Reverse Leakage Current: Definition, Causes & Formula

Reverse leakage current is an important parameter used to evaluate the performance of semiconductor diodes. It indicates how much unwanted current passes through a diode under reverse operating conditions. Its value depends on factors such as temperature, semiconductor material, and diode construction.

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What Is Reverse Leakage Current?

Reverse leakage current is the small amount of electric current that flows through a semiconductor device, such as a diode, when it is connected in reverse bias. Ideally, a reverse-biased diode should block current. However, a small current still flows because of the movement of minority charge carriers inside the semiconductor.

In a PN junction diode, reverse bias increases the width of the depletion region and creates a barrier that restricts the movement of majority charge carriers. Nevertheless, thermally generated minority carriers can cross the junction, producing a small reverse current.

Reverse leakage current is generally very low in silicon diodes, but its value depends on the semiconductor material, junction temperature, applied reverse voltage, and diode construction.

How Does Reverse Leakage Current Occur?

Reverse leakage current occurs because a real semiconductor diode cannot completely prevent the movement of charge carriers under reverse bias.

The process can be understood in the following steps:

Reverse bias is applied: The positive terminal of the supply is connected to the cathode, while the negative terminal is connected to the anode of a PN junction diode.
The depletion region widens: The applied voltage pulls majority carriers away from the junction, increasing the depletion-region width.
Majority carrier movement is restricted: The widened depletion region creates a barrier that prevents significant conduction by majority carriers.
Minority carriers move across the junction: Thermally generated minority carriers are swept across the depletion region by the electric field, producing a small reverse current.

Therefore, reverse leakage current exists even when the diode is operating below its reverse breakdown voltage.

Reverse Leakage Current Formula

The reverse current of a diode can be described using the Shockley diode equation:

I_D=I_S\left(e^{\frac{V_D}{nV_T}}-1\right)

Where:

$I_D$ is the diode current.
$I_S$ is the reverse saturation current.
$V_D$ is the voltage across the diode.
$n$ is the diode ideality factor.
$V_T$ is the thermal voltage, approximately 25.9 mV at 300 K.

When the diode is reverse biased and the reverse voltage is sufficiently greater than the thermal voltage in magnitude, the exponential term becomes very small. The current is then approximately:

I_D\approx-I_S

The negative sign indicates that the current flows in the reverse direction according to the usual diode voltage and current convention.

For practical purposes, the magnitude of the reverse leakage current is often approximated by the reverse saturation current before breakdown occurs. However, the actual current may vary with reverse voltage, temperature, and device construction.

Factors Affecting Reverse Leakage Current

Several factors influence the amount of reverse leakage current flowing through a semiconductor diode.

1. Temperature

Temperature is one of the most important factors affecting reverse leakage current. As junction temperature increases, more electron-hole pairs are generated inside the semiconductor. These additional charge carriers increase the reverse current.

This effect is particularly important in high-temperature electronic circuits and power semiconductor devices.

2. Reverse Voltage

Reverse leakage current generally increases as the applied reverse voltage increases, although the relationship depends on the diode design and operating region. As the reverse voltage approaches breakdown, the current can rise sharply.

The diode must operate within its specified reverse-voltage limits to avoid unwanted breakdown or damage.

3. Semiconductor Material

The semiconductor material affects the reverse leakage current.

Silicon diodes: Generally have low reverse leakage current at room temperature.
Germanium diodes: Typically have higher leakage current than comparable silicon diodes.
Schottky diodes: Can exhibit relatively high reverse leakage current, depending on their design and temperature.

These are general comparisons; the actual values depend on the specific device and its operating conditions.

4. Junction Area

A larger PN junction area can allow more leakage current because a greater area is available for charge-carrier generation and movement. However, the relationship also depends on the junction structure, material quality, and manufacturing process.

5. Semiconductor Quality

Defects, impurities, and imperfections within the semiconductor can increase leakage current. Proper fabrication techniques and good junction quality help minimize unwanted current flow.

Reverse Leakage Current in Silicon and Germanium Diodes

Silicon and germanium diodes differ in their semiconductor properties, which affects their reverse leakage current.

Parameter	Silicon Diode	Germanium Diode
Reverse leakage current	Generally low	Generally higher
Temperature sensitivity	Increases with temperature	Often more pronounced
Energy band gap	Larger	Smaller
Common applications	Rectification, switching, and protection	Specialized low-voltage detection and older circuits

Silicon diodes are widely used in modern electronic circuits because of their relatively low leakage current and good temperature performance. Germanium diodes may be selected for specific applications, but their higher leakage current can affect circuit accuracy and stability.

Reverse Leakage Current vs. Reverse Breakdown Current

Reverse leakage current and reverse breakdown current are different operating conditions of a diode.

Feature	Reverse Leakage Current	Reverse Breakdown Current
Operating condition	Below reverse breakdown voltage	At or beyond breakdown
Current magnitude	Usually very small	Can become very large
Main cause	Minority carriers and junction leakage mechanisms	Avalanche multiplication, Zener tunneling, or both
Effect on diode	Normally harmless within ratings	May damage a diode if current is not limited
Typical applications	Present in ordinary reverse-biased diodes	Used intentionally in Zener diodes and other breakdown devices

A diode operating below its breakdown voltage normally exhibits a small reverse leakage current. Once breakdown begins, the current can increase significantly. A suitable external circuit must limit the current to a safe value.

Reverse Leakage Current in a Zener Diode

A Zener diode is designed to operate in reverse bias, particularly in the breakdown region. When the applied reverse voltage is below its breakdown voltage, only a small leakage current flows through the diode. This current depends on the applied voltage, temperature, and device characteristics.

When the reverse voltage reaches the Zener breakdown voltage, the reverse current increases rapidly. The current flowing in this region is known as the Zener current and is different from the small leakage current observed before breakdown.

The datasheet of a Zener diode specifies its reverse current under particular test conditions. Understanding these specifications helps engineers select a suitable diode for voltage regulation and protection circuits.

How to Measure Reverse Leakage Current

Reverse leakage current can be measured by connecting a diode in reverse bias and measuring the current with a suitable microammeter or semiconductor parameter analyzer.

Follow these steps:

Identify the anode and cathode terminals of the diode.
Connect the positive terminal of a DC supply to the cathode and the negative terminal to the anode.
Set the supply to the specified reverse test voltage from the diode datasheet.
Connect a sensitive ammeter in series with the diode.
Increase the voltage carefully while remaining within the specified test conditions.
Record the measured current after the reading stabilizes.

Precaution: Use a current-limiting resistor or a current-limited source, select an appropriate meter range, and never exceed the diode’s maximum reverse-voltage rating. Leakage-current measurements should be performed at the specified temperature because even small temperature changes can affect the result.

Worked Example of Reverse Leakage Current

Problem: A silicon diode has a reverse leakage current of 2 µA at a specified reverse voltage. Calculate the charge that passes through the diode due to leakage over 30 seconds, assuming the current remains constant.

Given:

Reverse leakage current, $I_R=2\,\mu$
Time, $t=30\,\text{s}$

The charge is calculated using:

Q=I_Rt

Substituting the values:

Q=2\times10^{-6}\times30

Q=60\times10^{-6}\,\text{C}

Q=60\,\mu\text{C}

Therefore, the charge transferred by the reverse leakage current in 30 seconds is 60 µC.

Applications and Importance of Reverse Leakage Current

Although reverse leakage current is generally small, it is important in several electronic applications.

High-impedance circuits: Leakage current can introduce measurement errors in sensitive voltage-sensing circuits.
Photodiode circuits: Reverse current is used to detect light, although the measured current includes the photocurrent and dark current.
Low-power electronics: Leakage current can contribute to standby power consumption, especially in circuits containing many semiconductor devices.
Battery-powered equipment: Excessive leakage can gradually discharge a battery when equipment is switched off.
Precision analog circuits: Leakage can affect signal accuracy, particularly when high-value resistors are used.
Temperature-sensitive circuits: Monitoring leakage current under controlled conditions can help characterize semiconductor performance.

Conclusion

Reverse leakage current is the small current that flows through a reverse-biased semiconductor diode below its breakdown region. It mainly results from minority charge carriers and other leakage mechanisms within the semiconductor junction. Its magnitude depends on temperature, reverse voltage, semiconductor material, junction area, and device quality.

Understanding reverse leakage current helps engineers select suitable diodes, design reliable electronic circuits, and reduce unwanted current in low-power and precision applications. Always refer to the manufacturer’s datasheet for the maximum reverse leakage current under specified test conditions.

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Reverse Leakage Current: Definition, Causes, Factors, and Measurement