next up previous
Next: The Current Gain Up: 4.1.1 The Basic Model Previous: Contributions to  and Contributions to $ {\it\tau }$

A somewhat physical interpretation of $ {\it\tau }$ can be found, if the effective carrier velocity $ {\it v}_{\mathrm{eff}}$ used in the compact models for the transconductance $ {\mit g}_{\mathrm{m}}$ is interpreted as bias dependent group velocity. $ {\it\tau }$ then describes the phase information of this group velocity. This explains on one hand, why the absolute values of $ {\it\tau }$ are in the order of magnitude of extracted delay times, on the other hand why a direct interpretation as the delay time of the device is not appropriate. On the other hand, typical delay times are extracted from the inverse of $ {\it f}_\mathrm{T}$ values for analog or from ring oscillators for digital applications.