ALD110808A/ALD110808/

Advanced Linear Devices

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ALD110908A/ALD110908, Vers. 2.3

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. Rather, it decreases at a

fixed rate of approximately 116mV per decade of drain current de-

crease. Thus, if the threshold voltage is +0.20V, for example, the

current would decrease to 0.1µA. Extrapolating from this, the drain

and so forth. This subthreshold characteristic extends all the way

down to current levels below 1nA and is limited by other currents

such as junction leakage currents.

At a drain current to be declared “zero current” by the user, the

still hovers around 20nA when the gate is at zero volts, 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 the 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 these temperature

stable operating points is obtained by a ZTC voltage bias condi-

resulting in a temperature stable current level of about 68µA. For

other ZTC operating 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

old I-V curves to shift linearly as well. Accordingly, the subthreshold

operating current can be determined by calculating the gate volt-

Several of the EPAD MOSFETs produce a fixed resistance when

their gate is grounded. For ALD110800, the drain current is 1µA at

ALD114813 and ALD114835 produce drain currents of 77µA and

and 540Ω, respectively.

MATCHING CHARACTERISTICS

A key benefit of using a matched pair EPAD MOSFET is to main-

tain temperature tracking. In general, for EPAD MOSFET matched

pair devices, one device of the matched pair has gate leakage cur-

rents, junction temperature effects, and drain current temperature

coefficient as a function of bias voltage that cancel out similar ef-

fects of the other device, resulting in a temperature stable circuit.

As mentioned earlier, this temperature stability can be further en-

hanced by biasing the matched-pairs at Zero Tempco (ZTC) point,

even though that could require special circuit configuration and

power consumption design consideration.

PRECISION MATCHED PAIR MOSFET FAMILY (cont.)