Pseudomorphic In0.22Ga0.78As-Channel MOSFETs Using InAlP Oxide as the Gate Dielectric for RF Applications

In this work, wet thermal oxidation of epitaxially-grown InAlP in combination with a pseudomorphic high electron mobility In0.22Ga0.78As channel has been explored, as an alternative gate structure to conventional GaAs-based field effect transistors (FETs) as used in many radio frequency (RF) systems. This insulating metal-oxide-semiconductor (MOS) gate structure with higher effective gate potential barrier can suppress the gate leakage and extend the allowable gate voltage swing. Device structure engineering, including gate dielectric thickness and channel doping level, is shown to be able to result in heterostructure designs for both depletion-mode and enhancement-mode devices. The gate oxide formation does not require any special surface passivation or post-growth gate dielectric deposition, and is minimally disruptive relative to established high electron mobility transistor (HEMT) fabrication processes. The utilized process also results in self-alignment of the gate metallization and localized oxidation region, thus reducing the access resistance and enabling high RF performance to be obtained. The devices demonstrated here exhibit record RF performance for III-V channel MOSFETs with a measured cutoff frequency, fT, of 60 GHz obtained for a gate length of 0.25 ?m. Both the DC and RF performance are already comparable to commercialized pseudomorphic high electron mobility transistors (pHEMTs), while offering features that conventional technology cannot provide. Operation at high gate bias (to at least 1.5V) without significant gate leakage is demonstrated. A 3.5 nm thick InAlP oxide gate dielectric reduces the gate leakage below that of heterostructure field effect transistors (HFETs) based on the same epitaxial structure by more than 105 times. Based on a small-signal model and a two-noise-temperature model, this work also reports the microwave noise performance for III-V MOSFETs for the first time. A minimum noise figure (NFmin) of less than 1 dB at 8 GHz has been achieved by a 0.25 ?m GaAs-channel MOSFET, while In0.22Ga0.78As-channel MOSFETs exhibit an NFmin about 0.5 dB higher. The results suggest these devices are potentially viable candidates for low-noise RF and microwave applications. However, despite the excellent performance obtained, the demonstrated devices do not yet exhibit the full advantages of the new gate structure and high mobility channel due to un-optimized device designs. For example, the n+ AlGaAs donor layer beneath the channel causes short-channel effects, and the un-gated oxide region also leads to significant access resistance in enhancement-mode devices only due to the ?normally-off? operation. Therefore, investigation and discussion to provide directions for further device performance improvements in the future are also included.