A nanoscale vacuum-channel transistor (NVCT) is a

transistor A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

in which the electron transport medium is a

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

, much like a

vacuum tube A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...

. In a traditional solid-state transistor, a

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

channel exists between the source and the drain, and the current flows through the semiconductor. However, in a nanoscale vacuum-channel transistor, no material exists between the source and the drain, and therefore, the current flows through the vacuum. Theoretically, a vacuum-channel transistor is expected to operate faster than a traditional solid-state transistor, and have higher power output and lower operation voltage. Moreover, vacuum-channel transistors are expected to operate at higher temperature and radiation level than a traditional transistor making them suitable for space application. The development of vacuum-channel transistors is still at a very early research stage, and there are only limited study in recent literature such as vertical field-emitter vacuum-channel transistor, gate-insulated planar electrodes vacuum-channel transistor, vertical vacuum-channel transistor, and all-around gate vacuum-channel transistor.

History

The concept of using conventional field-emitted electron beam in a diode was first mentioned in a 1961 article by Kenneth Shoulders. However, due to the technological difficulty of fabricating a field-emitter electron source, such a diode was not implemented. As the field of microfabrication advanced, it became possible to fabricate field-emitted electron sources, thereby paving the way for vacuum-channel transistors. The first successful implementation was reported by Gary et al. in 1986. However, early vacuum-channel transistors suffered from high gate

threshold voltage The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important s ...

and couldn't compete with solid-state transistors. More recent advances in microfabrication have allowed the vacuum-channel length between the source and the drain to be shrunk, thereby significantly reducing the gate threshold voltage below 0.5V, which is comparable to the gate threshold voltage of current solid-state transistors. As the shrinking of solid-state transistors is reaching its theoretical limit, vacuum-channel transistors may offer an alternative.

Simplified operation

A nanoscale vacuum-channel transistor is essentially a miniaturized version of a

. It consists of a field-emitter electron source, a collector electrode, and a gate electrode. The electron source and the collector electrodes are separated by a small distance, usually of the order of several nanometers. When a voltage is applied across the source and the collector electrode, due to field-emission, electrons are emitted from the source electrode, travel through the gap and are collected by the collector electrode. A gate electrode is used to control the current flow through the vacuum-channel. Despite the name, vacuum-channel transistors do not need to be evacuated. The gap traversed by the electrons is so small that collisions with molecules of gas at atmospheric pressure are infrequent enough not to matter.

Advantages

The nanoscale vacuum-channel transistors have several benefits over traditional solid-state transistors such as high speed, high output power, and operation at high temperature and immunity to strong radiations. The advantages of a vacuum-channel transistor over a solid-state transistor are discussed in detail below:

High speed

In a solid-state transistor, the electrons collide with the semiconductor lattice and suffer from scattering which slows down the speed of the electrons. In fact, in silicon, the velocity of electrons is limited to 1.4×10⁷ cm/s. However, in vacuum electrons do not suffer from scattering and can reach velocities approaching the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

(3×10¹⁰ cm/s). Therefore, a vacuum-channel transistor can operate at a faster speed than a silicon solid-state transistor.

Operation at high temperature

The

band-gap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to the ...

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

is 1.11eV, and the

thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...

of electrons should remain lower than this value for silicon to retain its semiconductor properties. This places a limit on the operating temperature of silicon transistors. However, no such limitation exists in vacuum. Therefore, a vacuum-channel transistor can operate at a much higher temperature, only limited by the melting temperature of the materials used for its fabrication. The vacuum-transistor can be used in applications where a tolerance to high temperature is required.

Immunity to radiation

The radiation can ionize the atoms in a solid-state transistor. These ionized atoms and corresponding electrons can interfere with the electron transport between the source and collector. However, no ionization occur in the vacuum-channel transistors. Therefore, a vacuum-channel transistor can be used in a high radiation environment such as outer space or inside a nuclear reactor.

Disadvantage

The performance of a vacuum-channel transistor depends upon the field emission of electrons from the source electrode. However, due to the high electric field, the source electrodes degrades over time, thereby decreasing the emission current. Due to the degradation of electrons source electrode, vacuum-channel transistors suffer from poor reliability.