Equivalent Circuits For High Frequency Transistors

topology is determined by physical arguments and two-port characterization techniques. Mathematical functions are determined by physical device modeling or curve fitting to experimental data. The functions can then be analyzed to define a topology. In this manner, for example, it can be shown that the input circuit of an FET can be represented as a series RC circuit and the output network can be represented as a parallel RC circuit. The element values and their relationship to device parameters can be determined by analytic device modeling of the physical structure and the mechanisms responsible for device operation. Once the circuit topology is known the element values can also be extracted from experimental data taken from actual devices over a specified frequency band. The equivalent circuit is generally valid only over the frequency band for which the circuit has been determined. Attempts to extrapolate the response of the circuit beyond the characterized frequency band can produce misleading results, especially for circuits that have been simplified for convenience by removing certain elements. If an equivalent circuit is to be used to predict the upper frequency potential of devices (e.g., to determine ft or fmax) the equivalent circuit must be topologically accurate and based upon device physics. Elements representing the physical processes responsible for device operation must be present. The high frequency operation of four candidate transistors for mm-wave applications is compared in this paper. Physically based equivalent circuits are determined and used to predict high frequency potential. The element values are determined by extraction from measured dc and s-parameter data. The circuits are analyzed to determine the elements that limit high frequency operation. The four transistors investigated are listed in Table I and consist of a Hughes GaAs MESFET, the MIT Lincoln Labs PBT, a TRW AlGaAs/GaAs HEMT, and an Alpha AlGaAs/InGaAs/GaAs pseudomorphic HEMT.