I. INTRODUCTION

The AlGaN/GaN HEMTs are extremely promising devices for high power and high frequency device applications due to their properties of high breakdown voltage and high carrier velocity. The high-density and high-mobility two-dimensional electron gas (2DEG) generated at the AlGaN/GaN interface results in the realization of the power switching transistors having extremely low on-state resistance applicable to next generation power conversion systems ^[1-3]. In switching applications, the reverse gate leakage current will lead to high off-state power consumption and the forward leakage current limits the range of gate voltage swing, hence deteriorating the power handling capability. The gate leakage current is a key factor to assure good power added efficiency and linearity in high power amplifiers and noise figure in low noise amplifiers. However, the high gate leakage currents associated with the strong polarization electric field due to Al rich barriers layers, higher density of surface states limits the performance and reliability of AlGaN/GaN HEMTs ^[4]. The GaN HEMT (MOSHEMT) using a dielectric film as a gate insulator effectively reduces the high gate leakage current and improves the device performance ^[5,6]. The gate current is one of the most important parameters for HEMTs/MOSMEMTs. Hence, it is of prime necessary to fully analyze and properly engineer the reverse leakage current in GaN-based HEMTs/MOSHEMTs so as to improve their performance in the high-power device applications. The reverse leakage current is usually dominated by mechanisms such as Poole-Frenkel emission, Schottky emission, direct tunneling, Fowler-Norheim tunneling, and trap-assisted tunneling. Poole-Frenkel emission refers to electric field-enhanced thermal emission of electrons from a trap state into a continuum of electronic states rather than direct thermionic emission from the metal ^[7,8]. While in Schottky emission, the electron absorbs thermal energy and then emitted over the potential barrier at the interface ^[8]. In direct tunneling, the electrons tunnel all the way through the barrier and is more dominant in junctions having thinner oxide layers/barriers while the Fowler-Nordheim tunneling occurs in junctions having thicker oxide layers through the triangular barriers. Generally, the tunneling current remains unchanged with variation in temperature. It is reported that the Fowler-Nordheim tunneling possesses a temperature dependence and that the reverse leakage current by the tunneling could also increase with temperature as the ejected electrons follows the Fermi-Dirac distribution ^[9,10]. Trap-assisted tunneling is a current transfer mechanism in which electrons are assisted by defects/traps in the oxide, to propagate through the insulating layer. In contrast to single step tunneling process like Fowler-Nordheim tunneling or direct tunneling, the traps in oxide layer assist the electrons to tunnel from one electrode side to the other through a two-step process. The electrons are first captured from first electrode side into the traps and subsequently emitted to the other electrode side.

Until now, several attempts have been made to investigate the current transport mechanisms dominating the gate current in conventional AlGaN/GaN HEMTs ^[11-14]. For instance, Turuvekere $et$ $al$. ^[11] investigated the possible gate leakage mechanisms in AlGaN/GaN and AlInN/GaN HEMTS and that the thermionic emission and Poole-Frenkel emission mechanisms are observed in AlGaN/GaN HEMT, with an additional Fowler-Nordheim tunneling component existing in AlInN/GaN HEMT. Further, they observed trap-assisted tunneling current component existing in both the set of devices. Xia $et$ $al$. ^[12] studied the reverse leakage current characteristics of Ni Schottky contacts to GaN grown on Si using HEMTs in the temperature range of 273–428 K. They found that the reverse leakage current is dominated by Frenkel-Poole emission when the reverse electric field is < 1.4 MV/cm and by Fowler-Nordheim tunneling mechanism for electric field > 1.6 MV/cm. Yan $et$ $al$. ^[15] determined the field-dependent reverse gate leakage current characteristics of AlGaN/GaN HEMTs including the polarization effect within the AlGaN barrier into calculation of the near-surface electric field underneath the Schottky metal. They found that around zero bias, the reverse polarization-field-induced Frenkel-Poole emission current is balanced by a forward defect-assisted tunneling current, both of which follow the same temperature dependent characteristics.

Even though, there are numerous reports available on the conventional Schottky-gate AlGaN/GaN HEMT, the reports on the gate leakage current mechanism in AlGaN/GaN HEMT with gate oxide is quite scarce. In particular, the atomic layer deposited (ALD) Al$_{2}$O$_{3}$ was proven to be an excellent gate dielectric for GaN MOS-HEMTs exhibiting low leakage current, high breakdown voltage, strong accumulation and high effective 2D electron mobility under both low and high transverse fields ^[16]. Swain $et$ $al$. ^[17] has made an attempt to develop an analytical model for gate leakage current during forward bias in an AlGaN/GaN MOSHEMT. They reported that at high temperature ($\textit{T}$ > 388 K), the trap-assisted tunneling (TAT) mechanism dominates the gate leakage current at low electric field for bias from 0 to 2 V, while Poole-Frenkel emission component is dominant during medium and high electric field. During low temperature ($\textit{T}$ < 388 K), TAT alone is dominant and consistent in the whole range of electric field. Liu $et$ $al$. ^[18] investigated the mechanisms of the temperature-dependent forward gate current transport in the atomic-layer-deposited (ALD) Al$_{2}$O$_{3}$/AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor (MISHEMT). They found that Fowler-Nordheim tunneling dominates the forward current transport at low temperature ($\textit{T}$ < 0 $^{\circ}$C) and high electrical field and TAT mechanism was dominant at medium electrical field and high temperature ($\textit{T}$ > 0 $^{\circ}$C) as against the domination of thermionic field emission mechanism in conventional Schottky-gate AlGaN/GaN HEMT. Reddy $et$ $al$. ^[19] demonstrated dual-surface modification of GaN/AlGaN/GaN high-electron mobility transistors (HEMTs) using tetramethylammonium hydroxide (TMAH) and piranha solutions prior to gate metallization and investigated the gate leakage mechanisms considering Poole-Frenkel and Schottky emission mechanisms.

In this work, we investigate the gate current transport mechanisms of the 25 nm thick ALD Al$_{2}$O$_{3}$ gate oxide AlGaN/GaN MOSHEMTs at different measurement temperatures in the range of 300–400 K. The depletion depth profile and the interface state density have been investigated from the capacitance-voltage characteristics. It will be shown that the Poole-Frenkel emission and Schottky emission mechanisms dominates the reverse leakage current transport regardless of the temperature, in the lower and higher bias regions, respectively. The current transport mechanism dominating the forward bias current have been analyzed.

II. GROWTH AND DEVICE FABRICATION

Fig. 1. Schematic diagram and optical image of AlGaN/GaN MOS-HEMT fabricated using ALD Al$_{2}$O$_{3}$ gate oxide.

Fig.1(a) shows the schematic diagram, and (b) optical images of AlGaN/GaN MOSHEMT with Al$_{2}$O$_{3}$ gate oxide. The AlGaN/GaN heterostructure used in this work was grown by metal-organic chemical vapor deposition on Si (111) substrate. The epitaxial structure consists of a 2 nm thick cap layer, a 25 nm thick AlGaN barrier layer, 300 nm thick undoped GaN, a 20 μm thick buffer layer, and AlN spacer layer. The Al mole fraction of the barrier AlGaN was 22 % for the structures used in this study. Hall measurements at room temperature provided a carrier concentration of 8.6 ${\times}$ 10$^{12}$ cm$^{-3}$ and a carrier mobility of 1638 cm$^{2}$/V${\cdot}$s. The AlGaN/GaN wafers were initially cleaned with organic solvents. The sample surface was treated in a 30 %-HF solution for 5 min for the removal of native oxide. In this study, we employed the “ohmic first” process to prevent damage to the Al$_{2}$O$_{3}$ film during ohmic annealing. Ti/Al/Ni/Au (30/70/30/70 nm) source and drain electrodes were then deposited followed by rapid thermal annealing (RTA) at a temperature of 850 $^{\circ}$C for 2 minutes using N$_{2 }$ambient for alloying. After the contacts were formed, 150 nm thick SiN was deposited using plasma enhanced chemical vapor deposition (PECVD) as a surface protection layer. Then, the mesa isolation was performed in an inductively coupled plasma (ICP) chamber using Cl$_{2}$-based plasma at Radio frequency power/bias power of 200/50 W at 20 sccm flow rate with GXR 601 positive photoresist as mask. The SiN layer was then removed with a buffered HF. A 25 nm thick Al$_{2}$O$_{3}$ gate oxide was then deposited with ALD using trimethylaluminum and water vapor as precursors. Finally, an Au/Ni (50/100 nm) gate electrode was deposited on the Al$_{2}$O$_{3}$ layer. Later, the source-drain contacts have been opened with negative photoresist as mask through etching oxide layer on source-drain region using ICP etcher based on oxide layer etching conditions. In this work, the gate leakage current is measured between source and gate under the gate area of 8 ${\times}$ 300 μm$^2$, with the source-gate distance of 9 μm. The current-voltage ($\textit{I}$–$\textit{V}$) and capacitance-voltage ($\textit{C}$–$\textit{V}$) characteristics of Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode have been measured using precision semiconductor parameter analyzer (Agilent 4155C), and a precision LCR meter (Agilent 4284A), respectively.

III. RESULTS AND DISCUSSION

The energy band diagrams of Au/Ni/Al$_{2}$O$_{3}$/AlGaN /GaN MOS diode are presented in Fig.2. The parameters that are required in drawing energy band diagrams include work function, band gap, and electron affinity values of ${\Phi}$$_{Au}$ = 5.1 eV, ${\Phi}$$_{Ni}$ = 5.2 eV, ${\Phi}$$_{AlGaN}$ = 4.3 eV, ${\Phi}$$_{GaN}$$_{\mathrm{ =}}$ 4.02 eV, $\textit{E}$$_{g,Al2O3 }$= 7.0 eV, $\textit{E}$$_{g,AlGaN }$= 3.89 eV, $\textit{E}$$_{g,GaN }$= 3.4 eV, ${\chi}$$_{\mathrm{AlGaN }}$= 3.41 eV, and ${\chi}$$_{\mathrm{GaN}}$ =4.02 eV ^[20-23], respectively. The energy band diagram of this MOS structure consists of Au/Ni metal stacks for gate electrode, Al$_{2}$O$_{3}$ ALD gate oxide, and AlGaN/GaN semiconductor whose operating conditions depend on the applied voltage. Therefore, energy band diagrams have been determined under different bias conditions. As can be seen in Fig.2(a), the energy band diagram of Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode under gate bias is 0 V (flat-band conditions). When applied bias voltage is 0 V, no charge is present in oxide-semiconductor interface. Also, the Fermi level of Au/Ni gate metal is same as that of the Fermi level of AlGaN/GaN, indicating that the device is in equilibrium condition. Fig.2(b) shows the energy band diagram when the applied gate voltage is negative ($\textit{V}$$_{\mathrm{g }}$< 0 V). When the negative voltage is applied, Fermi level of Au/Ni gate metals will move up and its location is above the conduction band of AlGaN while Fermi level of AlGaN/GaN remains constant. Due to this, there is no current flow on AlGaN/GaN side. Therefore, the charge bends the band upwards on the AlGaN/GaN-Al$_{2}$O$_{3}$ interface. When voltage is applied to the Au/Ni gate metals, negative charge appears on the Au/Ni-Al$_{2}$O$_{3}$ junction, and excess holes are induced at the AlGaN/GaN-Al$_{2}$O$_{3}$ interface. In this case, holes accumulate near the semiconductor-oxide interface and can be considered as accumulation conditions. Fig.2(c) shows the band diagram for the positive voltage ($\textit{V}$ > 0). In this case, the Fermi level of Au/Ni gate metal, which has been above the valence band, has been moved down to a location below the valence band. The positive charge appears near the Au/Ni-Al$_{2}$O$_{3}$ junction causing the electrons to travel towards the gate creating a negative charge on the AlGaN/GaN-Al$_{2}$O$_{3}$ interface, and the charge bends the band downwards on the AlGaN/GaN-Al$_{2}$O$_{3}$ interface. The electrons will recombine with the holes that are present near the oxide leading to creation of depletion region. Surface voltage develops in depletion region and as an effect of this, the energy band bends in the depletion region.

Fig. 2. Energy band diagrams of Au/Ni/Al$_{2}$O3/AlGaN/GaN MOS diode under different bias conditions of (a) zero bias, (b) negative bias, (c) positive bias.

Fig. 3. Typical forward and reverse bias I-V characteristics of Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode measured in the temperature range of 300–400 K.

Fig.3 shows the typical semi-logarithmic reverse and forward bias $\textit{I}$–$\textit{V}$ characteristics of the Au/Ni/Al$_{2}$O$_{3}$/ AlGaN/GaN MOS diodes measured in the temperature range of 300 to 400 K at steps of 25 K by applying bias voltage from -5 to 3 V. The reverse gate leakage current at bias voltage of -1 V has been noted to be 6.60 ${\times}$ 10$^{-11}$, 9.92 ${\times}$ 10$^{-11}$, 1.25 ${\times}$ 10$^{-10}$, 1.90 ${\times}$ 10$^{-10}$, and 2.63 ${\times}$ 10$^{-10}$ A at temperatures of 300, 325, 350, 375 and 400 K, respectively. It is observed that the gate leakage current increases with increase in temperature. Then, in this study, the temperature dependence of current transport mechanisms at both reverse and forward bias voltage of Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode have been investigated. This is performed to determine the mechanisms dominating the current transport. The Poole-Frenkel emission, and Schottky emission has been analyzed in the context of the reverse bias voltage, and the power- law mechanism has been utilized for forward bias.

Fig. 4. $\textit{C}$-$\textit{V}$ characteristics of Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode structure in the frequency range of 1 KHz–1 MHz.

Fig.4 shows the $\textit{C}$–$\textit{V}$ characteristics of the Al$_{2}$O$_{3}$/ AlGaN/GaN diode structures with gate bias ranging from -20 to 5 V with a step of 0.05 V in the frequency range of 1 kHz to 1 MHz. The $\textit{C}$–$\textit{V}$ curves obtained were peculiar having a two-step capacitance change, which is the characteristic feature of the MOS-HEMT structure having two interfaces ^[24,25]. The nearly flat capacitance $\textit{C}$$_{\mathrm{2DEG}}$ indicates that the 2DEG accumulates at the AlGaN/GaN interface. On applying the positive bias, electrons start do distribute in the AlGaN layer, leading to an increase of capacitance to the insulator capacitance $\textit{C}$$_{\mathrm{a}}$$_{\mathrm{l2O3}}$. However, the capacitance of the gate oxide ($\textit{C}$$_{\mathrm{a}}$$_{\mathrm{l2O3}}$) corresponding to the 10-nm-thick Al$_{2}$O$_{3}$ was not observed till the measured bias of +5 V. The $\textit{C}$$_{\mathrm{2DEG}}$ is in accordance with the series capacitance of the 27-nm-thick AlGaN and the 25-nm-thick Al$_{2}$O$_{3}$. In negative bias range, as the gate bias nears the threshold voltage ($\textit{V}$$_{\mathrm{th}}$), the depletion of 2DEG leads to sharp decrease of capacitance, corresponding to 2$^{\mathrm{nd }}$$\textit{C}$–$\textit{V}$ step.

where $\textit{A}$ denotes the Schottky diode area, $\textit{q}$ is the electron charge, ${\varepsilon}$ is the dielectric constant and d$\textit{C}$/d$\textit{V}$ is the slope of the $\textit{C}$–$\textit{V}$ characteristics. The inset in Fig.5 shows the depletion depth profile for the Au/Ni/Al$_{2}$O$_{3}$/ AlGaN/GaN MOS diode determined from the $\textit{C}$–$\textit{V}$ characteristics (Fig.5) measured at 100 kHz. It can be seen that a plateau region of carrier concentration is prominent in the bulk GaN layer that marks enhanced background concentration at the interface of the two-step grown GaN. The 2DEG density (n$_{\mathrm{s}}$) at the AlGaN/GaN hetero-interface is found to be 7.15 ${\times}$ 10$^{14}$ cm$^{2}$.

Fig. 5. $\textit{C}$–$\textit{V}$ characteristics of Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode at 100 KHz with inset showing its corresponding depletion depth profile.

The interface state density ($\textit{N}$$_{ss}$) can be calculated from the capacitance-voltage measurements. At sufficiently high frequencies, the interface states are in equilibrium with the semiconductor and do not contribute to the capacitance as the charge at the interface states cannot follow the ac signal and any junction has only the space-charge capacitance ($\textit{C}$$_{sc}$) in this case. At low frequencies, the interface state capacitance ($\textit{C}$$_{it}$) adds up to the space-charge capacitance resulting in a higher total capacitance ($\textit{C}$). Hence, a subtraction of the space-charge capacitance obtained at high frequencies from the total capacitance measured at low frequencies yields the interface state capacitance ($\textit{C}$$_{it}$ = $\textit{C}$-$\textit{C}$$_{SC}$). The interface state density ($\textit{N}$$_{SS}$) can be obtained as ^[28]:

where $\textit{q}$ is the electron charge and $\textit{A}$ is the area of the diode. The interface state density ($\textit{N}$$_{SS}$) that is extracted from the measured high frequency (1 MHz) and low frequency (100 KHz) capacitance with applied bias is shown in Fig.6 for the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode. It is observed that the value of $\textit{N}$$_{ss }$is varied from 7.15 ${\times}$10$^{14}$ to 1.12 ${\times}$ 10$^{15}$ eV$^{-1}$ cm$^{-2}$ as the applied reverse bias varies from 5 to 0 V.

Fig. 6. Voltage dependence of interface state density ($\textit{N}$$_{SS}$) distribution of Au/Ni/Al/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode at room temperature determined from $\textit{C}$–$\textit{V}$ characteristics.

The reverse current did not saturate but exhibits an exponential dependence on applied reverse voltage ($\textit{V}$$_{R}$). The carrier generation in the depletion layer and the image force lowered barrier height often dominates the reverse current because the reverse bias increases the electric field in the junction. The image-force lowering of the barrier height being the most obvious. The dominance of the reverse leakage current by the image force lowering of the barrier height or the field enhanced process can be observed by considering the Poole-Frenkel and Schottky emission models and plotting ln($\textit{I}$$_{R}$) versus $\textit{V}$$^{\mathrm{1/2}}$. Fig.7(a) shows the plot of ln($\textit{I}$$_{R}$) versus $\textit{V}$$^{\mathrm{1/2 }}$at various measurement temperatures for the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode. The plot shows a linear relationship between ln($\textit{I}$$_{R}$) and $\textit{V}$$^{\mathrm{1/2}}$, implying the possibility of the dominance of the reverse current by Poole-Frenkel or the Schottky emission mechanism. In general, the dominance of Poole-Frenkel and Schottky emission can be differentiated by their field-lowering coefficient values. The reverse leakage current when dominated by Poole-Frenkel emission is given by ^[29-31]:

Fig. 7. (a) Plot of ln($\textit{I}$$_{R}$) versus $\textit{V}$$^{\mathrm{1/2}}$ of Au/Ni/Al$_{2}$O$_{3}$/ AlGaN/GaN MOS diode in the temperature range of 300–400~K and (b) the emission coefficient values versus temperature.

and when dominated by the Schottky emission effect is given as:

where $\textit{d}$ is the depletion width, ${\beta}$$_{\mathrm{PF}}$ and ${\beta}$$_{\mathrm{SC}}$ are the Poole-Frenkel and Schottky field lowering coefficients, respectively. The theoretical value of field lowering coefficient is given as ^[32,33]:

According to Eq.(5), the Poole-Frenkel emission field lowering coefficient is exactly twice the field lowering coefficient of Schottky emission. From Eq.(5), the theoretical values of ${\beta}$$_{\mathrm{PF}}$ and ${\beta}$$_{\mathrm{SC}}$ are determined to be 2.46 ${\times}$ 10$^{-5 }$and 1.23 ${\times}$ 10$^{-5 }$eVm$^{\mathrm{1/2}}$V$^{\mathrm{-1/2 }}$respectively. The plot of ln\textit{(I}$_{R}$$\textit{)}$ versus $\textit{V}$$^{\mathrm{1/2}}$of the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode structure in Fig.7(a) shows the existence of two linear regions, the lower bias and higher bias region represented as region I and region II. The emission coefficient values obtained from the linear portion of the plots are shown in Fig.7(b) for the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/ GaN MOS diode structure along with the theoretical values of ${\beta}$$_{\mathrm{PF}}$ and ${\beta}$$_{\mathrm{SC}}$ represented in dashed lines. The values obtained from region I are closer to the theoretical Poole-Frenkel emission coefficient regardless of the temperature. While at higher voltages (region II), the values obtained from the fit are closer to the theoretical Schottky emission coefficients. This indicates that the reverse leakage current in the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode structure is dominated by the Poole-Frenkel emission and Schottky emission mechanisms in the lower and higher voltage regions regardless of the temperature. A similar behavior of the domination of the reverse leakage current in TMAH-treated Au/Ni/Al$_{2}$O$_{3}$/GaN MIS diodes by Poole-Frenkel emission at low reverse voltages and Schottky emission at high reverse bias was reported ^[19]. The voltage range of the domination of the leakage current by Poole-Frenkel emission and Schottky emission may vary depending on the thickness and as well the magnitude of current varies with the thickness of the oxide ^[34,35]. Gupta et al. ^[35] investigated the current conduction mechanism of SiO$_{2}$/4H-SiC structure with varying thickness of the SiO$_{2}$ layer. They observed that as thickness of the SiO$_{2 }$increased, the electric field limit for the onset of Poole-Frenkel also increased.

Fig. 8. (a) Forward log I – log V characteristics of the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode at various temperatures in the range of 300–400 K, (b) Plot of the exponent m versus 1000/T.

An analysis of the forward current transport mechanism of the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode structure was made employing the log-log plot of the $\textit{I}$–$\textit{V}$ characteristics using power law mechanism. The power-law mechanism defines the power-law dependence of current on voltage as $\textit{I}$$_{F}$ ${\alpha}$ $\textit{V}$$^{m}$, and value of exponent $\textit{m}$ can be obtained from the slope of linear curve fitting. Fig.8(a) shows the log $\textit{I}$$_{F}$ versus log $\textit{V}$ plot for the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS diode structure in the temperature range of 300 to 400 K. The plot shows the existence of two linear regions (represented as region I and region II) having different values of the exponent m. At region I, the slope was close to unity showing a linear dependence as $\textit{I}$$_{F}$ ${\sim}$$\textit{V}$ with the value of $\textit{m}$ being 1.16, 1.02, 0.89, 0.77, and 0.71 for temperatures of 300, 325, 350, 375, and 400 K, respectively. Thus, the current transport in region I follows Ohm’s law controlled by thermally activated carriers predominant over the injected carriers ^[36,37]. Subsequently, at high bias voltage for region II, the slope values are found to be 3.69, 3.37, 3.05, 2.79 and 2.61 for temperatures of 300, 325, 350, 375, and 400~K, respectively. In this region, the values of slope are greater than 2 which match the behavior of power-law mechanism. Thus, the current transport mechanism in this region is governed by space-charge-limited current (SCLC) and is attributed to the presence of traps near the Fermi level in which the current flow occurs due to space charge limited charge carriers ^[38]. The SCLC conduction becomes dominant when the density of injected free-charge carriers is much larger than the thermally generated free-charge carriers. Further, it can be noted the values of exponent $\textit{m}$ decrease with increase in temperature. This is because of the fact that the properties of traps are affected by the operating temperature. The temperature dependence of the exponent can be used to determine the characteristic trap energy $\textit{E}$$_{t}$ from the temperature dependence of $\textit{E}$$_{t}$$=kT$$_{c}$, in which $\textit{E}$$_{t}$ and $\textit{T}$$_{c}$ are trap energy and characteristic temperature of traps, respectively ^[38]. Fig.8(b) shows the plot of $\textit{m}$ versus 1000/$\textit{T}$, fits to a straight line. The characteristic temperature $\textit{T}$$_{C}$ obtained from the slope of the straight line is about 1987 K and the corresponding trap energy ($\textit{E}$$_{t}$) is obtained to be 0.17 eV. $\textit{E}$$_{t}$ describes how rapidly the trap density distribution decays into the forbidden gap below the conduction band edge and the value of $\textit{E}$$_{t }$${\approx}$ 0.17 eV indicates that most of the traps are deep traps.

IV. CONCLUSION

The current transport mechanism dominating the reverse and forward gate leakage currents in Au/Ni/ Al$_{2}$O$_{3}$/AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) have been investigated in the temperature range of 300–400 K. The reverse gate leakage current of the Au/Ni/Al$_{2}$O$_{3}$/AlGaN/ GaN MOS structure was dominated by Poole-Frenkel emission mechanism and Schottky emission mechanism in the lower and higher bias regions, irrespective of the measurement temperature. The forward log $\textit{I}$–log $\textit{V}$ characteristics of Au/Ni/Al$_{2}$O$_{3}$/AlGaN/GaN MOS structure indicated that the ohmic conduction dominated the carrier transport in the lower bias range. While the space-charge-limited current mechanism dominated the current conduction in the higher bias range in which the density of injected charge carriers is much larger than the thermally generated carrier. The results obtained in this study could be helpful in improving the gate leakage current of MOS-HEMT device.