Printing GaN HEMTs Onto Silicon CMOS
A micro-transfer printing technique could boost the efficiency of integrated power electronics by uniting high-performance GaN HEMTs with highly integrated silicon CMOS by Stefan Eisenbrandt and Ralf Lerner from X-Fab
Lying at the heart of the majority of today's power ICs are three classes of silicon transistor "“ those with either a laterally diffused metal-oxide-semiconductor design, an insulated-gate bipolar architecture, or a super-junction configuration. All these workhorses can handle high voltages, high currents, or a combination of both.
It is known from Moore's law that miniaturising these transistors would deliver an evolutionary improvement in performance, but at the expense of spiralling development efforts. Better still is a revolutionary performance gain. This is possible, via the integration of III-V materials.
One of the most promising III-V devices for power electronics is the GaN HEMT. Compared to all forms of silicon device, it sports superior switching speeds, and it also has the upper hand when considering the trade-off between the on-resistance and the breakdown voltage. Thanks to these merits, the GaN HEMT can enable new, highly efficient power conversion topologies that would be unthinkable with state-of-the-art silicon based devices.
Let's not write silicon off just yet, however. Note that bipolar-CMOS-DMOS and high-voltage CMOS technologies have realised high levels of diversification, leading to the highest design complexities. These devices can be used in Smart power ICs, which enable an interface between digital control logic and the power load. By using monolithic integration to position output power devices next to digital and analogue circuitry, it is possible to combine signal processing, sensing and protection circuitry on the same chip. Further benefits of this approach are a trimming of the number of interfaces, the volume, and electromagnetic interferences. The upshot is increased efficiency, performance and reliability.
Engineers working with compound semiconductor technologies are also trying to realise the high-level of functional integration seen in these silicon-based, Smart power ICs. Several technologies for integrating GaN HEMTs with digital and analogue circuitry are currently being pursued: GaN-based Smart power ICs; monolithic integration of GaN on silicon; wafer bonding of GaN on silicon; and the approach that we are investigating at X-FAB Semiconductor Foundries of Erfurt, Germany: heterogeneous integration, enabled by micro-transfer-printing (see Figure 1).