2024-07-03 22:00:00
The world of semiconductors is dominated, in terms of market, by silicon. However, there are other semiconductors such as germanium, but especially III-V semiconductors, which allow to obtain better performances in specific fields of applications. The main ones are GaAs and InP, and more recently the so-called wide gap semiconductors, such as SiC and GaN with respective gaps of 3.2 and 3.4 eV. These semiconductors allow to realize components that combine breakdown voltages and high currents, which makes them suitable for power applications. This article deals with the aspects related to the technological realization of the components and their electrical and microwave characterization.
Regarding HEMT (high electron mobility transistor) technology, different steps are necessary starting from an electron lithography: the alignment marks which correspond to reference points on the sample in order to align and write the different mask levels; the ohmic contacts which can be annealed or unalloyed; the isolation of the components obtained either from a mesa structure or by ion implantation; the production of the gate electrodes which is one of the most critical steps for the frequency increase; the passivation of the component preceded by a pretreatment in order to improve the interface; the interconnection and finally the air bridges for components with more than two gate fingers. To obtain transistors operating at high frequencies, it is imperative to reduce the parasitic elements, so different types of gate have been proposed: T or mushroom, Γ, TT or Y. It is also necessary to reduce the dimensions, so self-aligned technology is an interesting alternative. HEMTs must be characterized in electrical and microwave conditions. This allows the quality of the ohmic contacts to be established, the voltage and current performances, the transconductance, and the frequency performances to be determined via the measurement of the S parameters. The power performances are determined from measurements of the type load pull.
GaN HEMTs are mainly intended for microwave power applications and the design of switches for power electronics converters. Microwave power applications concern the military and civil fields dedicated to telecommunications, for operating frequencies ranging from the S band to the W band. The main applications concern civil and military radars. In the military field, we can also cite jammers and missile seekers. In the telecommunications field, the applications are point-to-point and point-to-multipoint links, satellite links, transport network links (backhauling) up to 85 GHz. The potential of GaN technology also suggests the possibility of developing so-called “robust” low-noise amplifiers, with a noise figure and gain identical to those obtained by GaAs technology, while being able to withstand high electric fields, which can allow the elimination of limiters in radar reception chains. The last application concerns converters for power electronics where high dielectric robustness, high current density, the possibility of fast switching, low resistance Ron and the ability to withstand high operating temperatures. For this objective, it is imperative to develop normally-off HEMTs and also GaN diodes in order to simplify the design of power converters operating at high frequency (> 1 MHz).
At the end of the article, the reader will find a glossary and a table of acronyms and symbols used.
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