Highly robust microwave low-noise amplifiers

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The first component of a radio receiver that follows the antenna is commonly the low-noise amplifier (LNA). It is required to amplify the usually very weak signal to a power level high enough to ignore any additive noise of subsequent electronic stages. Therefore, an LNA is a very sensitive low-power component. However, a radio receiver usually also reveceives more signals than just the intended one. If a powerful transmitter is nearby, the power level easily exceeds the average receiver power level by a factor of 1000. This can easily destroy the LNA. Many communication applications integrate transmitter and receiver, for example RADAR or satellite communication. In these cases, a protection circuit is required, that blocks high-power signals at the expense of attenuating the signal, and thereby lowering the signal-to-noise ratio of the whole system. Furthermore, the protection circuit usually can't be integrated with the LNA, and the system gets bulky, more expensive in fabrication, and reliability might be reduced. It was shown in the literature recently, that GaH-HEMT technology allows to boost the maximum input powers to be increased from the typical 100 mW for GaAs-HEMT technologies to about 10 W. But it has to be stated, that this success is based on a rather simple circuit topology, and that no detailled theoretical analysis of the limiting factors regarding robustness was published so far. The improvements in ruggedness of GaN LNAs therefore mainly reflect the improvements in device technology. This marks the starting point of this investigation. A new circuit topology based on stacked GaN-HEMTs promises to improve maximum safe input powers, due to the higher effective breakdown voltage at the LNA input. But it has yet to be shown that also low noise performance is possible in this concept. This new approach, but also the traditional LNA circuit topology will be theoretically investigated in order to explore and explain the limiting factors for maximum robustness. Based on measurement, the degradation mechanisms in GaN HEMTs will be investigated in order to understand the limits of critical stress below the level of immediate destruction of the device. It is the goal of this investigation to present an analytical model to optimize the ruggedness of GaN LNAs on the basis of well understood critical stress condidtions for the transistors, and also to push the maximum safe input power significantly towards higher power levels though optimized circuit topologies.