In the past decades, wide band gap semiconductor devices, such as SiC and GaN, have been studied extensively for their superior properties. Nowadays, Ga₂O₃, with a wider band gap (4.8 eV), receives more and more attention.^[1,2] The Baliga merit of Ga₂O₃ is about 10 times that of 4H–SiC and four times that of GaN,^[3] suggesting that it is more suitable for high-power and high-voltage applications.

As a new semiconductor device, the Ga₂O₃ device is just beginning to be studied, and mainly in the Schottky barrier diodes (SBDs)^[4,5] and MOSFETs.^[6–8] For Ga₂O₃ SBD, edge termination is necessary because the electric field will concentrate on the edge of Schottky junction. In past studies, several ways of terminating edge have been applied to SiC SBDs, including field plates, guarding rings, mesa structures, high-resistivity layers or the combination of such techniques.^[9–11] Among these methods, the field plate is very simple and widely adopted when the voltage requirement is not that high, such as the case of 1 kV.^[12] For Si or SiC, SiO₂ is often used as the field plate dielectric, while for Ga₂O₃ whose dielectric is proper still needs to be investigated.

Recently, Cha et al.^[13] studied the field plate structure of Ga₂O₃ SBD with a p-type drift layer by simulation. They found that the dielectric should possess both high permittivity and a high breakdown field. This work focuses on a Ga₂O₃ SBD with an n-type drift layer. The field plate is studied to provide improved reverse electric field (E-field) distribution of Ga₂O₃ SBDs, which is conducive to breakdown performance. The work is based on simulations and three kinds of dielectrics. A series of field plate overlaps and dielectric thicknesses are studied systematically.

The simulations are carried out by using Silvaco TCAD software. The parameters of Ga₂O₃ are cited mainly from Ref. [14]. The Ga₂O₃ band gap and affinity are 4.8 eV and 4 eV, respectively. The work function of the Schottky electrode is 5 eV. The thickness and electron concentration of the Ga₂O₃ drift layer are 7 μm and 1×10¹⁶ cm^–3, respectively. The drift layer thickness is the same as that in Refs. [12] and [15]. Figure 1(a) indicates that the breakdown voltage for such a Ga₂O₃ drift layer is larger than 4000 V theoretically. The electron concentration of Ga₂O₃ substrate is 1×10¹⁹ cm^–3. SiO₂, Al₂O₃, and HfO₂ are employed as the field plate dielectric, respectively. The dielectric are the same as that in Ref. [13], their permittivity values are 3.9, 9.3, and 22, respectively. The maximum electric field intensity in Ga₂O₃ drift layer, at the edge of the Schottky junction (E_j) and the edge of the plate (E_p), as a function of the overlap are studied at different values of dielectric thickness and reverse voltage. In addition, the maximum electric field intensity in the dielectric (E_d), located at the edge of plate, as a function of dielectric thickness is also studied. The reverse bias voltages are 600 V and 1200 V, respectively. The range of field plate overlap is 1 μm–20 μm. The thickness ranges of SiO₂, Al₂O₃, and HfO₂ are 200 nm–500 nm, 300 nm–700 nm, and 300 nm–700 nm, respectively. The device structure is shown in Fig. 1(b).

Fig. 1.

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	Fig. 1. (color online) (a) Breakdown voltages versus doping concentration for different Ga2O3 thicknesses; (b) schematic diagram of the field plate termination of Ga2O3 SBD.

In addition, 4H–SiC and GaN counterparts with similar device structures are also presented for comparison. The thickness and electron concentration of the drift layer are the same as those of Ga₂O₃. The field plate overlap is 20 μm. The thickness range of SiO₂, Al₂O₃, and HfO₂ are 200 nm–300 nm, 300 nm–600 nm and 300 nm–900 nm, respectively. The SiC band gap and affinity are 3.2 eV and 3.5 eV, and the GaN band gap and affinity are 3.4 eV and 4 eV, respectively. The work function of the Schottky electrode is 4.5 eV for SiC and 5 eV for GaN, thus keeping the same Schottky barrier height as that in the case of Ga₂O₃.

Figure 2(a) shows the variations of E_j and E_p with overlap and SiO₂ thickness at 600 V. It is seen that, as the overlap increases from 1 nm to 5 μm, the E_j decreases quickly and the E_p increases slightly; as it exceeds 5 μm, their variations both become very slow. To save the termination area, 10 μm length overlap is sufficient. When the SiO₂ thickness is 200 nm, the E_j and E_p eventually tend to 2.66 MV/cm and 2.9 MV/cm, respectively. In this case, E_j is smaller than E_p. As SiO₂ thickness is 250 nm, both of the values tend to 2.81 MV/cm and 2.66 MV/cm, respectively. In this case E_j becomes greater than E_p. As SiO₂ thickness further increases, their gap becomes larger. Since the function of the field plate is to share the high termination electric field, it is ideal when E_j and E_p are both small and equal. Therefore, Fig. 1(a) reveals that the optimal SiO₂ thickness is between 200 nm and 250 nm at 600 V.

Fig. 2.

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	Fig. 2. (color online) Plots of Ej and Ep versus field plate overlap for different SiO2 thicknesses and reverse bias voltage of (a) 600 V and (b) 1200 V.

Figure 2(b) shows the variations of E_j and E_p with the overlap and SiO₂ thickness at 1200 V. It is seen that the variation tendency is similar to that in Fig. 2(a), but differences still exist obviously. In Fig. 2(b), when SiO₂ thickness is 200 nm, E_j is still smaller than E_p, while their gap becomes larger than that in Fig. 2(a). When SiO₂ thickness is 250 nm, E_p begins to be greater than E_j, which is diametrically opposite to that in Fig. 2(a). When SiO₂ thickness further increases, E_j is still greater than E_p, while their gap becomes smaller. Figure 2(b) reveals that the variations of E_j and E_p at 1200 V are different from those in the case of 600 V, and the optimal SiO₂ thickness will be a little greater than 250 nm at 1200 V.

Figure 3(a) shows the variations of E_j and E_p with the overlap and Al₂O₃ thickness at 600 V. It shows that when Al₂O₃ thickness is 300 nm, E_j is much smaller than E_p; as it increases to 400 nm, both of the values become almost equal; as it further increases to 500 nm, E_j begins to be larger than E_p. Figure 3(a) reveals that the optimal Al₂O₃ thickness is about 400 nm at 600 V.

Fig. 3.

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	Fig. 3. (color online) Plots of Ej and Ep versus field plate overlap for different Al2O3 thickness. The reverse bias is (a) 600 V and (b) 1200 V.

Figure 3(b) shows the variations of E_j and E_p with the overlap and Al₂O₃ thickness at 1200 V. Comparing with Fig. 3(a), it is seen that in the condition of 400 nm Al₂O₃ thickness, the gap between E_j and E_p turns larger, signifying that it is no longer the optimal thickness in this case. Moreover, the gap between E_j and E_p becomes very close when Al₂O₃ thickness is 500 nm. So it can be deduced that the optimal Al₂O₃ thickness is a little less than 500 nm at 1200 V.

Figure 4(a) shows the variations of E_j and E_p with the overlap and HfO₂ thickness at 600 V. It is seen that E_j is much smaller than E_p when HfO₂ thickness is 500 nm; as it increases to 600 nm, their gap becomes smaller; as it increases to 650 nm, E_j is just a little smaller than E_p; as it further increases to 700 nm, E_j starts to be greater than E_p. Figure 4(a) reveals that the optimal HfO₂ thickness is between 650 nm and 700 nm at 600 V.

Fig. 4.

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	Fig. 4. (color online) Plots of Ej and Ep versus field plate overlap for different HfO2 thickness. The reverse bias voltage is (a) 600 V and (b) 1200 V.

Figure 4(b) shows the variations of E_j and E_p with the overlap and HfO₂ thickness at 1200 V. Comparing with Fig. 4(a), it is seen that E_j is still smaller than E_p, their gap even becomes larger when HfO₂ thickness is 500 nm. The scenario for HfO₂ with 600 nm in thickness is similar to that with 650 nm in thickness. When HfO₂ thickness is 700 nm, E_j is just a little smaller than E_p. It means that at 1200 V, the optimal HfO₂ thickness is greater than 700 nm.

Figures 2–4 reveal that there is an optimal dielectric thickness for a certain circumstance. This is because, on the one hand, the dielectric thickness should be sufficiently thick so that the peak electric field inside the dielectric will not exceed its breakdown intensity; on the other hand, it should also be sufficiently thin so that the field plate can influence the electric field distribution inside the semiconductor and provide sufficient field relief at the edge.^[9] It is also observed that, as the reverse bias increases, the optimal thickness increases accordingly in the condition of same dielectric, which is very significant for designing the different voltage rate SBDs. Moreover, as the relative permittivity increases from 3.9 (SiO₂) to 22 (HfO₂), the optimal dielectric thickness also increases accordingly. In addition, the E_j and E_p both become smaller at optimal dielectric thickness as the permittivity increases. For example, at 600 V, the lowest values of E_j and E_p are both about 2.8 MV/cm for SiO₂, and decrease to about 2.4 MV/cm for Al₂O₃ and about 2.1 MV/cm for HfO₂, indicating that the higher the permittivity, the lower the breakdown risk of Ga₂O₃ will be.

Figure 5 shows the relationships between E_d and dielectric thickness for different dielectrics. In Fig. 5(a), it is seen that E_d decreases with SiO₂ thickness increasing. While it is worth noting that it is more than 8 MV/cm and 14 MV/cm at 600 V and 1200 V, respectively. Figure 2 shows that the maximum electric field in Ga₂O₃ is about 2.8 MV/cm and 4.5 MV/cm at 600 V and 1200 V, respectively. Since the critical breakdown electric field (E_c) of Ga₂O₃ and SiO₂ are 8 MV/cm^[3] and 15 MV/cm, respectively,^[16] suggesting that that SiO₂ is suitable to be the dielectric for the 600-V Ga₂O₃ SBDs but not quit suitable for the 1200 V ones.

Fig. 5.

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	Fig. 5. (color online) Variations of Ed with dielectric thickness, the dielectric is (a) SiO2, (b) Al2O3, and (c) HfO2.

Figure 5(b) shows that E_d also decreases with Al₂O₃ thickness increasing, while its decreasing rate is less than the decreasing rate of E_d that decreases as SiO₂ thickness increases. The value of E_d is about 5 MV/cm and 8.5 MV/cm at 600 V and 1200 V, respectively. The corresponding maximum electric field in Ga₂O₃ is about 2.4 MV/cm (Fig. 3(a)) and 3.8 MV/cm (Fig. 3(b)) at 600 V and 1200 V, respectively. Since the E_c of Al₂O₃ is 11.2 MV/cm–13.8 MV/cm,^[16] implying that Al₂O₃ is suitable for both the 600 V and 1200 V Ga₂O₃ SBDs.

Figure 5(c) shows that E_d increases slightly as HfO₂ thickness increases, which is different from the scenarios of SiO₂ and Al₂O₃. The E_d is about 3 MV/cm and 5.2 MV/cm at 600 V and 1200 V, respectively. The corresponding maximum electric field in Ga₂O₃ is about 2.1 MV/cm (Fig. 4(a)) and 3.3 MV/cm (Fig. 4(b)) at 600 V and 1200 V, respectively. Since the E_c of HfO₂ is 3.9 MV/cm–6.7 MV/cm,^[16] suggesting that it is only suitable for the 600-V Ga₂O₃ SBDs. All the results indicate that the dielectric permittivity of HfO₂ should approach that of Ga₂O₃. Reference [13] indicates that for Ga₂O₃ SBD with p-type Ga₂O₃ drift layer the best choice is HfO₂, thus our results also show that the scenario of the n-type drift layer is different that of the p-type one.

Figure 6 shows the variations of E_j and E_p with dielectric thickness for SiC SBDs at 600 V and 1200 V. Figure 6(a) shows that at 600 V, the optimal SiO₂ thickness is between 200 nm and 250 nm, the optimal Al₂O₃ thickness is between 400 nm and 500 nm, and the optimal HfO₂ thickness is about 800 nm. Figure 6(b) shows that at 1200 V, the optimal SiO₂ thickness is a little larger than 250 nm, the optimal Al₂O₃ thickness is a little larger than 500 nm and the optimal HfO₂ thickness is about 900 nm. Comparing with the results of Ga₂O₃, it is known that at the same reverse bias, the optimal dielectric thickness is larger for the SiC counterpart. Moreover, since the E_c of SiC is 2.5 MV/cm, suggesting that such a SiC drift layer is not able to withstand 1200 V (Fig. 6(b)). In Fig. 6(a), it can also be observed that such a drift layer is not suitable for the 600 V SiC SBD when the dielectric is SiO₂ or Al₂O₃.

Fig. 6.

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	Fig. 6. (color online) Plots of Ej and Ep versus dielectric thickness for SiC SBDs. The reverse bias voltage is: (a) 600 V and (b) 1200 V.

Figure 7 shows the variations of E_j and E_p with dielectric thickness for the GaN SBDs at 600 V and 1200 V. Figure 7(a) is for the case with a reverse bias of 600 V, an optimal SiO₂ thickness of nearly 250 nm, an optimal Al₂O₃ thickness is nearly 500 nm, and an optimal HfO₂ thickness larger than 800 nm. Figure 7(b) is for the case with a reverse bias of 1200 V, the optimal SiO₂ thickness is larger than 250 nm, the optimal Al₂O₃ thickness is larger than 500 nm, and the optimal HfO₂ thickness is a little larger than 900 nm. Comparing with the results of Ga₂O₃ and SiC, it is known that at the same reverse bias, the optimal dielectric thickness for the GaN SBDs is even larger than the SiC ones. Furthermore, since the E_c of GaN is 3.4 MV/cm,^[17] suggesting that such a GaN drift layer is also not able to withstand 1200 V (Fig. 7(b)).

Fig. 7.

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	Fig. 7. (color online) Plots of Ej and Ep with dielectric thickness for GaN SBDs. The reverse bias voltage is (a) 600 V and (b) 1200 V.

Figure 8 shows the relationship between E_d and dielectric thickness for the SiC and GaN SBDs. It is seen that at the same dielectric thickness and reverse bias, the E_d of GaN SBDs is less than that of the SiC ones. Combing Figs. 7(a), 8(a), and 8(b), it is clear that such a GaN drift layer is able to withstand 600 V when the dielectric is SiO₂ or Al₂O₃. It is because in such cases the maximum electric field does not exceed the E_c of GaN nor SiO₂ nor Al₂O₃ at 600 V. Figure 8(c) shows that the electric field intensity is about 5 MV/cm in HfO₂ at 600 V. Since the E_c of HfO₂ is 3.9 MV/cm–6.7 MV/cm, suggesting that it is not suitable for the 600-V rate SiC nor GaN power devices. Moreover, figure 6(a) already indicates that the drift layer is not appropriate for the 600 V SiC SBDs either, when the dielectric is SiO₂ or Al₂O₃, implying that the drift layer is not suitable for the 600-V rate SiC devices either, no matter which dielectric is chosen.

Fig. 8.

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	Fig. 8. (color online) Plots of Ed as versus dielectric thickness for SiC and GaN SBDs, the dielectric is (a) SiO2, (b) Al2O3, (c) HfO2.

Comparing Ga₂O₃ SBDs with the SiC and GaN counterparts, it is known that for Ga₂O₃ devices, the breakdown voltage bottleneck is the dielectric. While for SiC and GaN devices, the bottleneck is mainly the semiconductor itself.

In this work, the field plate termination in a Ga₂O₃ Schottky barrier diode (SBD) is studied systematically. Three kinds of oxides, i.e., SiO₂, Al₂O₃, and HfO₂, are used as the field plate dielectric, respectively. The influence of field plate overlap and dielectric thickness on the maximum electric field in Ga₂O₃ as well as the dielectric are studied. The results reveal that the optimal dielectric thickness increases with reverse bias increasing. The maximum electric field intensity decreases in SiO₂ and Al₂O₃, while it increases in HfO₂ with the increase of the dielectric thickness. Moreover, the results also indicates that SiO₂ and HfO₂ are suitable for the 600-V rate Ga₂O₃ SBD, the Al₂O₃ is suitable for both 600-V and 1200-V level Ga₂O₃ SBD. Furthermore, the comparison among Ga₂O₃, SiC and GaN devices reveals that, for the Ga₂O₃ device, the breakdown voltage bottleneck is the dielectric. While, for SiC and GaN devices, the bottleneck is mainly the semiconductor itself.