Scientists at the Fraunhofer Institute for Applied Solid State Physics (IAF) have developed a novel transistor with an extremely high cutoff frequency. Named a metal oxide semiconductor HEMT (MOSHEMT), the device uses an oxide instead of the Schottky barrier of a conventional HEMT, yielding a transistor that has already demonstrated a record oscillation frequency of 640 GHz.
The high frequency characteristics of high electron mobility transistors (HEMT) have steadily improved as the gate length has been reduced to 20 nm. However, the current transistor geometry of a conventional HEMT has reached its scaling limit. At such small sizes, HEMTs encounter a problem: the thinner the barrier material of InAlAs, the more electrons leak from the channel through the gate. These gate leakage currents hurt the efficiency and durability of the transistor, making further downscaling infeasible.
The secret is oxide
Faced with similar challenges, silicon MOSFETs have an oxide layer that prevents unwanted leakage currents more effectively than the traditional HEMT structure. So researchers at Fraunhofer IAF combined the advantages of III-V semiconductors with silicon MOSFETs, replacing the Schottky barrier of the HEMT with an isolating oxide layer. The resulting MOSHEMT is a new structure.
“We have developed a new device which has the potential to exceed the efficiency of current HEMTs by far. The MOSHEMT allows us to downscale it even further, thus making it faster and more efficient,” said Arnulf Leuther, a researcher in the field of high frequency electronics at Fraunhofer IAF.
With the new transistor technology, Leuther and his team have succeeded in achieving a record maximum oscillation frequency of 640 GHz.
“This surpasses the global state of the art for any MOSFET technology, including silicon MOSFETs," said Leuther.
High barrier to leakage
To overcome gate leakage, the scientists used a material with a significantly higher barrier than a conventional Schottky, replacing the semiconductor material with a combination of isolating layers of Al2O3 and HfO2.
“This enables us to reduce the gate leakage current by a factor of more than 1,000. Our first MOSHEMTs show a very high development potential, while current field effect transistor technologies have already reached their limit,” said Axel Tessmann, a scientist at Fraunhofer IAF.
Future MOSHEMT ICs
The extremely fast MOSHEMT is designed for applications above 100 GHz, such as novel communication systems, radars and sensors. Future high power devices will enable faster data transmission across radio links, imaging radar systems for autonomous driving and higher resolution, precision sensor systems.
While it may be some years before the MOSHEMT is widely adopted, the researchers at Fraunhofer IAF have already demonstrated what they say is the first amplifier MMIC using InGaAs MOSHEMTs, operating between 200 and 300 GHz.