Diodes

What are Diodes?

A diode is an electrical component that only allows current flow in one direction, as long as it is operated within its specified voltage levels. A diode lets charges through when the current is flowing from the anode end to the cathode end. The diode is known as being forward biased. When the current flows in the opposite direction, charges encounter is a very high resistance, blocking the flow of electrons. When this happens, the diode is described as reverse biased. When a diode is reverse biased, it blocks the flow of electricity until a certain voltage. This voltage is know as the reverse breakdown voltage. This is the point where the flow of electricity can "overpower" the barrier, allowing charges to through a reverse biased diode [1].

P-N Junction Diode

P-N junction diodes are diodes composed of a p-type semiconductor and an n-type semiconductor [4]. It is often referred to as a generic or standard diode. There is a wide range of diodes, with different specifications, but they usually fall into two categories, Signal Diodes and Power Diodes. Power diodes are built with larger P-N junctions compared signal diodes. This result in power diodes being able to handle much higher forward amperage and and have reverse blocking voltages [5].

Considerations

Signal Diode

As diodes create a small voltage drop when forward biased, they will generate heat. If their maximum forward current rating is exceeded, the diode can fail due to the excess heat generated. On the same note, it is important to keep diodes below their maximum operating temperature.
Another key consideration is the maximum reverse voltage [6]. 

Power Diode

Due to the large P-N junction, power diodes have slow reverse recovery time. It is slow enough that it is not suitable for high frequency application above 1 MHz, although specialized (expensive) high frequency diodes do exist [5]. 

Common Usage

• • • • Power Rectification

      • Data line Protection

      • Solid state switch protection (Freewheel diode)

      • Regulated Voltage Supply
      • Reverse Polarity Protection
      • More examples with diagrams

Light Emitting Diode

Light Emitting Diodes or LEDs are very similar to PN diodes [8]. As the name suggest, they emit light when power flows through them. They come in a variety of colors that range from infrared to visible light to ultraviolet.

Considerations

The main consideration when using LEDs is properly driving it, so that it does not burn out, and managing the heat output. Properly driving an LED may range from just limiting the current with a resistor to using a specifically designed driving circuit. As for heat management, the higher the power usage of the LED, the more heat it generates. Different types of heatsinks and heat pumps can be used to cool an LED.

Common Usage

• • • • Lighting
      • Status Indication
      • Remote Controls

Laser Diode

Laser Diodes are very similar to LEDs. The only difference is that they emit in phase coherent light.

Considerations

The performance of a laser diode is directly impacted by the current flow. As a result, depending on the requirements of the application, proper driver circuits must be used [9]. Heat is an other consideration. Just like LEDs, there are different ways to cool the diodes, from heatsinks to heat pumps.

When working near any Lasers, it is important follow any safety procedures or protocols as lasers have the capabilities of causing harm.

Common Usage

• • • • DVD/CD reader and burner
      • Laser range finder
      • Security systems
      • Laser etcher
      • Fiber Optic

Photodiode

Photodiodes are P-N diodes that can convert light energy into electrical voltage and current. They have a linear response to light levels [10]. They are used in two different modes, photovoltaic mode and photoconductive mode. In photovoltaic mode, also known as zero biased mode, the photodiode is not connected to any sources of electricity. Instead, it generates voltage from light sources. In photoconductive mode, the photodiode is put in reverse biased in a circuit. When it is hit by light, it will allow current to flow through it [11].

Considerations

There are four main considerations for when using photodiodes. The response speed or time, responsivity, dark current, and breakdown voltage. The responsivity is also known as the Quantum efficiency and it is the ratio of the current generated from incident light to the incident light's power. Dark current is a very small amount of leakage current that flows through the diode when it is not illuminated. Due to this, it might be difficult to detect low power incident light sources [11].

Common Usage

• • • Light Intensity Sensors
    • Optocoupler
    • Cameras
    • Smoke detectors

Schottky Diode

Schottky diodes are made out of a metal electrode and an n-type semiconductor. They produce a lower voltage drop than the p-n diodes. They also have a faster recovery time (or switching time) than p-n diodes. However, they do have higher reverse leakage currents and lower reverse breakdown voltages [12].

Considerations

Schottky diodes have very similar considerations to p-n diodes. However, It is especially important to take into account the higher reverse leakage currents and the much lower breakdown voltages. Depending on the application, it might introduce unwanted behaviors.

Also, depending on the application, the switching time and voltage drop may be key features to be considered.

Common Usage

• • • • High Frequency Digital Circuits
      • Schottky Transistor-Transistor Logic (TTL) digital logic gates and circuits

Zener Diode

Zener diodes are specifically designed p-n diodes to have a low and specific breakdown voltage. Thus, they are mainly reverse biased when used in circuits. The specific breakdown voltage is also referred as the knee voltage or Zener voltage [13].

Considerations

One key consideration is the maximum reverse current. If exceeded, the diode will overheat, which will damage the diode [13]. The other consideration is the zener voltage, depending on the application and circuit, will vary.

Common Usage

• • • • Voltage Regulator

      • Transient Suppressor

      • Voltage Clipping

      • Reference Voltages