High Performance Nitride-Based Heterojunction Bipolar Transistors on GaN Substrates
Kazuhide Kumakura and Toshiki Makimoto
Materials Science Laboratory
There are large mismatches between III-nitride semiconductors and conventionally used sapphire substrates in terms of the lattice constant or thermal expansion coefficient. Therefore, the most suitable substrate for GaN growth is no doubt the GaN substrate. From the viewpoints of device applications, the merits of using GaN substrates are as follows: a low dislocation of the substrate itself, resulting in improving the device performance, or a relatively large thermal conductivity, making it possible to spread heat generated under a high-power operation. In principle, heterojunction bipolar transistors (HBTs) have the ability to operate with uniform threshold voltages and high current densities. A normally off characteristic is advantageous for a fail-safe system. Therefore, nitride-based HBTs are one of the attractive devices for high-power electronics. In this work, we fabricated the pnp AlGaN/GaN HBTs on GaN substrates and showed their high performance at room temperature (RT).
Figure 1 shows current gains as a function of the collector current (Ic) of pnp AlGaN/GaN HBTs on sapphire and GaN substrates measured at RT. The HBTs on GaN substrates exhibited a high performance: a maximum current gain of 85 at a collector current of 30 mA and a maximum collector current density of 7.3 kA/cm2 at a collector-emitter voltage of 30 V, which corresponds to the maximum power dissipation density of 219 kW/cm2. The current gain and the collector current density increased compared to those on sapphire substrates. The calculated minority carrier diffusion length agreed well with that determined from electron beam induced current measurements . Therefore, these results indicate that the current gain was dominated by the minority hole diffusion in the neutral base at high Ic for the HBTs on the GaN substrates, and that the increase in the current gain is ascribed to the low dislocation density in the HBTs. For the HBT with the large emitter area, the current gain was still as high as 47 and the maximum collector current reached as high as 1 A, and this single HBT showed a high-power dissipation of 30 W as shown in Fig. 2. This high performance of the HBTs is ascribed to the low dislocation density and relatively high thermal conductivity of the GaN substrate.
 K. Kumakura et al., Appl. Phys. Lett. 86 (2005) 052105
Fig. 1. Current gain as a function of collector current of the HBTs measured at RT. The solid and broken lines correspond to the data for HBTs on GaN and sapphire substrates, respectively.
Fig. 2. Maximum collector current and the maximum power dissipation as a function of the emitter perimeter of the HBTs on GaN substrates.
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