HEMTs Devices based on GaN - Material and epitaxy

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 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 high current, making them ideal for power applications.

This article, dedicated to GaN, describes the material aspects and epitaxy techniques used to produce these components, whose main applications are in microwave and power electronics. We can manufacture high electron mobility devices (HEMTs), or monolithic millimeter-scale integrated circuits of the MMIC type, operating at up to 110 GHz, for telecommunications or military applications, as well as 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, AlInN and ScAlN, and quaternary semiconductors such as 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 AlGaN, AlInN, AIGaInN, AlN or ScAlN barrier region and the GaN active region, without the need to dop the barrier region.

For power applications, GaN 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. As a result, other types of host substrate, such as SiC and Si in particular, are commonly used. The former delivers the best performance, thanks to its low lattice mismatch with GaN. The latter is more available in larger sizes, and is less expensive.

MOCVD or MBE epitaxy includes :

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

• a GaN layer forming the buffer layer and the active zone;

• a thin AIN zone, which improves transport properties in the channel;

• a barrier zone in AIGaN, AlInN,...