Gallium-nitride-based high-frequency ICs with improved efficiency
Power amplifiers (PA) are key components of any communication, radar and satellite system. As the last element in the transmitter chain before the antenna they dominate the overall properties. The most important figures of merit are output power Pmax and power-added efficiency PAE (PAE: Power Added Efficiency). The PAE indicates how much of the consumed power is actually available for the application and how much is dissipated into (thermal) losses. High efficiency is a key issue in view of environment (CO2 emission) as well as system performance.
The combination of high power, high efficiency and high operating frequency is a problem for most common semiconductor technologies, which is due to physical constraints. In this regard, gallium nitride (GaN) outperforms most of its competitors as it offers both high breakdown electric fields and high electron mobility. This makes it the ideal choice for microwave power amplifiers. It allows realizing PAs with previously unattainable values for output power and PAE.
Various radar and satellite systems operate in the X-band, the frequency range from 8 to 12 GHz. In this frequency range, amplifiers are commonly built as monolithic circuits (MMICs, Monolithic Microwave Integrated Circuits), because through monolithic realization critical tolerances and parasitic properties can be reduced. A GaN MMIC process is available at FBH which also serves this purpose. Recently, the performance of this process has been further improved by reducing the gate length of the GaN transistors to 0.25 μm. Devices achieve efficiencies of 50% and more at 10 GHz in deep-AB operation.
Fig. 1 presents a recently designed power amplifier MMIC realized by using this process. Because the GaN semiconductor layers are grown on silicon carbide (SiC), the substrate is transparent and the metal structure of the circuit seems to float. In order to achieve high gain, a two-stage design is employed. As can be seen from the measurement data in Fig. 2, this circuit reaches a maximum output power Pmax of 11 W at 10 GHz. The maximum linear gain is approximately 25 dB and the efficiency of the final stage (PAE) almost 40%. These values can be enhanced further by appropriate circuit optimization. Work on this is ongoing.
Erhan Ersoy, Chafik Meliani, Serguei Chevtchenko, Paul Kurpas, Mathias Matalla, and Wolfgang Heinrich, "fbh-berlin/english/ver12/pub20.htm _blank internal-url-new-window>A High-Gain X-Band GaN-MMIC Power Amplifier", presented at 7th German Microwave Conference (GeMiC), Ilmenau, Germany, on 12-14 March 2012.