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

•Liquid crystal was applied on top of the SiN passivation surface of AlGaN/GaN HEMT.•Transmitted light in source–drain region was recorded through crossed polarizers.•Decrease in transmitted light intensity with increased source–drain bias was recorded.•Liquid crystal molecules shown to orientate with the horizontal electric field.•Experimental results confirm simulated field strengths. 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 4V 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=4V was simulated to reach or exceed 0.006MV/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.