# Homework Help: MOSFET confusion

1. Feb 8, 2014

### vermin

1. The problem statement, all variables and given/known data
The diagram:
http://images.fr1ckfr4ck.fastmail.fm/probC.jpg [Broken]
This MOSFET has the following parameters:
• |VT| = 0.6V
• μpCox = 100 (μa/V2)
• L = 0.25μm
• λ = 0

Establish a drain current of 0.8 mA and a voltage VD of 1.5V.

Determine the transistor width W, and the resistance of resistor R, that meet the above drain current and voltage.

2. Relevant equations

see below

3. The attempt at a solution

First I dove into my bag of equations for the one involving MOSFETS which had both μpCox and W/L in it.

The best looking one I found was;
ID=(1/2)μnCox(W/L)V2OV

and when I fill in the appropriate values, I get a width that just doesn't make sense. I don't think I'm using the right equation but I don't know where to look.

Last edited by a moderator: May 6, 2017
2. Feb 8, 2014

### vermin

Update, I think I figured out that I can use VGS-VT=VOV

because VGS should = VDD = 2.5V

so VOV=2.5-0.6 = 1.9V

Plugging that in to the above equation for ID gets me W=1.108

Also for the resistor I used
RD=(VDD-VD)/ID
= (2.5V-1.5V)/.08mA = 1250Ω

3. Feb 9, 2014

### rude man

Whence this? If the source voltage is +2.5V then Vgs can't be 2.5V.

4. Feb 9, 2014

### CWatters

Perhaps I've missed something but if Vd = 1.5 the voltage drop across the resistor is 1.5V. The drain current flowing through R is 0.8mA so R is easy to calculate from Ohms law. It's not 1250Ω.

5. Feb 9, 2014

### CWatters

Is the source voltage (the power supply voltage) specified in the problem statement?

6. Feb 9, 2014

### rude man

R is obvious, as you say

In the OP's second post he mentions Vdd = 2.5V so I assume that's the source voltage.

7. Feb 9, 2014

### vermin

Right sorry Vdd was given as 2.5V.

So. yeah. voltage across the resistor must be 1.5V, I_D is .8mA so R_D=V_D/I_D=1.5/.0008=1875ohms
That does seem obvious.

But now how do I go about solving for the Width? All I have are these equations I don't understand fully, and I know that I need V_OV to use the one I mentioned above. To get V_OV I need to know V_GS and I don't see how to find that...

8. Feb 9, 2014

### rude man

You do know Vgs.

Vg = Vd (look at the scematic) and Vs = Vdd = 2.5V. And by now you know Vd.

9. Feb 9, 2014

### vermin

so Vgs must = Vs-Vd = 2.5V-1.5V = 1V. Ok I will work with that and see where it gets me.

10. Feb 10, 2014

Good move.