Impedance spectroscopy of AlGaN/GaN HEMTs in contact with culture media

Abstract

AbstractTwo different high electron mobility transistor (HEMT) structures based on AlGaN/GaN heterojunctions in contact with electrolyte media of different pH‐values and conductances were characterized by impedance spectroscopy. The main differences between both structures are the arrangement of the metal electrodes to the electrolyte and the resistivity of the two dimensional electron gas (2DEG). In structure #1 the 2DEG current path causes dispersion at low frequencies within the imaginary plot which strongly depends on the electrolyte conductance but not the pH‐value. At higher frequencies an additional dispersion caused by the dispersion of the electrolyte solution itself is observed which decreases with increasing electrolyte conductance.With structure #2 a direct correlation between the pH‐value of the electrolyte, the DC‐current and the impedance at 1 kHz was verified. But, at higher frequencies the impedance only depends on the electrolyte conductance. This dependence on the electrolyte conductance is associated with a dominant current path from the contacts through the 2DEG and shows a similar behavior as the low frequency dispersion in structure #1. Consequently, both the pH‐value and the conductance of an electrolyte can be analyzed with one HEMT using different measurement frequencies. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)