Gate metallurgy effects in InAs/AlSb HFETs: Preliminary results and demonstration of surface Fermi level shifts

The effect of gate metallurgy on depletion-mode InAs/AlSb heterostructure field-effect transistors (HFETs) is studied for the first time by carefully comparing the characteristics of Al- and Ti/Au-gate transistors. HFETs fabricated simultaneously from the same molecular beam epitaxial layers and processed identically, but differing only in the metal used for the gate electrode, feature very different gate and drain I-V characteristics. The metal dependence indicates that the Fermi level is not completely pinned at the surface of InAs/AlSb quantum wells. We also show that the gate metal modifies the charge control properties of InAs/AlSb HFETs: Al-gate HFETs exhibit an enhanced kink effect accompanied by a marked transconductance compression at zero gate bias, whereas the Ti/Au-gate devices exhibit nearly kink-free drain characteristics. The gate metal dependence is shown to be a consequence of the increased channel equilibrium electron concentration accompanying the Al-metallization.