  Developing 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
micron-scale devices. This work both underscores and addresses the need to
image and understand spatial variations in the electronic properties of
nanoscale devices.
