Fig. 1. (a) Schematic diagram and (b) equivalent circuit model of the CPW. (c) The equivalent circuit model of the PD. Region 1 represents the transit time parameters, Region 2 represents the bulk and parasitic parameters of the PD, and Region 3 represents the n-section distributed parameter model of the CPW. The box on the left represents the intrinsic bandwidth parameters.

Fig. 2. Two different CPW structures: (a) CPW-1 with 50Ω impedance; (b) CPW-2 containing an inductive high-impedance part. The FEM simulated S11 and S21 parameters of (c) CPW-1 and (d) CPW-2 up to 40 GHz. The extracted distributed parameters (e) L and (f) C of the designed CPW-1 and CPW-2.

Fig. 3. Fitted S11 and S21 parameters of the 3-section circuit model of (a) CPW-1 and (b) CPW-2.

Fig. 4. Top view of the PDs with the (a) CPW-1 and (b) CPW-2 structures, (c) the schematic device structure^[16], and (d) the epitaxial layers of the MUTC-PD.

Fig. 5. (a) Measured and fitted frequency response at a reverse bias of 5 V and a photocurrent of 35 mA. (b) The output RF power versus the DC photocurrent at 40 GHz under a reverse bias of 5 V. (c) The 3 dB bandwidth under different biases and a fixed photocurrent of 35 mA. (d) The bandwidth improvement of PD-2 over that of PD-1.

Fig. 6. Measured and fitted S22 parameters (0–40 GHz) of (a) PD-1 and (b) PD-2 with different CPWs at a reverse bias of 5 V and a photocurrent of 35 mA. Also, the measured S22 parameters of (c) PD-1 and (d) PD-2 under different biases.

Fig. 7. 3 dB bandwidth limited by the transit time and RC constant of PD-1 and PD-2 under different bias voltages.

Fig. 8. (a) Frequency responses of the PDs under a bias voltage of 5 V. (b) Frequency responses of the PDs with and without the CPW electrodes.