= Solution
Use equal-width rectangular contact windows in a row with several different edge-to-edge spacings. For positive-resist \b[lift-off], the future metal pads correspond to clear windows in an otherwise chrome-covered mask: exposure removes resist there, and the unexposed background retains resist beneath unwanted metal.
For ordinary ohmic <transmission line contact measurement>, a conducting strip or isolated mesa of width $W$ connects adjacent pads. Measure several spacings $d$ so that
$$
R(d)=2R_c+R_\square\frac dW.
$$
The intercept separates two contact contributions from the sheet-resistance slope. Large enough pad length relative to the transfer length is needed for the simplest contact-resistivity extraction.
Prepare separate arrays on the appropriate surfaces: annealed source/drain metal on the n+ cap, and gate metal on cap-removed AlGaAs. A Schottky gate is nonlinear, so its interface cannot be assigned an ohmic zero-bias contact resistivity merely by fitting the same straight line. Use bias-dependent current–voltage or differential-contact analysis, accounting for the two rectifying contacts.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2023/paper-1-additional-tlm-mask.png]
{title=Positive-resist lift-off mask and variable-spacing contact array}
{description=Original schematic. Pale windows in the dark chrome background define future metal pads. The resulting equal-width pads have different edge spacings. Ohmic TLM analysis and bias-dependent analysis of a Schottky contact require distinct electrical models.}
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