High-Temperature Performance in AlGaN/GaN Heterostructure Field Effect Transistors

Narihiko Maeda, Tadashi Saitoh, Kotaro Tsubaki, Toshio Nishida, and Naoki Kobayashi
Physical Science Laboratory

@The GaN-based material system is very promising for high-temperature, high-voltage, and high-power microwave applications, because of the large breakdown electric field and the large electron saturation velocity in this material system. We have fabricated AlGaN/GaN heterostructure field effect transistors (HFETs) whose I-V characteristics have exhibited both excellent current saturation and sufficient pinch-off characteristics up to a high temperature of 400 C.
The layer structure of fabricated HFETs was 300 A Al0.15Ga0.85N/1 mm GaN/1000 A AlN/SiC(0001) [see Frontispiece]. The gate length and gate width were 2 and 20 mm, respectively. Since a large piezoelectric effect exists at the AlGaN/GaN heterointerface, unrelaxed AlGaN/GaN heterostructures with large lattice strains are capable of containing very high two-dimensional electron gas densities larger than 1 L 1013 cm-2 [1]. Figure 1 shows the I-V characteristics of the fabricated device at 400 C. Sufficient pinch-off characteristics have been obtained as well as excellent current saturation characteristics. Obtaining pinch-off characteristics are indispensable for practical uses of the devices, because reported insufficient pinch-off characteristics should lead to degradation in the RF performance, that in the breakdown voltage, and increase in the noise figure. Pinch-off characteristics shown in Fig. 1 have been obtained as the result of reduced crystal defects and reduced etching damage in the devices. Figure 2 shows the temperature (T) dependence of the transconductance (gm). Although gm decreases with increasing T, the degradation rate in gm is shown to be relatively small above 300 C, which is a favorable feature for high-temperature applications. We are especially interested in the application of the devices for satellite microwave communications.
[1] N. Maeda, T. Nishida, N. Kobayashi, and M. Tomizawa, Appl. Phys. Lett. 73(1998) 1856.
[2] N. Maeda, T. Saitoh, K. Tsubaki, T. Nishida, and N. Kobayashi, to be published in Jpn. J. Appl. Phys., Part 2 (1999).

Fig. 1:I-V characteristics at 400 C.

Fig. 2:Temperature (T) dependence of transconductance (gm)