What is it about?
HEMT (High Electron Mobility Transistors) could be fabricated with 2 or more conducting channels which could possess different electrical parameters like electron mobility’s or concentration. When you put these HEMT or simple AlGaN/GaN HEMT type heterostructures (without S, D and G contacts) into perpendicular magnetic field to measure mobility and concentration of 2DEG (2D electron gas) you will obtain an averages parameters. The reason is a difference in Hall Voltages in each channels and in the consequence flow of the new current (called it: compensatory current) .
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Why is it important?
During measurements of a Hall voltage on a casual semiconductor material an additional voltages: Ettingshausen, Nernts-Ettingshausen, Righi-Leduca, Tauca, Seebeca, or even by sample asymmetry occurs simultaneously. However when you will start to measuring a heterostructures with more than few conducting channels (even parasitic channel - with lower mobility’s and concentration) a new unique current will start to flow between these channels. Therefore the results obtained from Hall measurements are "averages" between these channels and could guide you to wrong conclusion because an true 2DEG mobility is hidden.
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This page is a summary of: The Origin and Influence of Compensatory Current in AlGaN/GaN Type High Electron Mobility Transistor Heterostructures with Two Conducting Channels on the Hall Measurements, physica status solidi (a), March 2020, Wiley, DOI: 10.1002/pssa.201900661.
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