On-Wafer Three-Port Characterization of Microstrip Transistors for Monolithic Microwave Integrated Circuits

The transistor, which is a three-port device, is generally characterized by its two-port s-parameters with one of its ports grounded. A finite source-to-ground impedance is unavoidable in two-port characterization, and is a significant source of error at microwave frequencies unless the common port is always grounded in the same manner as characterized. Three-port characterization is fundamentally better since no assumption is made on how the common port is grounded. This enables the circuit designer to manipulate the common port termination to the benefit of the design. An on-wafer microstrip transistor layout is presented that allows three-port microwave measurements to be made directly. Features employed to enhance measurement accuracy include use of on-wafer standards for the TRL calibration, correcting for the non-zero reflection at the terminations used for making three-port measurements, and changing the reference impedance used for the measurement system. We show that by accounting for measurement transitions and the common port parasitic impedance, transistor three-port characterization may be extended up to millimeter wavelengths. Three-port characterization is important to circuit designers who require increased flexibility in choosing circuit topologies and to MMIC designers to reduce costs attributable to CAD modeling errors.