Active and Passive Electronic Components

Volume 2012 (2012), Article ID 459043, 10 pages

doi:10.1155/2012/459043

Research Article

GaN-Based High-k Praseodymium Oxide Gate MISFETs with + UV Interface Treatment Technology

Chao-Wei Lin and Hsien-Chin Chiu

Department of Electronics Engineering, Chang Gung University, Taoyuan, Taiwan

Received 1 July 2012; Revised 28 September 2012; Accepted 26 October 2012

Academic Editor: Pei-Wen Li

Copyright © 2012 Chao-Wei Lin and Hsien-Chin Chiu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

This study examines the praseodymium-oxide- (Pr₂O₃-) passivated AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) with high dielectric constant in which the AlGaN Schottky layers are treated with P₂S₅/(NH₄)₂ + ultraviolet (UV) illumination. An electron-beam evaporated Pr₂O₃ insulator is used instead of traditional plasma-assisted chemical vapor deposition (PECVD), in order to prevent plasma-induced damage to the AlGaN. In this work, the HEMTs are pretreated with P₂S₅/(NH₄)₂ solution and UV illumination before the gate insulator (Pr₂O₃) is deposited. Since stable sulfur that is bound to the Ga species can be obtained easily and surface oxygen atoms are reduced by the P₂S₅/(NH₄)₂ pretreatment, the lowest leakage current is observed in MIS-HEMT. Additionally, a low flicker noise and a low surface roughness (0.38 nm) are also obtained using this novel process, which demonstrates its ability to reduce the surface states. Low gate leakage current Pr₂O₃ and high-k AlGaN/GaN MIS-HEMTs, with P₂S₅/(NH₄)₂ + UV illumination treatment, are suited to low-noise applications, because of the electron-beam-evaporated insulator and the new chemical pretreatment.

1. Introduction

Because of their inherent high breakdown voltage (), high two-dimensional electron gas (2-DEG) concentration, and high saturation velocity [1, 2], and AlGaN/GaN high electron mobility transistors (HEMTs) are suitable to high-power and low-noise applications. The major factors that limit the performance and reliability of GaN-based HEMTs at radio frequencies (RF) are their high gate leakage current and drain current collapse, which is associated with native oxide-induced surface states [3, 4]. Therefore, AlGaN/GaN metal-insulator-semiconductor HEMTs (MIS-HEMTs), in which SiO₂ [5], Si₃N₄ [6], Ga₂O₃ [7], Al₂O₃ [8], and Sc₂O₃ [9] are used as the gate dielectrics, are studied, in order to address these problems. Related works focus on the formation of a high-k insulator, which reduces the Schottky gate leakage current at high input signal swings and improves channel modulation. However, the treatment of the interface between AlGaN and the insulator has not been studied systematically. Pretreatment before the deposition of the passivating layer between the source, the drain, and the gate terminals is dominated by the effect of surface traps, which cause flicker noise and current collapse problems. For example, (NH₄)₂ sulfide treatment is known to eliminate native Ga₂O₃ and As₂O₃ dangling bonds on GaAs- and InP-related semiconductors, because of the formation of stable Ga-S and As-S bonds during immersion [10, 11]. In this work, a P₂S₅/(NH₄)₂ + UV treatment that suppresses surface traps is studied. Furthermore, to increase the efficiency of the P₂S₅/(NH₄)₂ treatment, the treatment is performed in a UV chamber and both the high-k   Pr₂O₃ gate insulator and the passivating layer are deposited using electron-beam evaporation, which effectively prevents the plasma-induced generation of surface states. A comparison of the flicker noise determined from the pulsed I-V of Pr₂O₃ AlGaN/GaN MIS-HEMT with that of traditional GaN HEMTs shows that the surface traps are markedly suppressed by P₂S₅/(NH₄)₂ + UV treatment. The　observed lower surface leakage current also improves the DC-RF dispersion of MIS-HEMT. X-ray photoelectron spectroscopy (XPS) measurement and secondary ion mass spectrometry (SIMS) are used to study the Ga-S energy bonds and the distributions of the depths of oxygen and sulfur atoms, following P₂S₅/(NH₄)₂ + UV treatment.

2. Device Structure and Fabrication

The AlGaN/GaN HEMT heterostructures used in this study were grown using atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) on 2 inch sapphire wafers. The 4000 nm-thick undoped GaN was grown first, to form the buffer and channel layers. Then, a 35 nm-thick undoped Al0.25Ga0.75N layer was grown as the Schottky layer. The designed structure had a sheet charge density of 1.65 × 10¹³ cm⁻² and a Hall mobility of 1060 cm²/V-s at 300 K. Figure 1 shows the cross-sections of a Pr₂O₃/AlGaN/GaN MIS-HEMT with P₂S₅/(NH₄)₂ + UV interface pretreatment. During fabrication of the device, the active region was protected by a photoresist and the mesa isolation region was removed using a BCl₃ + Cl₂ mixture gas plasma in a reactive ion etching (RIE) chamber. The ohmic contacts of the Ti/Al/Ni/Au (25 nm/125 nm/50 nm/100 nm) metal layers were deposited using electron-beam evaporation and patterned by conventional optical lithography and lift-off method, followed by 850°C RTA annealing for 30 s in an N₂ environment. Before the deposition of the high-k Pr₂O₃ insulator and the passivating layers, the samples were immersed in a standard treatment (dilute HCl), (NH₄)₂ and P₂S₅/(NH₄)₂ for 15 min. Figure 2 shows the pH value of the P₂S₅/(NH₄)₂ solution versus P₂S₅ weight. The pH of the saturated (NH₄)₂ solution is 11.2 and the pH of the mixed P₂S₅/(NH₄)₂ solution was adjusted to a value of 7 by adding 10 g of P₂S₅ into 30 mL (NH₄)₂ saturated solution. Although the (NH₄)₂ treatment effectively removes the native AlGaN surface oxide layer, it cannot prevent the increase in surface roughness that is caused by the alkaline (NH₄)₂ solution. After 15 min of pretreatment immersion, the AlGaN surface roughness was 0.24 nm for the standard treatment and 0.51 nm and 0.44 nm for the (NH₄)₂ and P₂S₅/(NH₄)₂ solution. Therefore, the P₂S₅/(NH₄)₂ solution was used as the interface pretreatment solution for the MIS-HEMT. In an earlier study by the authors, the reduction of the number of native oxide-induced dangling bonds on the AlGaN surface, which are difficult to remove at room temperature, was achieved by increasing the temperature of the solution temperature to 60°C, but this increase makes the procedure more complicated and time-consuming. In this study, UV illumination is used during P₂S₅/(NH₄)₂ pretreatment immersion, in order to rapidly eliminate the dangling bonds and form Ga-S bonds with high-binding energy. After interface treatment, a 10 nm-thick layer of praseodymium was firstly evaporated, using an optimal oxygen flow rate of 15 sccm. During this stage, the chamber pressure was increased to around 10⁻⁴ Torr. When the chamber pressure was reduced to Torr, the conventional Ti/Au (30 nm/150 nm) gate metals were deposited. For comparison, a traditional Ni/Au Schottky gate GaN HEMT was also fabricated. Finally, the Ti/Au (30 nm/300 nm) metals were deposited to form the interconnection and probe pads, and a 200 nm-thick SiO₂ layer was deposited to passivate the device. The complete process was also used to fabricate a traditional Ni/Au Schottky gate GaN HEMT, for the purpose of comparison.

Figure 1: The cross-sectional structure of a P₂S₅/(NH₄)₂+UV pretreatment Pr₂O₃ MISFET.

Figure 2: pH value of P₂S₅/(NH₄)₂ solution versus P₂S₅ weight.

The important optimization of the flow rate of the praseodymium oxide high-k layer was also considered. The electron-beam evaporation of the high-k Pr₂O₃ thin film is optimized by adjusting the oxygen flow rate in the chamber. Figure 3 shows the EDX measurements for Pr₂O₃ grown in the electron-beam evaporator using various oxygen flow rates. An analysis of the rare-earth metal atomic concentration in the oxide layer demonstrates that the optimal flow rate for the deposition of the praseodymium oxide layer is 15 standard cubic centimeters per minute (sccm), indicating that the dielectric constant is maximized, because a strong dipole is formed at the highest possible rare-earth metal concentration in the high-k oxide layer. The thermal stability of the insulator also plays an important role in high-power GaN MIS-HEMTs because the dc power is primarily dissipated near the gate contact, which causes local Joule self-heating [10]. The channel temperature can reach 100°C during high-output-power operation, which results in increased degradation and failure rates and a reduction in output power. In order to evaluate the thermal stability of Pr₂O₃ layer, the X-ray photoelectron spectroscopy (XPS) was used to measure the binding energy of the Pr₂O₃ thin films after 400°C, 600°C, and 800°C postannealing. Figure 4(a) shows the XPS 3d core-level spectra for Pr₂O₃ at various temperatures. The energy of the Ar-gun is 3 KeV, the operation current is 1 mA, and the analysis area is  mm². It is seen that the binding energies of the Pr₂O₃ after 400°C, 600°C, and 800°C postannealing are close to the standard value of 934 eV in the 3d core level, as recorded in the XPS handbook. Figure 4(a) also shows the high signal intensities of Pr₂O₃. Therefore, it is concluded that a high-quality and a highly thermally stable high-k insulator is obtained by using electron-beam-evaporated Pr with a high oxygen flow rate. The results of high-resolution cross-sectional transmission electron microscopy (TEM) prove that the Pr₂O₃ grows on the GaN in a planar fashion, as shown in Figure 4(b). The Pr₂O₃ equivalent oxide thickness, measured by the TEM, is 20 nm. A (Ga₂O₃)Pr₂O₃ compound film occurs between the interface of the GaN and Pr₂O₃ layers, which is generated by the native GaN surface oxide layer and praseodymium materials.

Figure 3: EDX analysis of Pr₂O₃ films with various oxygen flow rates.

(a)

(b)

Figure 4: (a) 3d core-level XPS spectra of Pr₂O₃ versus temperatures. (b) TEM cross-sectional photograph of Pr₂O₃ MIS-HEMT.

3. Experimental Results for the Device

Atomic force microscopy (AFM) or scanning force microscopy (SFM) has a very high resolution, with a demonstrated resolution of the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. It is also one of the most widely used tools for imaging, measuring, and manipulating matter at the nanoscale. The information is gathered by “feeling” the surface with a mechanical probe. Piezoelectric elements that facilitate tiny but accurate and precise movements on (electronic) command enable very precise scanning [11]. Figure 5(a) and Figure 5(b) show the 2D and 3D images of the surface roughness on the AlGaN/GaN surface with different treatments, as measured using a Park Systems XE-70. The P₂S₅/(NH₄)₂ + UV-treated surface exhibits a better roughness than the other sulfurization treatments and forms a superior interface between the AlGaN Schottky layer and the Pr₂O₃ high-k gate insulator layer.

(a)

(b)

Figure 5: (a) 2D images of the surface roughness on AlGaN/GaN surface evaluated with different treatment. (b) 3D images of the surface roughness on AlGaN/GaN surface evaluated with different treatment.

Table 1 shows the mobility, sheet charge density, and surface roughness for variously treated devices, characterized by Hall measurement at 300 K. The P₂S₅/(NH₄)₂ + UV-treated devices have a sheet charge density of 1.403 × 10¹³ cm⁻² and a Hall mobility of 1150 cm²/V-s at 300 K; these values are, respectively, 1.648 × 10¹³ cm⁻², 1060 cm²/V-s and 1.512 × 10¹³, 1087 cm²/V-s for the standard treatment and (NH₄)₂ + UV treatment for the devices. These results clearly demonstrate that P₂S₅/(NH₄)₂ + UV treatment improves the channel mobility by reducing the number of surface traps.

Table 1: The Hall measurement and surface roughness results for various treatment devices.

Pretreatment
Mobility

(cm²/V-sec)
Sheet charge density (cm⁻²)
Surface roughness (nm)

Standard
1060
1.648 × 10¹³
0.238

(NH₄)₂
985
1.784 × 10¹³
0.514

(NH₄)₂ + UV
1087
1.512 × 10¹³
0.471

P₂S₅/(NH₄)₂
1100
1.485 × 10¹³
0.443

P₂S₅/(NH₄)₂ + UV
1150
1.403 × 10¹³
0.382

The surface of composite semiconductor substrates was subject to heat treatment, using an ultraviolet (UV) light, in order to enhance the surface reaction [12]. The photoluminescence (PL) measurements in Figure 6 apply to the complete structure of AlGaN/GaN after sulfurization and demonstrate that P₂S₅/(NH₄)₂ + UV treatment yields a greater PL intensity for the AlGaN Schottky layer than either P₂S₅/(NH₄)₂ treatment or standard treatment. The increase in PL intensity is due to the elimination of surface states, which generates nonradiative recombination centers on the AlGaN surface [13]. No obvious Al0.25Ga0.75N signal is evident, because the laser used is He-Cd laser and the PL signal intensity of Al0.25Ga0.75N is much smaller than that for GaN. Because only 35 nm AlGaN was grown on GaN for the PL evaluation structure after sulfurization, the Al0.25Ga0.75N signals are not obvious in the PL measurements. Although the P₂S₅/(NH₄)₂ treatment can suppress the surface state density, as demonstrated by the PL results, a more concentrated P₂S₅/(NH₄)₂ solution with UV illumination treatment produces a stable phosphorus oxide layer and more Ga-S bonds, because the phosphorus and sulfur concentrations are higher [14].

Figure 6: PL measurement results for AlGaN/GaN full structure standard, P₂S₅/(NH₄)₂SX and P₂S₅/(NH₄)₂SX + UV treatments.

Figure 7 shows the and XPS spectra of the variously treated devices. The sulfur 2p core level and the binding energy of pure sulfur are all 163.8 eV. The binding energy of gallium is 160 eV at the signal peak in the spectrum of the standard treated GaN sample while the signal peak of the P₂S₅/(NH₄)₂-treated device is shifted to 160.3 eV, because the AlGaN surface contains Ga-S bonds (Ga-S = 163.2 eV) after this process. However, P₂S₅/(NH₄)₂ + UV treatment shifts the signal peak in the spectrum to 160.4 eV, with a linear distribution of intensity from 160 eV to 161 eV. The peaks are shifted to a higher binding energy after P₂S₅/(NH₄)₂ + UV treatment. This phenomenon is related to the Ga sulfurized states [15]. Therefore, more Ga-S bonds are generated on the AlGaN surface by P₂S₅/(NH₄)₂ + UV treatment than by P₂S₅/(NH₄)₂ treatment and these high-energy bonds are more stable than the Ga-O bonds that are formed in a moist environment.

(a)

(b)

Figure 7: The XPS spectra of (a) and (b) and core level of various treatment samples.

In order to investigate the material atomic composition of the AlGaN surface after various sulfide treatments, the samples were subject to secondary ion mass spectroscopy (SIMS). Figure 8 shows the sulfur and oxygen atom concentration profiles of the samples treated using the three aforementioned methods. Based on the measurement results shown in Figure 8(a), the concentration of S atoms in the P₂S₅/(NH₄)₂ and P₂S₅/(NH₄)₂ + UV-treated samples is relatively high near the AlGaN surface, dropping by two to three orders of magnitude in the channel. However, few S atoms are observed after the standard treatment. The S atoms produced from the sulfide solution are incorporated into the sample surface, which is similar to the effect of the immersion of CF₄ in plasma, used to enable a GaN HEMT to operate in enhancement mode [16]. In order to further investigate the mechanism for the removal of Ga-O bonds by sulfur treatments, an experiment was conducted to determine the oxygen concentration distribution; Figure 8(b) shows the results. The lowest curve in the figure shows that the P₂S₅/(NH₄)₂ + UV treatment removes a huge amount of AlGaN/GaN native oxide and reduces the number of oxygen atoms more effectively than the other treatments, because UV illumination provides enough energy to the sulfur atoms to replace the Ga-O bonds and form stable Ga-S bonds. Therefore, P₂S₅/(NH₄)₂ + UV treatment not only produces a smoother Schottky interface, reducing the gate leakage current, but also reduces the number of oxygen atoms more effectively. Both effects reduce the density of the surface states.

(a)

(b)

Figure 8: The SIMS measurements of the S atoms (a) and O atoms (b) distribution versus AlGaN/GaN depth for various treatment samples.

Figure 9 plots the gate-to-drain I-V curves for a standard GaN HEMT, a Pr₂O₃ MIS-HEMT, and a P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMT. A reduction in the number surface states and leakage current in the sulfide-treated sample means that P₂S₅/(NH₄)₂ + UV-treated MIS-HEMTs have a higher value than the others. The improved value of 1.71 V for the P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃-gate device results in a significant reduction in the gate leakage current at a high pumped gate voltage, which improves the linearity and results in a reduction in the signal dispersion of the device. The reversed gate-to-drain breakdown voltages (), defined as the voltage at which the gate leakage current is −1 mA/mm, are −131.3 V for the P₂S₅/(NH₄)₂ + UV-treated samples, which allows operation at high drain voltage, and only −128.2 V and −107.5 V for the Pr₂O₃ MIS-HEMT and standard-treated samples, respectively.

Figure 9: The device gate-to-drain I-V characteristics of standard HEMT, Pr₂O₃ MISFET, and P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MISFET.

In order to study the dc characteristics, the drain-to-source current () versus drain-to-source voltage () curves for three devices are shown in Figure 10. The drain current does not vary significantly between the treated devices. The turn-on resistance () is 3.958 Ω for the HEMTs subject to standard treatment, at of 0 V, and 4.675 Ω and 4.210 Ω for the Pr₂O₃ MIS-HEMTs and the P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMTs, respectively. This is primarily because the sulfide solution influences only the surface states, rather than the intrinsic parameters. However, since the traps within the interface between the gate and channel are suppressed by P₂S₅/(NH₄)₂ + UV treatment, the sample with this treatment has the highest output resistance, which results in an increase in device linearity and power gain cut-off frequency (). The output conductance () of the P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMTs is 0.46 mS/mm; the corresponding values for the Pr₂O₃ MIS-HEMTs and the standard device are 0.6 mS/mm and 1 mS/mm, respectively.

Figure 10: The -characteristics of various devices.

Figure 11 plots the transistor transconductance () versus curves for the three devices of interest at a of 8 V. The maximum drain-to-source currents () at are 923 mA/mm, 864 mA/mm, and 920 mA/mm, for the standard HEMTs, the Pr₂O₃ MIS-HEMTs, and the P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMTs, respectively. The maximum transconductance values (), biased at , are 144 mS/mm, 121 mS/mm, and 132 mS/mm, respectively. All of these values are reasonably favorable. The standard HEMTs exhibit a higher peak , because a high-k insulator is inserted in the MIS-HEMTs structure. The gate-to-channel improves channel modulation and effectively modulates any increase in the depletion region between the metal gate and the channel. Obviously, high-k GaN MIS-HEMTs demonstrate large swing voltage and low gate leakage current.

Figure 11: The - characteristics of various devices.

The measurement of on-wafer microwave S-parameters for  μm² devices was performed in a common-source configuration using an Agilent E8364C PNA network analyzer, from 0.1 GHz to 20.1 GHz. S-parameter measurements demonstrate a maximum current gain cut-off frequency () of 9.2 GHz and a maximum oscillation frequency of 16.8 GHz for standard HEMTs. These values are 7.9 GHz and 10.7 GHz for Pr₂O₃ MIS-HEMTs and 8.5 GHz and 16.2 GHz for P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMTs, at and , respectively. The superior RF characteristics for P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMTs prove their greater power and linearity. Table 2 presents dc and radio RF characteristics for various devices.

Table 2: The dc and RF characteristics comparisons of a standard HEMT, a Pr₂O₃ MISFET, and a P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MISFET.

Measurement results
Standard
Pr₂O₃
P₂S₅/(NH₄)₂ + UV Pr₂O₃

Schottky turn-on voltage ()
1.08 V
1.69 V
1.71 V

Breakdown voltage ()
−107.5 V
−128.2 V
−131.3 V

Pinch-off voltage ()
−5.8 V
−6.4 V
−6.8 V

Maximum drain current ()
923.4 mA/mm
864.4 mA/mm
920.04 mA/mm

Peak transconductance ()
143.58 mS/mm
121 mS/mm
132.36 mS/mm

Current gain cut-off frequency ()
9.2 GHz
7.9 GHz
8.5 GHz

Maximum oscillation frequency ()
16.8 GHz
19.2 GHz
16.2 GHz

Pulse measurements were made to characterize the carrier trapping phenomenon and heating effect in the device. With respect to the trapping carriers, the response time dominates the pulse measurement. The response time for these trapping carriers is typically of the order of μs longer than the ns of carrier transportation, especially for high-speed and high-power devices. In order to more easily observe the output signals from the drains of each device, a 50 Ω resistor was added between the drain electrode and the power supply, in order to determine the total drain-to-source current and to reduce the damping effect in the output square waveform. Pulse I-V measurements for the three devices were also made, in order to confirm their surface trapping effects. Figure 12 plots the dc to 1 μs pulse I-V measurement for  μm² devices at a of 0 V and a of 8 V, for the three devices. The gate width of the measured devices is 100 μm, so the heating effect could therefore be neglected. Since the surface states determine the dispersion effect, the current density at high current decays as the pulse width decreases. As shown in the figure, the standard GaN HEMTs demonstrate a larger slope with respect to the pulse period than do the Pr₂O₃ MIS-HEMTs and P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMTs. It is evident that P₂S₅/(NH₄)₂ + UV treatment produces reliable and stable surface performance and less load-line hysteresis and better linearity for the device in high-power applications are which both expected.

Figure 12: Pulse measurement characteristics of various devices.

To investigate the relationship between the flicker noise and the quantity of the variously treated devices, a low-frequency noise measurement was performed, because, at low frequency, this is sensitive to the semiconductor surface [17]. The bias point for low-frequency noise measurement was selected as V, which gives an of 100 mA/mm for all devices. Since the measurement is dominated by the series resistance of each device, identical bias point confirms the flicker noise characteristics. As shown in Figure 13, the P₂S₅/(NH₄)₂ + UV-treated MIS-HEMT demonstrates a lower 1/f spectral noise than both the Pr₂O₃ MIS-HEMT and the GaN HEMT, which proves the reduction in the number of surface dangling bonds due to P₂S₅/(NH₄)₂ + UV treatment.

Figure 13: The 1/f noise comparisons for various devices.

The fabricated 1 μm long gate GaN HEMT, Pr₂O₃ MIS-HEMT, and P₂S₅/(NH₄)₂ + UV-treated Pr₂O3 MIS-HEMT were tested on-wafer and the microwave power characteristics were evaluated using a load-pull system with automatic tuners, which simultaneously provides conjugate-matched input and load impedances for the maximum output power. The microwave load-pull power performance was conducted at 2.4 GHz, with a drain bias of 8 V, using various devices. The bias points for class AB operation of the standard HEMT, the Pr₂O₃ MIS-HEMT, and the P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMT must be biased at −3.1 V, −3.7 V, and −3.4 V (at 1/4 ), respectively. Figure 14 shows the output power (), power gain (), and PAE as a function of the input power (), for various devices with gate dimensions of  μm². The P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMT has better dc current and lower gate leakage current than that of the standard HEMT or the Pr₂O₃ MIS-HEMT, at high-input-power swing. The PAE values are 22.4%, 23.6%, and 24.2%, for the standard HEMT, the Pr₂O₃ MIS-HEMT and the P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMT, respectively. As a result, the microwave-power performance is improved by the MIS-gate structure, and the power-gain degradation is also improved in the high-input-power regime. The reduction in gate leakage current in the P₂S₅/(NH₄)₂ + UV-treated Pr₂O₃ MIS-HEMT allows a significant improvement in device linearity [18].

Figure 14: Power performances of three devices under 2.4 GHz and of 8 V operation.

4. Conclusion

In summary, Pr₂O₃ MIS-HEMTs with low gate leakage current and low flicker noise, as a result of P₂S₅/(NH₄)₂ + UV treatment, were developed and characterized. An electron-beam-evaporated high-k insulator and a passivating layer prevent the formation of surface states by plasma. P₂S₅/(NH₄)₂ + UV treatment represents a simple and efficient means of reducing the number of surface dangling bonds. Based on the results of Hall, XPS, and SIMS measurements, this P₂S₅/(NH₄)₂ + UV treatment prevents the formation of strong Ga-S bonds on the AlGaN surface. This reduction in surface states improves carrier mobility and simultaneously suppresses unstable native Ga-O bonds. This novel pretreatment is therefore proven to be eminently suitable to low-noise GaN MIS-HEMT applications.

Acknowledgments

The authors would like to thank the Nano Device Labs (NDL) for providing the low frequency noise measurements, and the National Central University (NCU) for providing the thermal image measurements. This work was financially supported by the National Science Council, ROC [NSC-97-2221-E-182-048-MY3 and NSC-100-2221-E-182-009], and High Speed Intelligent Communication (HSIC) Research Center of Chang Gung University, Taoyuan, Taiwan.

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