What is a diode

• pn junction diode
  pn junction diodes are the most common type of diode, and come in a wide range of products, from general-purpose types to high-voltage and high-frequency types. A slightly more unusual type is the fast recovery diode (FRD), which has improved reverse recovery time.
  As mentioned above, diodes only allow current to flow in one direction (forward direction), but if a large voltage is applied in the reverse direction, a continuous current will suddenly flow at a certain voltage. 
  This phenomenon is called Zener breakdown or avalanche breakdown. This behavior in ordinary diodes leads to deterioration and destruction, but Zener diodes are  specially designed to utilize this phenomenon.
  Zener diodes are known as constant-voltage diodes, but in recent years they have often been used for protection against electrostatic discharge (ESD). In particular, Zener diodes that specialize in ESD protection performance are called ESD protection diodes. We offer products that are suitable for various situations, such as high frequency and power sources.
  Please also refer to the following FAQ and e-learning
  • FAQ: What is diode reverse recovery time (trr)?
  • FAQ: What is a fast recovery diode (FRD)?
  • FAQ: What is a Zener diode (voltage regulator diode, constant-voltage diode)?
  • FAQ: What are TVS diodes (ESD protection diodes)?
  • e-learning: Basics of TVS Diodes (ESD protection diodes)
    
    
• Metal-semiconductor junction diode
  Schottky barrier diodes (SBDs) are characterized by their low forward voltage and lack of reverse recovery time. However, because they involve the junction of dissimilar materials, metal and semiconductor (such as silicon), the leakage is slightly larger than that of pn junction diodes.
  Please also refer to the following FAQs and e-learning materials.
  
  • FAQ: What is a Schottky barrier diode (SBD)?
    
  • e-learning: Basics of Schottky Barrier Diodes

The structure of a pn junction diode is shown as a junction of a p-layer and an n-layer, as illustrated in Figure-2(a). In practice, however, the p-layer is formed by ion implantation or thermal diffusion of impurities (such as boron, B) into an n-type substrate. In some cases, an n-type layer may instead be formed by introducing impurities such as phosphorus (P) into a p-type substrate. 
Schottky barrier diodes are generally constructed by evaporating metal particles onto an n-type. Although it is possible to configure an SBD with p-type, the leakage current is large and it is difficult to use it for general purpose, so n-type is mostly used.

As mentioned at the beginning, a diode only passes current in one direction. The direction in which current flows is called the forward direction, and the direction in which no current flows is called the reverse direction. Accordingly, voltage is defined as forward bias and reverse bias directions, as shown in the diagram.

Fig.3 shows a typical example of the current-voltage characteristics (I_F-V_F characteristics) of a diode, and a simple circuit configuration for measurement.
The measurement covers the low current region up to near the breakdown voltage (V_R) in the reverse bias direction, and near the rise time in the forward bias direction.

Diodes are the simplest semiconductor elements and have been used in a wide range of circuits. Some applications have been replaced by ICs, but here are some examples of where diodes are still used today.

• Rectification:
  
  Fig.4 shows a basic rectifier circuit consisting of a diode and resistor, and an example of the input and output waveforms for an AC input. When an AC signal is input, the output waveform has only one-directional components. This is called rectification.
  This circuit is the most basic circuit, utilizing the one-way conduction characteristics of diodes. When an AC signal centered on GND is input, the diode turns on at a voltage level that is forward voltage (V_F) higher than GND.
  Even today, full-wave rectification circuits (diode bridges) that apply this technique are used in many electronic devices.

Clipper circuits, limiter circuits:
Fig.5 shows examples of a forward clipper circuit, a reverse clipper circuit, and a limiter circuit that combines them, as well as an example configuration of an input protection circuit used in ICs, etc.
The basic idea of ​​using a diode to limit the amplitude of the signal voltage is shown together with the input and output waveforms.
There are forward clippers and reverse clippers in clipper circuits. One application of this is the limiter circuit. This is still used today for input protection (overvoltage protection) of ICs.

• Reverse battery protection / Reverse current protection:
  Fig.6 shows a typical circuit configuration example for protecting a circuit from reverse battery connection.
  This shows the basic idea behind inserting a diode in series with the power supply line to prevent reverse voltage from being applied to the main unit even if the polarity is accidentally connected in reverse.
  In cases where there is a possibility of accidentally connecting the polarity in reverse, such as with batteries, a diode is connected in series to the power line. Since normal pn junction diodes have a large insertion loss, Schottky barrier diodes with a small V_F are used. However, care must be taken regarding leakage current during reverse bias.

• ESD protection:
  Fig. 7 shows a typical circuit configuration example for protecting a circuit from electrostatic discharge (ESD) that intrudes from the outside. It shows the basic idea of ​​using protective elements to protect a circuit from ESD that intrudes through an external interface.
  ESD protection diodes are used by connecting between the signal line and GND of an externally exposed terminal (such as a USB terminal) into which ESD may penetrate. An ESD protection diode or Zener diode specially designed for this purpose is used.