Zinc Oxide: From Fundamental Properties Towards Novel Applications

After the invention of semiconductor-based recti?ers and diodes in the ?rst half of the last century, the advent of the transistor paved the way for semiconductors in electronic data handling starting around the mid of the last century. The transistors widely replaced the vacuum tubes, which had even been used in the ?rst generation of computers, the Z3 developed by Konrad Zuse in the 1940s of the last century. The ?rst transistors were individually housed semiconductor devices, which had to be soldered into the electric circuits. Later on, integrated circuits were developed with increasing numbers of individual elements per square inch. The materials changed from, e. g. , PbS and Se in rf-detectors and recti?ers used frequentlyin the ?rst halfof the last centuryoverthe groupIV element semicond- tor Ge with a band gap of 0. 7eV at room temperature to Si with a value of 1. 1eV. The increase of the gap reduced the leakage current and its temperature dependence signi?cantly. Therefore, the logical step was to try GaAs with a band gap of 1. 4eV next. However, the technology of this semiconductor from the group of III¿V c- poundsprovedto be muchmoredif?cult,thoughbeautifuldeviceconceptshadbeen developed. Therefore,GaAsanditsalloysandnanostructureswithotherIII¿Vc- poundslike AlGaAs or InP remained restricted in electronicsto special applications like transistors for extremely high frequencies, the so-called high electron mobility transistors (HEMT). The IT industry is still mainly based on Si and will remain so in the foreseeable nearer future.