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 ´ 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)

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