Owing to their high two-dimensional electron gas (2DEG) density, electron mobility, and high breakdown electric field, AlGaN/GaN high-electron-mobility transistors (HEMTs) have become excellent candidates in power electronics and high frequency applications.^[1–6] The value of gate length has influences on transconductance, cutoff frequency, and maximum frequency of oscillation.^[7,8] Numerous studies focused on reducing the gate length to improve high-frequency characteristics.^[9,10] Breakdown voltage is also a very important parameter for these applications. However, the effect of gate length L_G on breakdown voltage V_BR has rarely been investigated.

In this paper, the effect of gate length on breakdown voltage is investigated through experiments and simulations. With the increase of L_G, V_BR is first increased, and then saturated at L_G = 3 μm. In order to explain the relation between V_BR and L_G, leakage currents and software simulation are utilized to discuss the buffer leakage current and buffer-leakage-induced impact ionization.

Figure 1 shows the schematic cross section of AlGaN/GaN HEMT fabricated in this paper. The AlGaN/GaN heterostructure was grown on sapphire substrate in a metal–organic chemical vapor deposition (MOCVD) system. The epitaxial layers included a 100-nm AlN nucleation layer, 1.4-μm GaN buffer layer, 1-nm AlN interlayer, 21-nm AlGaN barrier layer, and 1.5-nm GaN cap layer. The sheet carrier density and 2DEG mobility were tested to be 1.01×10¹³ cm⁻² and 1915 cm²/V·s respectively by Hall effect measurements.

Fig. 1.

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	Fig. 1. Schematic cross section of AlGaN/GaN HEMT.

Source and drain electrodes were formed by evaporating Ti/Al/Ni/Au followed by annealing in N₂ ambient at 830 °C for 30 s. Then, mesa isolation was carried out by Cl-based reactive ion etching (RIE). Next, a Schottky gate was formed by evaporating Ni/Au/Ni. The devices were passivated with SiN layers to reduce surface states. The gate-source spacing and gate-drain spacing are all 3 μm. Six kinds of devices were fabricated with different gate length L_G values, and the gate length values are 1, 3, 5, 8, 13, and 20 μm.

The source–drain spacing increases with the increase of gate length from 1 μm to 20 μm, leading to the increase of on-state resistance. As a result, the maximum drain current I_Dmax decreases with the increase of gate length. As shown in Fig. 2(a), I_Dmax of the HEMT with L_G = 1 μm is 743 mA/mm. In contrast, I_Dmax is reduced to 342 mA/mm for the HEMT with L_G = 20 μm. In addition, the maximum transconductance G_m,max decreases with the increase of gate length as shown in Fig. 2(b). G_m,max decreases from 161 mS/mm for the HEMT with L_G = 1 μm to 82 mS/mm for the HEMT with L_G = 20 μm.

Fig. 2.

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	Fig. 2. I–V transfer characteristics of the AlGaN/GaN HEMTs with different gate length values: (a) IDS versus VGS and (b) Gm versus VGS.

Figure 3 shows the breakdown characteristics of the AlGaN/GaN HEMTs with different gate length values. For the HEMT with L_G = 1 μm, breakdown voltage V_BR is 117 V. In contrast, the values of V_BR are in a range of 140 V–156 V for the HEMTs with L_G = 3 μm–20 μm. V_BR is first increased, and then saturated at L_G = 3 μm. In order to explain the relation between V_BR and L_G, breakdown characteristics of the HEMTs with L_G = 1 μm and L_G = 3 μm are first analyzed.

Fig. 3.

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	Fig. 3. (a) Breakdown characteristics and (b) VBR–LG dependence of the AlGaN/GaN HEMTs with different gate length values.

Figure 4 shows three leakage currents, I_D, I_G, and I_S, in the HEMTs with L_G = 1 μm and L_G = 3 μm during the breakdown measurements. Source current I_S first decreases, and then increases, resulting in an inflection point.^[11] As shown in Fig. 5, I_S consists of source–gate leakage current I_sg and buffer leakage current I_ds, which flow in different directions. Before the inflection point, the source current is mainly composed of source–gate leakage current, which is part of gate leakage current. The buffer leakage current increases with the increase of the drain voltage. When the buffer leakage current exceeds the source–gate leakage current, the inflection point occurs. After the inflection point, the increase of I_S results from the increase of the buffer leakage current.^[11]

Fig. 4.

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	Fig. 4. Variations of ID, IG, and IS with VDS (drain–source voltage) during the breakdown measurements in HEMTs with LG = 1 μm and LG = 3 μm.

Fig. 5.

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	Fig. 5. Off-state leakage currents in AlGaN/GaN HEMTs during the breakdown measurements. Gate leakage current IG consists of source–gate leakage current Isg and drain–gate leakage current Idg.

As shown in Fig. 4, V_BR for the HEMT with L_G = 3 μm is 148 V, which is 31 V higher than that for the HEMT with L_G = 1 μm. For the HEMT with L_G = 1 μm, I_S increases quickly at V_DS = 82 V, and achieves 1 mA/mm at V_DS = 117 V, indicating that the device breakdown is induced by the buffer leakage current. In contrast, I_S increases slowly for the HEMT with L_G = 3 μm during the whole breakdown measurement. The breakdown for the HEMT with L_G = 3 μm is induced by the gate leakage current. The increase of V_BR for the HEMT with L_G = 3 μm compared with the HEMT with L_G = 1 μm is due to the suppression of the buffer leakage current. In order to explain the reason for suppressing the buffer leakage current, Silvaco-ATLAS software is utilized to investigate the depletion region and electric field distributions in the AlGaN/GaN HEMTs.

Figures 6(a) and 6(d) show the electron concentration distributions of the AlGaN/GaN HEMTs with V_GS = −6 V and V_DS = 0 V. With the increase of the drain terminal, the depletion region under the gate is shrinking, which results from the effect of drain induced barrier lowering (DIBL). For V_DS = 100 V, the depletion region in the HEMT with L_G = 1 μm is smaller than that in the HEMT with L_G = 3 μm as shown in Figs. 6(b) and 6(e). The comparison of these two depletion regions indicates that a larger gate length can suppress DIBL more effectively.

Fig. 6.

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Fig. 6. Off-state logarithmic electron concentration distributions of the AlGaN/GaN HEMTs respectively with (a) LG = 1 μm, VDS = 0 V; (b) LG = 1 μm, VDS = 100 V; (d) LG = 3 μm, VDS = 0 V; (e) LG = 3 μm, VDS = 100 V. Electric field distributions of the AlGaN/GaN HEMTs respectively with (c) LG = 1 μm, VDS = 100 V; (f) LG = 3 μm, VDS = 100 V. All the devices are biased at VGS = −6 V and simulated by Silvaco-ATLAS software.

With the drain biased at a high value, there exists an electric field peak at the drain-side edge of the gate terminal as shown in Figs. 6(c) and 6(f). Correspondingly, the GaN-based materials around the gate are in the high electric field region. As shown in Fig. 6(b), part of the high electric field region in the HEMT with L_G = 1 μm is not depleted, and electrons can be injected from the source to this part of the high electric field region, which may initiate impact ionization and the following avalanche breakdown. Hence, the buffer leakage current increases quickly due to the impact ionization at V_DS = 82 V, and achieves 1 mA/mm at V_DS = 117 V. For the HEMT with L_G = 3 μm, the high electric field region is almost completely depleted, and the electrons from the source are difficult to be injected into the high electric field region. The buffer leakage current of the HEMT with L_G = 3 μm does not show the abrupt increase during the whole breakdown measurement. Therefore, the gate length of 3 μm can alleviate the buffer-leakage-induced impact ionization compared with the gate length of 1 μm, and the suppression of the impact ionization is the reason for improving the breakdown voltage.

The breakdown for the HEMT with L_G = 1 μm belongs to buffer-induced breakdown. The suppression of the buffer leakage changes the breakdown type into gate-induced breakdown for the HEMT with L_G = 3 μm. For the gate length larger than 3 μm, the high electric field region is almost completely depleted, which is similar to the case of L_G = 3 μm. A similar suppression of the impact ionization exists in each of the HEMTs with L_G > 3 μm. For the HEMTs with L_G = 5, 8, 13, and 20 μm, the device breakdown types are all gate-induced breakdown. As shown in Table 1, the gate leakage current values are almost equal to each other for the HEMTs with L_G = 3, 5, 8, 13, and 20 μm. As a result, there is no obvious difference in breakdown voltages among the HEMTs with L_G = 3 μm–20 μm, and they are in a range of 140 V–156 V.

Table 1.

Table 1.

Table 1. Parameter values of AlGaN/GaN HEMTs. The threshold voltage Vth is defined as the gate voltage with drain leakage current reaching 1 mA/mm. The gate leakage current Igleak values are obtained at VGS = −15 V. . IDmax/(mA/mm) Vth/V Gm,max/(mS/mm) Igleak/(mA/mm) VBR/V LG = 1 μm 743 –3.69 161 33.0 117 LG = 3 μm 665 –3.64 139 46.7 148 LG = 5 μm 601 –3.63 126 44.6 147 LG = 8 μm 521 –3.56 112 42.8 140 LG = 13 μm 429 –3.49 96 43.6 142 LG = 20 μm 342 –3.41 82 45.2 156

Table 1. Parameter values of AlGaN/GaN HEMTs. The threshold voltage Vth is defined as the gate voltage with drain leakage current reaching 1 mA/mm. The gate leakage current Igleak values are obtained at VGS = −15 V. .

In order to find out the accurate gate length in which the buffer-leakage-induced impact ionization can be alleviated, the simulation for the HEMT with L_G = 2 μm is carried out, and the results are shown in Fig. 7(a). For the HEMT with L_G = 2 μm, part of the high electric field region is not depleted, which may result in impact ionization and avalanche breakdown. Then, the simulation for the HEMT with L_G = 2.5 μm is carried out, and the results are shown in Fig. 7(b). For the HEMT with L_G = 2.5 μm, the high electric field region is almost completely depleted. Therefore, the gate length value in which buffer-leakage-induced impact ionization begins to be alleviated is in a range between 2 μm and 2.5 μm.

Fig. 7.

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	Fig. 7. Off-state logarithmic electron concentration distributions of the AlGaN/GaN HEMTs respectively with (a) LG = 2 μm, VDS = 100 V; (b) LG = 2.5 μm, VDS = 100 V.

In this work, the effects of gate length on breakdown voltage are investigated in AlGaN/GaN HEMTs. The value of V_BR for the HEMT with L_G = 3 μm is 148 V, which is 31 V higher than that for the HEMT with L_G = 1 μm. For the HEMT with L_G = 3 μm, the high electric field region is almost completely depleted, and it is difficult to inject the electrons from the source into the high electric field region. The gate length of 3 μm can alleviate the buffer-leakage-induced impact ionization compared with the gate length of 1 μm, and the suppression of the impact ionization is the reason for improving the breakdown voltage. A similar suppression of the impact ionization exists in each of the HEMTs with L_G > 3 μm, and there is no obvious difference in breakdown voltage among the HEMTs with L_G = 3 μm–20 μm. Simulation results indicate that the gate length value in which buffer-leakage-induced impact ionization begins to be alleviated is in a range between 2 μm and 2.5 μm.