Vers des transistors de puissance à haut rendement en gamme d’ondes millimétriques

The growing demand for bandwidth and data transmission speed for future applications such as 5G networks, satellite communications or radar requires the manufacture of components delivering increased performance in the millimetre wave range. Silicon-based integrated circuits are limited to output powers of the order of a few watts in the Ka band (26-40 GHz). Combined with a silicon carbide (SiC) substrate with high thermal conductivity, gallium nitride (GaN) offers considerable advantages for power applications, such as a voltage withstand 5 to 10 times better than traditional semiconductors based on silicon or gallium arsenide, as well as better thermal dissipation. However, the main limitations of this technology are power added efficiency (PAE), which defines the energy efficiency of amplifiers, the control of trap effects, which is essential in RADAR applications, and the reliability of transistors with reduced dimensions (ultra-short sub-150 nm gates).

Researchers at the IEMN, in collaboration with SOITEC-Belgium, have developed an approach that makes it possible to reduce the dimensions of GaN transistors while fully exploiting the potential of this emerging material. This work was supported by the Renatech network, the European Defence Agency (EUGANIC project) and the DGA/AID (French Ministry of Defence). Support from these partners has enabled the acquisition of an expensive narrow-band (40 GHz) power amplifier delivering high power (> 10 W), as well as the associated components (connectors, couplers, pulsed generators, etc.). The functionalities of this NVNA power bench at the IEMN in this frequency band (i.e. high-power active load pull measurements in pulsed and CW mode) give the IEMN a status that is virtually unique in Europe.

In this context, a specific heterostructure has been developed to overcome the technological hurdles in terms of reliability, exacerbated trap effects and efficiency for 100 nm gate GaN transistors operating at voltages above 15 V. This solution is based on the implementation of an ultra-thin AlN barrier layer. With a thickness of less than 5 nm, this layer is conducive to frequency rise while delivering a high electron density. A layer of SiN, grown in situ, covers the barrier. This not only increases the robustness of the surface under high electric field and/or temperature conditions, but also greatly reduces the effects of traps. Finally, the buffer layers have been designed to promote heat dissipation. The combination of these elements, together with an optimised manufacturing process, has made it possible to demonstrate unprecedented efficiencies of more than 70 % at 40 GHz at high power density (> 5 W/mm). The absence of degradation during preliminary reliability measurements over several tens of hours at different temperatures is also noteworthy. These results pave the way for a new range of millimetre-wave power components. They offer real potential for applications in terrestrial and satellite communications, such as 5G, with optimal coverage of rural areas, for example.