The 2D metal contacts stop the transistor leakage current in its tracks

July 10, 2023

(Nanowerk News) Using two-dimensional materials to electrically connect to high-power semiconductor transistors improves device performance.

Computers, despite all their apparent complexity, are basically just a bunch of electronic switches, which turn on and off in the correct order to process digital information. Semiconductor technology makes these switches extremely small and very fast.

The semiconductor material gallium nitride promises to make it even faster. This is because the charge carriers in gallium nitride, such as electrons, can move through the material at high speeds. This makes GaN useful in so-called high electron mobility transistors, or HEMTs, for high-frequency and high-power applications, including cellphone chargers, 5G base stations, radar and satellite communications.

An important aspect of optimizing HEMT operation is making the electrical connections that turn the transistors on or off. These so-called Schottky gates can experience high leakage currents flowing even when the transistor is in the off state. This results in high power consumption and limits the voltage that can be applied before the device is damaged.

Chuanju Wang of Xiaohang Li’s team and Xiangming Xu of Husam Alshareef’s team, together with their co-workers and colleagues from India and China, have shown that this limitation can be minimized by fabricating Schottky gates from a class of materials known as MXenes: two-atom thin layer metallic dimensions of a transition metal carbide, nitride or carbonitride (Advanced Materials, “Ti3C2Tx MXene van der Waals gate contacts for GaN high electron mobility transistors”).

While conventional metals are the traditional choice for electrical contact to GaN, the chemical interaction between the two materials creates defects that can trap electrical charges and significantly limit gate control. “Traditional metal gate contact materials have been deposited using methods such as electron beam evaporation and sputtering, which have a direct chemical bond with the semiconductor substrate,” explained Wang.

“We show that our two-dimensional MXene makes so-called van der Waals contacts with a semiconductor substrate, which can significantly reduce interfacial entrapment and charge retention,” he says.

The KAUST team fabricated GaN HEMT with gate contacts made from an ultraclean MXene Ti film3C2QX. Their device shows an off-state current of only 10−7 milliamps per millimeter, which is roughly 1013 smaller than the current when the HEMT is “on”. This on-off ratio is a six notch improvement over devices with more conventional nickel-gold contacts.

“The next step is to use MXenes as the Schottky-gate contact material in other types of transistors, such as Ga2HI3In the2HI3NiO and AlN,” said Wang.

Alshareef said he and Professor Li were very proud of Chuanju and Xiangming for their creativity and hard work; “they deserve a lot of credit for the success of this project.”

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