Ohmic contacts to pseudomorphic HEMTs with low contact resistance due to enhanced Ge penetration through AlGaAs layers

摘要

AuGe/Ni ohmic contacts are used as source and drain electrodes of pseudomorphic HEMTs (pHEMTs). High alloying temperatures are generally believed to be necessary to enhance penetration of the alloy materials through the AlGaAs layers in order to establish a very low resistance path for the source-drain currents to access the two-dimensional electron gas (2DEG) layer. Here we have performed alloying experiments in the temperature range of 390-450 ℃, and the contact resistance was determined using transfer length method measurements. Germanium diffusion was studied using backside secondary ion mass spectrometry. During our study, we have observed that doping of the channel by germanium is possible even at lower temperatures. But alloying at lower temperatures does not appreciably enhance the concentration throughout the different device layers below the contact pads. Hence, unlike MESFET alloying, higher alloying temperatures are essential for increasing the doping concentration so as to reduce the contact resistance and overcome the resistance of the AlGaAs layers.