How to determine unCoxW/L value of a MOSFET?

In summary, the K value is not typically included in the datasheet, but can be determined by using a spice-model.
  • #1
atlbraves49
81
0
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

a713a6eb38e5a4f0531a014f183e9fc8.png


is that something that's 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,
 
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  • #2
- 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 [tex]\epsilon[/tex]0[tex]\epsilon[/tex]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
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 [itex]\mu_n C_{ox}\frac{W}{L}[/itex] 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
Corneo said:
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 [itex]\mu_n C_{ox}\frac{W}{L}[/itex] 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.

Im a student and this is an academic project.

See that's what we were planning on doing (biasing the gate), but isn't the K value still included in that?

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

?

because if i somehow don't need that K value, that would make my day =O
 

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

1. How do I calculate the unCoxW/L value of a MOSFET?

The unCoxW/L value of a MOSFET can be calculated by dividing the transconductance (gm) by the drain conductance (Gd). This can be represented by the equation unCoxW/L = gm/Gd.

2. What is the significance of the unCoxW/L value in a MOSFET?

The unCoxW/L value is an important parameter in MOSFET design as it determines the strength of the transistor's output current. A higher unCoxW/L value indicates a stronger output current, while a lower value indicates a weaker output current.

3. How can I measure the unCoxW/L value experimentally?

The unCoxW/L value can be measured experimentally by performing a DC analysis of the MOSFET. This involves applying a range of gate-source voltages and measuring the resulting drain current. The unCoxW/L value can then be calculated using the measured values of gm and Gd.

4. What factors affect the unCoxW/L value of a MOSFET?

The unCoxW/L value of a MOSFET can be affected by several factors, including the gate oxide thickness, channel length, and channel width. Other factors such as temperature, doping concentration, and material properties can also impact the unCoxW/L value.

5. How does the unCoxW/L value change with different operating conditions?

The unCoxW/L value of a MOSFET can change with different operating conditions, such as temperature and voltage. Higher temperatures can increase the unCoxW/L value, while higher voltages can decrease it. It is important to consider these operating conditions when designing and analyzing MOSFET circuits.

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