Due to variations during the device fabrication process the geometry and doping parameters of a MOS transistor are subject to uncertainties. Therefore, the electrical parameters of the transistor vary, as well. Devices should have electrical parameters that are rather insensitive against geometry fluctuations.
Traditionally, the relationship between threshold voltage and gate length,
also called ``
roll-off'', is used to measure the short channel effect
of a MOS transistor, as shown in Fig. 5.11 for the PCD device and
the uniformly doped device. The high-drain bias threshold voltages are
extracted using the constant current method which means that the gate-source
voltage is taken at
= 100 nA.
For larger gate lengths the PCD device has a rather constant
compared
to the uniformly doped device because the channel length of the effective
transistor region, that is the lateral length of the highly doped peak
(high-
region), stays unchanged and only the length of the lightly doped
region at the drain side (low-
region) increases [12,52].
The PCD device also shows a much better
roll-off for shorter channel
lengths for the same reason. At the nominal gate length of 0.25
m the
slope of the
-
curve is approximately 0.20 V/
m for the PCD
device and 0.56 V/
m for the uniformly doped device.