MOSHEMT—innovative transistor technology reaches record frequencies

The high frequency characteristics of high eElectron mobility transistors (HEMTs) have been steadily improved in the past years. The transistors have become increasingly faster by downscaling the gate length to 20 nm. However, a HEMT encounters a problem at such small structure sizes: The thinner the barrier material of InAlAs (indium aluminum arsenide) becomes, the more electrons leak from the current carrying channel through the gate.

These unwanted gate leakage currents have a negative impact on the efficiency and durability of the transistor, which renders further downscaling attempts impossible. The current transistor geometry of a conventional HEMT has reached its scaling limit.

Silicon MOSFETs (metal oxide semiconductor field effect transistors) are no stranger to this problem, either. However, they possess an oxide layer that can prevent unwanted leakage currents for longer than it is the case with HEMTs.

Combining advantages of both transistor technologies

Researchers at Fraunhofer IAF have combined the advantages of III-V semiconductors and Si MOSFETs and have replaced the Schottky barrier of a HEMT with an isolating oxide layer. The result is a new type of transistor: the metal oxide semiconductor HEMT, in short MOSHEMT.

»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,« explains Dr. Arnulf Leuther, 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 with a maximum oscillation frequency of 640 GHz. »This surpasses the global state of the art for any MOSFET technology, including silicon MOSFETs,« adds Leuther.

High barrier to overcome leakage currents

To overcome the gate leakage currents, the scientists had to use a material with a significantly higher barrier than the conventional Schottky barrier. They replaced the semiconductor barrier material with a combination of isolating layers consisting of aluminum oxide (Al2O3) and hafnium oxide (HfO2).

»This enables us to reduce the gate leakage current by a factor of more than 1000. Our first MOSHEMTs show a very high development potential, while current field effect transistor technologies have already reached their limit,« reports Dr. Axel Tessmann, scientist at Fraunhofer IAF.

The world’s first integrated circuit with MOSHEMTs

The extremely fast MOSHEMT is designed for the frequency range above 100 GHz and is therefore especially promising for novel communication, radar and sensor applications.

In the future, high-power devices will ensure a faster data transmission between radio towers and enable imaging radar systems for autonomous driving as well as higher resolution and precision of sensor systems.

While it will take some years until the MOSHEMT finds its way into commercial application, the researchers at Fraunhofer IAF have already succeeded to realize the world’s first amplifier MMIC (monolithic microwave integrated circuit) based on INGaAs MOSHEMTs for the frequency range between 200 and 300 GHz.