# High-Speed Photodetectors for Microwave Photonics
## 2 High-Power High-Speed Photodiodes  
Previously, we demonstrated back-illuminated charge-compensated MUTC PDs with 7.8 and 9.6 dBm RF output power at 110 and 100 GHz, respectively [12]. These devices included a high-impedance 200 \(\upmu\)m-long coplanar waveguide (CPW) between the RF pads and the PD that provided inductive peaking at 100 GHz. Recently, we flip-chip bonded similar MUTC PDs to a low-inductance transmission line (TL) on a AlN submount to increase the RF output power beyond the 3 dB bandwidth. Using thermo-compression Au-Au bonding, the PD's bonding pads were attachedto a CPW with a 130-\(\upmu\)m signal-to-ground gap. A short tapered TL connected the stub to a 50 \(\Omega\), 50 \(\times\) 154-\(\upmu\)m\({}^{2}\) pad with a 54-\(\upmu\)m signal-to-ground gap and 250-\(\upmu\)m pitch (Figure 1a). This optimized CPW design decreased the roll-off of the frequency response beyond 100 GHz and extended the usable frequency range up to 160 GHz. Figure 1b shows the measured RF output power and compression at 160 GHz using an optical heterodyne setup, a GGB Industries WR-6 waveguide probe with bias-T, and a VDI power meter PM5. A 9-\(\upmu\)m diameter PD reached a maximum RF output power of \(-\)2.6 dBm at 160 GHz and \(-\)3 V, and a saturation current of 40 mA. The dark current was 0.4 nA and the responsivity was 0.2 A/W at 1550 nm [13].  
It is well-known that a decrease in the absorption layer thickness in order to reduce the carrier transit time, results in a lower responsivity in a normal incidence photodiode. For example, in the 100-GHz PD in ref. [12], the absorption layer thickness was 180 nm (the drift layer thickness was 300 nm) to enable carrier transit times below 4 ps, however, the responsivity was only 0.17 A/W. To circumvent this trade-off, waveguide structures that decouple photon absorption from carrier transport can be employed. To this end, we have developed evanescently coupled waveguide-integrated MUTC PDs. Figure 2a shows the epitaxial layer structure that was grown on top of the InGaAsP input optical waveguide on the InP substrate [9]. The InGaAs absorber includes a 100 nm-thick p-type doped layer and a 100 nm-thick lightly n-type doped layer. A graded doping was designed to create a built-in electric field to support carrier transport in the un-depleted absorber. The 200 nm-thick charge-compensated InP drift layer was incorporated to reduce the junction capacitance. Intermediate-bandgap InGaAsP layers were added to prevent carrier pile-up at the band barriers. A quaternary cliff layer was designed to help increase the electric field in the depleted absorber and thus support electron transport across the heterojunction discontinuity into the drift layer.  
Evanescently-coupled MUTC PDs were fabricated on a three inch InGaAsP/InP wafer that was grown by metal-organic chemical vapor deposition. The fabrication process involved a double-mesa process using dry etching and was carried out in UVA's clean room facilities [9].  
The frequency responses of photodiodes with various areas are shown in Figure 2b. All three PDs had 3 dB bandwidths over 90 GHz and showed flat responses up to 80 GHz. The PD with an area of 24 \(\upmu\)m\({}^{2}\) had a bandwidth of over 105 GHz. For applications that require efficient light detection at high modulation frequencies, the photodiode bandwidth efficiency product is an important figure of merit, which was 29 GHz, 32 GHz and 38 GHz for 24 \(\upmu\)m\({}^{2}\), 35 \(\upmu\)m\({}^{2}\), and 50 \(\upmu\)m\({}^{2}\) PDs, respectively [9]. Waveguide photodiodes fabricated from the same wafer lot achieved an RF output power of 5 dBm at 120 GHz in ref. [14].  
Figure 1: (**a**) Photodiode (PD) die on submount after flip-chip bonding. (**b**) Radio frequency (RF) output power and compression at 160 GHz of modified uni-traveling carrier photodiodes (MUTC-PDs) with different diameters.