Aluminium gallium nitride (AlGaN) is a material that is growing in importance with its use in the next generation of RF Devices. AlGaN is a hard material and can be dry etched with either photoresist or hard mask using Inductively Coupled Plasma (ICP) and Reactive Ion Etching (RIE).
Electronics properties of AlGaN/GaN heterostructure are used for RF devices in devices such as HEMT, also called HFET. Our processing solutions for HEMTs focus on controlling state density introducing minimum defects at the 2 DEG.
Normally on and off operation refers to the 2DEG channel being able to conduct current or not from the Source to the Drain when the voltage applied at the gate is 0V. The 2DEG channel is shown below.
The concentration of electrons must be locally disrupted in the 2DEG in order to stop the current from flowing from the source to the drain. The 2DEG channel results from the difference in bandgap and polarization between the AlGaN and GaN. Additionally, the mechanical deformation created at the interface of the two layers due to their lattice mismatch generates additional polarization thanks to the piezoelectrical properties of GaN.
To locally suppress the 2DEG, a popular technique is to recess the gate. The depletion region present underneath the Schottky gate, therefore, gets closer to the 2DEG and reduces the concentration of electrons. Other techniques are also being developed to achieve normally-off operation.
2DEG formation in D mode AlGaN/GaN HEMTs
Source: ETH Zurich