Fig. 1. (a) Epitaxial layer structure of the DCL-UTC-PD. (b) Photocurrent induced electric field distribution in the 650-nm-thick p-doped absorption region. Electric field in the depletion region of (c) traditional UTC-PD and (d) DCL-UTC-PD structure under a photocurrent of 10 mA and different bias voltages.

Fig. 2. (a) Schematic and (b) equivalent circuit of the PD with high-impedance CPW structure. (c) RLC and transit time limited frequency responses of the PD with different CPW lengths. (d) Total frequency responses of the PD with different CPW lengths.

Fig. 3. (a) Schematic view of the PD structure. (b) Frequency responses at different photocurrents under a bias voltage of 2 V. (c) Output RF power versus the optical input power. The inset is the microscope image of the 6-µm-diameter PD.

Fig. 4. (a) Equivalent circuit of the PD with CPW structure. (b)–(d) Measured and fitted S₂₂ parameters (0–40 GHz) of the PD under different reverse biases.

Fig. 5. (a) Total bandwidth and (b) transit time limited bandwidth under different bias voltages and photocurrents.

Fig. 6. Responsivity versus frequency of back-illuminated PDs.