# Why we write W/L = 50/0.5 not W/L = 100?

1. Jun 17, 2013

### anhnha

Hi,
I want to ask a question.
In MOS transistors, the sizes of transistors often are given in the form:
W/L = 50/0.5 or W/L = 10/0.5
I think there are implies behind the writing. If not why they don't just write W/L = 100 or W/L = 20
And can you explain the difference between the two:
W/L = 100/0.5 and W/L = 200/1
Thanks.

2. Jun 17, 2013

### Staff: Mentor

I would expect that these values are the actual sizes (in ??) of width and length.

3. Jun 17, 2013

### phyzguy

As mfb says, these numbers are the actual width and length of the transistors, almost certainly in microns. So a 100/0.5 device is 100 microns wide and 0.5 microns long. It matters, because while to first order the current through a device with W = 100 and L = 0.5 is the same as a device with W = 200 and L = 1.0, the capacitance of the 200/1.0 device is 4X the capacitance of the 100/0.5 device. Also, because of short channel effects, even the currents through tehse devices will not be the same.

4. Jun 17, 2013

### TimeToShine

Some textbooks will give you the W/L value as 100 or 200 or whatever for the sake of simplicity, W/L is the width to length ratio and thus is expressed in the actual width and length values. The numbers you are getting in problems would not correspond exactly to real life MOSFETS.