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MOSFET confusion

  1. Feb 8, 2014 #1
    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. jcsd
  3. Feb 8, 2014 #2
    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
     
  4. Feb 9, 2014 #3

    rude man

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    Whence this? If the source voltage is +2.5V then Vgs can't be 2.5V.
     
  5. Feb 9, 2014 #4

    CWatters

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    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.
     
  6. Feb 9, 2014 #5

    CWatters

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    Is the source voltage (the power supply voltage) specified in the problem statement?
     
  7. Feb 9, 2014 #6

    rude man

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    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.
     
  8. Feb 9, 2014 #7
    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...
     
  9. Feb 9, 2014 #8

    rude man

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    You do know Vgs.

    Vg = Vd (look at the scematic) and Vs = Vdd = 2.5V. And by now you know Vd.
     
  10. Feb 9, 2014 #9
    so Vgs must = Vs-Vd = 2.5V-1.5V = 1V. Ok I will work with that and see where it gets me.
     
  11. Feb 10, 2014 #10

    rude man

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    Good move.
     
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