MOSFET Calculator: Drain Current, gm, and Power Dissipation

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N-channel devices conduct when V_GS exceeds V_th (positive V_th). P-channel devices conduct when V_GS falls below V_th (negative V_th). All voltages are entered as signed values referenced to the source.
Voltage from gate to source. For N-channel enhancement MOSFETs this is positive; for P-channel it is negative. The device turns on when |V_GS| exceeds |V_th|.
V
Voltage from drain to source. Positive for N-channel, negative for P-channel. Determines whether the device is in the triode (linear) or saturation region.
V
Gate-source voltage at which the channel begins to form. Typically 0.5 V to 5 V for power MOSFETs. Enter a negative value for P-channel devices (e.g., -2 V). Can also be negative for depletion-mode N-channel devices.
V
Process transconductance gain. K = (mu_n * Cox * W) / (2 * L). Typical values range from 0.0005 A/V^2 for small-signal devices to 0.05 A/V^2 for power transistors.
A/V²
Channel length modulation parameter (Early effect). Accounts for the slight increase in drain current with drain-source voltage in saturation. Set to 0 to ignore this effect. Typical values: 0.005 to 0.1 V^-1.
V⁻¹
Expand to compute K from individual device and process parameters: channel width W, channel length L, oxide capacitance C_ox, and carrier mobility mu_n.
Drain current I_DSaturation (active/amplifying)
9.9mA

Current flowing from drain to source

Operating regionSaturation
Transconductance g_m6.6mS
Output conductance g_ds99μS
Output resistance r_o10.1kΩ
Power dissipation P_D99mW
Overdrive voltage V_ov3V
9.9 mA
Micro-signal<1Small-signal1-10Medium power10-50High current50+

Device is in Saturation (active/amplifying region)

  • Drain current I_D = 9.900 mA (approximately constant with V_DS).
  • Transconductance g_m = 6.600 mS: this is the small-signal voltage gain per unit load resistance.
  • Power dissipated in the device is 99.00 mW. For power MOSFETs, verify the package thermal resistance keeps T_j within datasheet limits.
  • The square-law model (with channel length modulation) is valid here. For very short channels, velocity saturation effects reduce the exponent below 2.

Next stepUse this operating point for amplifier design. For switching use, drive V_GS well above V_th to minimize on-resistance and conduction losses.

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