Stepping toward standard methods of small-signal parameter extraction for HBTs

An improved HBT small-signal parameter extraction procedure is presented in which all the equivalent circuit elements are extracted analytically without reference to numerical optimization. Approximations required for simplified formulae used in the extraction routine are revised, and it is shown that the present method has a wide range of applicability, which makes it appropriate for GaAs and InP-based single and double HBTs. Additionally, a new method is developed to extract the total delay time of HBTs at low frequencies, without the need to measure h₂₁ at very high frequencies and/or extrapolate it with -20 dB/dec roll-off. The existing methods of finding the forward transit time are also modified to improve the accuracy of this parameter and its components. The present technique of parameter extraction and delay time analysis is applied to an InGaP/GaAs DHBT and it is shown that: (1) variations of all the extracted parameters are physically justifiable; (2) the agreement between the measured and simulated S- and Z-parameters in the entire range of frequency is excellent; and (3) an optimization step following the analytical extraction procedure is not necessary. Therefore, we believe that the present technique can be used as a standard extraction routine applicable to various types of HBTs