Charge Storage Elements
The current dependence of Δτf is simplified by neglecting the bias dependent collector current spreading formulation. However, the bias independent collector current spreading factor fcs remains included in the critical current ICK, resulting in a more physical geometry dependence of the low-field internal (epi-)collector resistance.
Series Resistance
The internal base resistance can be strongly bias dependent and is modeled as
where rBi0 is the geometry dependent zero-bias value, and the denominator represents the conductivity modulation. The above equation has been derived from HICUM level-2 by introducing certain simplifications. Note, that VroC is close to the Early voltage VEf for BJTs but generally differs from VEf for HBTs.
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