Zinc-based field effect transistors get performance boost from clever engineering

At the heart of a metal-insulator-semiconductor field effect transistor (MOSFET) lies a 2-D electron gas. These electrons are restricted to movement along the insulator-semiconductor interface and exhibit quantization of motion and energy that offers desirable properties to high-frequency electronic devices. Gallium nitride-based MOSFETs (GaN FETs), for example, are being explored for such applications.

Recent efforts show zinc oxide to be a promising candidate for better heterostructure FETs (HFETs). Here, the electron gas arises because of a polarization discontinuity between the ZnO and a ternary alloy like MgZnO. However, the performance of MgZnO/ZnO HFETs has not reached its full potential. Saturation velocity, the maxiumum possible velocity of the charge carriers, lingers substantially below the theoretical prediction. Fundamentally, saturation velocity is limited by the formation and accumulation of longitudinal optical (LO) phonons, or lattice vibrations, during operation that then scatter electrons.

Increasing the electron concentration until the plasma resonance has energy near the LO phonons offers a solution. Hadis Morkoç, principal investigator of the study, noted that his group’s previous research and that of his collaborator M. Arvydas, on high-speed GaN FETs, led to discovering that this was a viable way to optimize for performance speed.

Unfortunately, this requires high Mg content creating a range of other issues that limit performance. To improve on this, researchers developed BeMgZnO/ZnO, quaternary heterostructures, that exhibit high 2-D electron gas concentrations and report their findings in Applied Physics Letters. Using Be incorporation to engineer interfacial strain, they found just 5 percent Be concentration sufficient to reach the desired electron gas concentration with much lower Mg content.

“[This] is the only viable means of achieving sufficiently high electron concentration in ZnO to allow electron transport with the smallest effect of carriers scattering,” said Morkoç. The high concentration allows FETs to operate at higher frequencies.

Source: “Investigation of high density two-dimensional electron gas in Zn-polar BeMgZnO/ZnO heterostructures,” by K. Ding, M. B. Ullah, V. Avrutin, Ü. Özgür, and H. Morkoç, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.4993853 .