Types of Diodes and Their Applications

What is a Diode?

You can think of diodes as electrical check valves, allowing electricity to flow from positive to negative in only one direction. Symbolically, we represent this flow with an arrow pointing from the positive (anode) side of the diode to the negative (cathode). At the end of this symbol is a vertical line, which corresponds to a stripe on the negative side of many physical components.

What Does a Diode Do?

Beyond this basic functionality, diodes can perform a variety of specialized functions. These vary from:

1. Diodes that exhibit differing electrical behavior

2. Others that emit light (LEDs)

3. Those that can sense properties like temperature and light levels

We’ll explore several different types below, followed by a discussion of a few common diode uses.

Different Types of Diodes

Let’s explore some of the most notable diode categories of the past and present. With the exception of vacuum tube diodes, all the diodes on this list are semiconductor-based.

Vacuum Tube (Thermionic) Diode: The first vacuum diode was developed in the early 1900s at the same time as the first solid-state (semiconductor-based) diode. While vacuum diodes were commonplace until the middle of the 20th century, they eventually lost market share to semiconductor-based components. Vacuum tube diodes are still used in some high-power applications and specialty audio equipment, though they are relatively rare.

P-N Junction Diode: This type of diode consists of a p-type (positive per an excess of electron holes) semiconductor fused to an n-type (negative per an excess of electrons) semiconductor. The positive semiconductor attaches to a positive voltage source with respect to the negative semiconducting material, which pushes electrons and holes toward the junction area, allowing electricity to flow. When voltage is applied in the other direction, or “reverse biased,” this enlarges the depletion layer between the two areas, stopping electron flow.

Zener Diode: Normal p-n junction diodes will fail if enough voltage is applied in the reverse biased direction, allowing current to flow. However, Zener diodes are designed to exhibit this behavior at a set voltage level as part of their normal operation. These diodes also pass current in the forward bias region similarly to a standard p-n junction diode, but their reverse capability makes them suitable for applications like voltage regulation and AC waveform modification.

True Zener diodes operate at around 5 volts or less, while those that work in higher voltage regions operate using a different principle and are known as avalanche diodes. You will often see both diodes referred to as Zener diodes.

Schottky Diode: When a pure semiconductor diode conducts in only one direction, it will exhibit a 600-700 mV forward voltage drop. Schottky diodes work a little bit differently. Instead of using a p-n semiconductor junction, they use a semiconductor-metal junction to lower the voltage to the 150-450 mV range. N-type semiconductors (excess electrons) are common in Schottky diodes, but you will see p-type semiconductors employed in some situations. Schottky diodes are appropriate for several applications, including as components in DC power supplies and reverse current protection.

Practical Applications of Diodes

We touched on a few diode applications in the section above, but common diode uses also include the following:

1. Rectification: A diode’s most basic function is acting as a rectifier, straightening an alternating AC power source into a constant (or at least varying unidirectional) power source. Any diode can accomplish this task by blocking the flow of power in one direction or the other, though some diodes are better suited for the job than others.

Arranging multiple diodes can also form a full-wave bridge rectifier. Instead of blocking half of an AC signal, the diode allows each half to flow but reverses one side so that current only flows in one direction. You can use capacitors and other components to further smooth and produce power that approaches a linear signal.

2. Light emission: Up until a few years ago, if you needed lighting for your home or office, you’d buy an incandescent light bulb. These brilliant devices have worked well since the time of Edison, but besides light, much of the energy these coils use is converted into heat. LEDs provide a much more efficient source of light. These bulbs cost more than their incandescent counterparts, in part because they require additional control circuitry to work with household AC power.

LEDs are, of course, also available in much smaller form factors without the associated circuitry. These come as through-hole components, or as surface-mount devices meant for use with printed circuit boards. These surface-mount LEDs can come as small as the 0201 form factor, with a thickness of only .2mm.

3. Inductive Load Dissipation: When an inductive load switches off, its stored energy must go somewhere. Without the proper circuit protection, the stored energy can lead to voltage spikes that can arc across the switch or potentially overload a transistor. Diodes in a “flyback” configuration provide a simple solution: electricity flows in the proper direction across the inductor when in operation, then moves in a reverse path across the diode’s forward-biased direction when switched off. This configuration allows current to dissipate across the inductor and be fed back into the power supply, thus protecting the circuit.

4. Sensing and Control: Semiconductors can generate electrical charges based on optical effects. In general, these devices are packaged in such a way as to block out light to avoid unintended electrical activity. Photodiodes, however, are built to optimize this effect, and you’ll find them packaged in a light transmissive material that acts as a sensor. These photodiodes are often used in the infrared spectrum, such as inside consumer remote controls.

While diodes do their job of conducting current in only one direction, as seen here, we can also put them to work in many other applications. We’ve scratched the surface of how diodes operate in the real world, but there’s much more to learn.