GaN Technology on High-Resistivity Si-substrates for High-Power-Amplifiers
Das, J.; Derluyn, J.; Lorenz, A.; Oprins, H.; Xiao, D.; De Raedt, W.; Cheng, K.; Leys, M.; Degroote, S.; Germain, M.; Borghs, G.
IMEC

During the past decade, Gallium-Nitride (GaN) HEMT technology has attracted considerable interest because of its large potential for high power amplification at microwave and millimeter-wave frequencies. Today it is considered as a strategic superior technology mainly for space, defense and telecommunication applications. For system integration, important criteria are: low cost, high flexibility, high performance, short interconnects, low weight/volume ratio (for space), and optimal thermal management.

At IMEC, we are developing GaN technology both on Si(111) and SiC substrates. The main advantages of Si substrates are: low substrate cost, large substrate availability and compatibility with mainstream Si process technology. A disadvantage however is the lower thermal conductivity. Here we will discuss our recent advances concerning the GaN on Si device performance, Si wafer thinning and through wafer via technology, and GaN power bar design.

AlGaN/GaN HEMTs were grown onto high resistivity (HR) Si(111) substrates. The buffer was optimized to obtain a low substrate RF-loss which is an important requirement for RF application. On coplanar waveguides, a loss of -0.28 dB/mm was obtained at 4 GHz. Power cells, with 0.5μm gate length and 1.5mm gate width, were measured using pulsed load pull measurements at 4 GHz, resulting in an output power up to 6.3 W/mm at 60 V drain bias.

To realize a high power amplifier (HPA) module, we are using a system-in-a-package (SIP) approach. Here, the GaN power bar and passive components will be processed separately, but finally integrated into the same package. This approach has clearly the advantage that high flexibility, optimized performance and low cost can be combined together. For the passive components, a thin film multi-chip module (MCM) technology is used. The GaN power bar will be integrated on top of the MCM substrate. Thermal simulations were done to optimize the thermal dissipation towards the MCM substrate. To minimize the interconnect length between the power bar and the passives, a through-wafer-via technology was developed for Si(111) substrates. Note that due to the Si wafer thinning, the thermal performance significantly improves.

In conclusion, our GaN on HR-Si technology shows very promising power characteristics, in order to realize a C-band HPA using a SIP-approach. A through-wafer-via process is implemented to integrate the GaN power-bars onto the MCM substrate.

This work was supported by ESA (contract No. 20455/06/NL/GLC & 20073/06/NL/PA), and in collaboration with AMCAD-engineering and Thales Alenia Space.