Why does the current increase so sharply after about 0.7V in a silicon diode?

The exponential term in the Shockley equation dominates the current once the applied voltage V surpasses the thermal voltage (kT/q, ~26 mV at room temp) multiplied by the ideality factor. For silicon, the combination of the built-in potential and the exponential rise creates the common observation of a 'turn-on' voltage around 0.6-0.7V. This is not a fixed threshold but a region where the exponential function's value becomes very large very quickly.

What does the ideality factor (η) represent, and why isn't it always 1?

An ideality factor of 1 represents an ideal diode where current is dominated by pure diffusion of carriers in the neutral regions. In real diodes, factors like carrier recombination or generation within the depletion region contribute to the current. These processes have a different voltage dependence, requiring η to be between 1 and 2 (often ~1 for silicon, ~2 for high recombination) to fit the measured I-V curve to the Shockley model.

How does temperature affect a diode?

Temperature has two primary effects. First, it exponentially increases the reverse saturation current (I_0), as more electron-hole pairs are thermally generated. Second, it increases the thermal voltage (kT/q), which 'softens' the exponential curve. In forward bias, these competing effects mean that for a fixed forward voltage, the current increases with temperature, a critical consideration for thermal stability in circuits.

Does the diode conduct in reverse bias?

Yes, but very little. The Shockley equation predicts a small, essentially constant reverse saturation current (-I_0) for reverse biases larger than a few kT/q. This current is due to the minority carriers diffusing to the junction and being swept across by the built-in field. This model does not include reverse breakdown (Zener or avalanche), which occurs at much higher reverse voltages.

PN Junction & Diode

At the heart of modern electronics, the PN junction diode is a fundamental device whose behavior is governed by semiconductor physics. This simulator visualizes two core representations: the energy band diagram and the current-voltage (I-V) characteristic. The band diagram shows how the conduction and valence bands bend at the junction, creating a built-in potential (V_bi) and a depletion region where mobile charge carriers are absent. Applying a forward bias reduces this barrier, allowing majority carriers to diffuse across, while reverse bias widens it, suppressing diffusion current. The quantitative behavior is captured by the Shockley diode equation: I = I_0 [ exp( qV / (ηkT) ) - 1 ], where I_0 is the reverse saturation current, q is the electron charge, k is Boltzmann's constant, T is temperature, and η is the ideality factor. The simulator models this equation directly, allowing users to manipulate key parameters. Users can adjust the ideality factor (η) to see its effect on the 'sharpness' of turn-on, typically between 1 and 2 for real diodes. They can explore how temperature (T) affects both the saturation current I_0 (which has a strong T-dependence) and the thermal voltage (kT/q), shifting and stretching the I-V curve. The scale of I_0 can also be changed, modeling diodes made from different materials (like silicon vs. germanium). Key simplifications include neglecting series resistance, breakdown phenomena, and non-ideal recombination currents in the depletion region. By interacting, students learn to connect the microscopic band picture with macroscopic electrical behavior, understand the exponential nature of the diode law, and see how device parameters and operating conditions shape the characteristic curve.

Who it's for: Undergraduate students in introductory semiconductor physics, electronics, or solid-state device courses, as well as advanced high school physics students studying beyond ideal circuit components.

Key terms

Shockley Diode Equation
Ideality Factor
Reverse Saturation Current
Energy Band Diagram
Depletion Region
Built-in Potential
Forward Bias
Thermal Voltage

How it works

Forward bias shrinks the depletion region (cartoon width); reverse bias widens it and current saturates toward −I₀ in this ideal model.

Key equations

I = I₀ (e^{V/(nV_T)} − 1)

Frequently asked questions

Why does the current increase so sharply after about 0.7V in a silicon diode?: The exponential term in the Shockley equation dominates the current once the applied voltage V surpasses the thermal voltage (kT/q, ~26 mV at room temp) multiplied by the ideality factor. For silicon, the combination of the built-in potential and the exponential rise creates the common observation of a 'turn-on' voltage around 0.6-0.7V. This is not a fixed threshold but a region where the exponential function's value becomes very large very quickly.
What does the ideality factor (η) represent, and why isn't it always 1?: An ideality factor of 1 represents an ideal diode where current is dominated by pure diffusion of carriers in the neutral regions. In real diodes, factors like carrier recombination or generation within the depletion region contribute to the current. These processes have a different voltage dependence, requiring η to be between 1 and 2 (often ~1 for silicon, ~2 for high recombination) to fit the measured I-V curve to the Shockley model.
How does temperature affect a diode?: Temperature has two primary effects. First, it exponentially increases the reverse saturation current (I_0), as more electron-hole pairs are thermally generated. Second, it increases the thermal voltage (kT/q), which 'softens' the exponential curve. In forward bias, these competing effects mean that for a fixed forward voltage, the current increases with temperature, a critical consideration for thermal stability in circuits.
Does the diode conduct in reverse bias?: Yes, but very little. The Shockley equation predicts a small, essentially constant reverse saturation current (-I_0) for reverse biases larger than a few kT/q. This current is due to the minority carriers diffusing to the junction and being swept across by the built-in field. This model does not include reverse breakdown (Zener or avalanche), which occurs at much higher reverse voltages.

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