GaN-based high-electron-mobility-transistors (HEMTs) have been promising candidates for high voltage and high frequency applications due to their superior material and device characteristics.^[1–3] The critical electric fields for GaN and AlN are 3.3 MV/cm and 12 MV/cm, respectively, and the AlGaN material is a kind of ultra-wide bandgap (UWBG) semiconductor with better breakdown characteristics compared with the GaN material. The off-state breakdown voltage (BV) is one of the most important parameters for GaN-based HEMTs, and numerous studies concentrate on improving the breakdown characteristics.^[4–6] For GaN channel HEMTs, a record high breakdown voltage of 3000 V has been achieved in InAlN/GaN MOSHEMT with gate–drain spacing of 30 μm.^[7] In comparison, a BV of 1650 V has been obtained in AlGaN channel HEMT with gate–drain spacing of 10 μm.^[8,9] In order to improve BV, several methods were employed, including source field plate (FP), gate FP, AlGaN back barrier, and high-k passivation layer.^[10–12] However, the gate–drain average breakdown electric field E_BR values are much lower than the theoretical limitation ones for GaN channel and AlGaN channel HEMTs. For GaN-based HEMTs, it is difficult to achieve BV of 1 kV and E_BR of 2 MV/cm simultaneously. It is essential to further improve the BV and E_BR of GaN-based HEMTs.

In this paper, a simple method, large gate metal height, is proposed to enhance BV in GaN channel and AlGaN channel HEMTs. By using this method, breakdown voltage V_BR of 582 V and E_BR of 2.33 MV/cm have been achieved for GaN channel HEMTs with gate–drain spacing L_GD = 2.5 μm. For AlGaN channel HEMTs with L_GD = 7 μm, V_BR and E_BR can be improved to 1736 V and 2.51 MV/cm, respectively.

The GaN and AlGaN channel HEMTs structure adopted in this paper is shown in Fig. 1. The devices are simulated by using Silvaco-ATLAS software. For the fabricated GaN-based HEMTs, the drain and source metals should penetrate into the GaN buffer layer and be connected to 2DEG directly by an annealing process to form ohmic contacts. The drain and source contacts are designed as shown in Fig. 1 to simulate this process, and the contact length L_con = 0.5 μm. The simulated HEMTs have a gate length L_G = 1 μm, a gate–source spacing L_GS = 0.5 μm, and a gate–drain spacing L_GD = 2.5–9 μm. For AlGaN channel HEMTs, the aluminum compositions for the AlGaN buffer and AlGaN barrier layers are 40% and 70%, respectively.

Fig. 1.

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	Fig. 1. Device structure of the GaN and AlGaN channel HEMTs.

The electron mobility in the AlGaN barrier layer is set to 600 cm²/V⋅s.^[10] The GaN or AlGaN buffer layer consists of two parts, namely, 10-nm channel layer and 1.99-μm buffer layer. In the channel layer, the electrons belong to 2DEG and their mobility is set to 1500 cm²/V⋅s according to the 2DEG mobility in previous studies.^[13,14] The electron mobility in the buffer layer is set to 250 cm²/V⋅s, which is extracted from the GaN-based MOSFET.^[15] In the buffer layer, we consider a shallow donor, a deep donor, and a deep acceptor.^[12,16] The shallow donor is assumed to be ionized completely at room temperature, and its density is set to 1 × 10¹⁵ cm⁻³. The energy level and density of the deep donor are E_C – 0.5 eV and 2 × 10¹⁷ cm⁻³, respectively. The electron and hole capture cross sections for the deep donor are 1 × 10⁻¹³ cm² and 1 × 10⁻¹⁵ cm², respectively. The energy level and density of the deep acceptor are assumed to be E_C – 2.85 eV and 1 × 10¹⁷ cm⁻³, respectively. The electron and hole capture cross sections for the deep acceptor are both set to 1 × 10⁻¹⁵ cm². The positive polarization charge is modeled as a positive fixed sheet charge with a density of 1 × 10¹³ cm⁻² along the interface of the heterojunction. In this paper, surface states and gate tunneling model are not considered. The device breakdown is induced by the impact ionization. The impact ionization is modeled as α₀ exp (−E_C/E), where α₀ = 2.9 × 10⁸ cm⁻¹ and E_C = 3.4 × 10⁷ V/cm.^[17]

As shown in Fig. 2, AlGaN/GaN HEMTs with various relative permittivities ε_r are simulated. The gate height h and passivation layer thickness t are 0.2 μm and 0.1 μm, respectively. The breakdown voltage V_BR is defined as the drain voltage with the drain leakage current I_D reaching 1 mA/mm. V_BR increases with the increase of ε_r for one certain passivation layer thickness. For ε_r = 3.9 (SiO₂), the breakdown voltage is 99 V. In contrast, V_BR for the HEMT with ε_r = 28 (LaLuO₃) is 346 V, which is 2.5 times higher than that for the HEMT with ε_r = 3.9. When the passivation layer thickness increases to 0.5 μm, V_BR can be improved further to 518 V as shown in Fig. 3. The similar relationship between V_BR and ε_r and between V_BR and t is in agreement with the numerical calculation.^[12] For a high ε_r or t, the electric field at the drain-side edge of the gate is weakened, and the applied voltage tends to drop uniformly.

Fig. 2.

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	Fig. 2. Off-state ID–VDS curves for GaN channel HEMTs with various relative permittivities.

Fig. 3.

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	Fig. 3. Comparison of off-state ID–VDS curves for GaN channel HEMTs with εr = 28, t = 0.5 μm, and h = 0.2 μm or h = 0.4 μm.

The impact of gate metal thickness h on breakdown characteristics is shown in Fig. 3, the breakdown voltage V_BR increases from 518 V for h = 0.2 μm to 566 V for h = 0.4 μm. Figure 4 shows the electron concentration distribution in GaN channel HEMTs with h = 0.2 μm and h = 0.4 μm. The depletion region along the AlGaN/GaN interface can extend from X = 3.75 μm for h = 0.2 μm to X = 3.83 μm for h = 0.4 μm, indicating that a larger depletion region can be formed for a larger h. A larger h would increase the area of the drain-side wall of the gate and the gate sidewall capacitance. The increased depletion capacitance would enlarge the depletion region extension and improve the breakdown voltage. The relative permittivity of air is much lower than that of the passivation layer. In order to achieve a large gate sidewall capacitance, the passivation layer should be higher than the gate metal.

Fig. 4.

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	Fig. 4. Logarithmic electron concentration distribution in GaN channel HEMTs with εr = 28, t = 0.5 μm, VDS = 200 V, and (a) h = 0.2 μm or (b) h = 0.4 μm.

In order to investigate the impact of h on HEMTs with different L_GD, the dependence of V_BR on L_GD is shown in Fig. 5(a). L_GD varies from 2.5 μm to 9 μm while keeping t and h constant at 1.0 μm and 0.8 μm, respectively. The V_BR of the HEMT with L_GD = 2.5 μm is 582 V, which is 18 V higher than that of the same L_GD HEMT with t = 0.5 μm and h = 0.4 μm. The increase of V_BR results from the gate sidewall capacitance. For the HEMT with L_GD = 2.5 μm, the average electric-field strength at the breakdown point (V_DS = 582 V) is 2.33 MV/cm. The electric field between the gate and drain is very uniform for V_DS = 550 V as shown in Fig. 5(b). V_BR increases with the increase of L_GD for L_GD ≤ 5 μm and saturates at L_GD = 5 μm. The V_BR for the HEMT with L_GD = 5 μm is 922 V and the electric-field distribution at V_DS = 900 V is shown in Fig. 5(b). The breakdown strength decreases from 2.33 MV/cm to 1.84 MV/cm, indicating that the gate capacitance for t = 1.0 μm and h = 0.8 μm cannot form a sufficient depletion region to make the electric field even as in the HEMT with L_GD = 2.5 μm. The electric-field curve for the HEMT with L_GD = 7 μm almost overlaps with that for the HEMT with L_GD = 5 μm for X = 2–7 μm (X = 2 μm is the starting point of L_GD). Thus, the gate sidewall capacitance begins to saturate when L_GD reaches 5 μm for t = 1.0 μm and h = 0.8 μm. In order to improve V_BR further, t and h are increased to 2.0 μm and 1.6 μm, respectively. V_BR of the HEMT with L_GD = 7 μm is increased from 953 V to 1310 V. The average breakdown strength is increased from 1.36 MV/cm to 1.87 MV/cm. For a large L_GD, a larger h can increase the gate sidewall capacitance more effectively, resulting in a larger depletion region and a more uniform electric-field distribution.

Fig. 5.

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	Fig. 5. (a) Off-state ID–VDS curves and (b) electric-field distribution as a function of LGD for GaN channel HEMTs with t = 1.0 μm and h = 0.8 μm. Characteristics for the HEMT with LGD = 7 μm, t = 2.0 μm, and h = 1.6 μm are also drawn.

Figure 6(a) shows the output curves of the GaN channel HEMT with L_GD = 7 μm, ε_r = 28, t = 2.0 μm, and h = 1.6 μm. The on resistance R_on can be extracted from the output curves as shown in Fig. 6(a). The specific on-resistance R_ON is defined as R_ON = R_on × (L_GS + L_G + L_GD + 2L_con). The dependence of V_BR and R_ON on L_GD is shown in Fig. 6(b). V_BR increases with the increase of L_GD for L_GD ≤ 5 μm and saturates at L_GD = 5 μm. The V_BR is increased from 953 V to 1310 V by using a higher t and h for L_GD = 7 μm. R_ON increases with the increase of L_GD. For L_GD = 2.5 μm, R_ON = 0.137 mΩ⋅ cm². The R_ON is increased to 0.467mΩ⋅ cm² for L_GD = 7 μm. In addition, the h values have little effect on R_ON for a certain L_GD. The power figure-of-merit (FOM) is as high as 3.67 × 10⁹ V²⋅Ω⁻¹⋅cm⁻² for the GaN channel HEMT with t = 2.0 μm and h = 1.6 μm.

Fig. 6.

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	Fig. 6. (a) Output curves of the GaN channel HEMT with LGD = 7 μm, εr = 28, t = 2.0 μm, and h = 1.6 μm. (b) VBR and RON as a function of LGD.

Due to the higher critical breakdown electric field, AlGaN channel HEMTs have higher breakdown voltage compared with the conventional GaN channel HEMTs. Figure 7 shows the breakdown enhancement of gate metal height for Al_0.7Ga_0.3N/Al_0.4Ga_0.6N HEMTs. The breakdown voltage is increased from 1535 V to 1763 V by increasing from t = 1.0 μm, h = 0.8 μm to t = 2.0 μm, h = 1.6 μm. Due to the effect of alloy scattering in the AlGaN channel HEMT, the electron mobility would be reduced, leading to higher R_ON of 2.15mΩ⋅ cm² and lower FOM of 1.44 × 10⁹ V²⋅Ω⁻¹⋅cm⁻². The electron mobility can be improved by utilizing a buffer layer.^[18] The average breakdown electric field is increased from 2.19 MV/cm to 2.51 MV/cm, which is much higher than that of GaN channel HEMTs. The potential distributions in AlGaN channel HEMTs with t = 1.0 μm, h = 0.8 μm and t = 2.0 μm, h = 1.6 μm are shown in Fig. 8. With higher gate sidewall capacitance, the potential distribution is more even and BV is enhanced. Besides the gate metal thickness, the field plate thickness could reduce the peak at the FP edge and further improve the BV. The BV enhancement mechanism of large gate metal height is similar to that of slant FP, namely, using the sidewall capacitance.^[19] However, the slant FP structure requires a precise etching process to form the designed slope angle. Compared with the slant FP, the large gate metal height is more controllable. The large gate metal height would increase metal cost. In order to reduce the metal cost, metal pleating can be used instead of evaporating or sputtering process, and low cost metal can be employed instead of gold metal.

Fig. 7.

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	Fig. 7. Off-state ID–VDS curves of Al0.7Ga0.3N/Al0.4Ga0.6N HEMTs with h = 0.8 μm, t = 1.0 μm and h = 1.6 μm, t = 2.0 μm. LGD = 7 μm, εr = 28.

Fig. 8.

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	Fig. 8. Potential distribution of Al0.7Ga0.3N/Al0.4Ga0.6N HEMTs with (a) h = 0.8 μm, t = 1.0 μm and (b) h = 1.6 μm, t = 2.0 μm. LGD = 7 μm, εr = 28, and VDS = 1000 V.

A large gate metal height is proposed to enhance BV in GaN-based HEMTs. For GaN channel HEMTs with L_GD = 2.5 μm, V_BR and E_BR are 582 V and 2.33 MV/cm, respectively by using this method. The breakdown voltage enhancement results from the increase of the gate sidewall capacitance and depletion region extension. For larger L_GD HEMT, larger h is essential to improve V_BR effectively. For the HEMT with L_GD = 7 μm, V_BR increases from 953 V to 1310 V by increasing h from 0.8 μm to 1.6 μm. Compared with the GaN channel HEMT, V_BR of the Al_0.4Ga_0.7N channel HEMT is improved from 953 V to 1535 V. By increasing h from 0.8 μm to 1.6 μm, V_BR of the Al_0.4Ga_0.6N channel HEMT is further increased from 1535 V to 1763 V. Simulation and analysis indicate that the high gate metal height is an effective method to enhance breakdown voltage in GaN-based HEMTs, and this method can be utilized in all the lateral semiconductor devices. Besides the gate metal height, a larger field plate height can be used to enhance BV with the same mechanism.