Radio Frequency Integrated Circuit Design - Webs

A Brief Review of Technology
IC = IS e (V BE /v T )
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(3.1)
where IS is a constant known as the saturation current, VBE is the dc bias between
the base and emitter, and v T is the thermal voltage given by
v T = kT
q
(3.2)
where q is the electron charge, T is the temperature in Kelvin, and k is Boltzmann’s
constant. The thermal voltage is approximately equal to 25 mV at a
temperature of 290K, close to room temperature.
Figure 3.3 shows the collector characteristics for a typical bipolar transistor.
The transistor has two other regions of operation usually avoided in analog
design. When the base-emitter junction is not forward biased, the transistor is
cut off. The transistor is in the saturated region if both the base-emitter and
collector-emitter junctions are forward biased. In saturation, the base is flooded
with minority carriers. This generally leads to a delayed response when the bias
conditions change to another region of operation. In saturation, V CE is typically
less than a few tenths of a volt. Note that in the active region, the collector
current is not constant. There is a slope to the current versus voltage curve,
indicating that the collector current will increase with collector-emitter voltage.
The slopes of all the lines are such that they will meet at a negative voltage VA
called the Early voltage. This voltage can be used to characterize the transistor
output impedance.
The intrinsic transistor is connected through the diffusion regions to the
external contacts labeled e, b, and c. These connections add series resistance
and increase the parasitic capacitance between the regions. The series resistance
Figure 3.3 Transistor characteristic curves.