing a gate dielectric material  12 , which forms an MOS-type interface in the channel under the gate, as described in H. Kambayashi, Y. Satoh, S. Ootomo, T. Kokawa, T. Nomura, S. Kato, and T. P. Chow, “Over 100 A normally-off AlGaN/GaN hybrid MOS-HFET on Si substrate with high-breakdown voltage”, Solid State Elec., vol. 54 issue 6 pp. 660-664 (2010), and T. Oka and T. Nozawa, “AlGaN/GaN recessed MIS-Gate HFET with high threshold voltage normally-off operation for power electronics applications”, IEEE Elec Dev. Lett. vol. 29 no. 7 (2008). Away from the gate  16  the AlGaN barrier layer  14  induces a high-density high-mobility 2DEG  24 , which results in low on-resistance. Although the prior art “hybrid” MOS- or MIS-HFET devices have been shown to result in normally-off operation with high breakdown voltage, the prior art hybrid MOS-HFET devices have the disadvantage of a low electron mobility in the active region under the gate due to a poor quality interface between the gate dielectric  12  and the underlying GaN layer  18 , resulting in increased on-resistance compared to a traditional GaN HFET. The performance of such “hybrid” MOS-HFET devices therefore is extremely sensitive to the quality of the gate dielectric  12  and its interface with the underlying channel layer. As shown in  FIG. 1  the GaN layer  18  may be doped with magnesium (Mg). 
       FIG. 2  shows an elevation sectional view of a hybrid MOS-HFET  30  in accordance with the present disclosure. The AlGaN barrier layer  32 , which also may be formed of AlN, AlInN, a combination of AlN spacer and AlGaN barrier, or a combination of AlN spacer and InAlN barrier, in the gate  34  region of the device is completely etched, resulting in normally-off operation, while a low on-resistance is maintained by the presence of a polarization-induced 2DEG  36  in the access regions between the AlGaN barrier layer  32  and the GaN channel layer  38 , and away from the gate  34 . The channel layer  38  may also be formed of InN, or InGaN, may be a 0001 oriented GaN layer, and in a preferred embodiment is not doped with magnesium. A passivation layer  44 , which may be PECVD SiN, SiO 2 , Al 2 O 3 , HfO 2 , TiO 2 , amorphous AlN, or polycrystalline AlN, and which may be about 20-100 nm thick, is used to passivate the AlGaN layer  32  between the gate  34  and source  40  and drain  42 . 
     A gate dielectric  46 , which may be Al 2 O 3 , or may be hafnium oxide (HfO 2 ), titanium oxide (TiO 2 ), SiN, SiO 2 , amorphous AlN, or polycrystalline AlN, surrounds the gate  34  and also covers the passivation layer  44  in the embodiment shown in  FIG. 2 . In another embodiment the gate dielectric  46  only surrounds the gate  34  and does not cover the passivation layer  44 . Having a separate passivation layer  44  and gate dielectric  46  is in contrast to prior art devices which use the same thick layer  12 , as shown in  FIG. 1 , as both a passivation layer and gate dielectric. 
     In the present disclosure the gate dielectric and surface passivation layers are different materials, and may be deposited by different deposition techniques, allowing independent optimization of gate characteristics and current collapse, respectively. The gate dielectric  46  may be deposited using atomic layer deposition (ALD), which has advantages compared to PECVD, and may consist of a high-k material such as Al 2 O 3 . Al 2 O 3  has a higher dielectric constant of approximately 9-10, compared to SiO 2 , which has a dielectric constant of approximately 6-7. In addition Al 2 O 3  has a larger bandgap of approximately 7 eV, compared to approximately 5 eV for SiO 2 , making Al 2 O 3  a superior gate dielectric material. 
     As further described below, hybrid MOS-HFETs in accordance with the present disclosure have been fabricated and tested with a SiN surface passivation layer  44  in combination with an ALD Al 2 O 3  gate dielectric  46 , with the Al 2 O 3  gate dielectric  46  annealed after deposition for improved oxide/epi interface quality. The test results indicate that hybrid MOS-HFETs in accordance with the present disclosure are normally-off with low gate current, high g m , high drain current, low current collapse, low hysteresis, low on-resistance, and high breakdown voltage. Together the results indicate that hybrid MOS-HFETs in accordance with the present disclosure have an approximately seven times (7×) improvement in Vb 2 /R on  figure-of-merit over the prior art hybrid MOS-HFET structures. 
     As described above, the hybrid MOS-HFETs in accordance with the present disclosure have gate dielectric and surface passivation layers that are different materials and that may be deposited by different deposition techniques, allowing independent optimization of gate characteristics and current collapse, respectively. The characteristics which make good surface passivation layers and good gate dielectrics are very different. Due to uncontrolled oxidation of the epi surface during deposition, oxygen-containing materials such as SiO 2  typically result in inferior surface passivation of GaN devices and poor current collapse suppression compared to oxygen-free materials, such as SiN, as described by X. Hu, A. Koudymov, G. Simin, J. Yang, and M. Asif Khan, “Si3N4/AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors”, Applied Phys. Lett., vol. 79 no. 17 p. 2832 (2001). 
     Optimal surface passivation layers are typically at least 30-50 nm thick in order to remove the surface of the dielectric, which can be a source of charging due to ionization of air, from the active region of the device as described by Y. Pei, S. Rajan, M. Higashiwaki, Z. Chen, S. P. DenBaars, and U. K. Mishra, “Effect of dielectric thickness on power performance of AlGaN/GaN HEMTs”, IEEE Elec Dev. Lett. vol. 30 no. 4 (2009). 
     Finally, low dielectric constants are desired for surface passivation layers in order to reduce parasitic capacitances, particularly for high-frequency applications. Gate insulator dielectrics, on the other hand, ideally have high dielectric constants and low thickness to achieve high device transconductance, in addition to a large bandgap and large band offset with the channel layer in order to reduce gate current. Uniformity, thickness control, and a low interface density of states (Dit) are critical gate insulator properties—especially for MOS-type devices, in which the conduction electrons are confined directly by the dielectric. For low leakage current, gate insulator dielectrics should be amorphous, as grain boundaries have been shown to act as leakage paths. Suitable high-k dielectrics with large bandgaps for electron confinement include amorphous oxides such as Al 2 O 3 , HfO 2 , and TiO 2 , which have been studied extensively for GaAs III-V and Si MOSFETs. The atomic layer deposition (ALD) deposition technique is ideally suited to deposition of gate dielectrics due to excellent conformality, thickness control, low deposition temperature (thermal processing budget), and plasma-free deposition, which avoids plasma-induced damage of the underlying epi. The extremely low deposition rates in ALD (˜1 A/cycle) make it suitable for very thin (&lt;20 nm thick) films. In contrast, typically, SiO 2  and SiN passivation layers are deposited by plasma-enhanced CVD techniques (PEVCD), which result in much high deposition rates but relatively poor film quality due to the high-energy plasma environment during deposition. Other techniques for deposition include metal-organic chemical vapor deposition (MOVCD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), e-beam evaporation, and sputtering. 
     In the hybrid MOS-HFET of the present disclosure, the AlGaN barrier layer  32  may be etched away to the channel layer  38  in the gate region using an atomic layer etching (ALE) technique, eliminating the polarization-induced 2DEG  36  under the gate electrode  34  only. The atomic layer etching (ALE) technique is described in U.S. patent application Ser. No. 12/909,497 filed Oct. 21, 2010, which is incorporated herein as though set forth in full. The lack of a 2DEG  36  at zero-bias conditions on the gate  34  results in normally-off operation, while surface passivation  44  in the access region between the gate  34  and the source  40  and between the gate  34  and the drain  42  results in low current collapse. The high density 2DEG  36  remains in the access regions of the device under the AlGaN barrier layer  32 , resulting in a low on-resistance. The gate  34  is insulated from the channel by an amorphous gate dielectric layer  46 . The device is considered a “hybrid” MOS-HFET structure because the electrons under the gate  34  and in the GaN 38 channel are directly in contact with the gate dielectric  46  as in a MOS device, while the electrons in the access regions away from the gate are confined by the wide bandgap AlGaN layer  32  and form a high-mobility 2DEG  36 , as in an HFET device. 
     In one embodiment of the hybrid MOS-HFET of the present disclosure, the gate dielectric  46  may be 2-20 nm-thick ALD Al 2 O 3 , and the passivation layer may be 20-100 nm-thick PECVD SiN. An optimized post-deposition anneal process, in which the Al 2 O 3  is annealed immediately following deposition may be used. The anneal process improves the Al 2 O 3 /GaN interface and results in reduced electron trap density and increased channel mobility compared to unannealed Al 2 O 3 . Test results show a normally-off with low on-resistance, high breakdown voltage, very low current collapse, low gate current, and drain current and transconductance, and a figure of merit Vb 2 /R on,sp , where Vb is the breakdown voltage and R on,sp  is the on-resistance normalized by the area of the transistor, equal to 260 MW/cm2, which as discussed above is approximately a 7-fold improvement over prior art hybrid MOS-HFET devices, which typically use a 60 nm-thick PECVD SiO 2  as both a gate dielectric and surface passivation layer. 
       FIGS. 3A and 3B  show transfer curves for hybrid MOS-FETs according to the present disclosure.  FIG. 3A  shows transfer curves for hybrid MOS-FETs fabricated without the post-deposition anneal process described above.  FIG. 3B  shows transfer curves for hybrid MOS-FETs which underwent a post-deposition anneal (PDA) immediately following the Al 2 O 3  deposition. The PDA significantly reduced the interface trap density, reflected in reduced threshold voltage hysteresis, as indicated in the up and down arrows in  FIGS. 3A and 3B , and also increased the gm and maximum drain current due to an increase in the channel electron mobility. 
       FIG. 4  shows the resulting pulsed and DC current voltage for hybrid MOS-FETs according to the present disclosure. The device has very low current collapse at a quiescent bias of Vds=+30V, Vgs=−2V, indicating successful suppression of surface charge trapping by the SiN passivation layer. Common-source DC current voltage and breakdown measurements are shown in  FIG. 5 . In the embodiment tested the gate periphery was 200 μm and the gate-drain spacing was 12 μm. The on-resistance was measured at Vgs=+3V was 16.6 ohm-mm, while the off-state three-terminal breakdown (measured at zero gate bias) was 1132V. The specific on-resistance was 4.9 mohm-cm 2 , leading to a high-voltage device figure-of-merit, Vb 2 /R on,sp , of 261 MW/cm 2 , which is a good figure of merit for a normally-off GaN device, and is an excellent figure of merit for a normally-off insulated-gate GaN device. 
       FIG. 6  compares this result with results for prior art normally-off GaN devices. The result  50  for a hybrid MOS-HFET of the present disclosure significantly out-performs the result  52  for a prior art hybrid MOS-HFET device with a SiO 2  layer used for both a gate dielectric and a passivation layer, as described in H. Kambayashi, Y. Satoh, S. Ootomo, T. Kokawa, T. Nomura, S. Kato, and T. P. Chow, “Over 100 A normally-off AlGaN/GaN hybrid MOS-HFET on Si substrate with high-breakdown voltage”, Solid State Elec., vol. 54 issue 6 pp. 660-664 (2010). 
       FIG. 7  shows common-source DC current and voltage measurements for a larger-periphery (20 mm gate width) device. In these measurements, the maximum drain current is greater than 3 A at a gate bias of +3V, while the gate current is on the order of 10 uA/mm. This demonstrates that large area devices with an ALD Al 2 O 3  gate dielectric  46  and passivation layer  44  are also feasible. The gate periphery, which is the perimeter of the gate, may range from about 200 μm to as large as 5 meters in length for power electronic applications. 
       FIG. 8  is a flow diagram of a method of fabricating a hybrid MOS-HFET in accordance with the present disclosure. In step  100  a channel layer  38  is formed. Then in step  102  a barrier layer  32  is formed over the channel layer. Next in step  104  a passivation layer  44  is formed over the barrier layer. Then in step  106  a first area of the passivation layer is etched away for a source electrode  40  and second area is etched away for a drain electrode  42 . Next in step  108  a source electrode  40  and a drain electrode  42  is formed on the barrier layer. Then in step  110  a third area  47  of the passivation layer and a fourth area  48  extending through the barrier layer for a gate electrode  34  is etched away. Next in step  112  a gate dielectric  46  is formed over the surface of the third and fourth area. Then in step  114  a gate electrode  34  is formed in the third area and in the fourth area. In this method, as described in step  116 , the passivation layer is a first material and the gate dielectric is a second material and the first material is different than the second material. 
     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 . . . .”