I. Introduction
A promising step for widening the functionality and applicability of electronic devices is to develop electronics that are transparent in the visible spectral range. Transparent displays in car wind shields, cockpit windows, or visors, the implementation of augmented reality features [1] in our daily lives, and new devices that are still to be invented can be expected from this technology. Since 2003, research in the field of transparent electronics based on oxide wide band-gap semiconductors has gained large momentum [2]–[7]. Transistors and circuits constructed out of them have been realized either in -channel metal–oxide–semiconductor, PMOS, or CMOS technology [7]–[16]. All of the used transistors were based on the metal–insulator–semiconductor field-effect transistor (MISFET) technology. Recently, a promising new approach of fabricating high-quality metal–semiconductor FETs (MESFETs) using Schottky gates has been demonstrated [17]–[19]. Due to the absence of an insulating layer, only small gate voltage changes are necessary to turn the device on or off, respectively. Furthermore, using ultrathin highly rectifying AgxO gate contacts [20], [21], these MESFETs can be fabricated to be fully transparent for visible light and permit the fabrication of transparent high gain inverter circuits [22], [23]. In the following, the stability of such transparent inverter circuits at elevated temperatures and under illumination with visible light will be investigated.