*4.2. Binary High-κ Oxides for GaN-based MISHEMTs*

Standard AlGaN/GaN MISHEMTs (see Figure 8b) are obtained by insertion of the dielectric between the metal gate and the AlGaN layer. The introduction of the gate dielectric, instead of a standard Schottky barrier gate, gives the advantage of reducing the leakage current that could limit the off-state and the gate voltage swing of the device [118]. A typical example of gate current reduction observed in HfO<sup>2</sup> or CeO<sup>2</sup> MISHEMTs is displayed in Figure 12a. Indeed, a gate leakage reduction of several orders of magnitude can be observed in both forward and reverse characteristics. This achievement allows a

higher voltage swing in the device, which in turn results in a higher maximum drain current saturation value (IDSmax). Another great advantage is the very high ION/IOFF current ratio. Indeed, high ION/IOFF current ratios between 10<sup>6</sup> and 10<sup>8</sup> have been reported in AlGaN/GaN MISHEMTs. In Figure 12b, the ION/IOFF current ratio was plotted as function of the IDSmax. Interesting, two families of MISHEMTs can be observed depending on the leakage current level. Despite their non-outstanding IDSmax, some devices can exhibit very high ION/IOFF current ratios because of their very low leakage current. On the other hand, in other cases, despite slightly higher leakage current, extraordinary IDSmax values have been demonstrated. Table 5 summarizes a survey of the most promising results obtained in normally-on AlGaN/GaN MISHEMTs using different high-κ dielectrics. Indeed, not only are Al2O<sup>3</sup> [119–121] and HfO<sup>2</sup> [122–125] indicated as suitable dielectrics, but many other gate oxide layers (Y2O<sup>3</sup> [126], HZO [127], Ta2O<sup>5</sup> [128], La2O<sup>3</sup> [125], ZrO<sup>2</sup> [129–131], Gd2O<sup>3</sup> [132]) have shown promising results when integrated into GaN HEMT technology. κ

*κ*

**− −**

**−**

−

κ

**Figure 12.** (**a**) Comparison of the gate current–voltage characteristics of AlGaN/GaN HEMTs (Schottky gate) and MISHEMTs employing HfO<sup>2</sup> and CeO<sup>2</sup> gate insulators. The data are taken from [25,118]. (**b**) ION/IOFF versus IDSmax for MISHEMTs using different gate oxides. The data are taken from Table 6 and references therein.

**Table 5.** Survey of literature data on normally-on AlGaN/GaN MISHEMTs with different high-κ gate dielectrics.


A relevant concern often characterizing the behaviour of high-κ binary oxides is the occurrence of charge-trapping phenomena upon bias stress [39], which can be the cause of reliability issues in GaN insulated gate transistors. Nevertheless, the electron trapping inside the Al2O<sup>3</sup> gate insulator in GaN MISHEMTs can be used to intentionally induce a positive shift in the threshold voltage and finally obtain a normally-off operation [121]. In this context, Fiorenza et al. [133] recently studied the temperature stability of these effects, demonstrating the presence of two competitive electron trapping/de-trapping mechanisms in Al2O<sup>3</sup> films, which were likely related to the presence of oxygen vacancies in the material.

Slightly different is the case of normally-off recessed gate hybrid MISHEMTs (see Figure 8c). In this case, the AlGaN layer below the gate region is removed, interrupting the 2DEG channel and resulting in a positive threshold voltage. The gate region is formed by a metal/oxide/GaN (MOS) interface, which requires a positive gate voltage to accumulate electrons at the oxide/GaN interface to restore the channel device. Though this approach seems to solve the crucial issue of normally-off behaviour, the complexity of these systems generates additional concerns. As an example, the lack of a 2DEG channel in the gate region causes a notable increase the channel resistance, leading to a high final on-resistance (RON) and a reduced IDSmax. To avoid this problem, it is very important to achieve high electron mobility values [134]. Hence, the oxide/GaN interface quality is clearly a key aspect to ensure a high mobility, as are the morphology of the recessed gate region and the presence of electrically active defects [135]. In this context, the choice of dielectric gate becomes crucial. The use of SiO<sup>2</sup> resulted into a poor interface quality displaying fast (interface) and slow (border) traps [136]. Dielectrics such as AlN [137], SiN [138], and their combination [139] have been also investigated as beneficial solutions to passivate surface N-vacancy, especially after recess etching damage in the gate region [140]. However, despite the good quality of the achieved interface and improved electron mobility, it was very difficult to obtain positive threshold voltages Vth well beyond the zero [137]. For these reasons, an increasing number of studies are focused on high-permittivity binary oxide layers for normally-off behaviour of AlGaN/GaN MISHEMTs. Table 6 shows the most promising results obtained in recessed gate hybrid MISHEMTs. ALD-deposited Al2O<sup>3</sup> is one of the most diffused solutions for normally-off recessed gate hybrid MISHEMTs [141–147]. However, an excessive threshold voltage instability has been observed for Al2O<sup>3</sup> gate insulators [121,148]. This phenomenon has been attributed to the large number of negative fixed charges incorporated in the gate stack [148,149]. As an alternative solution, ALD gate oxides with even higher dielectric constants, such as HfO<sup>2</sup> [150] or ZrO<sup>2</sup> [151–153], have been investigated for normally-off recessed MISHEMTs. Furthermore, in this case, trapped or fixed charges result in Vth instability issues. Other opportunities have been found in ternary oxide layers, such as HfSiO<sup>x</sup> [154] or LaHfO<sup>x</sup> [155].

Another important challenge in normally-off recessed gate hybrid MISHEMTs is the possibility of obtaining a very high saturation current IDSmax with a well positive Vth value. In Figure 13, experimental values of IDSmax are plotted as a function of the threshold voltage Vth. However, the values of IDSmax seem to decrease in correspondence with an increase in Vth, thus suggesting the existence of a trade-off between a high output current and a more positive threshold voltage. In this context, a partial recession of the AlGaN barrier layer has also been explored to realize normally-off hybrid MISHEMTs. In this way, a higher 2DEG channel density is obtained. On the other hand, a more uniform and accurate AlGaN etching process is required to obtain a positive threshold voltage and normally-off devices.

Finally, to achieve normally-off behaviour in GaN-based HEMTs, the use of appropriate gate oxides with p-type semiconducting behaviour has been proposed. In fact, similarly to the most diffused p-GaN gate approach [156], the use of a p-type semiconducting oxide can lift up the conduction band at the AlGaN/GaN interface, resulting in the depletion of the 2DEG. By applying a positive gate bias VG, it is possible to realign the conduction band of the structures, restoring the 2DEG and the channel conduction. Among these p-type semiconducting oxides, oxides such as NiO and CuO have been taken in consideration for

normally-off HEMT fabrication [157–159]. The origin of the p-type doping of these oxides is still debated. The existence of negatively charged Cu or Ni vacancies and the presence of interstitial oxygen [160,161] have both been considered as possible causes. Moreover, the possibility of epitaxial CVD growth on an AlGaN or GaN template makes this approach for threshold voltage engineering in GaN technology interesting [23].

**Figure 13.** IDSmax versus Vth value for recessed hybrid MISHEMTs using different high-κ binary gate oxides. The data are taken from [141–147,150–153].

κ

**Table 6.** Survey of literature data on normally-off recessed gate hybrid MISHEMTs with different high-κ dielectrics.

