The development of van der Waals (vdW) homojunction devices requires materials with narrow bandgaps and simultaneously high hole and electron mobilities for bipolar transport, as well as methods to image and study spatial variations in carrier type and associated conductivity with nanometer spatial resolution. Here, we demonstrate the general capability of near-field scanning microwave microscopy (SMM) to image and study the local carrier type and associated conductivity in operando by studying ambiploar field-effect transistors (FETs) of the 1D vdW material tellurium in 2D form. To quantitatively understand electronic variations across the device, we produce nanometer-resolved maps of the local carrier equivalence backgate voltage. We show that the global device conductivity minimum determined from transport measurements does not arise from uniform carrier neutrality but rather from the continued coexistence of p-type regions at the device edge and n-type regions in the interior of our micrometer-scale devices. This work both underscores and addresses the need to image and understand spatial variations in the electronic properties of nanoscale devices.
Keywords: 2D materials; Atomic force microscope; field-effect transistor; microwave; near-field microscopy.