Npn-type InGaN/GaN Nitride Heterojunction Bipolar Transistor (HBT)

Toshiki Makimoto, Kazuhide Kumakura, and Naoki Kobayashi
Physical Science Laboratory

@Wide-gap nitride semiconductors are promising materials for electronic devices that require high power and/or operate under high temperatures. On the other hand, a heterojunction bipolar transistor (HBT), a kind of electronic devices, is suitable for high-power devices due to high breakdown voltages, high current densities, and good threshold voltage uniformity. Therefore, a nitride HBT is a promising electronic device in terms of both materials and devices. However, there are few reports about nitride HBTs with high common-emitter current gains. In the previous reports, p-GaN was used for a base layer of a nitride HBT and there are two major problems for the conventional p-GaN. One is its high resistivity and the other is its severe damage induced by HBT fabrication process. To solve these two problems, we have developed on p-InGaN layers and found that these p-InGaN layers show high hole concentrations above 1019cm-3 at room temperature, meaning that their resistivity is much lower than that of the conventional p-GaN [1]. Furthermore, we have also found that these p-InGaN layers are less damaged by the process [2].
@In this work, we have applied the low-resistivity and less-damaged p-InGaN layer to a base layer of a nitride HBT for the first time. Figure 1 shows an Npn-type InGaN/GaN nitride HBT structure. For a collector layer, wide-bandgap GaN was used instead of InGaN to increase a breakdown voltage. A graded InGaN layer was inserted between base and collector layers to obtain higher current gains. Figure 2 shows the common-emitter current-voltage (I-V) characteristics at room temperature. From these characteristics, the maximum current gain was as high as 20, meaning that the crystal quality of this p-InGaN layer is relatively good [3,4]. Furthermore, a high breakdown voltage over 20 V has been obtained due to wide bandgap GaN collector. These device characteristics will be improved further by reducing dislocation densities and process damage.

[1] K. Kumakura et al., Jpn. J. Appl. Phys. 39 (2000) L337.
[2] T. Makimoto et al., J. Cryst. Growth 221 (2000) 350.
[3] T. Makimoto et al., Appl. Phys. Lett. 79 (2001) 380.
[4] T. Makimoto et al., phys. stat. sol. (a) 188 (2001) 363.

Fig. 1. Schematic structure of an InGaN/GaN nitride HBT.
Fig. 2. Common-emitter current-voltage (I-V)characteristics at room temperature.