Black phosphorus-based transistors

Researchers have created a high-performance transistor using black phosphorus (BP).

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 at the IBS Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea, led in part by Director Young Hee Lee, have discovered that they can change its thickness and/or the contact metals and that will determine if it is high-performance n-type, p-type or ambipolar (functions as both n- or p-type) material.

What does this mean?

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. The 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.

Why is this important?

Technology manufacturers are in an arms race to make their devices lighter, smaller and more efficient. By using BP that is only several atomic layers thick, transistors can be made smaller and more energy efficient than what exists now.

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. BP may provide a thinner, more efficient alternative to silicon chips in electrical devices.

Another example is tiny autonomous data recording and transmitting devices which will make up the Internet of Things (IoT). A major constraint from preventing IoT from taking off immediately is the inability to scale down the component size and the lack of a long-term power solution. Two dimensional layered materials (such as black phosphorus) are interesting in this aspect, since both the electrical and mechanical properties are often enhanced compared to their bulk (three dimensional) counterparts. 

Is BP a good alternative to current semiconductor materials?

It is a great material for transistors since it has a high carrier mobility (how quickly an electron can move through it). This gives BP the ability to operate at lower voltages while also increasing performance, which translates to greatly reduced power consumption.

With aluminium as a contact, thicker BP flakes (13 nanometres) 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.

Perello explained, “The driving force in back phosphorus is the carrier mobility. Everything centres 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.”

Is it ready to compete with silicon?

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 yet. Tackling the scaling problem is already underway, with chemical vapour deposition (CVD) and other thin-film growth techniques being investigated in labs across the world. The lack of a monolayer fabrication technique isn’t necessarily a problem though. “We can probably operate with 3, 5 or 7 layers and that might actually be better in terms of performance,” said SKKU research fellow David Perello.

When asked if BP was ready to compete with silicon today, Perello said, “I don’t think it can compete with silicon at the moment, that’s a dream everybody has. Silicon is cheap and plentiful and the best silicon transistors we can make have mobilities that are similar to what I was able to make in these BP devices.” 

This doesn’t mean that BP isn’t worth exploring further though. 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.”

At present, BP isn’t ready for commercial use and its potential has just started to be recognised. If it continues to perform in further tests, it should be a strong contender as a chip material for future technology.