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Re: Can I make a MOS transistor's width longer than its leng
Definitely W can be more than L of a transistor.
In any semiconductor technology there will be minimum length and widths possible for manufacturing. With technology shrinking(people will be talking of 90nm, 65nm and recently 45nm technologies).
There is no limit on the maximum length and width. But there'll be some prblems like placing in the layout if we use large devices say 100um. So they'll be divided into fingers means no. of parallel devices. In 90nm technology devices >12 um width are not recomended.
Similarly L also have some limitation from fabrication point of view. In that case devices can be used in series.
Yes, if you need more current to drive high load or as per requirement. you can enlarge transistor's W. But in layout you will have to be careful about parasitic which will increase due to large W. So you have to do fingering of W.
I suppose you mixed up meaning of W and L -
W - transistor width
L- transistor length - Source to Drain distance
So most of the time in digital circuits W > L.
In analog world you can use W < L if for example you want to use it as a resistor...
Re: Can I make a MOS transistor's width longer than its leng
hi,
yes .In DSM(deep sub micron ) designs generally width is more than length.
Generally Designer will fix the width of transistors ,number of fingers to be used based on reliability problems like Electromigration ,hot electron effect tranisistors might experience.
I think there is no problem when W > L and most of the time is like that.
and you also have to remember about the minimum length for analog ic, it is depend on technology used. for example, it is recommended to use 5 x tech' length as your transistor's length for 0.8um tech.correct me if i am wrong.
Most of the time you want W > L for higher gm. L is usually set to a fixed value for a given technology. For example, for 0.18um technology you usually set L of most of the devices to be 0.35um.
But sometimes for special purposes you also want to make L > W, for example when you want a device to work in the triode region to act as a resistor with a big resistance, in that case you may want a large L for the device in order to obtain a higher resistance for the "resistor".
And when you are designing a current mirror, you may also want to make L larger than the usual value(but no necessarliy larger than W) so that you can obtain better current matching, because a larger L would mean weaker channel length modulation and thus lead to better current matching.
You can make a mosfet with either big L and small W, or small L and big W.
Both are fine, I 100% guarantee it and use them all the time.
If you are really talking about W (perpindicular to direction of current flow) then for very large W (greater than 5-10um) you should break it up into multiple fingers. For my 100,000um power switch I use 1000 fingers at 100um each in 0.35um process.
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