AsianScientist (Aug. 6, 2015) – Despite its ubiquity as a chip building material, silicon may be facing some competition from a new version of an old substance: black phosphorus (BP). In a paper published in Nature Communications, researchers describe a high performance transistor made of BP, which is thinner and lighter than silicon.
Transistors are made up of materials with semiconducting properties, which come in two varieties: n-type (excess electrons) and p-type (excess holes). With the BP crystal, researchers have discovered that they can change its thickness and/or the contact metals control whether the transistor is an n-type, p-type, or ambipolar (function as both n- or p-type) material.
Silicon has to be extrinsically doped (inserting another element into its crystal structure) to make it n-type or p-type in order for it to work in a semiconductor chip. In contrast, BP crystals can operate as both n-type and p-type or something in between, but don’t require extrinsic doping. This means that instead of having to fabricate a silicon-arsenic crystal sandwiched between silicon-boron crystals, a transistor can have a single, lightweight, pure black phosphorus logic chip, no doping required.
Additionally, changing the metals used to connect the chip to the circuit has an influence on whether BP will be n- or p-type. Instead of doping to make an n- and p-type material, both n- and p-type BP can be put all together on one chip just by changing its thickness and the contact metal used.
With aluminum as a contact, thicker BP flakes (13 nanometer) show ambipolar properties similar to graphene while thin 3 nm flakes are unipolar n-type with switching on/off ratios greater than 105. The thinner they can make the material, the better the switching performance.
The BP’s high carrier mobility—a measure of how quickly electrons can move through a material—also means that BP-based transistors can operate at lower voltages while also increasing performance, which translates to greatly reduced power consumption.
“The driving force in back phosphorus is the carrier mobility. Everything centers around that. The fact that the band gap changes with thickness also gives us flexibility in circuit design. As a researcher it gives me a lot of things to play with,” explains study first author Sungkyunkwan University (SKKU) research fellow Dr. David Perello.
Silicon chips exist in all of our electronic devices, and as manufacturers make devices smaller and more energy efficient, they begin to approach the threshold for just how small components can be. By using BP that is only several atomic layers thick, transistors can be made smaller and more energy efficient than what exists now.
Unlike other industry standard semiconductor materials, there isn’t a good method for making pure BP on a large scale. Currently, thin layers can be made only from scraping bulk crystalline BP samples, as no other manufacturing method exists. The lack of a monolayer fabrication technique isn’t necessarily a problem though.
“We can probably operate with three, five, or seven layers and that might actually be better in terms of performance,” Perello said.
Although BP is still unable to compete with silicon on price and abundance, its high carrier ability makes it an attractive research material.
According to Perello, “The fact that it was so simple to make such an excellent transistor without having access to state of the art commercial growth, fabrication and lithography facilities means that we could make it significantly better. We expect the upper bound for carrier mobility in black phosphorus to be much higher than silicon.”
The article can be found at: Perello et al. (2015) High-Performance n-Type Black Phosphorus Transistors with Type Control Via Thickness and Contact-Metal Engineering.
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Source: Institute for Basic Science.
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