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

[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.

 

[mfb]

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

 

[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.

 

[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.

 

[Nickie]

I can provide a response to your question about why W/L = 50/0.5 is often used instead of simply W/L = 100. The W/L ratio in MOS transistors refers to the width-to-length ratio of the transistor channel, which is a critical parameter in determining its performance. The reason for writing W/L = 50/0.5 is to emphasize that the width of the channel is 50 times larger than its length, which is an important distinction in MOS transistor design. This ratio is chosen based on the desired performance and characteristics of the transistor. Simply writing W/L = 100 would not convey this information and could potentially lead to confusion or misinterpretation.

Additionally, the two examples you provided (W/L = 100/0.5 and W/L = 200/1) have different W/L ratios, despite having the same overall width and length values. This is because the ratio is a relative measure and the performance of the transistor will be affected by both the width and length of the channel. Therefore, changing both the width and length in proportion (e.g. doubling both values) will result in a different W/L ratio and potentially different performance.

I hope this explanation helps to clarify the reasoning behind writing W/L = 50/0.5 and the difference between different W/L ratios. It is important to carefully consider the implications of the W/L ratio in MOS transistor design to ensure optimal performance.