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Equivalent circuit parameters of graded-bandgap base GaAs/AlGaAs heterojunction bipolar transistors are derived by analysing static and microwave characteristics. Here, the estimated base transit time of 1.4 ps indicates that the average electron velocity is enhanced under the built-in field of the graded base. Additionally, ECL ring oscillators are simulated using the obtained parameters. The simulated propagation delay time of ECL gates agrees well with an experimental result of as short as 65 ps/gate.