Shockley Diode Calculator

Created by Luis Fernando
Last updated: Sep 09, 2022

Welcome to the Shockley diode calculator, a tool created to calculate a diode current or voltage using the Shockley diode model.

In the rest of this article, you can read about Shockley diode characteristics, and if you need to learn more, in the last section, we present the equation for Shockley diode calculations. If you want to switch topics, check the capacitive reactance calculator to learn more about another characteristic metric of electronic circuits.

Shockley diode characteristics

Diodes are electronic components that conduct current in only one direction, using different materials, devices, and configurations. The semiconductor diode is the most common type, consisting of two semiconductor materials that form a p–n junction. Those two materials are known as p-type and n-type semiconductors. Unlike intrinsic semiconductors, the p-type and n-type are doped to provide the electrical properties that allow the unidirectional current. Attached to the semiconductors are two electrical terminals, which usually are electrodes.

The p–n junction that forms a semiconductor diode.
The p–n junction that forms a semiconductor diode. The Shockley and light-emitting diode (LED) are examples of semiconductor diodes. Attribution: Raffamaiden, CC BY-SA 3.0, via Wikimedia Commons

The Shockley diode is a particular type of semiconductor diode. Compared to conventional semiconductor diodes, it has two main characteristics:

  • It consists of four semiconductor layers forming three (instead of two) p-n junctions.
  • Another Shockley diode characteristic is that it presents a high resistance that impedes conduction unless we reach a trigger voltage (that diminishes that resistance).
Layer construction, equivalent circuit, and schematic symbol of a Shockley Diode.
Shockley Diode: a. layer construction, b. equivalent circuit, c. schematic symbol. Attribution: wdwd, CC BY 3.0, via Wikimedia Commons

Thanks to its ON/OFF behavior, it works as a switching device in semiconductor-controlled rectifiers.

Equation of the Shockley diode model

The equation to calculate a Shockley diode current is:

I = IS × (eVd/(nVₜ) - 1)

, where:

  • I — Diode current, calculated in amperes (A);
  • IS — Reverse saturation current, in A;
  • Vd — Voltage drop, in volts (V);
  • Vₜ — Thermal voltage, in V; and
  • n — Emission coefficient, dimensionless;

IS, Vₜ, and n are what we would call "diode parameters" or diode characteristics predefined by the manufacturer:

  • Emission coefficient (n), also known as ideality factor or quality factor, represents how the diode differs from an ideal diode. It accounts for imperfect junctions, and its value ranges from 1 ("ideal" diode) to a maximum of (usually) 2.

  • Reverse saturation current (IS) is a highly temperature-dependent property that refers to the small amount of current that occurs naturally when the diode is in reverse bias. Its direction is opposite to that of the conventional current.

  • Thermal voltage (Vₜ) is the internal voltage in the diode when disconnected from the circuit. Like IS, it's also temperature dependent. At 300 K (27 °C; 80 °F), Vₜ is approximately 25.85 mV, but we can calculate it for any temperature the following way:

    Vₜ = kT/q

    , where:

    • k — Boltzmann constant, whose value equals 8.617333262 × 10⁻⁵ electronvolts per kelvin (eV/K) (see what is an electronvolt);
    • T — Absolute temperature, in K; and
    • q — Elementary charge -the charge of an electron-, whose value equals 1.602176634 × 10⁻¹⁹ coulombs (C).

On the other side, voltage drop (Vd) and diode current (I) are the experimental values manipulated in the laboratory. I is the current flowing through the diode, and Vd is the voltage difference between its connections to the circuit.

Luis Fernando
Emission coefficient (n)
Saturation current (Iₛ)
mA
Thermal voltage (Vₜ)
mV
Voltage drop (Vd)
V
Current (I)
mA
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