Diode Forward Voltage
Shockley diode equation
Choose a direction; enter one value and the other is computed live.
Diode
How to use this calculator
The Shockley diode equation relates forward voltage and current: I = Is·(e^(V/(n·VT)) − 1), where Is is the saturation current, n is the ideality factor (1–2), and VT ≈ 25.85 mV at 25 °C. Choose a direction (V → I or I → V), enter one value, and the calculator computes the other, plus the small-signal dynamic resistance and power.
What it means in a real circuit
For Is = 1 nA, n = 1, V = 0.7 V: I ≈ 19 mA, and the dynamic resistance is ≈ 1.4 Ω. The exponential relationship means a small change in voltage causes a large change in current — this is why a diode's "forward voltage" is not a fixed value but depends on current. Use this to find the voltage drop across a diode at a given current, to estimate the small-signal resistance for AC analysis, or to check that a diode won't overheat.
Definitions
Saturation current (Is): the reverse-bias current, typically nA for silicon. Ideality factor (n): 1 for an ideal diode, 1–2 for real diodes. Thermal voltage (VT): kT/q ≈ 25.85 mV at 25 °C. Dynamic resistance (rd): the small-signal resistance, rd = n·VT/I. Power: P = V·I.
Design tips
For a typical silicon diode, Is is in the nA range and n ≈ 1–2. Schottky diodes have lower Is (higher leakage) and a lower forward voltage. Use the dynamic resistance for AC analysis (e.g. small-signal models). Check the power rating — a diode dissipates P = V·I, and exceeding the rating causes thermal runaway.
Common mistakes
Assuming the forward voltage is a fixed 0.7 V (it varies with current). Forgetting that Is is very small (nA), so the exponential term dominates. Using the wrong ideality factor (n = 1 for ideal, 1–2 for real). Ignoring the power rating, which can cause thermal runaway.
Component-selection / practical advice
Choose a diode with a forward voltage and current rating that fits your application. For low-voltage-drop applications, use a Schottky diode. For high-current applications, check the power rating and add a heat sink if needed. Use the dynamic resistance for AC analysis.
Limitations
The Shockley equation is an idealized model. Real diodes have series resistance, parasitic capacitance, and a non-ideal I-V curve at high currents. For power diodes, the series resistance becomes significant, and the simple exponential model underestimates the forward voltage. Use the datasheet's I-V curve for accurate results.
Example use cases
Finding the voltage drop across a rectifier diode; estimating the small-signal resistance for an amplifier; checking that a diode won't overheat in a power supply; designing a clamp circuit.
Frequently asked questions
The diode's I-V relationship is exponential, so the voltage depends on the current. At 1 mA it might be 0.6 V; at 100 mA it might be 0.75 V. The "0.7 V" is just a rough approximation.
n is a measure of how close the diode is to ideal. n = 1 is ideal; real diodes have n = 1–2. A higher n means a steeper I-V curve.
rd = n·VT/I is the small-signal resistance. It's used for AC analysis (e.g. small-signal models). At higher currents, rd is lower.
Ideal — they assume the Shockley equation with no series resistance or parasitics. Real diodes deviate at high currents. Use the datasheet's I-V curve for accurate results.
Assuming a fixed 0.7 V, forgetting that Is is very small, or ignoring the power rating (which can cause thermal runaway).