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enabling
innovative electronics
12
Logic
, V
can be any value between 2.2 and 5 V. For OFET/TFT, unless the operation voltage at
ON
which the DUT is expected to operate as component of some OFET/TFT-based device, V
assumed to be the maximum gate voltage at which the FET is swept in linear or saturation regime.
The ratio between the I
ON
transistor figure of merit, especially if the transistor is to be used as digital component.
Threshold voltage: second derivative method
From the definition of FET operation regions above, we saw that the transistor is entering the ON
region for V
> V
. The transition however is not step-like, and an extra ΔV voltage is required on top
GS
T
of V
to drive the transition to its operative ON state. For inorganic FETs, as the gate voltage crosses
Th
the threshold, the current increase is so steep that we can safely assume the FET to be fully ON for
V
> V
+ΔV, with ΔV/V
GS
Th
ON
fully ON region. Inorganic standard FETs are therefore characterised by a very narrow ON-OFF
transition region.
For OFET/TFT, the current increase for V
ON-OFF transition region. Nevertheless, it is still reasonable to approximate I
Eq. 13
where
is given by Eq. 1 or Eq. 6, while
For an ideal FET with
respect to the gate voltage would then be a step function i.e.
Eq. 14
with slope of the TC curve
function will be zero everywhere with the exception of the threshold voltage V
14
derivative is infinite
.
12
TTL (Transistor-Transistor Logic) is a protocol whereby a signal/state is assumed to be Low (0) if the voltage
is between 0 and 2 V, and high (1) if the voltage is between 2.2 and 5V. SuperFACT multiplexer is complying
with TTL logic.
13
Without loss of generality, we have assumed that the slope α is gate voltage independent.
14
For an ideal FET with constant slope over the ON region, the second derivative of the transfer characteristic
with respect to the gate voltage is a Delta of Dirac.
Ossila Ltd
and I
currents is referred to as the ON/OFF ratio. It is important
OFF
<<1, with ΔV extra voltage required to 'move' the DUT from the OFF to the
> V
can far more gradually (low mobility) with a wider
GS
T
is the current for
0 (steep ON/OFF transition), the derivative of
= 0 and
13
. If Eq 3 is differentiated again, and if
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is usually
ON
as
DS
.
with
is constant, the resulting
where the
Th
40
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