Nitride Heterojunction Bipolar Transistors for High-Power Applications
Toshiki Makimoto, Yoshiharu Yamauchi and Kazuhide Kumakura
Physical Science Laboratory
@Compared with Si and GaAs, nitride semiconductors have high breakdown voltage because of their wide bandgap. On the other hand, one of the characteristics of a heterojunction bipolar transistor (HBT) is high current density. Therefore, a nitride HBT is a promising electronic device in terms of both materials and device types. However, there are two issues for the nitride HBT. One is a relatively low current gain and the other is a high offset voltage in the common-emitter current-voltage (I-V) characteristics. As a result, high-power characteristics of a nitride HBT have not been reported, though they are theoretically expected.
@We have developed a GaN/InGaN HBT. Our GaN/InGaN HBT uses a low-resistivity p-InGaN layer as a base layer instead of the conventional p-GaN  and has a double heterostructure for a high breakdown voltage . To improve current gain and reduce the offset voltage, we used a base regrowth technique. The current gain increased 100 fold and the offset voltage was reduced to 1/10 compared with the previous record . This time, we have investigated the high-power characteristics of these GaN/InGaN HBTs.
@The base-collector junction is reverse-biased and the HBT power increases with increasing reverse voltage. Figure 1 shows typical I-V characteristics of a base-collector diode. The leakage current is small even at the reverse bias voltage of -50 V because of the wide bandgap of the GaN collector. On the other hand, the output power depends on the collector current in the common-emitter I-V characteristics. Figure 2 shows the maximum collector current as a function of the emitter size. As shown in this figure, the maximum collector current is proportional to the emitter size. Since the breakdown voltage is independent of the emitter size, the output power is proportional to the emitter size. Up to now, we have obtained a maximum output power exceeding 10 W for a 90 Êm x 50 Êm emitter. The maximum power density is as high as 270,000 W/cm2 . This is the first demonstration of high-power characteristics in a nitride HBT.
 K. Kumakura et al., Jpn. J. Appl. Phys. 39 (2000) L337.  T. Makimoto et al., Appl. Phys. Lett. 79 (2001) 380.  T. Makimoto et al., Appl. Phys. Lett. 83 (2003) 1035.  T. Makimoto et al., Appl. Phys. Lett. 84 (2004) 1964.
Fig. 1. Typical I-V characteristics of a
Fig. 2. Maximum collector current as a function of the emitter size.