# How to determine unCoxW/L value of a MOSFET?

1. Feb 5, 2009

### atlbraves49

So I'm going to be using a MOSFET as a voltage controlled resistor, but to do so,i need to know the value of the unCoxW/L, also known as k

is that something thats on datasheets? or is it generally the same for all MOSFETs?

(this is the mosfet i plan on using..)
http://www.irf.com/product-info/datasheets/data/irf630npbf.pdf

thanks,

2. Feb 5, 2009

### snapback

- W/L are the geometrical channel width and length of the MOSFET, this is strongly dependent on the type of MOSFET
- µn is the electron (in case of nMOS) mobility in the channel, this is technology dependent
- COX is the oxide capacitance per unit area, it is determined from $$\epsilon$$0$$\epsilon$$s/dox. dox is the thickness of the oxide (isolator); this number is also technology dependent

thus, µnCoxW/L value is in general different for different MOSFETS

Typically you will find various transconductances (given in siemens, S or A/V) in datasheets, you should select the proper one to calculate the drain current for a given gate voltage at a given drain voltage.
Or (what seems even better) try to find on the Int-Rect-website a spice-model for the MOSFET you are planning to use, the k-value you are looking for should be visible inside the model file.

3. Feb 5, 2009

### Corneo

hi atlbraves49, what is your background? Is this an academic project or one for work? I've only been working in the industry less than 2 years, so I may have my foot in my mouth when I say this. When dealing with discrete semiconductors, you shouldn't design or rely too much on the equations derived in textbooks. They are the foundations to understand how semiconductors work. But when it comes to practice you will hardly find too many vendors listing what $\mu_n C_{ox}\frac{W}{L}$ is for their products. You need to determine a way to bias the gate of your FET to adjust the drain-source resistance to match what you want.

4. Feb 6, 2009

### atlbraves49

Im a student and this is an academic project.

See thats what we were planning on doing (biasing the gate), but isnt the K value still included in that?

VDS/ID = 1 / [K(VGS-Vt)]

?

because if i somehow dont need that K value, that would make my day =O