Unified Scalable Model for HEMT-Based Planar Schottky Diodes

A unified, scalable Schottky diode model has been developed. Based on two different high-electron-mobility transistor (HEMT)-technologies from Fraunhofer IAF, test structures with diodes in series configuration were designed and manufactured. The geometrical dimensions of the Schottky contact are varied among the test structures to extract geometrical dependencies of the device parameters. All test structures were characterized by means of dc I - V curves and scattering parameters. From the obtained data, an equivalent circuit representation was derived. A new method to determine the junction capacitance is presented which is independent of the series resistance. If the latter is not negligible compared with the junction resistance at forward bias, this method proves to be more precise than a calculation from admittance parameters. In addition, this method allows to extract the frequency-and bias-dependent series resistance what has been subjected to significant simplifications in the past. For means of verification, the model was checked against the measured scattering parameters. Furthermore, a diode ring mixer was designed, manufactured, and characterized which uses four HEMT-based Schottky diodes with geometrical dimensions that differ from the evaluated test structures. A comparison of the measurement and a simulation based on the proposed model demonstrates excellent agreement at D-band frequencies.