High Power Photodiodes for Microwave Applications
Authors: Beling Andreas 
Published: February 2020

This talk reviews modified uni-traveling carrier photodiodes that have achieved high RF output power up to >1 Watt. Surface-normal discrete photodiodes, balanced detectors, and III-V on silicon-on-insulator (SOI) waveguide photodiodes are discussed.
Large saturation current high-speed photodiodes continue to be key components in microwave photonic applications including generation of low phase noise microwave waveforms [1], analog links, and antenna transmit applications [2]. However, two effects limit the saturation current in photodiodes: space-charge screening and thermal effects. Several photodiode (PD) structures including the uni-traveling carrier (UTC) PD [3] have been developed to reduce the space-charge effect. To further improve the high-power capability of UTC PDs the insertion of an un-doped InGaAs layer between the un-depleted InGaAs absorption layer and the InP drift layer has helped to maintain a high electric field at the heterojunction interface. The saturation current can also be increased by slightly doping the depletion region. The incorporated positive charges pre-distort the electric field to compensate the field change caused by the space charge in the presence of large photocurrents. In addition, incorporating a “cliff” layer between the absorber and transparent drift layers enhances the electric field in the depleted absorber layer. To improve thermal dissipation in these modified UTC (MUTC) PDs the chips were flip-chip-bonded to high-thermal conductivity substrates [4]. At 11 V, a 50 μm-diameter PD demonstrated a 1-dB compression current of 228 mA and a high output power of 30 dBm at 10 GHz (figure 1). The same PD reached 25.4 dBm at 20 GHz. Recently, device miniaturization and optimization of the on-chip microwave transmission lines enabled MUTC PDs with bandwidths up to 65 GHz [5]. In this design an air-bridge connected the photodiode to a high-impedance transmission line that was designed to provide slight inductive peaking. PDs with 14-μm diameter reached 16 dBm RF output power at 65 GHz. Similar photodiodes have been fabricated in balanced configuration bonded to a diamond substrate. Balanced MUTC photodiodes with 40 µm-diameter reached a bandwidth of 8 GHz, output power of 31.7 dBm (1.5 Watt), and a high saturation current of 320 mA [6]. Balanced detectors with 10 µm-diameter MUTC PDs achieved 50 mA saturation current and 16 dBm output power at 40 GHz.
To support heterogeneous silicon based photonic integrated circuits for microwave photonics the MUTC PD structure was also integrated on SOI waveguides using a wafer-bonding technology [7]. The PD structure is based on a previously demonstrated normal-incidence InGaAsP/InP MUTC PD that has achieved high saturation current and high linearity. Evanescently-coupled waveguide MUTC PDs (14x100 µm2) exhibited typical dark currents below 10 nA, 0.26 A/W fiber-coupled responsivity at 1.55 µm wavelength (no anti-reflection coating), and 15 GHz bandwidth. The output power was 8 dBm at 10 GHz at 8 V [8]. PDs with 25-μm length reached 30 GHz bandwidth with a maximum RF output power of 2.4 dBm at 40 GHz (figure 2). A comparison of waveguide and back-illuminated PDs with the same epitaxial layer stack reveals larger saturation current densities for back-illuminated PDs; a fact that can be attributed to the non-uniform photocurrent distribution in the waveguide MUTC PD that results in localized areas of strong saturation. To further enhance RF output power, PD arrays were developed in which the photocurrents of multiple waveguide PDs, integrated in parallel, are summed on the output pad. In one design the input signal is split in a multi-mode interference splitter to four waveguides, each terminated by a PD. An unsaturated output power of 10.2 dBm at 20 GHz and 5 V was measured.

Fig.1 Output power and compression of flip-chip MUTC PD.

Fig.2 Output power measured at 40 GHz. Inset: Micrograph of waveguide MUTC PD.

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