Environmental Engineering Reference
In-Depth Information
where is the emitter area and is a constant term that represents a
current density and is inversely proportional to the base width,
and its
doping concentration. The product
is often expressed in terms of the
current scale factor
In addition, the base current is exponentially related to the base-emitter
voltage and has an expression similar to (1.80). Consequently, at a first
approximation, the ratio between the collector and the base current is
constant and independent of both voltages and currents. This ratio is
commonly referred to as
that is
Due to the small amount of base current with respect to the large collector
current, the value of is typically between 50 and 200.
The ratio between the collector current and the emitter current,
is
denoted with
and results as
Since
the constant
can be expressed in terms of
that is
which is close to unity for high values of
1.4.2
Early Effect or Base Width Modulation
In (1.80) the collector current is independent of the collector voltage.
However, this is true only at a first order approximation since the
dependence in fact exists. Referring to Fig. 1.17, we note that the effective
base width, that should be used for evaluating is different from the
designed base width, due to the presence of two depletion regions. The
base-emitter depletion region is caused by a forward biasing. Therefore, it is
