Millimeter-wave load-pull techniques

In this talk, the challenges in the realization of on-wafer large-signal measurement systems in W-band will be summarized and a recent implementation of a W-band on-wafer load-pull system will be presented. The availability of W-band large signal set-ups opens several possibilities, ranging from technology assessment and large-signal device modeling at sub-THz frequencies, to W-band MMIC design and characterization. Direct and accurate load-pull measurements at W-band are thus crucial in the development of sub-THz integrated circuits. The main issue at these frequencies is the realization of highly reflective loads at the on-wafer reference planes. In-situ tuners can solve this problem, but they need to be integrated with the device under test and this is not always possible. Passive mechanical tuners could be a solution, but the losses between tuner and probe tip need to be compensated in some way, otherwise the reachable reflection coefficients will present severe magnitude limitations. An active load clearly overcomes this limit and in addition, with an active load, measurement directional couplers can be placed in real-time configuration and achieve higher accuracy than purely passive systems. Active loads can be implemented in open- or closed-loop configuration. The realized system is the first load-pull system to implement a 94 GHz load by means of an active loop exploiting frequency conversion techniques. This has advantages in cost and load stability. The system is based on a HP8510 vector network analyzer. Its performances in terms of reachable load, load stability, speed and residual uncertainty will be shown and compared to a 40 GHz load-pull system based on a PNA-X. Finally, the first measurements performed on high performance InP double heterojunction bipolar transistors (DHBTs) and GaN high electron mobility transistors (HEMTs) will be presented.