Environmental Engineering Reference
In-Depth Information
where all voltages are referred to the substrate. In (1.75), is a
characteristic current whose typical value is around 20 nA and
n
is a slope
factor whose value is about 1.5. Specifically,
n
depends on the surface
depletion capacitance,
as
The weak inversion condition is expressed as
Often, to be sure that a transistor is operating in sub-threshold mode,
(1.77) is roughly approximated to
Assuming and the source short-circuited to the bulk, the
drain current exponential relationship can be simplified into
Despite the exponential relationship, the drain current is very small and
so is the transconductance (equal to Therefore the device is very
slow. Moreover, it should be noted that transistor matching is very poor (due
to threshold voltage variations) and that minimum size devices are normally
avoided when working in weak inversion. This leads to large parasitic
capacitances that further decrease device speed.
1.4 BIPOLAR-JUNCTION TRANSISTORS
Bipolar transistors or BJT were widespread until the end of seventies
when MOS technology started to become popular thanks to the fact that a
