1.Introduction

Provided with both high blocking voltage and current handling capability,GaN-based devices have been expected to be a viable alternative to GaAs,silicon,and even to SiC ones in power electronics. The performances,especially of AlGaN/GaN high electron mobility transistors (HEMTs),have been tremendously improved over the past two decades. However,AlGaN/GaN HEMTs reported so far are usually operated as depletion-mode (D-mode). The development of enhancement-mode (E-mode) AlGaN/GaN HEMTs has become essential to expand the range of their applications. E-mode AlGaN/GaN HEMTs enable a circuit to be simple. This is important for high-frequency circuits,because all passive components tend to result in large loss when they operate in high-frequency range. E-mode AlGaN/GaN HEMTs have another benefit whereby various monolithic integrated D-/E-mode digital circuits can be realized. Furthermore,E-mode devices are particularly desirable for high-voltage and high-power switching applications in which safety is a critical issue^[1].

Common approaches to achieve E-mode AlGaN/GaN HEMTs are based on the conventional D-mode devices ($\sim$20-nm-thick barrier layer with an inherent two dimensional electron gas (2DEG) channel) and include growth of a p-type AlGaN cap layer^[2],recessed-gate^[3] or fluorine ion implantation^[4] to locally deplete the channel underneath the gate. These approaches suffer from non-uniformity in device performance,device instability and low $V_{\rm th}$,respectively^[5]. They are generally faced with the trade-off between the available drain current density and threshold voltage. An alternative method proposed by Higashiwaki \textit{et al.} is the combination of thin-barrier AlGaN/GaN heterostructure and SiN passivation^[6]. They found that passivation by SiN can dramatically increase the density of 2DEG in thin-barrier AlGaN/GaN heterostructures^[7]. This effect was utilized to fabricate E-mode GaN HEMTs by etching the deposited SiN in the gate region to deplete the 2DEG under it^[1]. As a result of Schottky contact,the gate voltage swing was restricted to a small value,thus limiting the available maximum drain current density. In addition,the reverse gate leakage current was quite large which deteriorated the device performances and long-term reliability^[8]. A metal-insulator-semiconductor (MIS) structure was preferred to solve the above problems^[9]. Owing to its large bandgap ($\sim$9~eV) and high dielectric constant ($\sim$10),Al$_{2}$O$_{3}$ was often chosen as the gate insulator and protective passivation layer for both D- and E-mode HEMTs^{[10, 11]}.

In this paper,we studied the passivation effect of atomic-layer-deposited (ALD) Al$_{2}$O$_{3}$ and found that it was a potential gate insulator for thin-barrier E-mode AlGaN/GaN MIS-HEMT with little influence on the electrical properties of 2DEG. The fabricated MIS-HEMT was compared with the corresponding E-mode HEMT (studied in Reference ^[1]) using the same material and process without ALD Al$_{2}$O$_{3}$ gate insulator. The DC performances had been greatly improved after the insertion of gate insulator. A simple method to realize E-mode operation with large positive $V_{\rm th}$ and high source-drain current density was feasible with this structure.

2.Device structure and fabrication

The AlGaN/GaN heterostructure used in this study was grown on a $c$-face sapphire substrate by metal-organic chemical vapor deposition (MOCVD) system. The epitaxial wafer consisted of,from bottom to top,a 100-nm-thick AlN nucleation layer,a 2000-nm-thick GaN buffer and channel layer,and a 5-nm-thick Al$_{0.3}$Ga$_{0.7}$N barrier layer. No intentional doping was performed during the MOCVD growth. Figure 1 shows a schematic cross-section view of the E-mode AlGaN/GaN MIS-HEMT. The source-drain spacing was 4 $\mu$m and the gate width was 100 $\mu$m.

The E-mode device was fabricated with the following procedures: (1) passivation. 10-nm SiN was deposited on the cleaned AlGaN surface by PECVD. A thinner SiN layer was preferred for better ohmic contacts^[12]. However,the thickness error was around $\pm$1 nm in our PECVD system and 10 nm was chose due to the equipment limitation. (2) Ohmic contact. After the definition of source and drain patterns by photolithography,Ti/Al/Ni/Au metals were deposited directly on SiN,followed by a lift-off process then annealed at 800℃for 30 s^[13]. Using on-wafer transfer length measurement (TLM) patterns,the specific contact resistance ($\rho_{\rm sc})$ was typically measured to be 4.6 $\times$ 10$^{-6}$ $\Omega$/cm$^{2}$. (3) Isolation. Isolation was achieved by B$^{+}$ implantation. (4) Etching of SiN in gate region. Following the opening of gate resist pattern by lithography,a two-step reactive ion etching (RIE) based on SF$_{6}$ was performed to remove the exposed SiN with little damage to the AlGaN surface. (5) Gate insulator. 8-nm-thick Al$_{2}$O$_{3}$ was deposited by ALD on the AlGaN in the gate region. (6) Gate and interconnection. The Ni/Au metals were evaporated for gate metallization and the Ti/Au bond-pad metals were evaporated for device characterization. We also fabricated an E-mode AlGaN/GaN HEMT without gate insulator using the same epi-wafer for comparison.

3.Results and discussion

The passivation effects of PECVD SiN and ALD Al$_{2}$O$_{3}$ on AlGaN/GaN heterostructures were first studied. Room-temperature Hall measurements were conducted to estimate the electrical properties of 2DEG. Table 1 shows summaries of sheet resistance ($R_{\rm sh})$,electron mobility ($\mu )$ and sheet charge density ($N_{\rm s})$ before and after the passivation respectively. $R_{\rm sh}$ of the as-grown epitaxial wafers was very high due to the thin AlGaN barrier layer^[14]. It decreased from 1842 to 546 $\Omega$/$\square$ after the SiN passivation,which was consistent with the results in Reference ^[7]. However,the passivation of ALD Al$_{2}$O$_{3}$ nearly had no influence on the properties of 2DEG. This made it a feasible gate insulator for E-mode MIS-HEMTs without increasing the 2DEG density in the gate region.

An Agilent B1500A was employed to measure the DC characteristics of the MIS-HEMT. The gate leakage characteristics of Schottky-gate HEMT and MIS-HEMT are shown in Figure 2. During the measurement,both source and drain electrodes were grounded and a varying voltage,from -10 to 10~V,was applied to the gate electrode. The leakage current was reduced by two orders of magnitude after the insertion of gate insulator,indicating the excellent insulation characteristics of ALD Al$_{2}$O$_{3}$. The fluctuation in leakage current of MIS-HEMT was due to the accuracy of the measuring equipment. For the Schottky-gate HEMT,the forward gate current density rapidly increased with the gate voltage and exceeded 10 mA/mm at a gate bias of 2 V,while in the MIS-HEMT,the forward gate current density at a gate bias of 10 V stayed as low as $\sim$1 $\times$ 10$^{-6}$ A/mm. For power switching applications,the larger gate input voltage swing may improve the safety and reliability of the device^[11].

Figure 3(a) shows typical DC current-voltage ($I$-$V$) curves of the E-mode AlGaN/GaN HEMT without Al$_{2}$O$_{3}$ gate insulator. The gate was biased from -1 to 2 V in steps of 1 V. The device exhibited a maximum drain current density ($I_{\rm DS})$ of 512 mA/mm at a gate bias of 2 V and a drain bias of 3.2~V. However,it had a large forward gate current (exceeding 3 mA) when the gate was biased at 2 V. This problem was solved by inserting an ALD Al$_{2}$O$_{3}$ layer between the gate and AlGaN (Figure 2(b)). The gate voltage ($V_{\rm G})$ could be biased at 6 V and $I_{\rm DS}$ was increased to 657 mA/mm which nearly reached the saturation current density. It was obvious that at the same gate bias,$I_{\rm DS}$ in MIS-HEMT was much smaller than the one in HEMT. This was because the gate capacitance was decreased by the insertion of gate insulator. When the same positive voltage was applied to the gate,the density of 2DEG under the gate was much smaller in the MIS structure. For the same reason,the pinch-off characteristic (bias at 0 V) of the MIS-HEMT was much better than the HEMT.

The DC transfer characteristics of the two devices are shown in Figure 4. For the device without gate insulator,a peak extrinsic transconductance ($g_{\rm m})$ of 268 mS/mm was measured at $V_{\rm G}$ $=$ 0.9 V. By defining the threshold voltage $(V_{\rm th})$ as the gate bias intercept of the extrapolation of drain current at the point of peak $g_{\rm m}$,$V_{\rm th}$ of the HEMT was determined to be 50~mV. After the insertion of gate insulator,$g_{\rm m}$ was decreased to 187 mS/mm (at $V_{\rm G}$ $=$ 2 V) and $V_{\rm th}$ was increased to 1 V. The increased distance between gate and channel was the main cause of the decrease of $g_{\rm m}$. Two main factors might contribute to the positive shift of $V_{\rm th}$: the Al$_{2}$O$_{3}$/AlGaN interface states^[15] and the fixed negative charges in the dielectric layer^[16]. Further experiments are needed to determine the mechanism.

Both devices' breakdown voltage ($V_{\rm br})$ at pinch-off state (with gate voltage of 0 V) was measured and shown in Figure 5. The maximum drain-source current $(I_{\rm D,br})$ was chosen to be 1 mA/mm to avoid permanent degradation of the devices. The MIS-HEMT showed great improvement against the HEMT in breakdown characteristics,with $V_{\rm br}$ increased from 41 to 94 V.

4.Conclusion

In summary,an E-mode AlGaN/GaN MIS-HEMT with excellent performances was achieved by combining thin barrier and passivation technology. Unlike SiN passivation,the ALD Al$_{2}$O$_{3}$ deposited on the AlGaN surface would not increase the 2DEG density and was used as the gate insulator for the thin-barrier AlGaN/GaN heterostructure which featured a high sheet resistance without SiN passivation. So we etched the SiN layer only in the gate region to deplete the 2DEG while keeping the 2DEG density of access region high and then deposited ALD Al$_{2}$O$_{3}$ as the gate insulator to fabricate an E-mode MIS-HEMT. Compared with the corresponding HEMT without gate insulator,the DC characteristics,such as maximum drain current density,gate leakage current and threshold voltage,have been greatly improved. Along with the D-mode AlGaN/GaN MIS-HEMT,these results demonstrate the great potential of thin-barrier AlGaN/GaN heterostructures for monolithic integrated E-/D-mode GaN-based circuits.