A New Impedance Technique to Extract Mobility and Sheet Carrier Concentration in HFET’s and MESFET’s

Conventional techniques to extract channel mobility, , and sheet carrier concentration, nS , in heterostructure fieldeffect transistors (HFET’s) do not account for the distributed nature of the device. This can result in substantial errors. To address this, we have developed a new technique that consists of measuring the gate-to-source impedance with the drain floating (Z11) over a broad frequency range. A transmission line model (TL model) is fitted to Re[Z11], thus obtaining the gate capacitance and channel resistance (and consequently (VGS) and nS(VGS)) in a single measurement. We demonstrate this technique in InAlAs/InGaAs on InP HFET’s. The TL model faithfully represents Z11 from 100 Hz to 15 MHz. Our technique can easily be automated and thus is a good tool for accurate charge control in an industrial environment.