Researchers in France, Britain and Japan have improved the channel mobility performance of diamond MOSFETs by doping with boron

Silicon offers tremendous benefits to the power electronics industry. However, the performance of silicon-based power electronics is approaching its limit, and the current power electronics industry has entered the era of wide bandgap (WBG) semiconductors. Wide bandgap semiconductor devices are more energy-efficient and have become a major competitor for field-effect transistors (FETs) in next-generation power electronics. This FET technology will be applied to a variety of renewable energy grids, enabling the benefits of renewable-powered car and train engines, among other things.

Researchers in France, Britain and Japan have improved the channel mobility performance of diamond MOSFETs by doping with boron, which can also be used for other wide bandgap semiconductor materials

Silicon offers tremendous benefits to the power electronics industry. However, the performance of silicon-based power electronics is approaching its limit, and the current power electronics industry has entered the era of wide bandgap (WBG) semiconductors. Wide bandgap semiconductor devices are more energy-efficient and have become a major competitor for field-effect transistors (FETs) in next-generation power electronics. This FET technology will be applied to a variety of renewable energy grids, enabling the benefits of renewable-powered car and train engines, among other things.

Left: Light microscope images of MOSCAP and diamond deep depletion MOSFETs (D2MOSFET). Top right: Scanning electron microscope image of a diamond D2MOSFET. S: Source, G: Gate, D: Drain. Bottom right: Concept art of D2MOSFET. The high mobility channel is a boron-doped diamond epitaxial layer.

Low channel mobility is a challenge

Diamond is widely regarded as the most ideal WBG material due to its excellent physical properties, the ability of diamond devices to operate at higher temperatures, voltages and frequencies with less losses. However, the biggest challenge in the implementation of MOSFETs is the ability to improve carrier mobility in the hole channel. This mobility is related to the current flow and is critical for the MOSFET to turn on the current.

New method of boron doping

Researchers from France, the United Kingdom, and Japan have solved this problem using a deep depletion approach of body-doped boron-doped diamond MOSFETs. The new method demonstrates the ability to fabricate simple diamond MOSFET structures using a single boron-doped epitaxial layer stack. This new approach to WBG materials is capable of increasing channel mobility by an order of magnitude. The research results have been published in the Journal of Applied Physics.

In a typical MOSFET structure, the top of the semiconductor material is covered with an oxide layer, followed by a metal gate, except that in this study the semiconductor material is diamond. By applying a voltage to the metal gate, the carrier density and conductivity in the diamond channel below the gate change significantly. The ability to use this electrical “field effect” to control channel conductivity and switch MOSFETs from on-(on-state) to high-insulation (off-state) has led to a wide range of applications in the field of power control. It has been shown that many diamond MOSFETs rely on hydrogen terminations on the diamond surface to transfer a positively charged carrier fluid, called a hole, into the channel. The operation of the oxygen-bonded diamond MOS structure has recently been demonstrated, similar to the common mode of operation of silicon MOSFETs. The on-current of a MOSFET is strongly dependent on the channel mobility, and in many MOSFET designs, the mobility is very sensitive to roughness and defect states at the diamond interface, which can easily cause carrier scattering.

To solve this problem, the researchers explored different modes of operation and established MOSFETs, in which a layer of alumina (Al2O3) was deposited on a thick diamond epitaxial layer at the oxygen terminal at 380°C. Holes are created by the incorporation of boron atoms into the diamond layer. Boron has one less valence electron than carbon, so there will be one electron missing in the bond, and it acts like adding a positive charge or hole. The epitaxial layer functions as a thick conductive cavity channel. Repels and depletes holes by applying voltage – creating a deep depletion region that switches the transistor from the ON state to the OFF state. In silicon-based transistors, this voltage will cause the formation of an inverted layer, and the transistor will not disconnect. The researchers were able to demonstrate that diamond has unique properties, particularly a large bandgap that inhibits the formation of an inverted layer, allowing the transistor to operate in a deeply depleted state.

Research implications

Julien Pernot, a researcher at the NEEL Institute in France and author of this article, said: “We have fabricated a transistor in which the conduction of the transistor is ensured by bulk channel conduction of a boron-doped diamond epitaxial layer. The shutdown state of the transistor is guaranteed by a thick insulation layer caused by the deep depletion region. Our research results pave the way for the use of diamond in MOSFETs. ”

Pernot adds: “This principle also applies to wide bandgap semiconductor materials of other materials. Boron is suitable for use with diamond materials, and other materials can be selected to create a stable deep depletion region for other wide bandgap materials. ”

Future outlook

The researchers plan to fabricate MOSFETs for these structures through their DiamFab startup.

bibliography

　　T. T. Pham, N. Rouger, C. Masante, G. Chicot, F. Udrea, D. Eon, E. Gheeraert, J. Pernot. Deep depletion concept for diamond MOSFET. Applied Physics Letters, 2017; 111 (17): 173503 DOI: 10.1063/1.4997975