III-Nitride field-emission vacuum transistors
Field-emission transistors featuring III-Nitrides unlock the door to
robust, compact vacuum-electronics-based circuits that deliver high
powers at high frequencies.
BY PAO-CHUAN SHIH AND TOMÁS PALACIOS FROM MIT
Vacuum based electronics have an incredibly rich and successful history. At the beginning of the twentieth century they enabled the first electric switch, based on a vacuum triode. Vacuum tubes also lay at the heart of the first programmable general-purpose electronic computer, known as ENIAC, which was built in the 1940s. Its construction revolutionised calculation speed, being about a thousand times faster than the electro-mechanical machines of the day. However, this trailblazing computer had a substantial footprint and lacked green credentials, being 2 m by 1 m by 30 m in size, occupying 170 m2, and consuming 150 kW of electricity.
Since the invention of solid-state transistors in 1947, semiconductor devices have gradually replaced vacuum electronics, due to lower production costs and ease of scaling. Due to this, vacuum electronics are now only used in few niche applications. Where they are still having an impact is in travelling wave tubes and klystrons, which produce intense high-frequency signals at 10-100 GHz and above 100 GHz, respectively, thanks to scattering-free electron transportation and a high breakdown field in the channel (see Figure 1). Additional merits of vacuum electronics are robustness at high temperatures and a capability to withstand radiation environments that degrade solid-state devices.
Figure 1. Electron transport in (a) semiconductor and (b) vacuum channels. Scattering-free transportation and a high breakdown field in vacuum make vacuum channels excellent candidates for high-power and high-frequency electronics. Theoretically, degenerately n-doped III-Nitrides, such as AlGaN alloys, can have a low electron emission barrier thanks to their low electron affinities, making them excellent vacuum emitters.
Key to leveraging the excellent potential of the vacuum channel is the injection of electrons into vacuum. A conventional approach for realising one form of electron emission, known as thermionic emission, is to heat the cathodes so that they give the electrons enough energy to overcome the work function barrier between the solid and vacuum. However, this requires high temperatures, hampering the construction of compact devices. Additional factors limiting the deployment of thermionic cathodes in large-scale circuits and systems are high power consumption, a need for cooling, and a relatively low switch speed.
Finding favour with field emission
Fortunately, all these issues can be addressed by turning to field emission. With this approach, electric fields control the tunnelling distance for the emission of electrons into vacuum. A number of semiconductors and metals have already been investigated as field emitters, but issues have often arisen, such as a limited gate control efficiency, less-than-ideal current densities, and device instability.
One class of material that promises to avoid these pitfalls is the III-Nitrides, such as GaN, AlGaN, and AlN. Recently, these compounds have been attracting attention due to their engineerable electron affinities (see Figure 1). Electron affinities reduce when the aluminium composition of the AlGaN alloy increases, and when the surface polarisation is changed from metal polar to N polar. According to theory, degenerately n-doped semiconductors with low electron affinities should have very low work functions, leading to a high emission current density and a low operating voltage.
Despite the great potential that III-Nitrides have for field-emission applications, research efforts have focused on ‘bottom-up’ approaches, growing these materials as nanostructures. Reports are often limited to two-terminal geometries with operating voltages usually larger than a few hundred volts. The lack of a third control terminal is a significant drawback, limiting their use in applications such as power amplifiers, high-voltage switches, and computation circuits for harsh environments.