4048 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 64, NO. 11, NOVEMBER 2016 Nonintrusive Near-Field Characterization of Spatially Distributed Effects in Large-Periphery High-Power GaN HEMTs Rui Hou, Student Member, IEEE, Martino Lorenzini, Member, IEEE, Marco Spirito, Member, IEEE, Thomas Roedle, Fred van Rijs, and Leo C. N. de Vreede, Senior Member, IEEE Abstract—This paper introduces an improved nonintrusive near-field technique for in situ characterization of distributed effects in GaN high-power transistors. Compared with previous passive probing approaches which sense electric fields induced by drain bondwires, the proposed method employs the position- signal difference method to measure the E-field fluctuations induced by transistor fingers. This allows a robust and detailed identification of in-circuit electrical quantities, such as voltages, currents, loading impedance, and output power, spatially dis- tributed over individual transistor cells and fingers. The E-fields needed for determining the distributed phenomena have been measured in situ above the fingers of a 100-W GaN power tran- sistor at fundamental and second-harmonic frequencies, while the device operates under realistic loading conditions. The deduced in-circuit quantities are compared with their counterparts from an independently developed distributed in-house model of the same device for validation. The practical value of the proposed method is further demonstrated by uniquely identifying device damage at the finger level (enforced by laser cutting). Index Terms— Device characterization, gallium nitride (GaN), high electron-mobility transistor (HEMT), near-field measurement. I. I NTRODUCTION P ERSISTENT growth of wireless data traffic relentlessly pushes communication networks toward higher frequencies and signal bandwidths, while still requiring high efficiency and output power. The efficiency and output power demands are the highest for the infrastructural transmitters, or more specifically, on their final stage radio frequency (RF) power amplifiers (PAs). Although conventionally powered by laterally diffused-metal–oxide– semiconductor (LDMOS) transistors, these demanding Manuscript received May 20, 2016; revised August 9, 2016 and September 17, 2016; accepted September 19, 2016. Date of publication October 19, 2016; date of current version November 3, 2016. This work was supported by the former RF Power Division of NXP Semiconductors, now Ampleon Netherlands. R. Hou is with Ericsson, Stockholm, Sweden (e-mail: rui.hou@ericsson.com). M. Spirito and L. C. N. de Vreede are with the Electronics Research Laboratory, Delft University of Technology, 2628 CD Delft, The Netherlands (e-mail: m.spirito@tudelft.nl; l.c.n.devreede@tudelft.nl). M. Lorenzini, T. Roedle, and F. van Rijs are with Ampleon, 6534 AV Nijmegen, The Netherlands (e-mail: martino.lorenzini@ ampleon.com; thomas.roedle@ampleon.com; fred.van.rijs@ampleon.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2016.2613525 Fig. 1. DUT: packaged Ampleon 100-W RF power GaN HEMT with its ceramic cap removed. The transistor die consists of ten identical cells in parallel to be characterized for their distributed effects. applications are increasingly adopting gallium nitride (GaN) high electron-mobility transistors (HEMTs) to profit from their high power density, thermal conductivity, and efficiency potential [1]. To meet the demand for high power amplification, GaN transistors, such as their LDMOS counterparts, are composed out of a large number of identical transistor cells placed in parallel (as shown in Fig. 1), yielding a large periphery trans- verse to the power propagation direction. When this aggregated periphery becomes thermally and electrically large, identical transistor cells no longer operate under equal (electrical and thermal) conditions. For example, cells at the center of a transistor die experience higher temperature compared with the cells at the edges of this die, due to the finite thermal conductance of the substrate [2]. Furthermore, center cells tend to experience more inductive loading than the outer ones, due to the magnetic mutual coupling of the parallel bondwires at the drain side [3]. Consequently, the distribution of voltage, current, and power across the transistor cells is not uniform [2]–[5]. These distributed effects may strongly affect transistor performance with respect to gain, power, and efficiency. Even worse, unequally operated cells may further load–pull each other, causing odd-mode device oscillations. 0018-9480 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.