Fig. 1. (a) Epitaxy structure of the proposed MUTC-PD. (b) Influence of high-impedance CPW inductance on frequency response of the 4-μm-diameter PD. (c) 3D schematic view of the PD. (d) SEM image of the fabricated device. (e) Frequency responses under various photocurrents of the 4-μm-diameter PD under 2 V bias voltage.

Fig. 2. (a) Top view of passive circuits consist of interdigital inductor-based RF-choke, interdigital capacitor-based DC-block and E-plane probe. Measurement and simulation results of the back-to-back (b) bias-tee and (c) probe package.

Fig. 3. (a) Schematic of the WR-5 packaged PD module. ESD: electro static discharge. (b) Wire-bonding interconnection of the passive circuits. (c) Microscope photo of the flip-chip bonded PD chip. (d) I-V characteristics before and after flip-chip bonding.

Fig. 4. (a) The packaged WR-5 MUTC-PD module. (b) Frequency response under various photocurrents and (c) frequency-dependent saturation characteristics at 2 V bias voltage of WR-5 PD module.

Fig. 5. Comparison of saturation output power against operation frequency for packaged UTC-PD modules. Open marks are for coaxial output modules, and closed ones for waveguide output modules. The rectangular mark indicates module with resonant matching circuit.

Fig. 6. Experimental setup of THz wireless communication with the proposed MUTC-PD. AWG: arbitrary waveform generator, ECL: external cavity laser, IQ Mod: IQ modulator, OC: optical coupler, SG: signal generator, LNA: low noise amplifier, EA: electrical amplifier, OSC: oscilloscope, SG: signal generator. (a) Spectrum of the transmitting signal with 25 G Baud. (b) Optical spectrum of the OC at a frequency difference of 150.5 GHz between two lasers. (c) Optical spectrum of the OC at a frequency difference of 210.5 GHz between two lasers.

Fig. 7. EVM performance at different baud rates: (a) 150.5-GHz and (b) 210.5-GHz.