Waveguide Uni-Traveling-Carrier Photodiodes for mmW Signal Generation: Space-Charge Impedance and Efficiency Limitations

Photonic generation of mmW signals using high-speed photodiodes is promising due to the potential for ultra-wide bandwidth [1]–[3]. Uni-traveling carrier photodiodes (UTC-PDs) based on the InGaAs/InP material system have demonstrated measured RF output powers of 10 dBm, 6 dBm, and −2.2 dBm at frequencies of 100 GHz, 170 GHz, and 300 GHz respectively [4]–[6]. However, the power conversion efficiency (PCE) is a metric that has seen relatively little investigation and is important for practical applications [7]. The PCE is given by the output RF power, $\frac{1}{2}R_{L}I_{RF}^{2}$ , divided by the sum of the input optical power, $P_{opt}$ , and the DC power, $I_{ph}\cdot V_{Bias}$ , applied to the diode. For the results in Refs. 4–6, the PCE is in the range of 1% to 10%. To investigate the efficiency limitations, we utilize the assumptions from Ref. 7 to write the PCE as $\eta_{RF}=\frac{1}{2}\cdot\frac{m^{2}R_{L}}{I_{DC}^{-1}(V_{th}+1/R)+\alpha^{-1}+m(R_{L}+\alpha^{-1})}(1)$ , where $R_{L}$ is the load impedance, $m$ is the optical modulation index, $I_{DC}$ is the average photocurrent, $V_{th}$ is the minimum bias to operate at high frequencies for low photo current, $\alpha^{-\mathit{1}}$ is the device series resistance, and $\mathscr{R}$ is the optical to electrical responsivity (A/W). The series resistance term, $\alpha^{-\mathit{1}}$ , includes resistances of the device such as contact resistance and sheet resistance, as well as an effective space-charge impedance. The space-charge impedance results from the electrical field of the photo-generated carriers in the junction that reduce the built-in field. Waveguide (WG) UTC-PDs were fabricated and here we characterize their space-charge impedance to demonstrate that this is the limiting factor for PCE and an important metric to quantify in state-of-the-art UTC-PDs.