Fig. 1. (a) (b) the cross-sectional structural schematic and the corresponding optical top view of the fabricated device (c) Raman spectrum of the monolayer graphene (d) transfer characteristic curve measured with V_ds = 0.2V

Fig. 2. (a) (b) Time response of the photocurrent under radiation of 25 GHz at modulation frequencies of 1kHz and 10kHz with zero bias (c) Photocurrent response as a function of incident power at different bias voltages (d) Time response of the photocurrent at different bias voltage

Fig. 3. (a) The extension of the rising edge and falling edge (inset) of the photocurrent in a single time-period (b) Photocurrent as a function of the gate voltage with V_ds of 0.2 V (c) Photoresponsivity at different bias voltage (d) Noise equivalent power variation with gate voltage

Fig. 4. (a) (b) (c) The time response of the device under radiation of 0.12 THz at different modulation frequencies of 1kHz, 5kHz and 10kHz under zero bias (d) The time response of the photodetector at different bias voltage at different modulation frequencies of 1kHz

Fig. 5. (a) The extension of the rising edge and falling edge (inset) of the photocurrent in a single time-period (b) Photocurrent variation as a function of gate voltage with V_ds of 0.4 V (c) Photoresponsivity obtained at different bias voltage V_ds (d) Noise equivalent power variation with the gate voltage V_g

Fig. 6. Schematic energy band diagrams of graphene /GaAs hetero-structure (a) without millimeter/THz radiation; and (b) under millimeter/THz radiation