Ultrafast Quantum-Well Photodetectors Operating At 10 Mu M With A Flat Frequency Response Up To 70 Ghz At Room Temperature

III-V semiconductor mid-infrared photodetectors based on intersubband transitions hold great potential for ultrahigh-speed operation up to several hundreds of GHz. In this work we exploit a similar to 350 nm thick GaAs/Al0.2Ga0.8As multi-quantum-well heterostructure to demonstrate heterodyne detection at lambda similar to 10 mu m with a nearly flat frequency response up to 70 GHz at room temperature, solely limited by the measurement system bandwidth. This is the broadest RF-bandwidth reported to date for a quantum-well mid-infrared photodetector. Responsivities of 0.15 and 1.5 A/W are obtained at 300 and 77 K. respectively. To allow ultrafast operation and illumination at normal incidence, the detector consists of a 50 Omega coplanar waveguide, monolithically integrated with a 2D array of subwavelength patch antennas, electrically interconnected by suspended wires. With this device architecture, we obtain a parasitic capacitance of similar to 30 fF, corresponding to the static capacitance of the antennas, yielding a RC-limited 3 dB cutoff frequency >150 GHz at 300 K, extracted with a small-signal equivalent circuit model. Using this model, we quantitively reproduce the detector frequency response and find intrinsic roll-off time constants as low as 1 ps at room temperature.