How does a diode work? Why does it mainly conduct current in one direction?

The diode is one of the simplest yet most important electronic components. It is found in power supplies, rectifiers, protective circuits, controllers, industrial devices, consumer electronics, automotive electronics, and almost any device where current flow direction needs to be controlled. Its basic function seems simple: a diode easily conducts current in one direction and blocks it in the other. However, it is not a perfect "valve" because a small leakage current can flow in the reverse direction, and breakdown can occur if a certain voltage is exceeded. In Zener diodes, this property is used in a controlled manner.

To understand why a diode behaves this way, one must look at the semiconductor structure, the p-n junction, and what happens in the diode when voltage is applied.

What is a diode?

A diode is a two-terminal electronic component that, in its basic version, is made of a p-n junction. It has two leads: an anode and a cathode. In a typical rectifier diode, conventional current flows from the anode to the cathode when the diode is forward-biased. If the voltage is applied in reverse, the diode goes into a reverse-bias state and conducts very little.

However, a diode is not an ideal switch. In the forward direction, current increases gradually and very quickly with increasing voltage, while in the reverse direction, there is a small reverse current. Nevertheless, within its normal operating range, its properties allow it to be used for rectifying, protecting, and stabilizing voltage.

The p-n junction, the heart of the diode

The operation of a semiconductor diode is based on the p-n junction. It is formed by joining two semiconductor regions with different doping types.

In an n-type semiconductor, the dominant charge carriers are electrons. This is achieved by introducing donor impurities, such as phosphorus, arsenic, or antimony. Atoms of these elements have more valence electrons than silicon, so one of the electrons can relatively easily become a free electron.

In a p-type semiconductor, the dominant carriers are holes, which are the absence of electrons. They are created by acceptor impurities, such as boron, aluminum, or indium. A hole is not a particle in the same sense as an electron, but in the crystal, it behaves like a positive charge carrier because electrons can jump between bonds, causing the apparent movement of the "absence of an electron."

It is important to note that an n-type semiconductor is not negatively charged as a whole, and a p-type semiconductor is not positively charged as a whole. Both regions are electrically neutral. The names "n" and "p" refer to the type of majority carriers: electrons in n-type and holes in p-type.

Depletion layer and potential barrier

When a p-type region meets an n-type region, carrier diffusion occurs at the boundary. Electrons from the n region move toward the p region, where they recombine with holes. Holes from the p region move toward the n region and also recombine. Near the junction boundary, some free carriers disappear.

On the n side, immobile, positively charged donor ions remain, and on the p side, immobile, negatively charged acceptor ions remain. This creates a region devoid of mobile charge carriers, called the depletion layer or barrier layer. This layer generates an internal electric field that opposes further diffusion of electrons and holes.

This creates a potential barrier. It should not be directly equated with the voltage drop measured across the terminals of a conducting diode. In a typical silicon diode, this drop often measures around 0.6–0.8 V at working current, but it is not an ideal, rigid threshold. The actual forward voltage depends on the diode type, temperature, current, semiconductor material, and element construction.

Forward bias

A diode is forward-biased when the anode is connected to the positive terminal of the voltage source and the cathode to the negative terminal. In this situation, the external electric field reduces the potential barrier of the p-n junction. The depletion layer narrows, and majority carriers can more easily cross the junction boundary.

In a typical silicon diode, noticeable current begins to flow at a voltage of a few hundred millivolts. At typical operating currents, the voltage drop across a conducting silicon diode often measures around 0.7V. However, this does not mean that below this value, the diode "does not work at all," and above it suddenly becomes an ideal conductor. The diode's characteristic is non-linear: the current increases exponentially with increasing forward voltage.

In practice, this means that a conducting diode still causes some voltage drop and generates heat. The power loss can be approximately calculated using the formula:

where UF is the forward voltage drop, and IF is the forward current. At high currents, attention must be paid to the allowable power loss, junction temperature, and heat dissipation method.

Reverse bias

If the voltage is applied in reverse, meaning the cathode is at the positive terminal and the anode at the negative terminal, the diode is reverse-biased. In this situation, the external electric field increases the potential barrier and widens the depletion layer. Majority carriers are pushed away from the junction, so current practically does not flow.

However, this does not mean a complete lack of conduction. In the reverse direction, there is a small leakage current, also called reverse or blocking current. It is usually very small – it can be in the order of nanoamperes, microamperes, or larger, depending on the diode type, temperature, and voltage. This current mainly results from the presence of minority carriers and thermal processes occurring in the semiconductor.

As the temperature increases, the leakage current usually increases. Therefore, in circuits operating in challenging thermal conditions, it cannot be completely ignored.

Current-voltage characteristic of a diode

The current-voltage characteristic of a diode shows how the current flowing through the diode changes depending on the applied voltage. In the forward direction, the curve rises slowly at low voltages and then increasingly faster. In the reverse direction, the current is very small until the breakdown voltage is reached.

The most important parameters of a diode are: forward voltage – the voltage drop across the diode during conduction; maximum forward current – the current the diode can conduct without damage;reverse current IR – the current flowing in the reverse direction; maximum reverse voltage VR or VRRM – the voltage the diode can withstand in the reverse direction; total power dissipation Ptot – the maximum thermal power the element can dissipate; recovery time trr – particularly important in switching diodes and high-frequency rectifiers.

Therefore, a diode is selected not only based on voltage and current but also on speed, package type, operating temperature, and cooling conditions.

Breakdown voltage

If the reverse voltage exceeds the allowable value, junction breakdown can occur. In a regular rectifier diode, this phenomenon is usually undesirable. The current increases rapidly, and if not limited, the semiconductor structure can overheat and become permanently damaged.

There are two main breakdown mechanisms: Zener breakdown and avalanche breakdown. In practice, both can coexist, but one usually dominates depending on the diode's construction and breakdown voltage value.

What is a Zener diode?

A Zener diode is a special type of semiconductor diode designed to operate in the reverse direction in the controlled breakdown region. In the forward direction, it behaves similarly to a regular silicon diode. Its special function is revealed only when connected in the reverse direction.

When the reverse voltage reaches a certain value, called the Zener voltage UZ or VZ, the diode begins to conduct in the reverse direction. If the current is properly limited, the voltage across the diode remains approximately constant. This allows the Zener diode to serve as a simple voltage stabilizer, reference element, or overvoltage protection.

It is very important, however, that a Zener diode cannot be connected directly to a voltage source without current limitation. Once in the breakdown region, the diode has a small dynamic resistance, so even a small voltage increase can cause a large current increase. For this reason, a series resistor or other current-limiting source is used in the classic parallel stabilizer circuit.

Zener breakdown and avalanche breakdown

In diodes with low stabilization voltage, usually below about 5V, the Zener phenomenon, or quantum tunneling, dominates. A heavily doped junction has a very thin depletion layer. With a sufficiently large electric field, electrons can tunnel through the energy barrier, causing a rapid increase in reverse current.

In diodes with higher voltages (usually above about 6V), avalanche breakdown dominates. In this mechanism, carriers accelerated in a strong electric field gain enough energy for impact ionization. Striking the crystal lattice atoms, they generate additional electron-hole pairs. New carriers are also accelerated, leading to an avalanche increase in current.

Around 5.1–5.6V, both mechanisms can partially compensate each other in terms of temperature. Therefore, Zener diodes with a voltage of about 5.6V often have relatively low temperature drift and are commonly used as simple reference voltage sources.

Applications of rectifier diodes

The most well-known application of rectifier diodes is converting AC voltage to rectified voltage, which is a waveform with constant polarity. A single diode can form a half-wave rectifier, which only passes one half of the sinusoidal waveform. In practice, a [bridge rectifier](/ca/en/katalog/mostki-prostownicze_112807/ (known as a Graetz bridge), built from four diodes, is more commonly used. It allows full-wave rectification, utilizing both halves of the AC waveform.

After rectification, filter capacitors are typically used to reduce voltage ripple. In more advanced power supplies, linear voltage regulators, switching converters or voltage regulation circuits are also used downstream of the rectifier.

Diodes are also used for reverse polarity protection. In the simplest variant, a diode can be connected in series with the power supply. When connected correctly, it conducts, and when reversed, it blocks current flow to the protected circuit. Schottky diodes are often used here because they have a lower forward voltage drop than typical silicon diodes.

Another important application is flyback or freewheeling diodes. They are used in parallel with relay coils, solenoids, and motors. During normal operation, the diode is reverse-biased. When the coil current is suddenly interrupted, the coil generates a voltage pulse of opposite polarity. The diode then conducts and allows the safe dissipation of energy stored in the magnetic field. This protects the transistor, microcontroller, or switch contact from overvoltage.

Applications of Zener diodes

A Zener diode is often used for simple voltage stabilization. In a typical circuit, it is connected in parallel with the load, in the reverse direction: cathode to the positive power rail, anode to ground. A current-limiting resistor must be placed before the diode. If the input voltage increases, the diode takes on more current and maintains the voltage across the load close to its Zener value.

This solution is simple and inexpensive but not always very precise. The Zener voltage depends on the current, element tolerance, and temperature. Therefore, in circuits requiring high accuracy, specialized reference voltage sources are used. Nevertheless, Zener diodes are still useful in simple stabilizers, voltage limiters, and protective circuits.

A Zener diode can also protect electronic circuit inputs from overvoltage. When the voltage exceeds a certain value, the diode begins to conduct and limits further voltage increase. In fast overvoltage protection, special TVS diodes are often used, which are structurally designed to absorb short pulses of high energy.

How to properly connect a diode?

The most important thing is to identify the anode and cathode. In many through-hole diodes, the cathode is marked with a stripe on the package. In the diode symbol, the cathode corresponds to the vertical line. In SMD diodes, markings depend on the manufacturer and package, so it is worth checking the datasheet.

In a typical rectifier diode, conventional current flows from the anode to the cathode. If the diode is to conduct, the anode should be on the higher potential side, and the cathode on the lower. If it is to block, the polarity is reversed.

A Zener diode in a stabilization circuit is connected in reverse to a regular conducting diode, meaning the cathode to the positive voltage, anode to ground. It then operates in the reverse direction. A series resistor is necessary because it limits the current flowing through the diode and protects it from damage.

What to watch out for when using diodes?

The first limitation is the maximum forward current. Exceeding this value causes excessive heating of the junction and can lead to permanent damage. The second limitation is the maximum reverse voltage. A regular rectifier diode should not operate in the breakdown region unless the manufacturer explicitly allows such operation.

Power dissipation must also be considered. Even a small voltage drop can cause significant heat dissipation if the current is large. For example, a diode with a 0.7V drop at 2A dissipates about 1.4W of power. This may require an appropriate package, larger copper areas on the PCB, or a heatsink.

In the case of Zener diodes, check the stabilization voltage, tolerance, minimum and maximum operating current, rated power, and temperature coefficient. An improperly selected diode may not stabilize the voltage, overheat, or not protect the circuit effectively enough.

Summary

A diode conducts current mainly in one direction due to the properties of the p-n junction. In the forward direction, external voltage reduces the potential barrier and allows carrier flow. In the reverse direction, the depletion layer widens, and the flowing current is limited to a small leakage current.

When the breakdown voltage is exceeded, the current can increase rapidly. In a regular rectifier diode, this is usually a dangerous state, while in a Zener diode, this phenomenon is intentionally used to stabilize or limit voltage. Thanks to their properties, diodes can also rectify voltage, protect circuits from reverse polarity, and suppress inductive surges.

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