Solve MOSFET Confusion: Establish 0.8mA & 1.5V with Parameters VT, μpCox, L, λ

In summary: So Vgs must = Vs-Vd = 2.5V-1.5V = 1V. Plugging that into the equation for ID gets W=1.108. So the transistor width is 108μm.
  • #1
vermin
22
0

Homework Statement


The diagram:
http://images.fr1ckfr4ck.fastmail.fm/probC.jpg
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.

Homework Equations



see below

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.
 
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  • #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 into 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
vermin said:
Update, I think I figured out that I can use VGS-VT=VOV

because VGS should = VDD = 2.5V

Whence this? If the source voltage is +2.5V then Vgs can't be 2.5V.
 
  • #4
Establish a drain current of 0.8 mA and a voltage VD of 1.5V.

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

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
Is the source voltage (the power supply voltage) specified in the problem statement?
 
  • #6
CWatters said:
Is the source voltage (the power supply voltage) specified in the problem statement?

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
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
vermin said:
Right sorry Vdd was given as 2.5V.

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...

You do know Vgs.

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

Good move.
 

FAQ: Solve MOSFET Confusion: Establish 0.8mA & 1.5V with Parameters VT, μpCox, L, λ

What is a MOSFET?

A MOSFET, or Metal-Oxide-Semiconductor Field-Effect Transistor, is a type of transistor that is commonly used in electronic devices. It is a three-terminal device that uses an electric field to control the flow of current.

What does it mean to establish 0.8mA and 1.5V with parameters VT, μpCox, L, and λ?

This means that the MOSFET is being operated at a current of 0.8mA (milliamps) and a voltage of 1.5V (volts) using the parameters of threshold voltage (VT), carrier mobility (μpCox), channel length (L), and channel length modulation (λ). These parameters have specific values that determine the behavior of the MOSFET.

How do I determine the values of VT, μpCox, L, and λ?

The values of these parameters can be found in the datasheet of the MOSFET or can be calculated using equations that are dependent on the physical characteristics of the device. These values may also vary depending on the operating conditions and the manufacturer.

What is the significance of establishing 0.8mA and 1.5V?

Establishing a specific current and voltage is important in determining the operating point of the MOSFET. This helps ensure that the device is being used within its safe and efficient operating range. It also allows for predictable and reliable performance of the MOSFET.

What is the relationship between the parameters VT, μpCox, L, and λ and the established current and voltage?

These parameters directly affect the current and voltage at which the MOSFET operates. VT determines the minimum voltage required to turn the device on, μpCox is related to the conductivity of the channel, L affects the resistance of the channel, and λ accounts for the changes in channel length due to the electric field. By manipulating these parameters, the current and voltage can be adjusted to the desired values.

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