Nitride-Based Semiconductor Light-Emitting Transistors


Kazuhide Kumakura, Hideki Yamamoto, and Toshiki Makimoto
Materials Science Laboratory

   Light-emitting transistors (LETs), which are based on high-speed heterojunction bipolar transistors (HBTs) with a quantum well (QW) in the base layer, are electrical devices as well as optical ones [1]. According to the principles of their operation, LETs can run faster than conventional light-emitting diodes (LEDs). Since nitride-based semiconductor LEDs are commercially available for lighting, it is expected that the nitride-based semiconductor LETs will work as light sources for visible light communication [2]. In this work, we fabricated nitride-based semiconductor LETs and investigated their optical output characteristics.
   We fabricated pnp AlGaN/InGaN/GaN LETs with a 3-nm-thick QW in the base layer. The In mole fraction of the InGaN QW was higher than that of the InGaN base layer. When we apply forward bias at the emitter-base junction of the LETs, holes are injected from the emitter to the base layer. The injected holes diffuse toward the collector and are captured at the QW. Some of the uncaptured holes reach the collector. Figure 1 shows a photograph of the emission from the LETs under forward bias conditions. We can observe the strong purplish emission with the peak wavelength of 410 nm. Next, we fabricated three types of LETs with the QW position varied in the base layer: near the emitter, at the center of the base, and near the collector. Figure 2 shows the optical output power as a function of the base current for the three LETs. We also show the output power for a QW-free LET (standard HBT) as a reference. The output power with the QW near the collector is weaker than with it near the emitter. This indicates that the probability of the hole capture depends on the position of the QW. The velocity of the holes diffusing in the base becomes larger as the holes approach the collector, which results in a lower hole capture probability near the collector. We achieved 0.3-mW optical output power from the LET with the QW near the emitter at the base current of 0.3 mA, corresponding to the external quantum efficiency of 3.3 %. We will investigate the electrical and optical modulation properties of the LETs for high-speed operation, which may open an avenue to their application as high-speed visible light sources.

[1] M. Feng et al., Appl. Phys. Lett. 84 (2004) 1952.
[2] Y. Tanaka et al., IEICE Trans. Commun. E86-B (2003) 2440.
[3] K. Kumakura et al., Int. Conf. Solid State Devices and Materials, 2011 Nagoya, A-4-2.

Fig. 1. Photograph of the emission from the LET under forward bias conditions.
Fig. 2. Optical output power as a function of the base current for the three LETs and a HBT.