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Diode Forward Voltage Calculator

Calculate Diode Current Using Shockley Equation I = I_S(e^(V/(nV_T))-1)

Diode Type Presets

Forward Voltage V_F (V)

Saturation I_S (A)

Ideality n

Temp (°C)

Shockley Diode Equation

I = I_S × (e^(V/(nV_T)) - 1)

V_T = kT/q = 25.85 mV at 25°C

Silicon: VF≈0.7V, n≈1.8

Schottky: VF≈0.3V, n≈1.05

The Shockley diode equation models the current through a PN junction as a function of applied voltage. The exponential relationship means current changes dramatically with small voltage changes. The thermal voltage V_T = kT/q depends on temperature (25.85mV at 25°C). The ideality factor n ranges from 1 (ideal) to ~2 (real diodes).

⚠ Above ~0.1mA, the series resistance R_S starts to dominate. The practical forward voltage for silicon diodes is ~0.6-0.7V. For Schottky: ~0.2-0.4V. For LEDs: 1.8-3.5V depending on color.

Diode Forward Characteristics

A forward-biased diode conducts current exponentially with applied voltage. Below the threshold (~0.6V for Si), very little current flows. Above threshold, current rises rapidly. The exact VF depends on the diode type, current level, and temperature. Diode forward voltage decreases by approximately -2mV/°C, making diodes useful as temperature sensors.

Silicon Diode

VF≈0.7V at 1mA. n≈1.8. I_S≈1e-14A. Max 1A typical. Used for rectification and general purpose (1N4148, 1N4007).

Schottky Diode

VF≈0.3V at 1mA. n≈1.05. Fast switching, low VF. Used in power supplies (low loss), RF detectors. Higher leakage than Si.

LED Forward Voltage

Red: 1.8V, Yellow: 2.0V, Green: 2.2V, Blue: 3.3V, White: 3.3V. I_S is higher (~1e-20 to 1e-7A). Current limited by series resistor.

Temperature Effect

VF drops ~-2mV/°C. V_T increases with T (25.85mV at 25°C, 26.9mV at 50°C). I_S doubles every ~10°C (exponential with T).

Teaching Example: Silicon diode: VF=0.7V, IS=1e-14A, n=1.8, T=25°C.
V_T = 25.85mV. V/(nV_T) = 0.7/(1.8×0.02585) = 0.7/0.04653 = 15.05.
I = 1e-14 × (e^15.05 - 1) = 1e-14 × (3,441,045 - 1) = 3.44e-8A = 34.4 nA.

Applications

Rectification LED Drivers Temperature Sensor Clamping/Protection RF Detection

Frequently Asked Questions

What is typical diode VF?▼

Si: 0.7V. Schottky: 0.3V. Ge: 0.3V. Red LED: 1.8V. Blue LED: 3.3V. VF varies with current and temperature.

How does temperature affect diode?▼

VF drops ~-2mV/°C. I_S doubles every ~10°C. Higher temp = more current at same VF. Used in silicon temperature sensors.

What limits diode current?▼

Series resistance R_S limits current at high VF. Power dissipation P = VF×IF must stay below rating. Use an external resistor to set LED current.

Schottky vs silicon diode?▼

Schottky: lower VF (0.3V), faster switching, higher leakage. Silicon: higher VF (0.7V), lower leakage, higher reverse voltage rating. Choose based on application.

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