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A de-embedding method using an electromagnetic (EM) simulator is proposed to extract field-effect transistor (FET) characteristics from FET test-pattern measurements. In the proposed method, the S-parameters of the parasitic circuit are analyzed using the EM simulator. Hybrid S/Z-parameters, converted from S-parameters, are used for the parasitic circuit to express waveguide ports with S-parameters and lumped-element ports with Z-parameters. The proposed method requires a high accuracy of the EM simulator; this method was verified by comparing calculated and measured frequency characteristics of the S-parameters of an open/short-pattern. It was shown numerically that the proposed de-embedding method has a better accuracy than the conventional method, which uses the open/short-pattern. For example, the extraction error is below 5% up to 75 GHz for the proposed method and a 5% error is exceeded around 30 GHz for the conventional method. The dominant error factor in the conventional de-embedding method using the open/short-pattern was investigated. It was established that the approximation of the parasitic circuit by an equivalent circuit topology is the cause of the error. The extraction of the FET characteristics by measured data is demonstrated and it is shown that the proposed method can be applied to extract active devices.