First demonstration of X-band AlGaN/GaN high electron mobility transistors using free-standing AlN substrate over 15 W mm⁻¹ output power density

GaN-based high electron mobility transistors (HEMTs) have made steady progress in high-frequency and high-power applications such as radio frequency (RF) amplifiers in wireless base stations and radar systems. ^1–3) These amplifiers are expected to be used for high data-rate wireless communications using millimeter-waves, ^4–6) and higher output power density is essential to extend their communication distance. Here, both large drain current (I_d) and high breakdown voltage (V_BD) are required to improve the output power density. However, the drain-leakage current causes the degradation of V_BD of the device, which limits the high power operation. ^7–9) Device technologies such as source-field plate and optimization of device dimension are known to be promising to reduce the electric field and improve the V_BD of GaN HEMTs. ¹⁰⁾ However, the epitaxial design is indispensable to fundamentally improve the device performance. For example, a thin GaN channel is effective in reducing the drain-leakage current. However, for the HEMT using conventional SiC substrate, the difference in lattice constant between the substrate and nitride epitaxial layer is remarkable. Thus, dislocations in the buffer and channel layers are inevitable. Moreover, the effect of point defects because of dislocations is remarkable and deteriorated electrical properties of HEMT such as I_d. ^11,12) On the other hand, in the case of HEMT structures grown on AlN free-standing substrate, the difference in lattice constant between AlN and GaN is relatively small. Consequently, the dislocations in the epitaxial layer can be reduced to 10⁴–10⁵ cm², ^13–17) and the degradation of electrical properties due to the thin GaN channel will be suppressed. A direct approach to reducing dislocations is the use of free-standing GaN substrate. ^18–22) However, the thermal conductivity of GaN is 230 W mK⁻¹, which is significantly lower than that of conventional SiC substrates (420 W mK⁻¹). On the other hand, it has been reported that the AlN substrate grown by hydride vapor phase epitaxy (HVPE) has good thermal conductivity of 341 W mK^−1 23,24) and is expected to improve the heat dissipation of the substrate. However, HEMTs on AlN substrates have so far reported use in AlGaN channels for high temperature operations, and there is an issue that the electron mobility is lower than that of the GaN channel HEMTs due to the alloy scattering. ^25,26) In this letter, we developed GaN channel HEMTs using AlN free-standing substrate, and evaluated its advantages in DC and output power characteristics.

Figure 1 shows the schematic illustration of AlGaN/GaN HEMT used in this work. The gate length, gate width, and gate-to-drain length were 0.25, 50, and 3 μm, respectively. Table I shows epitaxial structures grown by metal-organic vapor phase epitaxy on AlN (sample A) and SiC substrate (B-F). AlN substrate used for Sample A was grown by physical vapor transport. The specifications of AlN substrate were as follows; diameter: 1 inch, dislocation density: < 10³ cm⁻², orientation: (0001), root mean square of surface roughness: ∼ 1 nm. To reduce the drain-leakage current, the channel thickness was reduced from 1000 to 200 nm, and the pinch-off characteristics were compared between the HEMTs on AlN and SiC substrate (samples A and B). Furthermore, AlGaN buffer with high Al composition effectively act as a back-barrier by the polarization charge at the channel/buffer interface and the large conduction-band offset. ^27,28) However, for the HEMTs on SiC substrates, AlGaN buffers with an Al composition of more than 15% have a large difference of lattice constant from the substrate with remarkable dislocations in HEMT structures. ^29–31) Nevertheless, AlGaN buffers with a high Al composition can be grown with low dislocations on AlN substrates. In this experiment, AlGaN buffer with an Al composition of 30% was applied for HEMT structures using AlN substrates and the back-barrier effect was evaluated. The thicknesses of GaN cap and AlGaN barrier layers were 5 and 15 nm, respectively. To evaluate the effect of carrier density on the DC characteristics, the Al composition of AlGaN barrier was varied from 17% to 40%. A 2 nm thick AlN spacer was applied to samples A and F to increase two-dimensional electron gas (2DEG) density. Ti/Al bilayer ohmic electrodes were formed on AlGaN barrier in the device fabrication after the removal of GaN cap by Cl₂-based dry etching. SiN films were deposited as passivation films, and the gate region was selectively etched with SF₆-based dry etching. Then, the Ni/Au bilayer gate electrode was patterned.

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Figure 2 shows the drain current versus gate-to-source voltage (I_d–V_gs) characteristics of AlGaN/GaN HEMTs fabricated on AlN (sample A) and SiC substrates (samples B and C) at a V_ds of 20 V. The I_d was plotted as a function of gate over drive voltage (V_gs–V_th) to compare the pinch-off characteristics. For the HEMT on SiC substrate with a thick channel (sample C), the slope of I_d–V_gs curve was increased due to the drain-leakage current when gate over drive voltage was in the range of −4 V to 0 V. This indicates that there is a leakage current path under the gate electrodes, probably because of the thick GaN channel. In fact, the drain leakage current was effectively suppressed using a 200 nm thick GaN channel, as shown in Fig. 2. Furthermore, the I_d of the HEMT on AlN substrate with a thin channel sharply decreased in the pinch-off state, although the HEMT on AlN substrate has a higher carrier density than the HEMT on SiC substrate with the same channel thickness (sample B). We believe that the current path under the gate electrode was blocked by decreasing the channel thickness. It was confirmed that the AlGaN buffer with a high Al composition on AlN substrate effectively reduced the leakage current, as shown in Fig. 2.

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For better insight into this behavior, we simulated the band profiles when the channel thickness is decreased in the HEMT structure using the AlGaN buffer with an Al composition of 30% assuming Sample A on AlN substrate (Fig. 3). In the simulations, the polarization charge at the channel/buffer interface was set to be −9 × 10¹² cm⁻² assuming the AlGaN buffer with an Al composition of 30%. The V_gs–V_th and V_ds were set to be −3, −1 and 20 V, respectively. The thickness of the GaN channel was reduced from 1000 to 200 nm, and the current density and potential distribution in the pinch-off state was calculated. At a zero bias (V_ds = 0 V and V_gs = 0 V), band profile of channel was increased with reducing the channel thickness without decrease in the 2DEG density. Accordingly, the 2DEG densities in these structures were attributed to the polarization charge at the barrier/channel interface. Thus, Sample A on AlN substrate shows high carrier density of 1.13 × 10¹³ cm⁻² in spite of using the AlGaN buffer with an Al composition of 30% for back-barrier. As shown in Figs. 3(a) and 3(b), decreasing channel thickness suppressed the current path in the channel under the gate electrode. The band profiles under the gates in Figs. 3(c) and 3(d) are shown in Fig. 3(e). When the channel thickness is reduced, the electric field in the AlGaN barrier layer is reduced. Thus, we believe that the electron tunneling in the barrier ^32–35) can be suppressed and reduced the gate leakage current, as shown in Fig. 2. Moreover, the electric fields in the channels were reduced under the edge of gate electrode and gate-field plate by reducing the channel thickness, as shown in Fig. 3(f). It is confirmed that the AlGaN buffer with high Al composition effectively act as a back-barrier by the polarization charge at the channel/buffer interface and the large conduction-band offset, leading to the relaxation of the electric field in the barrier and channel layers of the HEMT on AlN substrate with thin channel. Contrary, in the case of AlGaN buffer with low Al composition such as 5%, back-barrier effect will be weakened as Sample B shown in Fig. 2.

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To improve the output power characteristics of HEMT, both large I_d and high V_BD are required. Figure 4 shows the comparison of the relationship between the maximum drain current (I_dmax) and the off-state breakdown voltage (V_BD) using AlN and SiC substrates by changing the carrier density of HEMT as shown in Table I. For the HEMTs on SiC substrate (samples B-F), the V_BD was decreased by increasing the I_dmax, i.e., 2DEG density due to the electric field crowded by increasing carrier density. On the other hand, the HEMT on AlN substrate (sample A) achieved both large I_dmax and high V_BD, despite higher carrier density than those of the sample B, which has the same channel thickness. For the HEMT on AlN substrate, the electric fields in the barrier and channel layers are relaxed by the back-barrier effect of the AlGaN buffer with high Al composition shown in Fig. 3, resulting in the improvement of V_BD.

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Figure 5 shows the output power characteristics of the HEMTs on AlN substrate at the X-band. The total gate width of device was 1 mm, and the pulse width and the duty cycle of load-pull measurement were 10 μs and 1%, respectively. In this case, the operating voltage (V_ds) was 70 V. The peak power-added efficiency (PAE) of 49.1% had an associated output power (P_out) of 41.7 dBm, equivalent to 14.7 W mm⁻¹, with a gain of 9.6 dBm. The saturated outputs power densities (P_sat) were plotted against the V_ds, as shown in Fig. 6. The P_sat was linearly improved by increasing the V_ds and successfully achieved 15.2 W mm⁻¹ at the X-band. We believe that the influence of the current collapse because of electron capture ^6,36) was reduced by high 2DEG density ³⁷⁾ of the HEMT on AlN substrate, especially at high V_ds. If the same number of electrons were captured by current collapse, the ratio of decreased electrons would be reduced with increasing 2DEG density. Our results obtained without device technologies such as source-field plate and optimization of device dimension indicate that the HEMT on AlN substrate is promising to improve the output power characteristics of AlGaN/GaN HEMTs as next-generation high-power RF devices.

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In summary, we successfully achieved high-power RF operation of AlGaN/GaN HEMT fabricated on free-standing AlN substrate at X-band. The developed HEMT structure on AlN substrate comprised a 200 nm thick GaN channel and AlGaN buffer with an Al composition of 30%. For DC measurements, the HEMT on AlN substrate exhibited both large I_d and high V_BD, which is difficult to achieve for the HEMT on SiC substrate as increasing 2DEG density for high drain current impedes high voltage operation because of severe electric-field crowding. Consequently, P_sat of 15.2 W mm⁻¹ was achieved at an operating voltage of 70 V at the X-band. To the best of our knowledge, this is the first demonstration of the output power characteristics for X-band HEMT using AlN substrate.

Acknowledgments