HEMTs Devices based on GaN Technology and characterization

The world of semiconductors is dominated, in market terms, by silicon. However, there are other semiconductors such as germanium, but above all III-V semiconductors, which enable better performance in specific fields of application. The main ones are GaAs and InP, and more recently the so-called large-gap semiconductors, such as SiC and GaN, with gaps of 3.2eV and 3.4eV respectively. These semiconductors enable the production of components combining breakdown voltages and high currents, making them ideal for power applications. This article deals with aspects linked to the technological realization of transistors and their electrical and microwave characterization.

As far as HEMT technology is concerned, different steps are required from an electronic lithography: alignment marks, corresponding to reference points on the sample for aligning and writing the various mask levels; ohmic contacts, which may be annealed or unalloyed; component insulation, obtained either from a mesa structure or by ion implantation; the production of gate electrodes, one of the most critical steps for frequency rise; passivation of the component, preceded by pre-treatment to improve the interface; interconnection and, finally, air bridges for components with more than two gate fingers. To obtain transistors operating at high frequencies, it is imperative to reduce parasitic elements, so different types of gate have been proposed: T or mushroom, Γ, TT or Y. Dimensions also need to be reduced, so self-aligned technology is an interesting alternative. HEMT transistors need to be characterized in electrical and microwave regime. This makes it possible to establish the quality of ohmic contacts, determine voltage and current performance, transconductance, and frequency performance by measuring S-parameters. Power performance is determined by load pull measurements.

GaN HEMTs are mainly used in microwave power applications and in the design of switches for power electronics converters. Microwave power applications concern military and civil telecommunications, at operating frequencies ranging from S-band to W-band. Military applications include jammers and missile seekers. Telecommunications applications include point-to-point and point-to-multipoint links, satellite links and backhauling up to 85 GHz. The potential of GaN technology also points to the possibility of developing "robust" low-noise amplifiers, with a noise figure and gain identical to those obtained with GaAs technology, yet capable of withstanding high electric fields, which could make it possible to do away with limiters in radar reception chains. The final application concerns converters for power electronics, where high dielectric strength, high current density, fast switching capability, low R _on...