**2. Gate Dielectrics on (Al)GaN**

The design of a MIS gate structure for insulated-gate GaN-based transistors requires consideration of the properties of the bandgap, the band offset to (Al)GaN, the permittivity and the chemical stability of the insulators [7,29,49–51]. For a sufficient suppression of the gate leakage currents, even at forward gate bias operation, a large bandgap material as well as large band offsets to (Al)GaN are necessary, in particular for power switching devices. On the other hand, a high value of permittivity is favorable to obtain high transconductance [55]. In particular, in the case of MIS-HEMTs, since the introduction of a dielectric leads to a reduction of the gate-to-channel capacitance with respect to Schottkygate HEMTs, a high permittivity dielectric reduces the capacitive contribution of the gate dielectric, enabling it to obtain a stronger coupling between the gate and the 2DEG channel, and hence to maintain a high transconductance, which is especially important for RF devices. At the same time, in normally on MIS-HEMTs, high-permittivity materials can minimize the shift of the threshold voltage towards negative values when compared to Schottky-gate HEMTs, which is beneficial to reduce the static power consumption and to improve the energy efficiency of the device [7].

Various insulator materials have already been considered as gate dielectrics in insulatedgate GaN-based transistors. Figure 2 reports the relationship between the bandgap and permittivity for the relevant insulators and nitride compounds. Figure 3a shows the band offsets of the insulators on the GaN as calculated by Robertson and Falabretti, who first predicted the band alignment of the GaN and the insulators based on the calculation of the charge neutrality levels (ECNL) [56]. The band offsets of the dielectrics on Al0.3Ga0.7N, recently determined by Reddy et al. using the same method, are illustrated in Figure 3b [57]. Note that, as shown from the comparison of Figure 3a,b, the different values of the energy bandgap of the same insulators are used in the calculations performed by Robertson and Falabretti [56] and by Reddy et al. [57].

**Figure 2.** Energy bandgap versus permittivity for major insulators and GaN compounds. Data taken from [7,29,49–51].

<sup>∆</sup> <sup>∆</sup> **Figure 3.** Conduction band offset (∆EC) and valence band offset (∆EV) of various dielectric materials with respect to (**a**) GaN, calculated by Robertson and Falabretti [56], and to (**b**) Al0.3Ga0.7N, calculated by Reddy et al. [57]. Note that, in (**a**,**b**), the different energy bandgaps of the insulators were assumed in the calculations. The conduction band (EC) and valence band (EV) of GaN and AlGaN are marked as dashed lines. The energy bandgap (EG) of GaN and AlGaN is also indicated.

SiO<sup>2</sup> is an attractive insulator due to its large bandgap, large band offset to (Al)GaN and chemical stability. In fact, after Khan and coworkers first applied SiO<sup>2</sup> to AlGaN/GaN MIS-HEMTs to control the gate leakage currents and improve the gate voltage swing capability [24], further high-performance MIS-HEMTs using SiO<sup>2</sup> have been demonstrated [58,59]. Nevertheless, the relatively low dielectric constant of SiO<sup>2</sup> represents a disadvantage compared to other dielectrics. From this perspective, various high-permittivity dielectrics such as HfO2, ZrO2, Ta2O5, La2O3, CeO2, TiO2, etc., have been applied to the MIS gate structures of GaN HEMTs [60–72]. Although higher g<sup>m</sup> values have been achieved in some cases, most of these insulators have reported to be relatively susceptible to leakage problems due to the relatively small band offsets with respect to (Al)GaN [49,68,73,74]. Similar observations of high gate leakage currents were reported for MIS gate structures employing dielectrics such as SiN<sup>x</sup> and Ga2O<sup>3</sup> due to the small conduction band offsets [51,75–77]. Ga2O<sup>3</sup> would be appealing as a native oxide grown by thermal or chemical processes. However, in addition to the small band offset to GaN, Ga2O<sup>3</sup> grown by thermal oxidation at low temperatures has a slow growth rate, while surface damage can be caused at higher growth temperatures [51]. Moreover, the growth of Ga2O<sup>3</sup> is even more difficult on AlGaN since Al is more easily oxidized than Ga. Differently, SiN<sup>x</sup> deposited by in situ metal organic chemical vapor deposition (MOCVD) or by low-pressure chemical vapor deposition (LPCVD) has emerged as a promising candidate as a gate dielectric as well as a passivation layer [42,78]. Similarly, AlN has also been reported in a few studies to be suitable as a gate insulator and passivation layer, especially due to its small lattice mismatch to (Al)GaN [51,79–81]. Other attempts have also used dielectrics like NiO, MgO and Sc2O<sup>3</sup> [82–87], stacked dielectric layers like SiNx/SiO2, SiNx/Al2O<sup>3</sup> and HfO2/Al2O<sup>3</sup> [88–90] or engineered alloys such as SiON, HfSiO<sup>x</sup> and LaLuO<sup>3</sup> in order to tune the dielectric constant and band gap of the insulators [90–93]. A comprehensive overview and comparison of the various insulators which have been considered as gate dielectrics for insulated-gate GaN-based devices is given in [7,29,49–51].

Among the insulators, Al2O<sup>3</sup> remains one of the most attractive insulators as a gate dielectric because of its large bandgap and conduction band offset to (Al)GaN, relatively high permittivity (~9) as well as high breakdown field (~10 MV/cm) and thermal and chemical stability against (Al)GaN [75,94,95]. Additionally, the considerable technological progress in the atomic layer deposition (ALD) process enables the deposition of high-quality Al2O<sup>3</sup> films to the gate structures in GaN transistors. In the next section, the status of the gate dielectric technology using Al2O<sup>3</sup> for GaN-based devices is reviewed.
