= von Klitzing-Dorda-Pepper quantum Hall experiment
{c}
<Klaus von Klitzing>, <G. Dorda> and <M. Pepper> reported a silicon <MOSFET> measurement in 1980. Its gate controlled an <semiconductor inversion layer>; transverse contacts measured the Hall voltage and separate longitudinal contacts measured dissipation. One sweep held $B=18\,\mathrm T$, $T=1.5\,\mathrm K$ and $I=1\,\mu\mathrm A$ fixed while varying gate voltage. Hall plateaus accompanied small longitudinal voltages. Device-geometry comparisons and high-input-impedance voltage measurements helped test whether contact placement or electrical loading produced the result.<New Method for High-Accuracy Determination of the Fine-Structure Constant Based on Quantized Hall Resistance by Klaus von Klitzing and others>[pp. 494–496, Fig. 1 and precision-measurement discussion]{ref}
\Image[/quantum-hall-bar-voltage-contacts.png]
{title=Hall-bar contacts and gate-controlled channel}
This original functional diagram separates transverse and longitudinal readouts; it does not reproduce the historical device's dimensions. The gate inset explains how a <field-effect transistor> provides an adjustable <two-dimensional electron gas>. A longitudinal voltage close to zero is a useful check because accidental longitudinal separation between Hall contacts can otherwise contaminate the transverse measurement.
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