Terahertz (THz) imaging has promising applications in chemical and biomedical systems, industrial defect inspection, security screening, and radio astronomy owing to its favorable resolution and nondestructive features [1]. The existing atmosphere absorption notches in the 3-to-4THz frequency band provide a convincing opportunity for high-resolution THz imaging and its practical application. Unfortunately, no 3-to-4THz band CMOS image sensors have been reported so far because of the following difficulties. First, it is a challenge to trade off on-chip antenna radiation efficiency and bandwidth to implement 3-to-4THz pixels with sufficient sensitivity. Second, large pixel pitch and complex readout circuitry are not friendly to high-resolution image sensing due to unacceptable silicon area. Although several sub-THz (< 1 mathsfTHz) imaging chips have been repo mathsfrted, the sensor in [2] adopts a pixel-level voltage-controlled-oscillator (VCO) based ADC architecture resulting in large pixel size and low resolution; or, the sensor in [3] decreases chip area by employing a chip-level ADC at a cost of image speed. The 3mathsfTHz detector in [4] has relatively low sensitivity and small bandwidth coverage that do not meet the aforementioned wideband requirements.