ALD110800/ALD110800A/ALD110900/ALD110900A Advanced Linear Devices 4 of 11
SUB-THRESHOLD REGION OF OPERATION
Low voltage systems, namely those operating at 5V, 3.3V or less,
typically require MOSFETs that have threshold voltage of 1V or
less. The threshold, or turn-on, voltage of the MOSFET is a voltage
below which the MOSFET conduction channel rapidly turns off. For
analog designs, this threshold voltage directly affects the operating
signal voltage range and the operating bias current levels.
At or below threshold voltage, an EPAD MOSFET exhibits a turn-
off characteristic in an operating region called the subthreshold re-
gion. This is when the EPAD MOSFET conduction channel rapidly
turns off as a function of decreasing applied gate voltage. The con-
duction channel induced by the gate voltage on the gate electrode
decreases exponentially and causes the drain current to decrease
exponentially. However, the conduction channel does not shut off
abruptly with decreasing gate voltage, but decreases at a fixed rate
of approximately 116mV per decade of drain current decrease. Thus
if the threshold voltage is +0.20V, for example, the drain current is
1uA at V
GS
= +0.20V. At V
GS
= +0.09V, the drain current would
decrease to 0.1uA. Extrapolating from this, the drain current is
0.01uA (10nA) at V
GS
= -0.03V, 1nA at V
GS
= -0.14V, and so forth.
This subthreshold characteristic extends all the way down to cur-
rent levels below 1nA and is limited by other currents such as junc-
tion leakage currents.
At a drain current to be declared “zero current” by the user, the Vgs
voltage at that zero current can now be estimated. Note that using
the above example, with V
GS(th)
= +0.20V, the drain current still
hovers around 20nA when the gate is at zero volt, or ground.
LOW POWER AND NANOPOWER
When supply voltages decrease, the power consumption of a given
load resistor decreases as the square of the supply voltage. So
one of the benefits in reducing supply voltage is to reduce power
consumption. While decreasing power supply voltages and power
consumption go hand-in-hand with decreasing useful AC bandwidth
and at the same time increases noise effects in the circuit, a circuit
designer can make the necessary tradeoffs and adjustments in any
given circuit design and bias the circuit accordingly.
With EPAD MOSFETs, a circuit that performs a specific function
can be designed so that power consumption can be minimized. In
some cases, these circuits operate in low power mode where the
power consumed is measure in micro-watts. In other cases, power
dissipation can be reduced to nano-watt region and still provide a
useful and controlled circuit function operation.
ZERO TEMPERATURE COEFFICIENT (ZTC) OPERATION
For an EPAD MOSFET in this product family, there exist operating
points where the various factors that cause the current to increase
as a function of temperature balance out those that cause the cur-
rent to decrease, thereby canceling each other, and resulting in net
temperature coefficient of near zero. One of this temperature stable
operating point is obtained by a ZTC voltage bias condition, which
is 0.55V above a threshold voltage when V
GS
= V
DS
, resulting in a
temperature stable current level of about 68uA. For other ZTC op-
erating points, see ZTC characteristics.
PERFORMANCE CHARACTERISTICS
Performance characteristics of the EPAD MOSFET product family
are shown in the following graphs. In general, the threshold voltage
shift for each member of the product family causes other affected
electrical characteristics to shift with an equivalent linear shift in
V
GS(th)
bias voltage. This linear shift in V
GS
causes the subthresh-
old I-V curves to shift linearly as well. Accordingly, the subthreshold
operating current can be determined by calculating the gate volt-
age drop relative from its threshold voltage, V
GS(th)
.
RDS(ON) AT VGS=GROUND
Several of the EPAD MOSFETs produce a fixed resistance when
their gate is grounded. For ALD110800, the drain current at V
DS
=
0.1V is at 1uA at V
GS
= 0.0V. Thus just by grounding the gate of the
ALD110800, a resistor with R
DS(ON)
= ~100KOhm is produced.
When an ALD114804 gate is grounded, the drain current I
DS
= 18.5
uA@ V
DS
= 0.1V, producing R
DS(ON)
= 5.4KOhm. Similarly,
ALD114813 and ALD114835 produces 77uA and 185uA, respec-
tively, at V
GS
= 0.0V, producing R
DS(ON)
values of 1.3KOhm and
540Ohm, respectively.
MATCHING CHARACTERISTICS
A key benefit of using matched-pair EPAD MOSFET is to maintain
temperature tracking. In general, for EPAD MOSFET matched pair
devices, one device of the matched pair has gate leakage currents,
junction temperature effects, and drain current temperature coeffi-
cient as a function of bias voltage that cancel out similar effects of
the other device, resulting in a temperature stable circuit. As men-
tioned earlier, this temperature stability can be further enhanced by
biasing the matched-pairs at Zero Tempco (ZTC) point, even though
that could require special circuit configuration and power consump-
tion design consideration.
PERFORMANCE CHARACTERISTICS OF EPAD®
PRECISION MATCHED PAIR MOSFET FAMILY (cont.)
