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High-Q AlGaN/GaN Varacators for Mobile Communication Systems

: Amirpour, Raul
: Ambacher, Oliver; Reindl, Leonhard; Manoli, Y.

Freiburg/Brsg., 2020, XIX, 147 S.
Freiburg/Brsg., Univ., Diss., 2020
Fraunhofer IAF ()

In recent years, adaptive circuits are investigated to tackle the increasing complexity of modern mobile communication systems, due to new standards like LTE and 5G. The crucial element of an adaptive circuit, like a tunable filter, is the tunable component. However, available tunable capacitors – or varactors – are not able to fulfill the stringent requirements of modern mobile communication systems concerning quality factor, tuning ratio, frequency, and linearity. Since the aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterojunction is used successfully for power amplifiers that need to handle high power at high frequencies, AlGaN/GaN high electron-mobility varactors are investigated recently. In this work, linear high-Q AlGaN/GaN varactors are developed based on the Fraunhofer IAF technology. This device is proposed as a powerful alternative to existing varactor concepts, as it displays superior performance especially at frequencies exceeding 2 GHz. To understand the device better and to analyze the influence of individual modifications, a new accurate device model is developed. To distinguish between intrinsic and extrinsic effects, first, a bias-independent scalable parasitic shell is derived. This parasitic shell consists of ideal lumped elements. Then, an analytic large-signal model for the intrinsic core is developed. For the first time, this model takes forward conduction, breakdown, and dispersion into account. DC and S-parameter measurements with a large range of biasing points are used to extract and verify the model. Measurements on multiple device geometries validate the scaling of the parasitic shell. In addition, two-tone measurements verify the large-signal behavior of the model and show a high device linearity. The high electron-mobility varactor is developed based on a standard high electron-mobility transistor. By modifying the device parameters, the AlGaN/GaN varactor is improved and a tuning ratio of five in combination with a quality factor QVar >= 150 is presented for the first time. Especially the gate cross section and the barrier are employed to set the tuning ratio and to reduce losses and dispersion. To increase the linearity, a novel anti-series varactor is presented. Additionally, this device reduces the footprint size by a factor of five, which leads to less parasitic effects and to higher maximum operating frequencies. Two-tone measurements verify a reduction of second-order intermodulation distortion by at least 20 dB in comparison to a single device.
High electron-mobility varactors reach the best linearity and lowest losses if operated in an on-state or off-state. To increase the number of possible capacitance values, multiple anti-series devices are combined in parallel and additionally in series for the first time. Furthermore, the series combination increases the power handling capabilities into the range of watt level applications. It is shown by measurements and simulations that the AlGaN/GaN varactors developed in this work, especially the cascaded anti-series devices, outperform the state-of-the-art by a large amount. A GaN on silicon technology enables large-volume production due to large diameter wafers and allows hetero integration with silicon CMOS. Therefore, it reduces the cost significantly and enables mass-market applications like filters in mobile handsets. The analysis of this technology shows increased substrate losses and dispersion compared to the silicon carbide substrate. Nevertheless, the influence on single varactors is low. A comparison of the first GaN on silicon varactor to GaN on silicon carbide devices shows a similar performance if the increased losses of the ohmic contacts are neglected.
To proof the usability and performance of the high electron-mobility varactor and to verify the large-signal model, tunable circuits are analyzed. Microstrip resonators, which are a simple type of a tunable filter, are used to verify the quality factor and the device model. Then, a power amplifier with varactor based dynamic load modulation at 3.5 GHz is demonstrated for the first time. The varactor developed in this work enables load tuning at much higher frequencies than it was done before. With this, the back-off efficiency is improved by 13%. Finally, a monolithically integrated phase shifter, operating at Ka-band frequencies from 27 GHz to 35 GHz, is presented. A relative phase shift of up to 42° is obtained. It demonstrates the usability of the AlGaN/GaN high electron-mobility varactor in integrated circuits for millimeter-wave 5G applications.