*3.2. Al2O3-Gated MIS-HEMTs*

Among the issues facing the MIS gate toward the improvement of the performance of AlGaN/GaN MIS-HEMTs, the dynamic Vth instability caused by the trapping mechanisms involving the gate dielectric is the one major concern [1]. The instability of the Vth has been reported under various bias conditions [31,32,34,78,160,161]. In particular, the large Vth shift induced by forward gate bias stress due to electron trapping at the dielectric/(Al)GaN interface is one of the most serious problems for the operational stability and reliability of the device [34–37]. For this reason, many groups have focused their efforts on studying the origin of the Vth instability and various fabrication processing strategies to overcome this issue.

For Al2O3-gated MIS-HEMTs, Lu et al. [32] reported that a larger Vth shift towards the forward bias direction was induced by increasing the gate positive bias stress in the pulsed current-voltage (I-V) measurements. Similar results were obtained by other groups [28,31,35,151,160,162,163]. Bisi et al. [160] pointed out that the large positive shift of the Vth can also promote the current collapse of MIS-HEMTs. Regarding the origin of the Vth instability, Tapajna et al. [ ˇ 105] discussed the effect of interface states and bulk traps on the Vth shift in Al2O3-gated MIS-HEMTs. Wu et al. [153] and Zhu et al. [33] pointed out that the Vth shift during a positive gate bias stress was highly correlated to the trap states at the dielectric/(Al)GaN interface but also to the border traps near the interface. Fixed charges within the dielectric are also involved in the Vth shift mechanism [107,116].

A reduction of the interface and/or border traps by means of annealing and surface treatments can lead to an improvement of the dynamic Vth instability of MIS-HEMTs. In addition, as mentioned before, the current linearity and the saturation of current at forward bias of MIS-HEMTs can be also affected by a change in the density of the electronic states at the dielectric/(Al)GaN interface [38]. Hori et al. [27] reported that the reduction of the interface states obtained by applying an N2O-radical treatment on the AlGaN surface prior to the ALD-Al2O<sup>3</sup> deposition led to a higher maximum drain current of the MIS-HEMTs at the positive gate bias and a suppressed Vth instability under the negative gate bias stress even at 150 ◦C. Nishiguchi et al. [38] showed that the improvement of the Al2O3/AlGaN interface by the reverse-bias anneal at 300 ◦C in air for 3 h of Al2O3-gated MIS-HEMTs gave a better gate control of the current even at forward gate bias, effectively enhancing the current linearity, subthreshold behavior and the maximum drain current of the device. Moreover, reduced gate leakage currents and more stable Vth under forward bias stress and at higher temperatures were obtained. Similarly, Ando et al. [147] recently reported on the improved gate controllability and current linearity of MIS-HEMTs with the Al2O<sup>3</sup> gate dielectric as a result of a reduction of the electronic states at the Al2O3/AlGaN interface after PMA in N<sup>2</sup> at 300 ◦C. A subthreshold slope of 68 mV dec−<sup>1</sup> and excellent Vth and operation stability up to 150 ◦C were also achieved, as shown in Figure 6. Note that in this case Ando et al. [147] pointed out that the improvement of the device performance also benefited from using epitaxial GaN layers grown on free-standing GaN substrates from hydride vapor phase epitaxy (HVPE) with a low dislocation density. Very recently, Calzolaro et al. [151] reported that the reduction of interface trap states by a remote O<sup>2</sup> plasma-based surface treatment before the ALD-Al2O<sup>3</sup> deposition combined with a PMA in N<sup>2</sup> at 350 ◦C resulted in a better Vth stability in pulsed I-V measurements. It is worth mentioning that, despite the benefits of the PMA treatments, specific attention has to be paid to the employment of higher PMA temperatures, as it can affect the gate leakage currents of the devices using ALD-grown Al2O<sup>3</sup> films [119,164]. Therefore, a trade-off must be considered when using the PMA treatment between the quality of the Al2O3/(Al)GaN interface and the gate leakage currents in a certain voltage range of operation.

**Figure 6.** Transfer characteristics of Al2O<sup>3</sup> -gated AlGaN/GaN MIS-HEMTs fabricated on freestanding HVPE GaN substrates and subjected to PMA at 300 ◦C in N<sup>2</sup> atmosphere, reported by Ando et al. [147]. In (**a**,**b**), the transfer characteristics of MIS-HEMTs with and without PMA are compared in a semi-log scale and as a function of the gate overdrive voltage, respectively. Transfer curves in (**c**,**d**) were obtained after applying an initial gate voltage stress up to 10 V and by increasing the temperature up to 150 ◦C, respectively.

As in the case of the surface and annealing treatments, various strategies in the fabrication process of the devices can also be adopted to influence the trap states at the interface and, therefore, suppress the Vth instability. Szabó et al. [31] reported that for MIS-HEMTs where the deposition of the Al2O<sup>3</sup> gate dielectric was performed before the ohmic contacts formation and at annealing temperature of 650 ◦C resulted in an improvement of the Vth stability compared to devices where the Al2O<sup>3</sup> was deposited after the ohmic contacts formation was obtained with a high temperature anneal of 850 ◦C. It was suggested that this result was a consequence of a better Al2O3/(Al)GaN interface quality. Nakazawa et al. [165] applied an interesting approach based on the selective area regrowth of AlGaN to reduce the impact on the ALD-Al2O3/AlGaN interface of the dry etching process used for the fabrication of normally off AlGaN/GaN MIS-HEMTs with recessed gate structures. With this approach, they reported a reduced Vth instability compared to Al2O3-gated MIS-HEMTs with dry-etched recessed gates.

Trapping mechanisms related to the gate dielectric can lead to the failure of the device. For this reason, reliability tests of the gate dielectric are also essential to bring the MIS-HEMT devices to industrial maturity. In this regard, Meneghesso et al. [30] performed an extensive analysis of trapping mechanisms and the reliability issues of AlGaN/GaN MIS-HEMTs using different insulators. They reported a significant correlation between the dynamic Vth shift and gate leakage currents under forward gate bias stress and suggested that trapping effects were determined by the electrons trapped in the gate insulator or at the AlGaN/insulator interface. Wu et al. [166] investigated the positive bias temperature instability (PBTI) in hybrid GaN MIS-FETs. Since the defect distribution inside the ALD-Al2O<sup>3</sup> was found to be centered at about 1.15 eV away from the conduction band of the GaN with a narrow spread in energy, the ALD-Al2O<sup>3</sup> gate dielectric was suggested to be very promising to improve the PBTI reliability. Meneghesso et al. [30] also measured the TDDB characteristics of MIS-HEMTs with Al2O<sup>3</sup> as gate dielectrics. Since the timeto-failure of devices indicated a Weibull distribution with slopes larger than 1.0, they demonstrated high robustness for ALD-Al2O3. Similarly, a Weibull distribution with a slope of 2.87 was extracted from the TDDB measurements of the Al2O3-gated MIS structures by Wu et al. [134]. Huang et al. [127] also achieved good TDDB behavior and a high breakdown electric field of 8.5 MV cm−<sup>1</sup> in recessed-gate MIS-HEMTs with a gate dielectric stack consisting of 13 nm of ALD-Al2O<sup>3</sup> deposited using O<sup>3</sup> as an oxygen source and grown on top of 2 nm of ALD-Al2O<sup>3</sup> deposited using a H2O oxygen source. For the ALD-Al2O<sup>3</sup> films on the GaN, Kachi et al. [167] reported a TDDB lifetime at RT and 150 ◦C of more than 20 years at an electric field of 3 MV cm−<sup>1</sup> . Kikuta et al. [168] obtained a time-to-breakdown for the ALD-Al2O<sup>3</sup> on a dry-etched GaN of more than 40,000 years at 3 MV cm−<sup>1</sup> and RT. In contrast, a time-to-breakdown of only 102–10<sup>3</sup> s was obtained at 250 ◦C, which was suggested to be caused by large TAT leakage currents.

As mentioned before, the dielectric layer employed in MIS-HEMTs can be used both as a gate dielectric and a passivation layer to reduce current collapse. Hashizume et al. [53,75] first demonstrated the use of an Al2O<sup>3</sup> layer as a gate dielectric and a passivation scheme to control the current collapse in AlGaN/GaN HEMTs. Moreover, comparing the effects of surface passivation on MIS-HEMTs and Schottky-gate HEMTs, Tajima and Hashizume [169] showed a more pronounced reduction of the current collapse in Al2O3-gated MIS-HEMTs in contrast to Schottky-gated HEMTs, with Al2O<sup>3</sup> serving only as a surface passivation. The suppression of the current collapse with a passivation layer, arising from negative surface charges, injected from gate edges to surface states was generally attributed to a reduction of electronic states at the AlGaN surface and of the peak field near the gate edge. Park et al. [94] reported for the first time on the use of Al2O<sup>3</sup> deposited by ALD as a gate dielectric and passivation layer for AlGaN/GaN MIS-HEMTs. Park et al. [94] and Ye et al. [23] reported on the excellent electrical characteristics of AlGaN/GaN MIS-HEMTs using ALD-Al2O<sup>3</sup> as a gate dielectric and passivation layer. Despite the improvements obtained by Al2O3-based passivation schemes for MIS-HEMT devices, further work is still required to limit and fully understand the current collapse phenomena in GaN transistors [1]. A more detailed overview about surface passivation for GaN-based transistors can be found in [29,49–51].
