While diamond is believed to be the most ideal wide band gap material, one of the challenges in creating diamond MOSFETs is increasing the hole channel carrier mobility. However, researchers from France, the UK and Japan believe they may have solved this problem using the deep-depletion regime of bulk-boron-doped diamond MOSFETs. The proof of concept enables simple diamond MOSFET structures to be produced from single boron-doped epilayer stacks. Using this approach, mobility is said to be increased by an order of magnitude.
According to the team, many of the diamond MOSFETs demonstrated to date have relied on a hydrogen-terminated diamond surface to transfer positively charged carriers – holes – into the channel. More recently, operation of oxygen terminated diamond MOS structures in an inversion regime, similar to the common mode of operation of silicon MOSFETS, has been demonstrated.
To build their MOSFET, the researchers deposited a layer of aluminum oxide at 380°C over an oxygen-terminated thick diamond epitaxial layer. Holes were created in the diamond layer by incorporating boron atoms into the layer; boron has one less valence electron than carbon, which acts like the addition of a hole. The bulk epilayer functioned as a thick conducting hole channel. The transistor was switched from the on- to the off-state by application of a voltage which repelled and depleted the holes – the deep depletion region. In silicon-based transistors, this voltage would have formed an inversion layer and the transistor would not have turned off.
“We fabricated a transistor in which the on-state is ensured by the bulk channel conduction through the boron-doped diamond epilayer,” said Julien Pernot, a researcher at the NEEL Institute in France. “The off-state is ensured by the thick insulating layer induced by the deep-depletion regime. Our proof of concept paves the way in fully exploiting the potential of diamond for MOSFET applications.”
The researchers plan to produce these structures through their new startup called DiamFab.
Collaborating in the project were researchers from Universite Grenoble Alpes, CNRS Institut NEEL, CNRS G2Elab, Université de Toulouse, University of Cambridge, University of Tsukuba and Institut Universitaire de France.