Appendix A: Gummel-Poon Models and ft

Appendix A: Gummel-Poon Models and ft

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Another reason to change the actual Gummel-Poon parameters (while keeping ft the same) is to see the effect on the circuit of a particular parameter (the usual one being Cjc) without upsetting the over ft of the device. Cjc affects things outside the immediate transistor. For example, it is the main component in s12; typically, if Cjc were zero, s12 would be zero. It is also the principal contributor to preshoot. It supplies the capacitive sneak path that causes preshoot. To see if these effects are dominant in your circuit, it is sometimes necessary to absorb the effects of Cjc on ft into one or both of the other parameters (Cje or τf) so that the transistor ft does not change if Cjc is set to zero. The outside circuit effects will be changed with Cjc equal to zero. In this way, a valid comparison can be made and we can determine the importance of Cjc. The author used this technique when talking about preshoot in earlier chapters to illustrate certain points.

Inspec keywords: transistors; SPICE; integrated circuit modelling

Other keywords: immediate transistor; principal contributor; capacitive sneak path; preshoot; Gummel-Poon models; circuit effect

Subjects: Semiconductor integrated circuit design, layout, modelling and testing; Digital circuit design, modelling and testing; Computer-aided circuit analysis and design; Semiconductor device modelling, equivalent circuits, design and testing

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