How it Works
The MOSFET I-V plot shows drain current I_D versus drain-source voltage V_DS for a family of gate voltages V_GS. Each curve transitions from the linear (triode) region, where I_D rises steeply, to the saturation region where I_D flattens. The boundary between regions is the pinch-off locus V_DS = V_GS − V_th (dashed curve).
Increasing V_GS (or reducing V_th) raises the entire family of curves. The channel length modulation parameter λ creates a slight upward slope in saturation. Adjust the sliders to see how device parameters shift the characteristics.
Saturation: I_D = (k/2)(V_GS−V_th)²(1+λ·V_DS)
Cutoff: I_D = 0 (V_GS < V_th)
g_m = k(V_GS−V_th) = √(2k·I_D)
Frequently Asked Questions
What is a MOSFET?
A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled transistor. The gate voltage controls the channel conductivity between drain and source through the electric field across a thin oxide layer, with virtually no gate current.
What are the three operating regions of a MOSFET?
The three regions are: (1) Cutoff — V_GS < V_th, no channel forms and I_D ≈ 0; (2) Linear (triode) — V_GS > V_th and V_DS < V_GS − V_th, I_D increases with V_DS; (3) Saturation — V_DS ≥ V_GS − V_th, I_D is nearly constant and controlled only by V_GS.
What is the threshold voltage V_th?
The threshold voltage V_th is the minimum gate-to-source voltage needed to create a conducting inversion channel under the gate oxide. Below V_th the transistor is off. Typical values are 0.5–2V for silicon MOSFETs.
What is the drain current equation in saturation?
In saturation, I_D = (k/2)(V_GS − V_th)² where k = μ_n × C_ox × W/L is the transconductance parameter. This square-law relationship means doubling (V_GS − V_th) quadruples the drain current.
What is channel length modulation?
In reality, increasing V_DS slightly shortens the effective channel length, causing I_D to increase slightly even in saturation. This is modeled by I_D = (k/2)(V_GS − V_th)²(1 + λ·V_DS). It causes the finite output resistance in saturation.
What is transconductance g_m of a MOSFET?
Transconductance g_m = ∂I_D/∂V_GS = k(V_GS − V_th) = √(2k·I_D) in saturation. It represents how much drain current changes per volt of gate voltage. Higher g_m means higher gain in amplifier circuits.
What is the difference between NMOS and PMOS?
NMOS (n-channel) uses electrons as carriers; it turns on when V_GS > V_th (positive). PMOS (p-channel) uses holes; it turns on when V_GS < V_th (negative). NMOS is faster due to higher electron mobility (~3× vs holes) and is more common in digital circuits.
How does C_ox affect MOSFET performance?
C_ox = ε_ox/t_ox is the gate oxide capacitance per unit area. Higher C_ox (thinner oxide) increases k = μ_n × C_ox × W/L and thus drive current and transconductance. Modern transistors use high-k dielectrics (HfO₂) to increase C_ox while keeping physical thickness large enough to prevent leakage.
What is the body effect in a MOSFET?
When the source is not at the same potential as the body (substrate), the threshold voltage increases. This matters in stacked transistor circuits and can reduce overdrive voltage and effective drive current.
How are MOSFET I-V curves used in circuit design?
I-V curves determine bias points for amplifiers (operating in saturation for high gain), switch resistance in digital circuits (operating in linear region), and transconductance for RF design. Load-line analysis on I-V curves graphically finds the quiescent operating point.