CHARACTERIZATION AND MODELING OF LOW FREQUENCY DISPERSIVE EFFECTS IN III-V ELECTRON DEVICES

In this thesis, three years are enclosed of research activity in the topic of non linear characterization and modelling of microwave devices. I investigated various issues related to those topics which are closely related. In fact, to obtain good predictions, empirical models require accurate measurements. This aspect is particularly important when we want to predict the behavior of devices in nonlinear regime. More and more applications take advantage of devices operation in non-linear regime. For such a reason, non linear characterization is an hot topic and research activities have focused particular attention on the need of characterize the nonlinear behaviour of electron devices to obtain more accurate model prediction under actual operating conditions. The importance of this theme can be clearly understood by considering how, in recent years, microwave technologies have become attractive for communication applications and a number of commercial devices which are largely used in everyday life (e.g., cell phone, GPS, wireless communication and so on). In the first chapter the most important properties of devices and technologies used in microwave electronics circuits will be dealt with. Particular attention is devoted to the comparison between two III-V semiconductors for the fabrication of these devices: GaAs, proven technology and used for years, and GaN, a youngest technology still being tested. After this some of the most interesting issues related to III-V electron devices, are discussed, such as low frequency dispersion. Finally a brief look will be given at the non linear models for these devices. In the second Chapter the most important microwave measurement systems exploited to characterize the non linear dynamic behaviour of electron devices will be discussed: pulsed setups, load / source-pull measurement systems, and Large Signal time domain characterization systems. In particular, for each measurement technique, it has been described the principle of operation and the application they are used for. In chapter III an alternative, technology-independent large-signal measurement setup, developed during the PhD studies, is proposed for the experimental investigation on the low frequency dispersion of current/voltage characteristics in micro- and millimetre-wave electron devices and for their modeling. The proposed measurement technique will be presented describing its hardware and software implementations and showing different experimental examples. In Chapter IV a new modeling approach will be presented accounting for the nonlinear description of low-frequency dispersive effects (due to thermal phenomena and traps) affecting electron devices. The model will be identified by exploiting measurements carried out with the measurement system described in chapter III. In the last Chapter a new, low-cost technique will be described for drawing “load-pull contours” which are a powerful tool for power amplifier design. By exploiting the lowfrequency measurement system described in chapter III and conventional descriptions of device parasitic elements and nonlinear reactive effects, the proposed approach allows to obtain the same information gathered by expensive highfrequency load pull measurement systems.