Nitride Heterojunction Bipolar Transistor with Regrown Base Layer
Toshiki Makimoto, Kazuhide Kumakura, and Naoki Kobayashi
Physical Science Laboratory
A high-power microwave transistor is getting more important, since it is used for the communications between the base stations for cellular phones. Compared with Si and GaAs, nitride semiconductors have high breakdown voltage due to their wide bandgap. On the other hand, a heterojunction bipolar transistor (HBT), a kind of electronic devices, is suitable for a high-power microwave transistor due to its capabilities such as the high breakdown voltage, the high current density, and the good threshold voltage uniformity. Therefore, a nitride HBT is a promising electronic device in terms of both materials and devices.
However, there are two issues to be addressed for this nitride HBT. One is a relatively lower current gain, compared with HBTs composed of other semiconductors such as Si/SiGe and AlGaAs/GaAs. The other is that the offset voltage in the common-emitter current-voltage (I-V) characteristics is much higher than the value expected from the conduction-band discontinuity between the emitter and the base layers. To solve these two issues, we have used the base regrowth technique, resulting that the nitride HBT characteristics have been drastically improved.
Figure 1 shows a schematic illustration of an HBT structure fabricated in this work. This structure has two features. One is that the low resistive p-InGaN layer is used as a base layer instead of the conventional p-GaN . The other is that a double heterostructure is used to obtain a high breakdown voltage . As shown in Fig. 1, many defects were produced on the base surface during the HBT fabrication process and they are considered to degrade the nitride HBT characteristics. In this work, p-InGaN was regrown on the surface to eliminate the effect of these defects. Figure 2 shows the common-emitter I-V characteristics at room temperature. The maximum current gain is 2000, which is 100 times as high as the previously reported value for the nitride HBTs [3,4]. Furthermore, the minimum offset-voltage (the turn-on voltage of the collector current in the common-emitter I-V characteristics) is 0.3 V, which is one tenth of the previously reported value [3,4].
 K. Kumakura et al., Jpn. J. Appl. Phys. 39, L337 (2000).  T. Makimoto et al., Appl. Phys. Lett. 79, 380 (2001).  T. Makimoto et al., Proceeding of The Fifth International Conference on Nitride Semiconductors (ICNS-5), Nara Japan, May 2003.  T. Makimoto et al., Proceeding of 2003 Device Research Conference (2003 DRC), Salt Lake City USA, June 2003.
Fig. 1. Schematic illustration of an HBT structure.
Fig. 2. Common-emitter I-V characteristics at room temperature.
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