Highly reliable InP‐based HBTs with a ledge structure operating at high current density

For the InP HBT being developed for 40 Gbit/s optical communication system realization, the effectiveness of the ledge structure for enhancement of device lifetime due to suppressing generation of the surface recombination current is confirmed. An accelerated life test for this structure shows that the activation energy of the mode in which the current gain decreases gradually is 1.7 eV, that the device life extrapolated at 125 ° C is more than 1×10 8 hours, and that this degradation does not depend on the operating current density up to 2 mA/µm 2 . Thus, high reliability at a high current density allowing high‐speed design is realized. Also, the degradation mechanism of the current gain is analyzed. It is found that sudden degradation can be controlled by increasing the layer thickness of the emitter and can consequently be eliminated. The above results confirm that the authors' InP HBT has sufficient reliability for practical implementation.