The addition of these components will enable the design of chips with extended functionality and increased performance and enhance monolithically-integrated GaN power ICs, paving the way towards smaller and more efficient DC/DC convertors and Point-of-Load convertors.
GaN power electronics are still dominated by discrete components driven by an external driver IC that generates the switching signals. However, to take full advantage of the fast-switching speed GaN is able to offer, monolithic integration of power devices and driver functions is recommended. Imec has already successfully demonstrated the monolithic co-integration of a half-bridge and drivers together with control and protection circuits that are key to an integrated all-GaN power IC in one chip.
One of the main hurdles to boost the full performance of GaN power ICs has been finding a suitable solution for the lack of p-channel devices in GaN with acceptable performance.
CMOS technology uses complementary and more symmetrical pairs of p- and n-type field effect transistors (FETs), based on the mobilities of holes and electrons for both types of FETs. However, in GaN, the mobility of holes is about 60 times worse than that of electrons. That means that a p-channel device, where holes are the principal carriers, would be 60 times larger than the n-channel counterpart and highly inefficient. A widespread alternative is replacing the P-MOS by a resistor. Resistor-Transistor Logic (RTL) has been employed for GaN ICs but shows trade-offs between switching time and power consumption.
“We have improved the performance of GaN ICs by using a combination of enhancement-mode and depletion-mode switches, called e-mode and d-mode HEMTS. By extending our functional e-mode HEMT platform on SOI with co-integrated d-mode HEMTS, we can now take the step from RTL to direct-coupled FET logic which is expected to improve the speed and reduce the power dissipation of the circuits,” said Stefaan Decoutere, program director GaN power systems at imec.
ABOVE: Process cross-sections of the high-voltage components fabricated on 200 mm GaN-on SOI substrates (a) e-mode pGaN-HEMT (b) d-mode MIS-HEMT, (c) Schottky barrier diode. All devices include metal field plates based on front-end and interconnect metal layers and separated by dielectric layers.
Another important component for co-integration on GaN power ICs is a Schottky barrier diode. Compared to their silicon counterparts, GaN Schottky diodes combine higher blocking voltages with reduced switching losses.
“We have successfully extended our 200 V GaN-on-SOI e-mode HEMT GaN ICs platform with monolithically-integrated high-performance Schottky barrier diodes and d-mode HEMTs which brings us a step closer to smart power ICs based on GaN. This GaN-IC platform is available for prototyping through our multi-project-wafer (MPW) service,” added Stefaan Decoutere. “Our platform is ready for transfer to partners. We’re looking for foundries, but also design houses and end-users. The next step will be to develop and release a 650 Volt version of the platform. Target applications for GaN-on-SOI technology include high-voltage power switching and power conversion, fast chargers for mobile phones, tablets and laptops, and on-board chargers for electric cars, and invertors for solar panel connections to the grid”.
