GaN-based HEMT devices - Materials and epitaxy

The world of semiconductors is dominated, in market terms, by silicon. However, there are other semiconductors, such as germanium, and above all III-V semiconductors, which enable better performance in specific fields of application. The main ones are GaAs and InP, and more recently so-called "large-gap" semiconductors such as SiC and GaN, with gaps of 3.2 eV and 3.4 eV respectively. These semiconductors enable the production of components combining high breakdown voltage and current, making them ideal for power applications. This article on GaN describes the materials and epitaxy techniques used to produce these components, whose main applications are in microwave and power electronics. High electron mobility devices (HEMTs) or monolithic MMICs operating at up to 100 GHz can be manufactured for telecommunications or military applications, as can transistors combining high voltage and high current for the design of high-frequency switching converters.

GaN offers many advantages, as it allows the combination of ternary semiconductors such as AIGaN and AlInN, and quaternary AIGaInN, enabling the design of heterojunction devices such as the HEMT transistor. In this structure, a two-dimensional (2D) gas of electrons is created at the heterojunction interface, resulting in high carrier densities characterized by good mobility. Among III-V semiconductors, III-N materials with a wurtzite-type crystal structure, such as GaN, AIN and InN, exhibit both spontaneous and piezoelectric polarization. These polarizations are responsible for the 2D gas at the heterojunction between the AIGaInN barrier zone and the GaN active zone, without the need to dop the barrier zone. For power applications, the GaN die offers other advantages, such as high temperature resistance and the ability to operate in hostile environments. However, one limitation is the limited availability of semi-insulating GaN substrates, so other types of host substrate such as SiC and Si are commonly used, the former providing the best performance thanks to its low lattice mismatch with GaN, and the latter for its availability in large size and low cost. Epitaxy by MOCVD or MBE includes :

• a nucleation layer deposited on the substrate to ensure good mesh matching with the GaN,

• a layer of GaN to buffer the active zone,

• a thin AIN zone to improve transport properties in the channel,

• a barrier zone in AIGaN or AlInN or even AIN,

• then a surface layer (cap) of GaN or SiN.

One limitation of GaN HEMTs is the density of defects due to lattice mismatch, which leads to the creation of traps that can limit performance.

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