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Base-Emitter and Base-Collector Current
The base-emitter current in the BJT has been changed significantly from the Gummel-Poon and other earlier models. These models assume that the non-leakage base-emitter current is related to the collector-emitter current by a simple constant, known as beta. Observation of base-emitter current in both silicon and AlGaAs devices has shown that this assumption is incorrect. Difficulties with this method of modeling base current have been observed for many years. A large, very bias-dependent base resistance in the modified Gummel-Poon model in Berkeley SPICE has been used to attempt to correct the problem with the base-emitter current expressions. This base resistance value and its variation is often extracted from DC data only, with the result that the behavior of the device over frequency is often poorly modeled. This problem is then solved by assigning some fraction of the base-collector capacitance to either side of the base in a distributed manner.
Agilent EEsof's experience with Agilent EEBJT2 has shown that properly modeled base-emitter current and conductance renders both the large bias-dependent base resistance and distributed base-collector capacitance unnecessary and greatly improves both the DC and AC accuracy of the resulting model.
Agilent EEBJT2 models the base-emitter current with 2 non-ideal exponential expressions, 1 for the bulk recombination current (usually dominant in silicon devices), and 1 for other recombination currents (usually attributed to surface leakage).
Note that NBF is not necessarily 1.0, which is effectively the case in the Gummel-Poon model.
The base-collector current is similarly modeled:
Virtually all silicon rf/microwave transistors are vertical planar devices, so the second current term containing ISC and NC is usually negligible.
The total base current Ib is the sum of Ibe and Ibc. Note that this method of modeling base current obsoletes the concept of a constant beta.