I. INTRODUCTION

The demand for wide bandgap semiconductors has dramatically increased in application fields such as electric vehicles, the military, and national defense. Therefore, the development of next-generation power devices based on wide bandgap materials has become an important issue ^(1,2). Gallium-nitride (GaN) has the high critical electric field and the high electron mobility due to the two-dimensional electron gas (2-DEG) in the heterostructure of AlGaN/GaN, and it is suitable for power electronics ^(3,4). However, GaN devices with the lateral channel are not suitable for extremely high-power devices. Because the lateral GaN devices require a long gate-drain distance to achieve high threshold voltage (V_th) and high breakdown voltage (V_B), it results in the increase of device area. Otherwise, the vertical device has the advantage of high V$_{\mathrm{B}}$ and wide drift current region without increasing the chip size ^(5-8). Therefore, vertical GaN devices could be promising candidate device for high-power electronic applications. For general vertical devices, the cylindrical device is adopted for its excellent gate controllability ^(9,10). However, for vertical GaN devices, the performance is highly dependent on the orientation of the sidewall plane. GaN has a wurtzite crystal structure and has various sidewall planes such as m-plane (1-100), a-plane (11-20), and c-plane (0001) ⁽¹¹⁾. Many studies have reported that the m-plane outperforms the a-plane ^(12,13). However, a direct comparison between a cylindrical device with mixed planes and a hexagonal device with a single crystal plane has not yet been reported for GaN-based vertical trench MOSFETs.

In this study, we present the electrical characteristics of GaN-based vertical trench MOSFETs based on cylindrical and hexagonal structures. Through the 3-D simulations, the electrical performances of GaN-based vertical trench MOSFETs are extracted and evaluated in terms of the current density, V$_{\mathrm{th}}$, and on-resistance (R$_{\mathrm{on}}$) for both cylindrical and hexagonal devices.

II. DEVICE STRUCTURE AND SIMULATION

Fig. 1 shows the schematic diagrams of the hexagonal and cylindrical cell structures of GaN-based vertical trench MOSFETs ⁽¹⁴⁾. The geometric parameters are summarized in Table 1. The epitaxial structure including the GaN substrate consists of a 13 μm-thick n$^{{-}}$-GaN drift layer, a 700 nm-thick p-GaN channel layer, and a 200 nm-thick n$^{+}$-GaN layer. The n-type doping concentration of n$^{+}$-GaN and n$^{{-}}$-GaN are 6 ${\times}$ 10$^{18}$ cm$^{{-}3}$ and 9 ${\times}$ 10$^{15}$ cm$^{{-}3}$, respectively. The p-type doping concentration of p-GaN is 2 ${\times}$ 10$^{18}$ cm$^{{-}3}$. 80 nm-thick Al$_{2}$O$_{3}$ is used for the gate dielectric ⁽¹⁵⁾. This simulation assumes that all acceptors in the p-GaN layer are ionized. In addition, a channel is formed in the p-GaN layer to operate as a conventional enhancement mode transistor. The radius of both devices is equal to 7.27 μm to compare the electric field at the same corner.

The c-plane of GaN is not suitable for the normally-off operation because spontaneous polarization occurs as a polar plane. Generally, the a-plane or m-plane is used as a non- polar plane when fabricating the vertical GaN devices because they can achieve high V$_{\mathrm{th}}$ ^(16,17). Fig. 2 shows the Al-$_{2}$O$_{3}$/GaN interface trap densities for the m-plane and the mixed-plane ⁽¹⁸⁾. The hexagonal device consists of six m-plane sidewalls, and the cylindrical device consists of sidewalls with a mixed-plane of m-plane and a-plane, rather than a single crystal plane. The mixed-plane has higher interface traps than the m-plane because the a-plane has a higher trap density than the m-plane ⁽¹³⁾. These simulations include various physical models such as the Fermi-Dirac model, the Shockley-Read-Hall recombination model, the low field mobility model, the high field mobility model, and the shock ionization model, which were applied to increase simulation’s accuracy.

III. RESULT AND DISCUSSION

Fig. 3 shows the drain current (I$_{\mathrm{d}}$)-gate voltage (V$_{\mathrm{g}}$) characteristics for the hexagonal and the cylindrical GaN-based trench MOSFETs at drain voltage (V$_{\mathrm{d}}$) = 0.5 V. The I$_{\mathrm{d}}$ is normalized by the cross-sectional area of the device. Fig. 3(a) is the I$_{\mathrm{d}}$-V$_{\mathrm{g}}$ curve for devices without the interface trap. The V$_{\mathrm{th}}$ for the hexagonal and the cylindrical devices in this case are 4.37 V and 4.99 V, and the I$_{\mathrm{d}}$ are 0.450 kA/cm$^{2}$, and 0.464 kA/cm$^{2}$ at V$_{\mathrm{g }}$= 40 V. The hexagonal device has the smaller perimeter and cross-sectional area than the cylindrical device, so the channel forms faster and has a lower V$_{\mathrm{th}}$. In addition, the hexagonal device such as angled device, the V$_{\mathrm{th}}$ is lower because the electric field is focused on the corner due to the corner effect, and channel forms quickly at the corner ⁽¹⁹⁾. The cylindrical device has higher I$_{\mathrm{d}}$ than the hexagonal device due to excellent gate controllability. Fig. 3(b) is the I$_{\mathrm{d}}$-V$_{\mathrm{g}}$ curve for devices with the interface trap. The V$_{\mathrm{th}}$ of the hexagonal and the cylindrical devices in this case are 5.12 V and 11.15 V, and the I$_{\mathrm{d}}$ are 0.425 kA/cm$^{2}$ and 0.335 kA/cm$^{2}$ at V$_{\mathrm{g}}$ = 40 V. Due to the influence of the interface traps on both devices, the V$_{\mathrm{th}}$ is increased and the I$_{\mathrm{d}}$ is decreased. The cylindrical device is more affected by interface traps than the hexagonal device, due to the inclusion of an a-plane with high interface traps. Therefore, despite the excellent gate controllability of the cylindrical device, the electrical performance is inferior to the hexagonal device.

Fig. 4 shows the electric fields of the hexagonal and the cylindrical devices at V$_{\mathrm{g}}$ = 40 V. The cylindrical device has a smooth perimeter, which leads to a uniform electric field along the surface. On the other hand, the hexagonal device, the electric field is concentrated at the corners due to the sharp corners, as a result having a non-uniform electric field along the surface. Specifically, the electric field of the hexagonal device is 1.02 MV/cm at the corner and 0.46 MV/cm at the edge. Although the electric field of the cylindrical device is slightly different, the result is almost uniform at 0.33 MV/cm.

Fig. 5(a) shows the logarithmic I$_{\mathrm{d}}$-V$_{\mathrm{g}}$ and transconductance (g$_{\mathrm{m}}$)-V$_{\mathrm{g}}$ characteristics of the hexagonal device. Fig. 5(b) shows the cross-sectional electron concentration at each point. As shown in Fig. 5(a), the hump shape occurs in the I$_{\mathrm{d}}$-V$_{\mathrm{g}}$ curve of the hexagonal device. The inversion layer is not formed and the current does not flow well when V$_{\mathrm{g}}$ is 0 V, as shown in Fig. 5(b). The inversion layer begins to be formed at the corner when V$_{\mathrm{g}}$ increases until just before the formation of the hump. When additional V$_{\mathrm{g}}$ is applied and the hump shape occurs, a sufficient inversion layer is formed on the edges as well as the corners, allowing more current to flow. Finally, at V$_{\mathrm{g}}$ = 40 V, the inversion layer of high electron concentration is formed at the edges and corners. This is due to the corner effect shown in Fig. 4, which is the reason the channel is formed rapidly at the corners of the hexagonal device, resulting in lower V$_{\mathrm{th}}$ and a difference in the formation of an inversion layer between edges and corners, resulting in the hump shape occurs.

Fig. 6 shows the I$_{\mathrm{d}}$-V$_{\mathrm{d}}$ characteristic of the hexagonal and the cylindrical devices. The R$_{\mathrm{on}}$ is calculated in the linear region of V$_{\mathrm{d}}$ = 0.5 V and V$_{\mathrm{g}}$ = 40 V. Fig. 6(a) for devices without the interface trap shows that the cylindrical device has a higher current density than the hexagonal device. Also, the R$_{\mathrm{on}}$ is 1.16 mΩ·cm$^{2}$ of the cylindrical device, which is lower than 1.20 mΩ·cm$^{2}$ of the hexagonal device. However, Fig. 6(b) for devices with the interface trap shows that the hexagonal device has a higher current density than the cylindrical device. This means that the hexagonal device has higher channel mobility and lower R$_{\mathrm{on}}$ than the cylindrical device. The calculated R$_{\mathrm{on}}$ is 1.24 mΩ·cm$^{2}$ of the hexagonal device and 1.66 mΩ·cm$^{2}$ of the cylindrical device. This is because the a-plane included in the mixed-plane of the cylindrical device has higher trap density and lower interface quality than the m-plane. These results indicate that the GaN-based vertical trench MOSFET is highly dependent on the orientation of the channel sidewall plane.

Fig. 7 shows the off-state I$_{\mathrm{d}}$-V$_{\mathrm{d}}$ characteristics of the hexagonal and the cylindrical devices when V$_{\mathrm{g}}$ is 0 V. The V$_{\mathrm{B}}$ of the hexagonal and the cylindrical devices are 2,660 V and 2,682 V, respectively. The V$_{\mathrm{B}}$ of the hexagonal device is slightly lower than that of the cylindrical device because the electric field is focused on the corners. The hexagonal device has not only better electrical performances than the cylindrical device but also the V$_{\mathrm{B}}$ of only about 1\% lower than the cylindrical device.

The V$_{\mathrm{th}}$, I$_{\mathrm{d,}}$ and R$_{\mathrm{on}}$ of the analyzed hexagonal and cylindrical devices are summarized in Table 2. As shown in Table 2, the cylindrical device shows superior electric performances when there are no interface traps. However, considering the interface traps along the crystal plane orientation, the electrical performances of both devices are degraded. Specifically, the cylindrical device shows inferior electrical performances as compared to the hexagonal device due to many interface traps.

IV. CONCLUSIONS

This study analyzed the effect of crystal orientations for the sidewall plane on the electrical performances of GaN-based vertical trench MOSFETs with hexagonal and cylindrical structures. The GaN-based vertical trench MOSFET’s electrical performances are highly dependent on the orientation of the sidewall plane of the channel. Electrical performances decreased in both devices due to the influence of the interface traps. However, it is shown that the hexagonal device consists of m-plane and has fewer interface traps than the cylindrical device consists of mixed-plane, it has better electrical performances despite the negative effects due to the corner effect. Therefore, to enhance the performances of the device, it is advantageous to design the crystal orientation toward the m-plane. The results of this study provide insight into the electrical properties of GaN-based vertical trench MOSFETs, as well as design guidelines.