tesi R. Miscioscia.pdf - EleA@UniSA

Chapter 4 131
Ea(eV)
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
PFTEOS:TEOS/PMMA 273nm
PFTEOS:TEOS 270nm
PMMA 115nm
PMMA 225nm
PMMA 400nm
-20 -18 -16 -14 -12 -10 -8 -6 -4 -2
V GS (V)
figure 15: Activation Energy (Ea(VGS)) graph for charge mobility in considered samples versus VGS
gate bias.
As it can be seen in figure 15, in every sample, activation energy
decreases with gate bias but the relationship between sample’s
physical features and activation energy appears unclear and all the
curves tend to collapse, at high gate fields to a common value of
0.12eV of activation energy and a precise trend of TMN is not
identified.
Our hypothesis is that a thermally-activated non-ideality
phenomenon is hiding the real thermal behavior of charge mobility
from our analysis. Between the main causes of device’s drift from the
nominal behavior there are the gate leakages as we seen in chapter 3.
Nevertheless it cannot easily be eliminated by means of the
circuital equivalent we proposed in previous works [24] because the
wide variation of mobility with gate field cannot be addressed in the
model without losing the physical meaning of the extracted
parameters. This is due to the introduction of an additional variable in
the fitting of output curves in the triode region.
Thus, it appears clear that we have to extract the channel current
Ich from the measured drain current ID and gate leakage IG by
following an alternative path. For these reasons we decided to change
the approach and to apply the Esseni’s [26] method to clear the