Dear Respected Researchers,
For high performance FET devices, specifically for Organic FETs, we require high charge mobility, in addition to low threshold voltage and high on-current/off-current ratio.
I wanted to know if threshold voltage is different than what is known as the sub-threshold voltage?
In addition, why is that current ration desired to be high ?
how do each of those parameters effect the performance of FETs? and is it the same effect for the Organic FETs?
Your contributions are greatly appreciated
This paper should address many of your questions, as the same concerns exist in all FETs, as sizes are reduced over time.
http://www-mtl.mit.edu/researchgroups/icsystems/pubs/theses/narendra_phd_2002.pdf
The sub-threshold region refers to gate voltages that should ideally result in 0 drain current. If the FET is used as a switch, you would normally want a gate voltage below VT to result in 0 drain current, and above VT to result in a current which depends on gate voltage. As in any switch, you would prefer off to be truly off.
In fact, as FET sizes shrink, the subthreshold region doesn't behave this way anymore, and the drain current is no longer 0 or some function of gate voltage above VT. In fact, the threshold voltage itself exhibits variability. This behavior has to be accommodated, when the device expected to operate as a simple on-off switch does not behave as a simple on-off switch. Think of it like a switch used to operate a lamp that never truly shuts off power to that lamp. Now you have to get more clever, and try to exploit the shortcomings of such a switch.
Dear Mr. Albert, thank you for your answer, i will read the paper you recommended for sure.
The reason for a growing importance of subthreshold conduction is that the supply voltage has continually scaled down, both to reduce the dynamic power consumption of integrated circuits (the power that is consumed when the transistor is switching from an on-state to an off-state, which depends on the square of the supply voltage), and to keep electric fields inside small devices low, to maintain device reliability. The amount of subthreshold conduction is set by the threshold voltage, which sits between ground and the supply voltage, and so has to be reduced along with the supply voltage. That reduction means less gate voltage swing below threshold to turn the device off, and as subthreshold conduction varies exponentially with gate voltage (see MOSFET: Cut-off Mode), it becomes more and more significant as MOSFETs shrink in size)
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