An ion transistor is a field-effect transistor (FET) in which the current between the source and drain electrodes is modulated by an applied voltage to an electrolyte-gated gate electrode. The electrolyte may be either a liquid or a solid, and the ions may be either cations or anions. Ion transistors are one type of electrochemical transistor.
Ion transistors were first demonstrated using mercury as the electrolyte and cesium as the ionic species in 1967 by Bockris, et al.. However, it was not until 1974 that the first reports of solid-state ion transistors appeared in the literature. In these early devices, the gate electrode was typically made of platinum, and both n-type and p-type semiconductor materials were used for the source and drain electrodes. The first practical application of ion transistors was reported by Nakano et al. in 1977, who used them as sensors for detecting trace amounts of organic compounds in air.
Since then, much progress has been made in understanding the underlying physics of ion transport through solid state interfaces, and in developing new materials and device architectures for improved performance. For example, it has been shown that by using high dielectric constant gate insulators such as hafnium oxide (HfO2), it is possible to achieve very high levels of control over the channel doping density without resorting to extreme measures such as implantation or lithography. This has led to a new generation of so-called “metal–oxide–semiconductor field effect transistor” (MOSFET) devices with extremely small feature sizes (<10 nm) and very low power consumption (<1 mW/cm2). These devices are finding applications in areas such as biomedical sensing, where they can be used to measure minute concentrations of bioactive molecules; energy storage, where they can be employed as ultrafast switches for controlling discharge from capacitors;and computing, where their low power consumption makes them attractive for use in portable electronic devices such as laptops and cell phones. The basic operation of an ion transistor is illustrated schematically in Figure 1. When a positive voltage Vapp is applied to the gate electrode with respect to both source (S)