  In a GaN/AlGaN field-effect terahertz detector, the directional photocurrent
is mapped in the two-dimensional space of the gate voltage and the drain/source
bias. It is found that not only the magnitude, but also the polarity, of the
photocurrent can be tuned. A quasistatic self-mixing model taking into account
the localized terahertz field provides a quantitative description of the
detector characteristics. Strongly localized self-mixing is confirmed. It is
therefore important to engineer the spatial distribution of the terahertz field
and its coupling to the field-effect channel on the sub-micron scale.
