Halide semiconductors stand at the very beginning of semiconductor science and technology. CuI was reported as the first transparent conductor, and the first field effect transistor was made from KBr. Although halogens are frequently used in semiconductor preparation, little use is currently made from halide semiconductors in electronics and photonics. We review past reports on the metal halide semiconductor CuI and related alloys and discuss recent progress with regard to this material including its use in organic electronics and solar cells as well as our own work on fully transparent bipolar heterostructure diodes (p‐CuI/n‐ZnO) with high rectification of several 107 and ideality factors down to 1.5. γ‐CuI(111) thin film on glass (1 × 1 cm2) and IV‐characteristics of p‐CuI/n‐ZnO/a‐Al2O3 bipolar heterojunction diode.

Transparent and electrically conducting p-type copper(I)-iodide thin-films form highly rectifying p-CuI/n-ZnO diodes. Sputtered copper thin films on glass were transformed into polycrystalline γ-CuI by exposing them to iodine vapor. The electrical parameters extracted from Hall effect are p=5×1018 cm−3, μh,Hall=6 cm2/Vs, and ρ=0.2 Ωcm for hole concentration, mobility, and electrical resistivity, respectively. Heterostructures consisting of p-CuI and pulsed-laser deposited n-ZnO were fabricated on a-plane sapphire substrates. The p-CuI/n-ZnO diode exhibits a current rectification ratio of 6×106 at ±2 V and an ideality factor of η=2.14.

CuI is a p-type transparent conductive semiconductor with unique optoelectronic properties, including wide band gap (3.1 eV), high hole mobility (>40 cm2 V−1 s−1 in bulk), and large room-temperature exciton binding energy (62 meV). The difficulty in epitaxy of CuI is the main obstacle for its application in advanced solid-state electronic devices. Herein, room-temperature heteroepitaxial growth of CuI on various substrates with well-defined in-plane epitaxial relations is realized by reactive sputtering technique. In such heteroepitaxial growth the formation of rotation domains is observed and hereby systematically investigated in accordance with existing theoretical study of domain-epitaxy. The controllable epitaxy of CuI thin films allows for the combination of p-type CuI with suitable n-type semiconductors with the purpose to fabricate epitaxial thin film heterojunctions. Such heterostructures have superior properties to structures without or with weakly ordered in-plane orientation. The obtained epitaxial thin film heterojunction of p-CuI(111)/n-ZnO(00.1) exhibits a high rectification up to 2 × 109 (±2 V), a 100-fold improvement compared to diodes with disordered interfaces. Also a low saturation current density down to 5 × 10−9 Acm−2 is formed. These results prove the great potential of epitaxial CuI as a promising p-type optoelectronic material.

Cuprous iodide has been investigated since 1907 when Karl Bädeker prepared this material from metallic copper thin films with subsequent iodization and reported it as fully transparent conductor. Nowadays CuI is recognized as p‐type wide bandgap, transparent semiconductor, offering rather high hole mobilities of so far up to 10 Vs∕cm2 in thin films. The charge carrier density is primarily controlled via the amount of copper vacancies. CuI has been prepared as bulk material and substrate and thin film as well as in the form of various nanostructures. Thin films can be prepared by various techniques including iodization of copper and by thermal evaporation, sputtering or pulsed laser deposition of CuI. Recent progress is represented by the epitaxy on other semiconductors, in particular zinc oxide. CuI has found use as intermediate layer between ITO and organic absorbers in solar cells. Recently, bipolar heterostructure diodes prepared from p‐CuI∕n‐ZnO layers on sapphire were found to exhibit very high rectification. This makes CuI interesting for use in transparent electronics. For further details see the Review Article by M. Grundmann et al. on pp. http://doi.wiley.com/10.1002/pssa.201329349.

Metal-semiconductor field-effect transistors (MESFETs) are widely known from opaque high-speed GaAs or high-power SiC and GaN technology. For the emerging field of transparent electronics, only metal-insulator-semiconductor field-effect transistors (MISFETs) were considered so far. This article reviews the progress of high-performance MESFETs in oxide electronics and reflects the recent advances of this technique towards transparent MESFET circuitry. We discuss design prospects as well as limitations regarding device performance, reliability and stability. The presented ZnO-based MESFETs and inverters have superior properties compared to MISFETs, i.e., high channel mobilities and on/off-ratios, high gain, and low uncertainty level at comparatively low operating voltages. This makes them a promising approach for future low-cost transparent electronics.