Liquid crystal electrography: electric field mapping and detection of peak electric field strength in AlGaN/GaN high electron mobility transistors

The liquid crystal mixture E7, based on cyanobiphenyl, has been successfully employed to map electric
field strength and distribution in AlGaN/GaN high electron mobility transistors. Using a transmitted light
image through crossed polarizers the optical response of the liquid crystal deposited onto the surface of
the devices was recorded as a function of source–drain bias, Vds. At a critical voltage of 4 V the preferred
direction of orientation of the long axes of the liquid crystal molecules in the drain access region aligned
with one of the polarizers resulting in reduced transmitted light intensity. This indicates that at this
electric field strength molecule orientation in most of the liquid crystal film is dominated by the electric
field effect rather than the influence of surface anchoring. The experimental results were compared to
device simulations. Electric field strength above the surface at Vds = 4 V was simulated to reach or exceed
0.006 MV/cm. This electric field is consistent with the field expected for E7 to overcome internal elastic
energy. This result illustrates the usefulness of liquid crystals to directly determine and map electric
fields in electronic devices, including small electric field strengths.