microelectronics GaN
Figure 4: Devices made for the M3 milestone of the GREAT2
project include an L-band discrete GaN HEMT.
Operating at 50 V and a baseplate temperature of 50°C,this transistor delivers approximately 10W of output power for >15 dB associated gain and a power added efficiency (PAE) >40 percent at a frequency of 1.7
GHz.The PAE performance was not optimised for the reliability test campaign,but with appropriate matching and a deep class AB quiescent bias point the typical PAE value easily exceeds 60 percent with the UMS GH50 process
hours, respectively, and fulfilling the final goal, M7, requires demonstration of device operation of at least 20 years at a junction temperature higher than 230 °C. In addition, the device must be resilient to space environmental effects, including radiation, hydrogen poisoning and electro-static discharge. This level of performance must be realised for both an L-band discrete power transistor and an X-band MMIC for the GH50 and GH25 foundry processes respectively. What’s more, specific performance and manufacturing yield targets must be met at each milestone.
When the project kicked-off in 2008, engineers at UMS put together a technology development plan that defined the methodology for technology transfer within the consortium. This included a patent survey and the signature of an intellectual property agreement to allow exploitation of key results.
At the outset, a great deal of effort was devoted to establishing a common methodology for routine device assessment, reliability testing and the analysis and display of measurement data. To allow a fair and sensible cross- comparison of data, all mask sets featured common test structures, such as 1 mm gate-width RF transistors, process control monitoring structures and dedicated radiation test cells. All the partners within the consortium evaluate these structures consistently – this was confirmed to be the case with a ‘round-robin’ test campaign on an evaluation wafer. The multiple batches of epitaxial wafers that have been used for experimental processing trials within the consortium were grown by IAF and imec (see figure 3 for an example of the growth tools used by IAF). SiC and silicon substrates provided a foundation
Figure 5: The X-band evaluation vehicle built for the M3 milestone is a two-stage MMIC. Operating at 30 V and a baseplate temperature of 50°C,this device delivers more than 6W of RF output power over the 8 to 8.5 GHz frequency band,with around 18 dB associated gain and a PAE of typically 30-40 percent depending upon the quiescent bias condition
October 2011
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