Pmos Mosfet

  • For MOSFET transconductance parameter (p. 255): k’ n (W/L) = n C ox (W/L) = k n (n-channel) and k’ p (W/L) = p C ox (W/L) = k p (p-channel). Overdrive Voltage V OV (p. 251): Defined at the triode-to-saturation point of MOSFET I-V curve where v DS = V OV and v GD = V t (note that V t is either V tn or V tp) at channel pinch-off V DS,sat = V.
  • The MOSFET's model card specifies which type is intended. The model card keywords NMOS and PMOS specify a monolithic N- or P- channel MOSFET transistor. The model card keyword VDMOS specifies a vertical double diffused power MOSFET. Monolithic MOSFETS are four terminal devices.

MOSFET Transistors or Metal Oxide-Semiconductor (MOS) are field effect devices that use the electric field to create a conduction channel. MOSFET transistors are more important than JFETs because almost all Integrated Circuits (IC) are built with the MOS technology.

These devices are off at zero gate–source voltage. NMOS can be turned on by pulling the gate voltage higher than the source voltage, PMOS can be turned on by pulling the gate voltage lower than the source voltage. In most circuits, this means pulling an enhancement-mode MOSFET's gate voltage towards its drain voltage turns it ON.


There are two kinds of MOSFET transistors:

  • N channel MOSFET Transistor or NMOS
  • P channel MOSFET Transistor or PMOS

At the same time they can be enhancement transistors or depletion transistors. In the present days the last ones are not used. In these tutorials we will describe only the enhancement MOS transistor.

NMOS and PMOS Symbols

The following image shows the different symbols used to describe the MOS transistor.


The image shows the curves of electrical characteristics of an NMOS transistor with the different regions of operation. These regions of operation are briefly described below.

NMOS FET Cutoff region

We can verify that VGS < VT and the current ID is zero.

NMOS FET Linear region

The transistor behaves as a nonlinear resistive element, controlled by voltage. Check the following equations:


is a characteristic parameter of the MOS transistor, which depends on the k constant and the size of the transistor gate (width W and length L).

NMOS FET Saturation region

The NMOSFET transistor behaves as a voltage controlled current source VGS. Check the following equations:

where ß = K (W/L)


In this region, the quadratic relationship between VGS and ID is shown in the left part of the picture. In a similar way to the JFET transistors, it can be used to identify, by graphical methods, the bias point of the transistors. This method is rarely used.

NMOS FET Breakdown region

A MOS transistor can be affected by the avalanche phenomena in the drain and source terminals. The MOS transistor can also be affected by breaks in the thin oxide layer of the gate that may destroy the device.


Mosfet Pmos Nmos

Finally, note that the table above shows the differences in sign and direction of the currents and voltages between NMOS and PMOS transistors.

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