# The Frequency Response Characteristics of Ge-on-Si Photodetectors Under High Incident Power
## 3 Experiments and Discussion
This observation is consistent with the simulation results shown in Figure 3. When the incident light power is 4 mW, the simulation results in Figure 3 show that the light absorption region has a significant carrier-shielding effect. We eliminated the carrier-shielding effect by increasing the bias voltage during the experiment, and the bandwidth of the photodetector was significantly improved. When the incident light power is 0.5 mW, the simulation results of the absorption region electric field show that the carrier-shielding effect does not influence the carrier  
Figure 4: Normalized \(S_{21}\) response of the photodetectors under different bias voltages at incident light powers of 0.5 mW and 4 mW. saturation drift velocity. In the experiment, by increasing the bias voltage to eliminate the carrier-shielding effect, the bandwidth of the photodetector remains almost unchanged. The simulation results and experimental results show good consistency. When the bias voltage is \(-5\) V and the optical power is 4 mW, the photodetector bandwidth reaches 18.62 GHz, which remains lower than the 38.56 GHz bandwidth observed at an optical power of 0.5 mW with a \(-1\) V bias. These experimental results indicate that at high incident light power, the carrier-shielding effect reduces the device bandwidth by lowering the electric field intensity. However, the impact on the bandwidth is limited and is significant only when the incident light power is less than 2.5 mW. Both high optical power and bias voltage can compromise the device's reliability. The increase in carrier concentration at high incident light power leads to a decrease in the carrier saturation drift velocity [20; 21], which is another key factor that contributes to the reduction in bandwidth. A higher carrier concentration increases the scattering probability during carrier drift, thereby decreasing the carrier saturation drift velocity. An increase in the carrier concentration leads to an increase in the scattering probability, which mainly includes scattering between electrons and holes, carriers and lattices, phonons, and defects [22; 23]. The scattering between electrons and holes primarily arises from their interaction through Coulomb force, leading to a change in the carriers' direction of motion. Phonon scattering occurs due to collisions between lattice vibrations and moving carriers, resulting in dispersion. Additionally, defects and impurity centers may capture carriers in motion, thereby diminishing their velocity. By lowering carrier concentration, operating at reduced temperatures, and minimizing impurities and defects, one can effectively decrease carrier scattering and enhance the migration rate of carriers nearing saturation. Intervalley scattering reduces the carrier saturation migration rate, thereby decreasing the device's bandwidth. Phonon-mediated relaxation leads to an increase in scattering between phonons and carriers, which in turn lowers the mobility and reduces the device's bandwidth. The scattering during carrier migration changes the direction of carrier movement, reducing the rate of carrier migration. Therefore, the rate of carrier movement along the electric field direction is reduced. We experimentally measured the changes in the carrier saturation drift velocity at various photogenerated carrier concentrations using the Hall effect, as shown in Figure 5. The carrier saturation drift velocity in germanium was measured with an Ecopia HMS-3000 Hall tester (Ecopia, Anyang-city, Republic of Korea). The sample was clamped onto a sample plate with a probe and injected with light through an optical fiber.