## 2 Technologies  
The III-V Lab develops both UTC-PD and InP DHBT technologies within its clean rooms. UTC-PDs and DHBTs share the same base-collector interface heterostructure and therefore, for this interface, the physical models are similar despite differences in thicknesses and doping levels between optimized UTC-PD structures and optimized DHBT structures. This technological similarity opens up the way to integrate DHBT and UTC photodiodes on the same substrate, in order to suppress the parasitic on the interface between them (Figure 1). This could be done either by finding a shared epitaxial structure with the best compromise between UTC-PD and DHBT structures or by a selective regrowth of UTC-PD layers on a DHBT layer or vice versa.  
The InP DHBT structure is grown externally by solid source molecular beam epitaxy on 3 wafers. The vertical structure is optimized in order to achieve balanced frequency performances for 0.7 mm emitter DHBTs with \(f_{T}\) and \(f_{max}\) around 400 GHz. The structure is similar to the one described in [9] except for the low doped collector region for which Si-doping level is reduced to 1.5 \(\times\) 10\({}^{16}\) at/cm\({}^{-3}\) to improve the breakdown voltage. The 0.5 and 0.7 mm InP DHBTs with emitter length varying from 5 to 10 mm are processed using a wet-etched triple mesa technology. Figure 2a presents a simplified cross-sectional view of the transistor.  
Figure 2: (**a**) Simplified schematic cross-sectional view of submicrometer InP/InGaAs double heterojunc-tion bipolar transistors (DHBTs) and (**b**) Scanning electron microphotographs of a 0.7 \(\times\) 5 mm 2 InP DHBT before metal interconnection level; (**c**) Simplified schematic cross-sectional view of a waveguide uni-traveling carrier photodiode (UTC-PD), (**d**) Scanning electron microphotographs of a 5 \(\times\) 25 μm2 InP UTC photodiode after metal in-terconnection level.  
Emitter and base contacts as well as the base plug needed for interconnection are defined by e-beam lithography whereas stepper lithography is used for other technological steps. TiPdAu (titanium-palladium-gold), deposited by vacuum evaporation, is used for all contacts. After encapsulation with silicon nitride (SiN) for higher robustness and planarization with polyimide, emitter, base and collector contacts are opened by etching to form device interconnects. Figure 2b presents a scanning electron microphotograph of a \(0.7\times 5\) um\({}^{2}\) InP DHBT.  
The UTC photodiode structure is grown either by gas-source molecular beam epitaxy (GS-MBE) [10] or by metal-organic vapor-phase epitaxy (MOVPE) as in this paper. A multimode diluted waveguide is used for optimal fiber coupling [11]. The process begins with the etching of the top InP layers, followed by base and collector etching using a mixed of dry and wet etching. Pt is used for a P contact followed by a rapid thermal annealing and TiPtAu are used for the N contacts and electrodes (Figure 2c). Figure 2d shows a \(5\times 25\) um\({}^{2}\) InP UTC photodiode with the metal interconnections.  
To evaluate the performances of this integrated co-design approach with the UTC-PDs and the DHBTs, accurate and physics-based compact models are required. This is even more important if the same epitaxy is shared, in order to obtain a precise overview of the impact on the performances of individual UTC-PDs and DHBTs as well as the potential gain offered by the co-design on the circuit performances. To that end, in this paper, we present two compact models compatible with UTC-PD and DHBT structures which are first validated on individual and optimized UTC-PD and DHBT chips.