## 3 Compact Model Validation
### Validation of RF Characteristics
#### 3.2.2 Intrinsic Series Resistance Extraction  
The overall series resistance can be extracted from the real part of the intrinsic \(Z_{11}\)-parameter. In the case of the rectangular InGaAs/InGaAsP UTC-PDs, the contact covers the entire mesa, which makes it easy to determine the scaling law by analyzing the effective area of the device geometry. Figure 12 shows the overall series resistance extracted at 80 GHz for the three geometries at various biases. Since the collector is 425 nm thick, it is not completely depleted. As the reverse voltage increases, the collector resistance decreases until it disappears at full collector depletion.  
The resistance values are extracted for the GaInAsSb/InP UTCPDs from the real part of \(Z_{11}\)-parameter at a frequency of 60 GHz. The extracted values are: 11.6 \(\Omega\), 10.7 \(\Omega\) and 12.4 \(\Omega\) for the UTC-PD geometries of 64 \(\upmu\)m\({}^{2}\), 79 \(\upmu\)m\({}^{2}\) and 100 \(\upmu\)m\({}^{2}\) active area respectively. Contrary to the rectangular waveguide InGaAs/InGaAsP UTC-PDs, the metallic contacts of the GaInAsSb/InP UTCPDs are located at the edge of the mesa. In addition, the contact width varies for each geometry. It is therefore not surprising that the resistance does not scale with the mesa area. It is important to note that no bias dependence was observed as the collector is already fully depleted at zero bias, due to the small thickness of the collector.  
The InGaAs/InP UTC-PDs have a structure similar to the one of the GaInAsSb/InP UTC-PDs where the metal contact is located at the edge of the mesa. A series resistance of 51.3 \(\Omega\) was extracted at 80 GHz from the real part of \(Z_{11}\)-parameter. Since real \(Z_{11}\) of the UTC-PDs tends toward the series resistance at high frequencies, we chose the highest possible value of the frequency range for extracting the series resistance (80 GHz for InGaAs/InGaAsP and InGaAs/InP UTC-PDs and 60 GHz for GaInAsSb/InP UTC-PDs) to ensure reliable parameter extraction.  
\begin{table}
\begin{tabular}{c c c} \hline \(R\) & \(L\) & \(C\) \\ \hline
1.5 \(\Omega\) & 106.74 pH & 48.06 fF \\ \hline \end{tabular}
\end{table}
Table 4: Extracted RLC lumped element values from InGaAs/InP UTC-PD Open and Thru test structures.  
Figure 11: UTC-PD junction capacitance as functions of the active device area and the applied reverse bias: (**a**) InGaAs/InGaAsP UTC-PD; (**b**) GaInAsSb/InP UTC-PD.  
Figure 10: Measured (symbol) and simulated (lines) intrinsic S11-parameter for all the UTC-PDs technologies: (**a**,**c**,**e**) magnitude in dB of InGaAs/InGaAsP UTC-PD with actives areas of 100, 125 and 150 \(\upmu\)m\({}^{2}\); (**b**,**d**,**f**) phase in degree of InGaAs/InGaAsP UTC-PD with active area of 100, 125 and 150 \(\upmu\)m\({}^{2}\); (**g**,**i**,**k**) magnitude in dB of GaInAsSb/InP UTC-PD with active area of 64, 79 and 100 \(\upmu\)m\({}^{2}\); (**h**,**j**,**l**) phase in degree of GaInAsSb/InP UTC-PD with active area of 64, 79 and 100 \(\upmu\)m\({}^{2}\); (**m**) magnitude in dB of InGaAs/InP UTC-PD with a 13 \(\upmu\)m\({}^{2}\) active area; (**n**) phase in degree of InGaAs/InP UTC-PD of 13 \(\upmu\)m\({}^{2}\) of active area.