Contents I. Introduction
As well-known, GaAs HEMT has demonstrated to be a mature and well-established technology, which plays a central role in microwave and millimeter-wave applications requiring very low noise performance (e.g., front-ends). The physical properties of III-V compound semiconductors as well as the typical layer structure make these devices extremely attractive for integrated communication systems, where transistors are optically controlled to perform various functions of microwave electronic circuits (e.g., gain control of amplifiers, frequency tuning or locking in oscillators, and phase shifting in phase shifters). The investigation of the impact of the light exposure on the HEMT behavior is therefore mandatory. To accomplish this task, many studies have been conducted over the years [1]–[14]. Different results have been reported as the light sensitivity may critically depend on the specific case under consideration (e.g., device physics and structure, bias point, frequency range, optical power, and radiation wavelength). Nevertheless, the attention has been mainly focused on the analysis of the dc characteristics and scattering (S-) parameters [1]–[12], while so far only a limited amount of results have been reported on the noise (N-) parameters [13], [14]. In [13] it has been observed a reduction of the minimum noise figure close to the pinch-off, whereas only a slight increase at higher gate voltage. On the other hand, in [14] the minimum noise figure has not been affected at all by the illumination.
