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An analytical method has been developed that gives a simple and practical means of extracting small-signal equivalent circuit parameters (ECP's) of GaAs FET's with negligibly small bond-pad capacitances. Only the S-parameter measurement of the pinched-off cold field-effect transistor (FET) is enough to determine the extrinsic FET ECP's. The intrinsic FET ECP's of a medium-power Ku-band GaAs FET chip with a total gate width of 800 Irm have been analytically extracted for two types of eight-element intrinsic FET models; Model 1 (Curtice model) andModel 2 that differ in the control voltage (li:) definition. Model 2 with 17, defined across the gate-source capacitance is found more appropriate judging from the smaller frequency dependence of the ECP's and a better agreement between the calculated and measured S-parameters over 2-20 GHz.

Multiple high-field domains have been observed with a capacitive probe in relatively long samples (700∼1100 µ) of n-GaAs with the resistivity of about 3 \varOmega-cm at room temperature. They usually appear during the first cycle of the current oscillation if the bias voltage V b is applied abruptly (d V b /d t\gtrsim1×1012 V/s). While moving toward the anode, the most dominant one among them survives as a single domain in a few nanoseconds. From the comparison between the two kinds of contacts, it is deduced that the cathode contact is not essential to domain nucleation, but its inhomogeneity often gives rise to domain nucleation. In certain samples, anomalous time-dependent current saturations and large voltage drops appearing close to the cathode have been observed below the current instability threshold. They are well explained by the presence of large contact inhomogeneities.

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