Abstract:
A two stage plasma etching technique is described that allows the fabrication of an enhancement mode GaN HFET/HEMT. A gate recess area is formed in the Aluminum Gallium Nitride barrier layer of an GaN HFET/HEMT. The gate recess is formed by a two stage etching process. The first stage of the technique uses oxygen to oxidize the surface of the Aluminum Gallium Nitride barrier layer below the gate. Then the second stage uses Boron tricloride to remove the oxidized layer. The result is a self limiting etch process that uniformly thins the Aluminum Gallium Nitride layer below the HFET&#39;s gate region such that the two dimensional electron gas is not formed below the gate, thus creating an enhancement mode HFET.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the area of fabrication of enhancement mode Gallium Nitride (GaN) Heterogeneous Field Effect Transistors (HFETS) or High Electron Mobility Transitors (HEMTs). In particular a method is described of fabricating a normally off device by selectively suppressing the formation of the two dimensional electronic gas (2DEG) below the gate area by thinning an Aluminum Gallium Nitride (AlGaN) barrier layer with a two stage plasma etching process such that the conduction band of the material structure is greater than the Fermi energy. 
     BACKGROUND OF THE INVENTION 
     This invention involves a two-step etch process that can be used to sequentially remove layers of material. This digital etching technique is particularly useful for the fabrication of semiconductor devices that require the ability to very accurately remove a few nanometers of material. One particular application is the conversion of AlGaN/GaN HFET/HEMT devices from depletion mode (normally-on operation) to enhancement mode (normally-off operation). Because of the polarization effects at the AlGaN—GaN interface, a 2DEG is formed at the interface [1], which creates a conduction path for normally-on, or depletion-mode (D-mode) devices, as shown in  FIG. 1 .  FIG. 1  shows device  100  comprising a D-mode AlGaN/GaN HFET structure, with continuous 2DEG  126  between the source  150  and drain  170  contacts. The device  100  comprises a buffer layer  130 , a GaN channel layer  120  and an AlGaN barrier layer  110 . The device  100  in  FIG. 1  further comprises a source  150 , drain  170 , gate  160  and passivation material  180 , which may be SiN. The vertical energy band diagram of the device is also shown on the right, with the charge density plot below showing the concentration of the electrons  11  in the 2DEG  126  along the length of the 2DEG  126 . The upper diagram on the right is the band energy diagram showing the conduction band  13 , valence band  12  and Fermi level  14 . 
     Depletion mode devices use a negatively biased gate on top of the AlGaN barrier layer to deplete the 2DEG of charge, and thus turn off the device. These devices are called “normally-on” because under no gate bias, there is a conduction path between the source and drain contacts.  FIG. 1  shows the GaN channel layer of a depletion mode device, in contact with an approximately 200 angstrom AlGaN layer. The associated band diagram shows the Fermi energy level as function of depth of the AlGaN layer measured from the upper surface of the AlGaN layer and the corresponding charge density as a function of this depth. The charge density plot shows the presence of the 2DEG at the interface between the AlGaN and GaN layers. Note at this depth, the Conduction band dips below the Fermi energy level. 
     A “normally-off” device would have negligible conduction between the source and drain under no gate bias. A normally-off device would be useful for power applications for increased safety and lower power consumption. GaN enhancement mode (E-Mode) HFETs/HEMTs have been fabricated by (a) growing a thinner AlGaN barrier layer [2], (b) implanting fluorine into the barrier under the gate [3], and (c) etching the barrier layer under the gate [4]. The numbers in square brackets refer to the references that follow. 
     By growing a thinner AlGaN barrier layer  210 , as shown in  FIG. 2 , the conduction band of the energy band diagram is shifted higher than the Fermi energy so that the 2DEG  226  does not exist, making the device normally-off However, the drawback to this approach is that the 2DEG  226  does not exist anywhere in the device, so there is no low-resistance, high-mobility conduction path between the source and drain. Furthermore, it is not practical to make a gate  260  long enough to span the entire source  250 -drain  270  gap to modulate a channel  220 . 
     By implanting fluorine ions into the AlGaN barrier under the gate, as shown in  FIG. 3 , the 2DEG is depleted under the gate near the fluorine ions, creating a break in the 2DEG channel between the source and drain. This break in the 2DEG channel results in a normally-off device, but because only the area under the gate is affected the gate is still able to modulate the channel by building up charge in that area and therefore turning the device on again. However this method suffers from a few drawbacks: (1) control of the fluorine dose is difficult, so repeatable fluorine treatment is an issue, (2) fluorine is usually implanted using a non-uniform low-power plasma hence the uniformity of the threshold voltage across a wafer is an issue when long gates are constructed, (3) the implant process inherently causes damage to the material, reducing performance, and (4) the stability of the implanted fluorine ions is unknown, so the reliability of the device is an issue. 
     The final method to create enhancement mode devices is to selectively remove the AlGaN barrier layer under the device gate, as shown in  FIG. 4 . This selective removal or “gate recess etch”, thins the AlGaN barrier sufficiently to shift the conduction band higher than the Fermi energy, as with the thin-grown AlGaN barrier method, depleting the 2DEG in the etched area under the gate. However, unlike the thin-grown AlGaN barrier method, the 2DEG is only depleted under the gate and continues to exist everywhere else in the device. Therefore, the device is normally-off, but the gate is still able to modulate the channel. Gate recess etching is usually achieved with a plasma dry etching system, such as RIE (Reactive Ion Etching) or ICP (Inductively Coupled Plasma). However, problems with the traditional method of gate recess etching include (1) that the plasma is usually not very uniform resulting in non-uniform threshold voltages as with fluorine treatment, and (2) the etches are usually done at sufficiently high plasma powers to cause deep damage in the material that is not removed by the etching process. 
     SUMMARY OF THE INVENTION 
     The purpose of the digital etching technique described in this invention disclosure is to have better control and uniformity of etching compared to other etching methods. The technique is particularly useful for creating normally-off devices from normally-on HFETs. This method is better than thin-grown AlGaN barrier layers because it does not sacrifice the beneficial properties of the 2DEG across the entire wafer. This method is better than the fluorine-treatment method because it does not depend on implanted ions in the barrier below the gate that may be unstable. Since it does not implant ions it results in less lattice damage, and it is semi-self limiting so it is more uniform. It is better than standard gate recess etching because it uses a semi-self-limiting etching process and therefore results in more uniform etching. 
     Use of this digital etching technique will have wide-spread application due to the improved control and uniformity it allows. However, it is particularly applicable for power electronics for car, aircraft and solar power manufacturers. Finally, as solar power becomes more prominent, efficient power distribution networks will also benefit from power electronics made using this digital etching technique. 
     GaN enhancement mode HFETs/HEMTs have been fabricated by (a) growing a thinner AlGaN barrier layer [2], (b) implanting fluorine into the barrier under the gate [3], and (c) etching the barrier layer under the gate [4]. By growing a thinner AlGaN barrier layer, as shown in  FIG. 2 , the conduction band of the energy band diagram is shifted higher than the Fermi energy so that the 2DEG does not exist, making the device normally-off. However, the drawback to this approach is that the 2DEG does not exist anywhere in the device, so all of the advantages of high-speed and high-power GaN HFETs do not exist either. Furthermore, it is not practical to make a gate long enough to span the entire source-drain gap to modulate a channel. By implanting fluorine ions into the AlGaN barrier under the gate, as shown in  FIG. 3 , the 2DEG is depleted under the gate near the fluorine ions, creating a break in the 2DEG channel between the source and drain. This break in the 2DEG channel results in a normally-off device, but because only the area under the gate is affected the gate is still able to modulate the channel by building up charge in that area and therefore turning the device on again. However this method suffers from a few drawbacks: (1) control of the fluorine dose is difficult, so repeatable fluorine treatment is an issue, (2) fluorine is usually implanted using a non-uniform low-power plasma in a dry-etching system, so the uniformity of threshold voltage across a wafer is an issue, (3) the implant process inherently causes damage to the material, reducing performance, and (4) the stability of the implanted fluorine ions is unknown, so the reliability of the device is an issue. The final method to create enhancement mode devices is to selectively etch away the AlGaN barrier layer under the device gate, as shown in  FIG. 4 . This selective gate etch, or “gate recess etch”, thins the AlGaN barrier sufficiently to shift the conduction band higher than the Fermi energy, as with the thin-grown AlGaN barrier method, thus depleting the 2DEG in the etched area under the gate. However, unlike the thin-grown AlGaN barrier method, the 2DEG is only depleted under the gate and continues to exist everywhere else in the device. Therefore, the device is normally-off, but the gate is still able to modulate the channel. Gate recess etching is usually achieved with a plasma dry etching system, such as RIE or ICP. However, problems with the traditional method of gate recess etching include (1) that the plasma is usually not very uniform resulting in non-uniform threshold voltages as with fluorine treatment, and (2) the etches are usually done at sufficiently high plasma powers to cause deep damage in the material that is not removed by the etching process. There are a few established methods for digital etching for GaAs [5], but very little work has been done for GaN. A digital etching technique has been presented that uses a dry plasma step and a wet etch step to remove 5-6 Angstroms per step [6], but the drawback of this technique is that it could take more than 6 hours to remove 100 Angstroms for a dry-wet processing step that took 20 minutes per cycle. 
     These problems and others are overcome by using a two step plasma etching process to thin the AlGaN layer where the Gate is formed. In the first step the AlGaN is oxidized with an O 2  plasma. In the second step, the Gate area in the AlGaN is etched with a Boron tricloride plasma. Since the oxidation of the AlGaN/GaN is self limiting in that only the top layer of the AlGaN/GaN is oxidized and the Boron tricloride etches the oxide layer at a faster rate than the non-oxidized AlGaN/GaN, the result is a self limiting etching of the AlGaN/GaN layer. By repeating cycles of O 2  and BCl 3  the AlGaN/GaN may be etched in a very controlled manner one or a few atomic layers at a time. Furthermore, since moderate powers and gas flow rates are used, the etching does not introduce deep damage to the surface of the AlGaN. The thickness of the oxide layer in the AlGaN/GaN material depends on the plasma power used to oxidize the material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
         FIG. 1 : Depletion mode AlGaN/GaN HFET structure, with continuous 2DEG between the source and drain contacts. The vertical energy band diagram of the device is also shown on the right, with the charge density plot below showing the concentration of the electrons in the 2DEG. 
         FIG. 2 : Enhancement mode AlGaN/GaN HFET structure fabricated with a thin AlGaN barrier layer, resulting in no 2DEG. 
         FIG. 3  Enhancement mode AlGaN/GaN HFET structure fabricated with implanted fluorine ions in the AlGaN barrier under the gate, which depletes the 2DEG under the gate, allowing the gate to modulate the channel but making the device normally-off. 
         FIG. 4  Enhancement mode AlGaN/GaN HFET structure fabricated with a gate recess etch, which depletes the 2DEG under the gate much like the thin-grown AlGaN barrier structure, but only under the gate. Therefore, the excellent properties of the AlGaN/GaN 2DEG still exist everywhere but under the gate, and the gate is able to modulate the channel in that area. 
         FIG. 5  Measured data of etch rate per cycle for a BCl 3  plasma time per cycle and varying oxygen plasma time per cycle. 
         FIG. 6  Timeline of digital etching technique as described in this invention disclosure. For each cycle, an oxygen plasma is exposed to the AlGaN/GaN surface for an amount of time, followed by a BCl 3  plasma for a separate amount of time. The diagram shows two cycles of the process. 
         FIG. 7  Atomic Force Microscopy (AFM) image showing effects of two stage O2-BCl 3  plasma etching on surface roughness. The AFM image shows a step edge feature showing the surface remains smooth. 
         FIG. 8  Device characteristics of a standard fluorine-treatment enhancement mode device compared to a device processed using the ALE technique with oxygen and BCl 3 , showing higher peak transconductance (gm) for the ALE-processed device. 
         FIG. 9  Output characteristics of devices in the off-state (Vgs=−1V) showing the wafer processed with the ALE process has almost 2 orders of magnitude lower leakage current than the wafer processed with the standard fluorine-treatment. 
         FIG. 10  Histogram of threshold voltage distribution for the ALE-processed devices and fluorine-processed devices showing a narrower distribution of threshold voltage (i.e. more uniform) for the ALE-processed devices. 
         FIG. 11  Correlation between threshold voltage and sheet resistance for ALE-processed wafer compared to the standard-fluorine-treatment wafer. 
         FIG. 12 : Pulsed IV data comparing ALE-processed wafer with standard fluorine treatment wafer, showing ALE wafer has slightly lower current collapse. 
         FIG. 13   a - d  Process flow for fabrication of enhancement mode GaN HFET with two stage O 2  BCl 3  plasma edging. 
     
    
    
     Figures use consistent reference numbers in that the two least significant digits of a reference number refers to a component with the same description. For example, barrier layer  230  in  FIG. 2  has the same description as barrier layer  130  in  FIG. 1 . 
     The following papers are incorporated by reference as though fully set forth herein:
     [1] O. Ambacher, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Murphy, W. J. Schaff, L. F. Eastman, R. Dimitrov, L. Wittmer, M. Stutzmann, W. Rieger, and J. Hilsenbeck, “Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N-And Ga-face AlGaN/GaN heterostructures,”  Journal of Applied Physics , vol. 85, pp. 3222-3233, 1999.   [2] M. A. Khan, Q. Chen, C. J. Sun, J. W. Yang, M. Blasingame, M. S. Shur, and H. Park, “Enhancement and depletion mode GaN/AlGaN heterostructure field effect transistors,”  Applied Physics Letters , vol. 68, pp. 514-516, 1996.   [3] Y. Cai, Y. Zhou, K. J. Chen, and K. M. Lau, “High-performance enhancement-mode AlGaN/GaN HEMTs using fluoride-based plasma treatment,”  IEEE Electron Device Letters , vol. 26, pp. 435-437, 2005.   [4] J. S. Moon, D. Wong, T. Hussain, M. Micovic, P. Deelman, M. Hu, M. Antcliffe, C. Ngo, P. Hashimoto, and L. McCray, “Submicron enhancement-mode AlGaN/GaN HEMTs,”  Device Research Conference,  2002. 60 th DRC. Conference Digest , pp. 23-24 2002.   [5] A. Ludviksson, M. Xu, and R. M. Martin, “Atomic layer etching chemistry of C12 on GaAs (100),”  Surface Science , vol. 277, pp. 282-300, 1992.   [6] D. Buttari, S. Heikman, S. Keller, and U. K. Mishra, “Digital Etching for Highly Reproducible Low Damage Gate Recessing on AlGaN/GaN HEMTs,” in  Proceedings IEEE Lester Eastman Conference on High Performance Devices , Newark, Del., 2002, pp. 461-469.   [7] D. Buttari, A. Chini, T. Palacios, R. Coffie, L. Shen, H. Xing, S. Heikman, L. McCarthy, A. Chakraborty, S. Keller, and U. K. Mishra, “Origin of etch delay time in C12 dry etching of AlGaN/GaN structures,”  Applied Physics Letters , vol. 83, pp. 4779-4781, 2003.   

     DETAILED DESCRIPTION 
     In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the described exemplary embodiments may be modified in various ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive. 
     While the invention has been described in connection with certain exemplary embodiments, it is to be understood by those skilled in the art that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications included within the spirit and scope of the appended claims and equivalents thereof. 
     A thin-grown AlGaN barrier layer is undesirable because of the inability to conduct from source to drain. Fluorine-treatment is undesirable because of non-uniformity, implant damage and reliability concerns. Therefore, gate recess etching is the most promising technique for fabricating enhancement mode HFET/HEMT devices, but the non-uniformity and plasma damage issues must be addressed. It would be ideal to have a self limiting etch process that would allow precise control over the etching, remove non-uniformity issues, and potentially minimize damage to the material. There are a few established methods for digital etching for GaAs [5], but very little work has been done for GaN. A digital etching technique has been presented that uses a dry plasma step and a wet etch step to remove 5-6A per step [6], but the drawback of this technique is that it could take more than 6 hours to remove 100A if the dry-wet processing steps took 20 minutes per cycle. Therefore, there has been no repeatable method for a semi-self-limiting etching process of GaN for improved uniformity in a short time period until now. 
     This disclosure describes a self-limiting, two-step etching technique that can be applied to AlGaN/GaN HFET structures for a more uniform, less damaging gate recess etch process for enhancement mode, or normally-off devices. It has been shown that a low-power oxygen plasma creates an oxidized layer on an AlGaN/GaN surface [6]. It has also been seen that this oxidized AlGaN/GaN layer has a self-limiting depth with time, but is linear with plasma power. However, this oxide layer is not easily removed with conventional Cl 2  plasma etching. On the other hand, it has been shown that a low-power BCl 3  plasma is very effective at removing the surface AlGaN/GaN oxide (better than Cl 2 ), but less effective at etching GaN than Cl 2  [7], which provides some selectivity for a semi-self-limiting etch process. By combining the AlGaN/GaN oxidation step with the BCl 3  oxide etch step, as shown in  FIG. 6 , a digital, or atomic layer etching (ALE) technique is created which can be used to remove angstroms of material for each cycle. 
     Pieces of GaN on Si masked with photoresist or SiN were processed in a RIE plasma etcher with oxygen and BCl 3 . For the oxygen plasma step of the cycle, the oxygen flow rate was 45 sccm (standard cubic centimeters per second), the chamber pressure was 100 mTorr, the RIE power was 50 W, and the time was varied. For the BCl 3  plasma step of the cycle, the BCl 3  flow rate was 10 sccm, the chamber pressure was 10 mTorr, the RIE power was 15 W, and the time was fixed. GaN samples were etched the necessary number of cycles to result in approximately 100 nm of etched material. After removing the photoresist or SiN mask, the depth of the etch was measured using atomic force microscopy (AFM), and the etch rate was found by dividing the etch depth by the number of cycles. For a BCl 3  time of 60 sec per cycle, the data shown in  FIG. 5  shows that the etch rate per cycle increases with oxygen plasma time per cycle up to 15 seconds of oxygen plasma per cycle. Above 15 seconds of oxygen plasma per cycle, the etch rate per cycle remains fairly constant at almost double the etch rate per cycle of no oxygen plasma per cycle. Therefore, for BCl 3  etch step conditions, this technique is shown to be a self-limiting etch process, which produces improved uniformity across a wafer compared to fluorine implantation treatment, or standard gate recess etching using a plasma etcher. Furthermore, since the BCl 3  etch step uses such a low power (15 W), and mostly etches only the oxidized layer, this process results in low damage to the remaining material. An AFM image of an etched sample is also shown in  FIG. 7 , which shows the roughness of the etched surface  712  is similar to the roughness of the unetched surface  711 . 
     To show the advantages of the ALE process compared to the standard fluorine-treatment process, two GaN on Si wafers from the same chemical vapor deposition (CVD) growth were processed side-by-side using the same process steps except for the depletion-to-enhancement mode conversion step. Input characteristics of representative devices from each wafer are shown in  FIG. 8 . This data shows that the ALE-processed wafer has almost 3 times higher peak g m  than the standard fluorine-treatment process (gm is the transconductance of the transistor). Transconductance is the derivative of the drain current with respect to the gate to source voltage. The characteristics of devices in the off-state (Vgs=−1V) from each of the two wafers are shown in  FIG. 9 .  FIG. 9  shows that the leakage current (measured when the Gate to Source voltage is such that the device is biased in the off state) of the ALE-processed wafer is about 2 orders of magnitude lower than the standard fluorine-treatment devices, and the ALE data is more uniform than the fluorine-treatment data. Furthermore, pulsed IV data shown in  FIG. 12  shows that the ALE-processed wafer has slightly better current collapse than the wafer with standard fluorine treatment. 
       FIG. 3  shows the cross section of a prior art enhancement mode HFET  300  fabricated using fluorine implantation to prevent the 2DEG under the gate region. The structure  300  is substantially the same as the structure  100  in  FIG. 1  except fluorine ions  327  have been implanted in the barrier layer  310  below the gate  360 . The result is an enhancement mode device because the break in the 2DEG  326  requires a positive gate source voltage above a threshold to supply the carriers needed for conduction. 
       FIG. 4  shows an embodiment  400  of the present invention comprising a buffer layer  430  which may be AlGaN, a GaN channel layer  420 , an AlGaN barrier layer  410 , a source  450 , gate  460  and drain  470 . The AlGaN barrier layer  410  is etched a distance  424  such that the remaining distance  422  between the Gate  460  and the GaN channel  420  is insufficient for the formation of the 2DEG  426 , and the device  400  channel  420  has no continuous path at zero bias. This is shown by the 2DEG  426  absence under the gate  460  as a result of thinned barrier  410 , which reduces polarization-induced charge. In a preferred embodiment, the distance  422  is approximately 10 nm. The thickness of the AlGaN barrier layer  410  is approximately 20 nm. The AlGaN layer  410  may range in thickness between 5 and 50 nm. The AlGaN thickness  422  under the gate  460  is generally half the thickness of the AlGaN barrier layer  410 . A person skilled in the art will realize the thicknesses  422  and  424  used will depend on the concentration of aluminum in the AlGaN layer  410 . Not shown in  FIG. 4  is the substrate which may be silicon or silicon carbide. The buffer layer  430  is chosen such that it lattice matches the GaN layer  420 . One such buffer  420  material is AlGaN with an appropriate concentration of aluminum. 
       FIG. 5  shows the etch rate per cycle of Oxygen and Boron tricloride. The Boron tricloride plasma etch time is substantially 60 seconds while the oxygen plasma oxidation time is varied. Note that beyond 15 seconds of oxygen plasma oxidation time, the etch rate per cycle does not substantially change. While the data in  FIG. 5  is for calibrating the etch rate of GaN, AlGaN will etch substantially identically because the Aluminum does not prevent the oxidation or the etching by BCl 3 .  FIG. 5  shows etching 2.5 nm of GaN will take 20 seconds for the O 2  and 60 for the BCl 3  or 80 seconds. To etch 100 angstroms or 10 nm will take four cycles or 320 seconds. 
       FIG. 6  shows a preferred embodiment of the two phase plasma etch process. In the first stage the GaN is oxidized with oxygen at approximately 100 mTorr, approximately 45 sccm (standard cubic centimeters per minute) and approximately 50 watts. In the second stage the oxidized AlGaN layer is etched or removed with Boron tricloride plasma at approximately 10 mTorr, approximately 10 sccm and approximately 15 watts. Preferred Oxygen plasma oxidation times are greater than 15 to 20 seconds at 50 watts and 45 to 90 seconds for Boron tricloride plasma etching at 15 watts. A person skilled in the art will appreciate the oxidation rates and etch rates may vary with the plasma power. 
       FIG. 7  shows a sample of GaN etched according to the process of the present disclosure as shown by Atomic Force Microscopy. The surface  712  was etched according to embodiment of the present disclosure and the surface  711  was not. The surface roughness of  711  and  712  is substantially the same. 
       FIG. 8  shows the characteristics of an HFET device fabricated according to embodiment of the present disclosure and of a device fabricated using Fluorine implantation. The drain current  82  for the device fabricated according to an embodiment of the present disclosure has a greater slope than the device fabricated using the fluorine implantation process  84  as well as a greater value for a given gate-source voltage. This is also seen in the respective transconductance curves  81  and  86  where the transconductance  81  of the device fabricated according to an embodiment of the present invention has a greater slope and peak value than the transconductance curve  86  of the device fabricated using Fluorine implantation. The drain current is measured in units of milli amps per mm of gate width. In  FIGS. 3 and 4  the gate width is normal to the figure. The transconductance is the derivative of the drain current with respect to the gate to source voltage. 
       FIG. 9  shows the drain current of a device fabricated according to an embodiment of the present invention (curves  95 ) and of a device fabricated using fluorine treatment (curves  93 ), when the gate to source voltage is minus one volt such that the device is off. The dispersion of the drain currents  95  fabricated with ALE is approximately 28 nanoamps/mm. For the Fluorine fabricated HFETs the dispersion in the drain currents  93  is approximately 2 microamps/mm.  FIG. 9  shows an approximately two order of magnitude decrease in leakage current, from 2E-6 to 2.8E-8 amps/mm. 
       FIG. 10  shows a measure of the uniformity of the ALE fabricated HFETs and the fluorine fabricated HFETs in terms of the threshold voltages. A smaller standard deviation is preferred since it allows for more predictable device performance.  FIG. 10  shows a histogram of threshold voltage distribution for devices from the ALE-processed wafer along with devices from the fluorine-treatment wafer. The histogram shows that the ALE process has a narrower threshold voltage distribution compared to the standard fluorine-treatment wafer, meaning that the ALE process results in more uniform threshold voltages compared to the fluorine-treatment method. The ALE processed Enhancement mode HFETs have a median threshold voltage of 0.2 volts with a standard deviation of 0.063 volts. The median threshold voltage for the HFETs produced with the Fluorine process is 1.17 volts with a standard deviation of 0.2 volts. The value of the device threshold voltage is dependent on the thickness of the thinned ALGaN barrier below the gate. 
       FIG. 11  shows the correlation between the threshold voltage and the sheet resistance for the two devices, showing the narrower distribution of device characteristics for the ALE fabricated device. The correlation coefficient “con” indicates the spread of the data around the best fit straight line. Ideally “con” is one. The “Fit Slope” is the slope of the best fit line to the data and has units of volts/ohm/sq. As can be seen from  FIG. 11 , devices fabricated according to an embodiment of the present invention have a lower sheet resistance and threshold voltage than those fabricated using Fluorine implantation. 
       FIG. 12  shows pulsed IV data comparing ALE-processed wafer with standard fluorine treatment wafer, showing ALE wafer has slightly lower current collapse. Current collapse is the difference in drain current measured with a static drain-source voltage versus a pulsed drain-source voltage. In the pulsed drain-source voltage the drain-source voltage is decreased from a quiescent bias point to the value used in a static measurement for a time short enough that current transients persist. Current collapse is a measure of the bandwidth of the device. The greater the current collapse, the lower the bandwidth. In addition, the drain current versus the drain-source voltage for a device is lower, the transconductance of the device is lower, and the on resistance is higher for a device operating with a dynamic drain-source and gate-source voltage. 
       FIGS. 13   a  through  13   d  illustrate a preferred embodiment for fabricating an enhancement mode HFET in GaN. In the first step a lattice matching buffer layer  1330  is placed by methods known in the art on a substrate (not shown). The substrate may be silicon or silicon carbide or other material compatible with the lattice matching buffer layer. On top of lattice matching buffer layer  1330  a GaN layer  1320  is fabricated such that the GaN layer is Ga Polar towards the AlGaN barrier layer  1310 . A barrier layer  1310  of AlGaN is then formed. Next a gate recess  1340  is fabricated by masking of the gate region then using a two step plasma etching process to etch the AlGaN such that a desired thickness of AlGaN remains between the gate and the GaN channel layer  1320 . The device is further fabricated by standard techniques, as shown in  FIG. 30   c , by fabricating a gate  1360 , source  1350  and drain  1370  on the device. The source and drain are connected to the GaN channel  1320  by known techniques such as annealing or etching the ohmic contacts. Finally, the device is passivated using know techniques by adding a passivation coating  1380 .