Fig. 1. Epitaxy structure of our DDL-UTC-PD (a) and (b) energy band diagram of PD under reverse bias

Fig. 2. Electric field distribution within the device under various reverse biases. (a) Typical UTC-PD; (b) DDL-UTC-PD

Fig. 3. (a) Frequency response at photocurrent of 10 mA; (b) 3 dB bandwidth at different reverse biases

Fig. 4. Voltage-dependent saturation characteristics at 105 GHz

Fig. 5. Epitaxial structure of MUTC-PD for optimization

Fig. 6. Transit-time-limited bandwidth varies with thickness of depleted absorber

Fig. 7. Circuit model for frequncy response simulation

Fig. 8. Tuning effects of high impedance transmission line on frequency response

Fig. 9. Optical microscopic photo of fabricated PD

Fig. 10. Measurement results of frequency response of 4.5 μm-diameter PD