Beginner - Couldn't get the P-Channel MOSFET turned ON

[devonsc]

Hi there,

I was trying to turn ON the P-Channel MOSFET and has been following the test conditions given in the datasheet. By the way, it is a logic MOSFET.

Am I right to say that the following is what I need to do to turn ON the P-Channel MOSFET? Mind correcting if I'm wrong? Thanks in advance...

a.) Apply 5V to the Gate.
b.) Gnd the Drain.
c.) 5V to the Source.

Am I suppose to be able to turn ON the MOSFET and obtain a zero voltage drop across Drain and Source in this way?

Thanks a lot in advance. Advice needed.

 

[khouly]

no this will not turn on the transistor "p channel"
to get the drain connected to source apply 0 vlot ot gnd to the gate
if u apply 5 v on the gate this turn of the device

i wish this help

khouly

 

[devonsc]

I see, thanks. Will try testing again.

But, does it work in this manner? Say, for N-Channel MOSFET =>

Applying 12V to the Drain and 5V to the Gate and Gnd the Source. Then, 12-5 = 7V. And if 7V is higher than the threshold voltage, it turns ON, else, it doesnt?

 

[easytarget]

Hi devonsc,

It doesn't work this way, to turn on an nmos or a pmos, you don't need to consider gate to drain voltage, only gate to source voltage matters for turning on and off both types, so:

Vgs > Vth -----> nmos turns on
Vgs < Vth -----> pmos turns on

Know that there are limits to the voltages that you can use, depending on the technology, so make sure you know these. Good luck with electronics.

 

[Humungus]

N- and P-MOS are complementary. Think of the physics that makes them work.

Generally, N-MOS are made on a P-doped substrate. So to make them work, you create an inversion layer just on the silicon surface beneath ther gate. To make so, the Gate-source potential must be positive. That is why your N-MOS works when you apply a Vgs=5V. The Vds voltage only gives you the amount of current going through the channel.

For a P-MOS, generally made on an N-doped substrate, you need to apply a negative Vgs in order to create an inversion layer on top of the silicon. To make current flow through the channel, you also need a negative Vds. If you apply a positive Vds, you'll forward bias the drain-body diode.

Remember, think always in terms of the materials used to make the tranaistors and the physiscs behind.

 

[Yun Lin]

The pmos turn on at 0v at gate!

 

[rhesusmonkeyboy]

I don't like all that negative notation, so I learned from a professor to do everything in terms of positive quantities.

Let's say you went to a magical foundry where they tell you

the threshold voltage for the NMOS is 0.6v and for the PMOS is -0.9v ( negative 0.9 volts ) ... the K' for the NMOS is 100uA/V² and for PMOS is 30uA/V²...

... on my sheet of paper, I'd take the magnitudes and write

VTHN = 0.6v
Kn' = 100 uA/V²
VTHP = 0.9v
Kp' = 30 uA/V²

so then when figuring out currents, neglecting output impedance and assuming I'm in pinch-off / ( mosfet ) saturation / active-mode I'd write their respective equations as

Inmos ~ ½ * ( W / L ) * Kn' * ( Vgs - VthN )²
and similarly:
Ipmos ~ ½ * ( W / L ) * Kp' * ( Vsg - VthP )²

Notice I write VGS for nmos ( gate minus source voltage ) and VSG for pmos ( source minus gate voltage )!

If you draw a PMOS where the source is an arrow pointing in the direction of the gate ( ie not with the body drawn ) or

PMOS:

      S
       |
       |
    |<--
    |
 G -|
    |---
       |
       |
       |
      D

Then you just have to forward bias the SOURCE-TO-GATE DIODE, or in other words, have the source higher in voltage than the gate. The opposite / complement to nmos.

The ">" is pointing towards the cathode of a diode, it's a diode symbol, just subtley different!

DIODE:

anode ---|>|--- cathode

Draw an NMOS and PMOS in this manner and take a little time to convince yourself biasing the diodes properly makes sense. It does!

^-^