The newly developed logic circuits are equipped with diamond-based MOSFETs (metal-oxide-semiconductor field-effect-transistors), and they have two different operation modes. The different modes are a depletion mode (D mode) and an enhancement mode (E mode). The longer-term aim is to construct diamond integrated circuits that will operate under extreme environmental conditions. The development has been reported by the National Institute for Materials Science and it is the idea of two independent research scientists: Jiangwei Liu and Yasuo Koide.
Critical to the ability to work in extreme environments are the diamonds. Diamonds possess special properties; in this case those of importance are: high carrier mobility, a high breakdown electric field and high thermal conductivity. This means diamond based materials can be used in the development of current switches and integrated circuits. These are necessary to operate stably under conditions of high-temperature, high-frequency, and high-power.
Logic gates are implemented using diodes or transistors acting as electronic switches; most gates are made from field-effect transistors (FETs), particularly metal–oxide–semiconductor field-effect transistors (MOSFETs). Many electronic circuits have to make decisions. They look at two or more inputs and use these to determine the outputs from the circuit. The process of doing this uses electronic logic, which is based on digital switches called gates.
Up until now it has proved too challenging to enable diamond-based MOSFETs to control the polarity of the threshold voltage. To address this, the two researchers produced a logic circuit equipped with both D- and E-mode diamond MOSFETs. This was achieved by fabricating them on the same substrate by using a specially developed threshold control technique. This control technique came about after the researchers pinpointed the electronic structure in the interface between various oxides and hydrogenated diamond by using the method photoelectron spectroscopy. Photoelectron spectroscopy refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect; the method involves surface analysis by measuring ejected electrons.
From this, a diamond metal-oxide-semiconductor capacitor with very low leakage current density was fashioned. Tests on the logic circuits with diamond-based transistors act as promising devices that can be used for the development of digital integrated circuits under high-temperature conditions. The circuits should also be able to withstand high exposure to radiation and cosmic rays, leading to another application with deep space missions.
The research, which will pave the way for new technologies for operating computers in extreme environments, is published in the journal IEEE Electron Device Letters. The research paper is headed “Logic Circuits With Hydrogenated Diamond Field-Effect Transistors.”
