Improvements in the Maximum THz Output Power and Responsivity in Near-Ballistic Uni-Traveling-Carrier Photodiodes With an Undercut Collector

We demonstrate a novel ultra-fast photodiode structure that fundamentally relaxes the trade-offs between the speed, responsivity, and saturation power at sub-THz regime. Our device, with its 3 μm wide undercut collector profile, exhibits a DC responsivity (0.11 A/W) which is close in value to that reported for uni-traveling-carrier photodiodes (UTC-PDs) with similar sized active diameters (∼10 μm), but has a much larger 3-dB optical-to-electrical (O-E) bandwidth (220 GHz vs. around 75 GHz). This leads to much less optical power being required to deliver a close value of THz output power at the same operating frequency. Furthermore, the responsivity (0.11 vs. 0.1 A/W) and output power (0.4 vs. −2.4 dBm@165 GHz) are higher than those of reference devices without the undercut collector layer but with a miniaturized active diameter as small as 3 μm.