NMOS Transistor

Symbol

Information

A N-channel MOSFET (NMOS) is a three-terminal semiconductor device used for switching and amplification. The current flow between the drain and source is controlled by the gate-to-source voltage.

The MOSFET operates in three regions:

• Cutoff Region: No conduction occurs when \(V_{GS} \leq V_T\).

• Triode Region: The MOSFET behaves like a resistor when \(V_{GS} - V_T \geq V_{DS}\).

• Saturation Region: The MOSFET acts as a current source when \(V_{GS} - V_T < V_{DS}\).

Ports

• d: Drain terminal

• g: Gate terminal

• s: Source terminal

Model

The NMOS model follows the standard MOSFET equations to describe its behavior.

Attributes:

• Vgs (signal): Gate-Source voltage.

• Vds (signal): Drain-Source voltage.

• Id (signal): Drain current.

• Ig (signal): Gate current (idealized to zero).

• Kp (param): Transconductance coefficient (default: 200 µA/V²).

• W (param): Channel width (default: 100 µm).

• L (param): Channel length (default: 100 µm).

• Vt (param): Threshold voltage (default: 0.5 V).

• λ (param): Channel-length modulation (default: 0).

Methods:

analog(): Defines the NMOS behavior in cutoff, triode, and saturation regions.

\[I_D = 0, \quad \text{if } V_{GS} \leq V_T \quad (\text{Cutoff})\]

\[I_D = \frac{K}{2} (V_{GS} - V_T)^2 (1 + \lambda V_{DS}), \quad \text{if } (V_{GS} - V_T) < V_{DS} \quad (\text{Saturation})\]

\[I_D = K ((V_{GS} - V_T) - \frac{V_{DS}}{2}) (1 + \lambda V_{DS}) V_{DS}, \quad \text{if } (V_{GS} - V_T) \geq V_{DS} \quad (\text{Triode})\]

from pyams.lib import model, signal, param, voltage, current

class NMOS(model):
    """
    NMOS model with cutoff, triode, and saturation behavior.
    """

    def __init__(self, d, g, s):
        # Signal declaration
        self.Vgs = signal('in', voltage, g, s)
        self.Vds = signal('in', voltage, d, s)
        self.Id = signal('out', current, d, s)
        self.Ig = signal('out', current, g, '0')

        # Parameter declaration
        self.Kp = param(200e-6, 'A/V^2', 'Transconductance coefficient')
        self.W = param(100.0e-6, 'm', 'Channel width')
        self.L = param(100.0e-6, 'm', 'Channel length')
        self.Vt = param(0.5, 'V', 'Threshold voltage')
        self.lambd = param(0.000, '1/V', 'Channel-length modulation')

    def analog(self):
        """Defines the MOSFET's behavior in different operating regions."""
        K = self.Kp * self.W / self.L
        self.Ig += 0.0

        # Cutoff Region: Vgs <= Vt
        if self.Vgs <= self.Vt:
            self.Id += 0.0
        # Saturation Region:  Vgs - Vt < Vds
        elif (self.Vgs - self.Vt) < self.Vds:
            self.Id += K * (self.Vgs - self.Vt) ** 2 * (1 + (self.lambd * self.Vds)) / 2
        # Triode Region:  Vgs - Vt >= Vds
        else:
            self.Id += K * ((self.Vgs - self.Vt) - (self.Vds / 2)) * (1 + (self.lambd * self.Vds)) * self.Vds

Command syntax

The syntax for defining an NMOS in a PyAMS simulation:

# Import the model
from pyams.models import NMOS

# Mname: is the name of the NMOS instance
# d, g, s: The connection points in the circuit
Mname = NMOS(d, g, s)