# Advances in High-Speed, High-Power Photodiodes: From Fundamentals to Applications
## 2 Overall Design Considerations for Photodiodes
### Saturation Current and Output Power  
High-power PDs with a broadband frequency response are key components serving as microwave photonic antenna systems [123; 124; 125], phased array antennas, MMW imaging and sensing systems [5], and analog fiber-optic links wireless communications [8; 126; 127; 128; 129]. The high-speed and high-power PD is a promising candidate, such as the UTC-PD which can be monolithically integrated with a matching circuit using a coplanar waveguide short stub to achieve saturation output power, obtaining a value of \(-13.2\) dBm (\(20.8\) mW) at 100 GHz with a \(-3\) V bias voltage [90]. Furthermore, narrow bandwidth high-power also serves as an analog photonic emitter and oscillator systems. The charge-compensated MUTC-PD (CC-MUTC-PD) with a cliff  
Figure 14: Equivalent circuit models for MUTC–PD integrated with antennas. (**a**) Classical equivalent circuit models of MUTC–PD with Vivaldi antenna and matching network [25]; (**b**) improved equivalent circuit models of CC–MUTC–PD with planer antennas [22]. Figure 15: Two CPW schematic diagrams, equivalent circuit models, and simulated results [107]. (**a**) Conventional CPW–1 with 50 \(\Omega\) impedance and the new CPW–2 with 115 \(\Omega\) impedance, and the fabricated CPW–1, and CPW2; (**b**) PD equivalent circuit model (Region 1: transit time; Region 2: RC–delay time; Region 3: CPW pad) and the simulated series inductance and shunt capacitance. layer [78] is an excellent candidate to achieve high power and high frequency which are realized by the flip-chip techniques on AlN [36,37] or diamond [38] submounts, as shown in Figure 16a, for thermal dissipation. Figure 16b shows that the dissipated power of the device bonded on the diamond submount is faster than that of the devices bonded both on AlN and no bonding. Similarly, the simulated results indicate that the lowest junction temperature is produced from the device bonded on the diamond submount when using the same input power, as shown in Figure 16c. Considering to use the well-designed high impedance CPW signal transmission line, bonding on AlN submount, a 9 \(\upmu\)m CC-MUTC-PD with 70 GHz bandwidth achieves an RF output power of \(-2.6\) dBm at 160 GHz [121]. To further improve the RF output power, high thermal conductivity substrates, including AlN and diamond, are employed bonding with CC-MUTC-PDs using flip-chip techniques [114], which shows 3-dB bandwidth of 105 GHz, 117 GHz, 129 GHz, and 150 GHz for PDs with diameters of 10 \(\upmu\)m, 8 \(\upmu\)m, 6 \(\upmu\)m and 4 \(\upmu\)m at a \(-3\) V bias voltage, achieving an RF output power of \(-3\) dBm at 150 GHz and \(-5.7\) dBm at 165 GHz. Using the same methods for heat dissipation, the record RF output powers for gated modulation at 1 GHz and 10 GHz are separately 41.5 dBm (14.2 V) and 40 dBm (10 W) for 40 \(\upmu\)m CC-MUTC-PD at a \(-36\) V bias voltage [122], as shown in Figure 17.  
Figure 16: (**a**) Flip–chip bonded CC–MUTC–PD; (**b**) PD junction temperature vs. dissipated power; and (**c**) junction temperature comparisons for PD with and without submounts [38]. In brief, the PD saturation current and RF output power is affected by several factors, including device area, PD types, bias voltage, fabrication quality, CPW design, junction heat with high-bias voltages, short microwave stub [131], subomounts, measurements and so on, which need to be considered comprehensively. Furthermore, the superior conductivity of SiC substrate also is an excellent candidate for heat dissipation. Using the wafer-bonding technique, InP-based UTC-PD bonded on the SiC substrate [39] can increase the RF output power to a mW-level, i.e., 0.22 dBm (1.05 mW) at 300 GHz with 15.5 mA photocurrent and 2.01 dBm (1.59 mW) with the optimized bias at 18 mA photocurrent [132; 133], while the output power is up to 4.04 dBm (2.53 mW) at 273 GHz with