AlGaN/GaN HEMTs Epitaxially Grown on Single-Crystal Diamond
Kazuyuki Hirama, Yoshitaka Taniyasu, and Makoto Kasu
Materials Science Laboratory
@AlGaN/GaN high-electron mobility transistors (HEMTs) are being developed extensively for RF high-power applications. However, the output power of AlGaN/GaN HEMTs is limited by the thermal conductivities of the substrate materials. Among all materials, single-crystal diamond has the highest thermal conductivity of ` 22 W/cmK. Therefore, diamond is expected to be the ideal substrate for high-power devices. However, due to the difference in the crystal structures of nitride semiconductors and diamond, single-crystal nitride growth on diamond substrates is difficult. Recently, using (111) surface orientation, we have successfully grown single-crystal AlN (0001) layers on diamond substrates [1, 2].
@Here, using the single-crystal AlN as a buffer layer, we epitaxially grew the AlGaN/GaN HEMT structure on diamond (111) substrate. As shown in Fig. 1, first, a 180-nm-thick AlN buffer layer was grown, followed by the growth of 20-period AlN(3 nm)/GaN(17 nm) multi layers, a 600-nm-thick GaN layer, a 1-nm-thick AlN spacer layer, a 30-nm-thick Al0.25Ga0.75N barrier layer, and a 4-nm-thick GaN cap layer. From X-ray diffraction measurements, we confirmed the single-crystal growth of the AlGaN/GaN HEMT structure. Next, we confirmed the formation of two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure by Hall-effect measurement. The sheet electron density at room temperature was 1~1013 cm-2. On the other hand, the electron mobility at room temperature was 730 cm2/Vs.
@A 3-µm-gate-length AlGaN/GaN HEMT on the diamond substrate showed the maximum IDS of 220 mA/mm. Figure 1 shows the high-frequency small-signal characteristics. From the frequency dependence of the current gain (|H21|2), maximum stable gain (MSG) and maximum power gain (MAG), the cut-off frequency (fT) of 3 GHz and maximum frequency of oscillation (fmax) of 7 GHz were obtained. Then, we compared the device temperature of AlGaN/GaN HEMTs on diamond and SiC substrates under high-DC-power operation. Figure 2(a) shows the setup for measuring the device temperatures. Figures 2(b) and (c) show the temperature distributions from the side of the AlGaN/GaN HEMTs on the diamond and SiC substrates at a dissipated power of 2 W (3.2 W/mm). The device temperature at 2 W increased from 23 to 36ºC (temperature rise ĒT = 13ºC). On the other hand, on the SiC substrate, the device temperature increased from 23 to 46 ºC (ĒT = 23ºC). From the dissipated power dependence of the device temperature rise [Fig. 2(d)], we estimated the thermal resistance of the AlGaN/GaN HEMT on the diamond substrate to be 4.1 Kmm/W, which is about half of that on a SiC substrate (7.4 Kmm/W) . The low thermal resistance for the AlGaN/GaN HEMT on the diamond substrate is attributed to the high thermal conductivity of the single-crystal diamond. These results show that AlGaN/GaN HEMTs on diamond are promising for high-frequency and high-power applications.
 Y. Taniyasu and M. Kasu, J. Cryst. Growth 311 (2009) 2825.
 K. Hirama, Y. Taniyasu, and M. Kasu, J. Appl. Phys. 108 (2010) 013528.
 K. Hirama, Y. Taniyasu, and M. Kasu, Appl. Phys. Lett. 98 (2011) 162112.
Fig. 1. High-frequency small-signal characteristics of AlGaN/GaN HEMT on diamond (111).
Fig. 2. (a) Setup for measuring the temperature distributions. Temperature distribution of the AlGaN/GaN HEMTs on (b) diamond and (c) SiC substrates at a dissipated power of 2 W (3.2 W/mm). (d) Dissipated-power dependence of device temperature for AlGaN/GaN HEMTs on the diamond and SiC substrates. Closed and open squares indicate the temperatures for diamond and SiC substrates, respectively.