UC Santa Barbara

This work summarizes the efforts made to extend the current gain cutoff frequency of InP based FET technologies beyond 1 THz. Incorporation of a metal-oxide-semiconductor field effect transistor (MOSFET) at the intrinsic Gate Insulator-Channel interface of a standard high electron mobility transistor (HEMT) has enabled increased gm,i by increasing the gate insulator capacitance density for a given gate current leakage density. Reduction of RS,TLM from 110 Ωμm to 75 Ωμm and Ron(0) from 160 Ωμm to 120 Ωμm was achieved by removing/thinning the wide bandgap modulation doped link regions beneath the highly doped contact layers. Process repeatability was improved by developing a gate metal first process and Dit was improved by inclusion of a post-metal H2 anneal. InxGa1-xAs / InAs composite quantum wells clad with both InP and InxAl1-xAs were developed for high charge density and low sheet resistance to minimize source resistance.

With these improvements a Lg = 8 nm, tch = 6.5 nm transistor with fτ = 511 GHz, fmax = 283 GHz, a Lg = 18 nm, tch = 2.5 nm transistors with fτ = 350 GHz, fmax = 400 GHz, a Lg = 40 nm, tch = 7.0 nm transistor with fτ = 420 GHz, fmax = 592 GHz were demonstrated. Power gain was sometimes limited by RG more than 10 Ω due to poor T-Gate stem filling.

Based on these results and those reported in [1-3], a theory on the effects of channel thickness on quantum well ballistic devices was proposed. Simply, thin quantum wells have large Eigen-state energies limiting the amount of available (EF – E1) beyond threshold.

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Because a quantum well device’s maximum conductance occurs when EF is equal to or nearly equal to the conduction band minimum of the barrier material, current in thin channel devices is limited not by effective mass effects (i.e. density of states or injection velocity) but by band-offset limitations.