As a key factor representing the bipolar transistor characteristics

the Early effect influences the output transconductance

the transfer current

the base transit time

the current gain

the diffusion capacitance

and so on.In this paper

we begin with the primary definition of the Early effect

overview the origin of the Early voltage

the development of the model

and the applications to Si and SiGe circuit simulators

with details as follows:(1) Summarize the basic Early effect model of the Si bipolar transistor

and the introduction into SPICE

then describe the improvement of VBIC model in view of limitations of SPICE.(2) As SPICE and VBIC are unable to describe the introduction of Ge profile into the base

we review the modeling methods of the Early voltage with SiGe HBTs

based on the modeling ideas of Mextram and HICUM

two SiGe HBT standard models.(3)Sum up the strengths and weaknesses of present models for the Early effect.