This study investigates the band bending profile of gallium-nitride-based heterostructures with respect to the change in the direction of the induced current in the out-of-plane plane direction, which gives the phase of the terahertz emission peak and can be detected by laser THz emission microscopy. We use a wavelength-tunable laser THz emission spectroscope to observe the transport of charges within the band bending regions, excitation of nearby photocarriers, and carrier scattering. We observe flip-flop peak-to-peak THz emission waveforms for a GaN heterostructure field-effect transistor (FET) compared to a strong THz emission radiated from an Al0.3Ga0.7N/GaN high-electron-mobility-transistor (HEMT). The flip-flop THz emission indicates the emission from various interfaces inside the FET structure, and intense THz emission from the HEMT structure indicates band-edge excitation. Our results provide a valuable perspective for characterizing complex heterostructures that provide insight into possible defects, carrier mobilities, and band bending of multilayer interface electronic devices.

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