At extremely small feature sizes, High Electron Mobility Transistors (HEMTs) encounter a problem: The thinner the barrier material of InAlAs (indium aluminum arsenide) becomes, the more electrons flow from the charge-carrying channel through the gate. These unwanted gate leakage currents have a negative effect on the performance and lifetime of the transistor - making further transistor scaling impossible. The classic HEMT thus has reached its scaling limit with this transistor geometry. Conventional silicon MOSFETs are also familiar with this problem. However, they have an oxide layer that can prevent unwanted leakage currents for longer than is the case with the HEMT.
Researchers at Fraunhofer IAF now combined the advantages of III/V semiconductors and Si-MOSFETs and replaced the Schottky barrier of the HEMT with an insulating oxide layer. The result is a new type of transistor: the metal oxide semiconductor HEMT, MOSHEMT for short. According to Fraunhofer researcher Arnulf Leuther, this technology opens up the potential to scale HEMTs further and thus raise the cut-off frequency even further. For the start, Leuther and his team reached an oscillatator frequency of 640 GHz, setting a new record.
To overcome the increasing gate leakage currents, the researchers had to use a material with significantly higher barriers than the classical Schottky barrier. They replaced the semiconductor barrier material with a combination of insulating layers consisting of aluminium oxide (Al2O3) and hafnium oxide (HfO2). "This enabled us to reduce the gate leakage current by more than a factor of 1000. The first MOSHEMTs produced demonstrate a very high development potential, while existing FET technologies have already reached their limits," reports Axel Tessmann, also a researcher at Fraunhofer IAF.