Environmental Engineering Reference
In-Depth Information
small and almost independent of the voltage applied. In contrast, reverse
biasing creates the base-collector depletion region, which, consequently, is
larger and strongly depends on the voltage applied. Specifically, the
collector
voltage
modulates the base-collector depletion
region, thus
decreasing and influencing the effective base width,
To take this effect into account, a corrective term is introduced in (1.80)
that becomes [6]
where the constant is commonly referred to as the
Early Voltage
and has
a typical value between 50 and 100 V. In most of the applications, especially
for large signal analysis, this effect is negligible.
1.4.3
Saturation Region
When the collector-emitter voltage, approaches the value of about
0.2-0.3 V, commonly referred to as the base-collector junction
becomes forward biased. In such a situation, holes from the base start to
diffuse into the collector, and the collector current is no longer related to the
base current by (1.81). Specifically, the base-emitter junction behaves like a
diode whose current exponentially depends on while the base-
collector junction behaves like a voltage source whose value is set to
1.4.4
Charge Stored in the Active Region
In the active region, the base-emitter junction is forward-biased so, like in
a forward-biased diode, the charge stored across this junction is derived from
two contributions. The first takes into account the charge in the small
depletion region, and can be modeled by substituting to in
(1.12). This charge leads to a junction capacitance whose value can be
approximated by where has an expression similar to (1.14) or
(1.17) depending on whether the junction is assumed to be abrupt or not.
The second contribution is given by the minority carrier concentration in
both the base and the emitter. As in a forward-biased diode, this contribution
leads to a diffusion capacitance,
expressed by