The application of stacked-FETs in power amplifiers allows for a supply voltage higher than supported by the breakdown voltage of a single transistor. Potential benefits of the increased supply voltage are reduced supply currents and a lower matching ratio at the output of the amplifier. Furthermore, an increased output power per chip area is obtained due to the reduction in passive structures resulting in more area-efficient power combining. In this paper, the procedure for the design of integrated microwave stacked-FET is discussed. Several options for the correct distribution of RF voltage and current swings are investigated and the relationship between the number of stacked transistors and bandwidth is addressed. The procedure is demonstrated by the design of an S-band GaAs stacked-FET containing three transistors. This stacked-FET is applied in an S-band HPA that has a PAE of more than 40% at an output power of 20 W, which is more than twice the output power of any previously reported GaAs stacked-FET HPA.
The front-end circuitry of transceiver modules is slowly being updated from GaAs-based monolithic microwave integrated circuits (MMICs) to Gallium-Nitride (GaN). Especially GaN power amplifiers and T/R switches, but also low-noise amplifiers (LNAs), offer significant performance improvement over GaAs components. Therefore it is interesting to also explore the possible advantages of a GaN mixer to enable a fully GaN-based front-end. In this paper, the design-experiment and measurement results of a double-balanced image-reject mixer MMIC in 0.25 μm AlGaN/GaN technology are presented. First an introduction is given on the selection and dimensioning of the mixer core, in relation to the linearity and conversion loss. At the intermediate frequency (IF)-side of the mixer, an active balun has been used to compensate partly for the loss of the mixer. An on-chip local-oscillator (LO) signal amplifier has been incorporated so that the mixer can function with 0 dBm LO input power. After the discussion of the circuit design the measurement results are presented. The performance of the mixer core and passive elements has been demonstrated by measurements on a test-structure. The mixer MMIC measured conversion loss is <8 dB from 6 to 12 GHz, at 1 GHz IF and 0 dBm LO power. The measured image rejection is better than 30 dB.
In this report two different aspects in the development of AlGaN/GaN power amplifiers will be discussed. In the first part of this paper we report on the optimization of the Ti/Al/Ni/Au metallization scheme on a doped AlGaN/GaN FET structure. By a systematic investigation we were able to reduce the contact resistance to 0.2 ωmm (7.3×10×7ωcm2). The Al/Ti thickness ratio for this contact was 6, which according to the Al-Ti binary phase diagram, does not result in excess Ti which should react with nitrogen in the AlGaN layer to render the surface heavily doped. Preliminary results on Schottky contacts indicate an improvement in the reverse leakage current if a RIE oxygen plasma in combination with a NH4OH dip is performed prior to metallization.Coplanar waveguides on AlN are discussed in the second part of this paper. These transmission lines can be used in AlGaN/GaN power amplifiers if no via-hole technology is available or if a hybrid solution is pursued. The signal line should have a large metal cross- sectional area (> 5 × 50 [.proportional]m2) in order to carry enough current in the output stage of an amplifier. It is shown that CPWs with large dimensions show non-quasi TEM behavior related to propagation of parallel plate modes.
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