Traveling-wave structure improves phototransistor performance
Researchers in the electrical engineering department of the University of California (Los Angeles, CA) have applied traveling-wave (TW) concepts from optical-modulator technology to achieve high optical coupling efficiency and high-frequency operation of heterojunction phototransistor (HPT) detectors. Traditional lumped-element HPT designs must be scaled down in size for high-frequency operation, which causes the devices to saturate at relatively low levels of optical power input, thereby limiting their effectiveness as components in microwave fiberoptic links.
The traveling-wave design overcomes the small-device saturation problem by distributing the optical power of the incident light signal over the length of an integrated polyimide waveguide that runs across the top of the active region of the HTP. Light is coupled into one end of the waveguide and leaks into the active region of the HTP while transiting the waveguide. The researchers tested device lengths at 20, 200, and 2000 µm. The 200-µm-long device was tested at up to 50 mA of dc photocurrent and 60-GH¥operating frequency without any observed saturation in output power. Input optical coupling frequencies were in the 70% range.