High-voltage AlInN/GaN superjunction ﬁn-gate high electron mobility transistor for power-switching application

An AlInN/GaN superjunction ﬁn-gate high electron mobility transistor (SJFin-HEMT) is proposed in this work. A superjunction region with GaN/AlInN/GaN/AlInN/GaN structure is deﬁned between the gate and drain, and two-dimensional-hole-gas/two-dimensional-electron-gas/two-dimensional-hole-gas/two-dimensional-electron-gas(2DHG/2DEG/2DHG/2DEG)arerespectivelyinducedduetothepolarizationchargesattheheterojunctioninterfaces.The2DHGsand2DEGscompensateeachother,result-ingachargebalancedsuperjunctioninthegate-drainspacing,thusinducingauniformelectricﬁelddistributionunderoff-statemaximizingthebreakdownvoltage.Additionally,becausethecurrentﬂowsthroughboththe2DEGs,higheroutputcurrentandloweron-stateresistanceareobservedincontrasttotheconventionalFin-HEMT.Simulationresultsshowbreakdownvoltageof2957V(


INTRODUCTION
Due to the high-voltage sustaining capability (>3 MV/cm), high two-dimensional electron gas (2DEG) density (> 1 × 10 13 cm -2 ) and high electron mobility (> 1000 cm 2 /V⋅s), gallium nitride (GaN)-based high electron mobility transistor (HEMT) is a promising candidate for high-power, high-speed and high-temperature applications [1][2][3]. Comparing with the mature AlGaN/GaN heterostructure, AlInN/GaN system has also attracted much attention. The AlInN/GaN heterostructure can be strain free and address better reliability than the conventional AlGaN/GaN devices [4]. Additionally, due to the strong polarization in the AlInN layer, the AlInN/GaN HEMT always has higher 2DEG density and thinner barrier, leading to higher output power, transconductance and cut-off frequency [5][6][7][8][9][10][11][12]. However, the conventional planar HEMTs suffer from the intrinsically depletion-mode operation and buffer leakage current. HEMTs) [13][14][15][16][17][18]. The Fin-HEMTs show significant positive shift of threshold voltage by narrowing the fin width, and better leakage current suppress capability due to enhanced channel control from the gate sidewalls. Although excellent device performances of Fin-HEMTs have been reported, the breakdown voltage (V br ) is still much lower than that of planar devices, which restricts their application in high-power region. On one hand, under off-state, the electric force lines from the non-fin depletion region would crowd to the gate around the thin fin. This aggravates the curvature effect of the depletion region boundary, resulting an increase in peak electric field and thus a decrease in V br . On the other hand, conventional junction terminal technologies, such as field plate, reduced surface field and so on, which flat the electric field distribution and enhance the V br , are hard to be applied to Fin-HEMTs due to the nanoscale fin.
In this paper, a novel superjunction Fin-HEMT (SJFin-HEMT) is proposed to achieve both high V br and low onstate resistance (R on ) by introducing a superjunction between the gate and drain. By carrying out a numerical simulation, channel electric field tends to be optimized, leading to a flat electric field distribution along the channel, and the V br is significantly improved. Moreover, an extra 2DEG channel is formed in the superjunction region, which leads to higher output current and lower R on , comparing with the conventional Fin-HEMT. The gate-to-source spacing and gate-to-drain spacing are set to be 5 µm and 10 µm, respectively. Acceptor-type deep-level traps with energy level of 0.5 eV under the conduction band minimum and density of 2 × 10 16 cm -3 are defined in the 2-µmthick GaN buffer to achieve a high-resistive (HR) buffer layer [19]. In the superjunction region between the gate and drain, a 30-nm-thick GaN cap layer is defined on the 7-nm-thick Al 0.92 In 0.08 N barrier, and a 10-nm-thick Al 0.92 In 0.08 N layer is inserted 30 nm below the barrier layer, otherwise the thickness of GaN cap is 2 nm, and all the layers are not-intentionally doped. Finally, a 20-nm-thick Al 2 O 3 passivation layer is defined on the surface of the device and around the fin, which is not shown in Figure 1(a).

DEVICE STRUCTURE AND CONCEPT
Due to the discontinuity of polarization charge between the GaN and AlInN, two 2DEGs and two two-dimensional hole gas (2DHGs) accumulate at the AlInN/GaN interface in the superjunction region [20,21], as shown in Figure 1(b). Under on-state, current flows through the two 2DEGs in the superjunction region, which leads to a higher output current and To better demonstrate the performance of the proposed device, a conventional AlInN/GaN Fin-HMET using the same geometry parameters reported in [16] is also simulated.

RESULTS AND DISCUSSIONS
3-D device simulation using Synopsys Sentaurus TCAD is carried out to demonstrate the performance of the proposed device. Material parameters reported in [22] are used in our simulation. The degree of relaxation of the AlInN barrier is set to be 0.21 by fitting the density of 2DEG channel to the experimental result (2.02 × 10 13 cm -2 ) [16], and the same value is applied to the AlInN insert layer. Figure 3 shows the simulated band structure and carrier density of the conventional Fin-HEMT (a) and SJFin-HEMT (b). Due to the thick GaN cap (30 nm) used in the superjunction region of SJFin-HEMT, a 2DHG accumulates at the GaNcap/AlInN-barrier interface. Similarly, extra 2DHG and 2DEG also accumulate at the upper and lower surface of the AlInN insert layer. The sheet density of the 1st 2DHG, 1st 2DEG, 2nd 2DHG and 2nd 2DEG are found to be 1.53 × 10 13 cm -2 , 1.31 × 10 13 cm -2 , 1.24 × 10 13 cm -2 and 1.50 × 10 13 cm -2 , respectively. It is noted that the sheet density of the 1st 2DEG is lower than that of the 2DEG channel, because the part of the 1st 2DEG is depleted by the negative polarization charge at the upper surface of AlInN barrier and AlInN insert layer. Moreover, the sum of the 1st and 2nd 2DHGs equals with that of the 1st and 2nd 2DEGs, because all the 2DEGs and 2DHGs are generated due to polarization effect. The charge balanced superjunction formed between the 2DEGs and 2DHGs can maximize the V br [23,24]. Figure 4 plots the simulated and experimental transfer characteristics of the SJFin-HEMT and conventional Fin-HEMT [16] at different V DS . The simulation results of Fin-HEMT show a good agreement with the experimental ones, which confirms the feasibility and accuracy of our simulation. The SJFin-HEMT and conventional Fin-HEMT have the same threshold voltage of -3.48 V since they share the same fin structure, close to the experimental value of -3.5 V [16]. It is noted that the SJFin-HEMT shows a higher drain current than the conventional device because the current flows through both the 2DEGs in the superjunction region, or in other words, the resistance of the superjunction region is reduced due to the 2nd 2DEG. Under the bias of V DS = 7 V and V GS = 2 V, the SJFin-HEMT (1.35 A/mm) shows ∼14% increase in I DS comparing with the conventional Fin-HEMT (1.18 A/mm, simulation). Moreover,  [16] are also plotted the SJFin-HEMT (0.31 mΩ⋅cm 2 ) shows ∼13% decrease in R on comparing with the Fin-HEMT (0.36 mΩ⋅cm 2 , simulation) at V DS = 0.1 V and V GS = 0 V. Figure 5 plots the simulated electric field distribution of the SJFin-HEMT and conventional Fin-HEMT at V br , and the V br is defined as the drain voltage when the off-state current is 1 mA/mm at V GS = -6 V. For the conventional device, a typical triangular shape electric field is observed due to the electric field crowding near the drain-side of the gate. The 2DEG channel cannot be fully depleted at breakdown, which leads to a V br of 241 V, close to the experimental value of 249 V [16], far from the material limit of GaN. However, a flat electric field distribution is observed in the superjunction region of the SJFin-HEMT, which leads to a significant increase of V br to 2957 V, over twelve times higher than that of the conventional device. The proposed device shows a figure of merit (FOM = V br 2 /R on ) of 28.21 MW⋅cm -2 , which is much higher than that of the conventional device (0.16 MW⋅cm -2 ), due to the decrease in the R on and dramatical enhancement in V br simultaneously. Figure 6 shows a feasible process flow of the proposed SJFin-HEMT. First, the GaN channel layer and AlInN insert layer are grown on the HR-GaN buffer layer (Figure 6a), then AlInN insert layer in the non-superjunction region is etched (Figure 6b). Second, selective area growth of GaN is carried out to fill the trench (Figure 6c), followed by deposition of 30 nm GaN layer, 7 nm AlInN barrier layer and 30 nm GaN layer (Figure 6d), then part of the GaN layer is etched (Figure 6e) using the same mask used in Figure 6(b). Third, the fin-gate is formed by applying reactive ion etching (Figure 6f). Finally, the device is completed with the formation of source, drain and gate electrodes, and the growth of Al 2 O 3 passivation layer, as shown in Figure 6(g).

CONCLUSIONS
In this paper, an AlInN/GaN superjunction Fin-HEMT is proposed. Two 2DHGs and two 2DEGs are induced at the interfaces of GaN/AlInN/GaN/AlInN/GaN structure in the gateto-drain spacing due to the polarization effect. Charge balanced superjunction formed between the 2DHGs and 2DEGs achieves the device with uniform electric field and thus high breakdown voltage. Moreover, since the current flows through both the 2DEGs, the proposed device shows higher output current and lower on-state resistance. Comparing with the conventional Fin-HEMT, the SJFin-HEMT shows 14% increase in I DS , 13% decrease in R on and 1127% increase in V br . Since the SJFin-HEMT achieves high V br and low R on simultaneously, it can be a promising candidate for power-switching application.