Reverse bias is a way of connecting a voltage source to a diode (or any PN junction) so that current is blocked instead of flowing freely. You achieve it by connecting the positive terminal of the voltage source to the n-type (negative) side of the junction and the negative terminal to the p-type (positive) side. This is the opposite of forward bias, where current flows easily. Understanding reverse bias matters because it’s not just about blocking current. It’s the operating principle behind photodiodes, voltage regulators, variable capacitors, and many other components.
How Reverse Bias Works
Every PN junction has a built-in electric field at the boundary where p-type and n-type materials meet. This boundary region, called the depletion zone, is naturally stripped of free charge carriers. When you apply a reverse bias voltage, you’re reinforcing that built-in field. The negative terminal pulls holes in the p-type material away from the junction, and the positive terminal pulls electrons in the n-type material away from the junction. The depletion zone widens, and the energy barrier that charge carriers need to overcome gets taller.
The wider depletion zone and stronger electric field are what make current flow essentially stop. In forward bias, the opposite happens: the external voltage shrinks the depletion zone, lowers the barrier, and lets carriers flood across. Reverse bias pushes the junction further into its “off” state.
The Small Current That Still Flows
Reverse bias doesn’t block current completely. A tiny amount, called reverse saturation current or leakage current, still trickles through. This happens because a small number of minority carriers (electrons on the p-side, holes on the n-side) are always being generated by thermal energy. These carriers wander into the depletion zone, where the strong electric field sweeps them across to the other side.
In silicon diodes, this leakage current is typically in the nanoampere range, so small it’s negligible in most circuits. But it’s sensitive to temperature. As a device heats up, more thermal energy creates more minority carriers, and leakage current rises. The exact rate depends on the mechanism involved, but temperature is always the dominant factor in how much current sneaks through a reverse-biased junction.
What Happens at Breakdown
If you keep increasing the reverse voltage, eventually the junction can’t hold. Current suddenly surges through in what’s called breakdown. This isn’t necessarily destructive (some components are designed for it), but in a standard diode it can be. There are two distinct breakdown mechanisms, and which one occurs depends on how heavily the semiconductor is doped.
In heavily doped junctions, the depletion zone is very thin. At a certain voltage, the electric field becomes strong enough that electrons can quantum-tunnel straight through the energy barrier without needing extra energy. This is Zener breakdown, and it typically dominates below about 6 volts. In lightly doped junctions, the depletion zone is wider, making tunneling unlikely. Instead, the few electrons passing through get accelerated so violently by the electric field that they slam into atoms and knock loose new electrons, which then do the same. This chain reaction is avalanche breakdown, and it’s the dominant mechanism above roughly 6 volts.
These two types respond to temperature differently. Zener breakdown voltage decreases as temperature rises because the energy barrier shrinks. Avalanche breakdown voltage increases with temperature because heat makes atoms vibrate more, slowing down carriers so they’re less likely to trigger the chain reaction. This distinction is useful when designing circuits that need stable voltage references.
Reverse Bias as a Tunable Capacitor
The depletion zone in a reverse-biased junction behaves like the gap between the plates of a capacitor. The p-type and n-type regions on either side act as the plates, and the depletion zone is the insulating layer between them. Increasing the reverse voltage widens the depletion zone, which is equivalent to pulling the plates apart, so capacitance drops. Decreasing the voltage narrows the gap and raises capacitance.
Varactor diodes are specifically designed to exploit this effect. The relationship between voltage and capacitance is nonlinear and depends on the doping profile, but it’s predictable enough to use for precision tuning. Varactors are found throughout radio-frequency electronics, particularly in voltage-controlled oscillators and RF filters. They’re the reason modern radios can tune digitally: a microprocessor adjusts the reverse voltage through a digital-to-analog converter, changing the capacitance and shifting the tuned frequency, with no mechanical parts involved.
Reverse Bias in Photodiodes
Photodiodes are one of the most important applications of reverse bias. These sensors convert light into electrical current, and they work best when reverse-biased for several reasons.
First, the wider depletion zone created by reverse bias gives incoming photons a larger target area where they can generate electron-hole pairs. The strong electric field then sweeps those carriers out quickly, which means faster response times. This speed is critical in applications like fiber-optic communications and optical disk readers, where the photodiode needs to track signals changing millions or billions of times per second. Second, reverse bias improves linearity, meaning the output current tracks the incoming light intensity more accurately over a wider range. Without reverse bias, a photodiode still works, but its response becomes nonlinear at higher light levels.
The tradeoff is dark current. Even with no light hitting the sensor, the reverse-biased photodiode produces a small leakage current. This dark current creates shot noise that sets a floor on how faint a signal the sensor can detect. High-quality photodiodes are engineered to minimize dark current while maximizing the benefits of reverse-bias operation.
Voltage Ratings on Real Components
Every diode has a maximum reverse voltage it can safely handle, listed on its datasheet as peak repetitive reverse voltage. Exceeding this rating pushes the diode into breakdown, which in a standard rectifier causes damage. The common 1N400x rectifier family illustrates how this works: the 1N4001 is rated for 50 volts of reverse bias, while the 1N4007 handles up to 1,000 volts. They’re otherwise nearly identical, differing mainly in how thick and lightly doped their junctions are to withstand higher reverse voltages.
Choosing the right reverse voltage rating is one of the most basic decisions in circuit design. You want enough margin above the highest voltage the diode will actually see in the circuit, including any transient spikes. Undersizing this rating is a common cause of diode failure in power supplies and rectifier circuits.