Types of Diodes

Rectification Diode (REC): Structure and Features

Structure Symbol Applications・ Characteristics
Figure: Rectification Diode Structure Figure: Rectification Diode Symbols
  • Used for rectification (i.e. primary side of the power supply block)
  • Mainly 1A-class and higher, high breakdown voltage (400V/600V)

Rectifier diodes, as their name implies, are designed to rectify common AC frequencies. Rectification primarily involves conversion from AC to DC and may include high voltages and currents. Conversion efficiency can vary greatly depending on the operating frequency and conditions. As such, different types are offered, including low VF(forward voltage), high-speed switching, and low-noise models.

Figure: Converting AC to DC
Figure - Rectification Circuit Configuration

[Rectification Circuit Configuration]

Switching Diode (SW): Structure and Features

Structure Symbol Applications・Characteristics
Figure - Rectification Diode Structure Figure - Rectification Diode Symbols
  • Ideal for a variety of switching applications
  • Switching speed:Short reverse recovery time trr

These diodes provide switching operation. Supplying voltage in the forward direction will cause current to flow (ON). Conversely, applying voltage in the reverse direction will stop current flow. Switching diodes are normally characterized by shorter reverse recovery times (trr), resulting in better switching performance.

Switch ON Switch OFF
Figure - ON Current Figure - OFF Current

What is Reverse Recovery Time (trr)?

The reverse recovery time trr refers to the time it takes for the switching diode to turn completely OFF from an ON state. Generally, electrons cannot be stopped immediately after operation turns OFF, resulting in some current flow in the reverse direction.The higher this leakage current is, the greater the loss. However, reverse recovery time can be shortened through heavy metal diffusion, material optimization, or by developing FRDs (Fast Recovery Diodes) that suppress ringing after recovery.

Graph - High switching speed →Smaller area (power consumption)→ Less heat
Key Points
  • Trr refers to the time it takes the current to disappear after voltage switches to the opposite direction.
  • A shorter trr translates to lower loss and higher switching speeds

Switching Diode (SW): Structure and Features

Structure Symbol Applications・Characteristics
Figure - Rectification Diode Structure Figure - Rectification Diode Symbols
  • Ideal for a variety of switching applications
  • Switching speed:Short reverse recovery time trr

These diodes provide switching operation. Supplying voltage in the forward direction will cause current to flow (ON). Conversely, applying voltage in the reverse direction will stop current flow. Switching diodes are normally characterized by shorter reverse recovery times (trr), resulting in better switching performance.

Switch ON Switch OFF
Figure - ON Current Figure - OFF Current

What is Reverse Recovery Time (trr)?

The reverse recovery time trr refers to the time it takes for the switching diode to turn completely OFF from an ON state. Generally, electrons cannot be stopped immediately after operation turns OFF, resulting in some current flow in the reverse direction.The higher this leakage current is, the greater the loss. However, reverse recovery time can be shortened through heavy metal diffusion, material optimization, or by developing FRDs (Fast Recovery Diodes) that suppress ringing after recovery.

Graph - High switching speed →Smaller area (power consumption)→ Less heat
Key Points
  • Trr refers to the time it takes the current to disappear after voltage switches to the opposite direction.
  • A shorter trr translates to lower loss and higher switching speeds

Schottky Barrier Diodes (SBD): Structure and Features

Structure Symbol Applications・Characteristics
Figure - SBD Structure Figure -SBD Symbol
  • Used for secondary power supply rectification
  • Low VF(low loss),large IR
  • Fast switching speed

Unlike conventional diodes that provide diode characteristics through a PN (semiconductor-semiconductor) junction, Schottky barrier diodes utilize a Schottky barrier consisting of a metal-semiconductor junction.This results in much lower VF characteristics (forward voltage drop) compared with a PN junction diodes, enabling faster switching speeds. However, one drawback is larger leakage current (IR), making countermeasures necessary to prevent thermal runaway.

Figure - Schottky barrier diodes feature low VF but large IR

SBDs, which are often used for secondary power supply rectification, feature characteristics that can vary greatly depending on the type of metal used. ROHM offers a broad lineup of industry-leading SBDs that utilize a variety of different metals.

  • RB**1 series low VF type
  • RB**0 series low IR type
  • ROHM offers the RB**8 series of ultra-low-IR diodes for automotive applications
Key Points
  • Low VF and IR types can be achieved by simply changing the type of metal.

Thermal Runaway

Schottky barrier diodes are susceptible to excessive heat generation during large current flow. As a result, the combination of high heat with increasing IR (leakage current) can cause both the case and ambient temperature to rise. Consequently, implementing incorrect thermal design may result in the amount of heat generated to exceed the amount dissipated, which can lead to increasing heat generation and leakage current and eventually result in damage. This phenomenon is referred to as "thermal runaway."

Figure - Heat generation > Heat dissipation→Stable IR/Heat generation <Heat dissipation→Thermal Runaway
Key Points
  • High ambient temperatures can cause thermal runaway

Zener Diode (ZD):Structure and Features

Structure Symbol Applications・Characteristics
Figure - Zener Diode Structure Figure - Zener Diode Symbol
  • Used in constant voltage circuits
  • Protects ICs from damage due to surge currents and ESD
  • Generates a constant voltage when voltage is supplied in the reverse direction

Zener diodes are typically used in constant voltage circuits to ensure constant voltage even if the current fluctuates or as protection elements against surge currents and ESD. Unlike standard diodes that are employed in the forward direction, Zener diodes are designed to be used in the reverse direction. The reverse breakdown voltage of a Zener diode is referred to as the Zener voltage VZ, and the current value at this time is called the Zener current (IZ). In recent years, with the continuing miniaturization and increasing performance of electronic devices comes a need for more advanced protection devices, bringing about the emergence of TVS (Transient Voltage Suppression) diodes.

Graph - Zener diodes maintain a constant voltage even with fluctuating currents
Key Points
  • Only Zener diodes are operated in the reverse direction

High-Frequency Diodes (PIN Diodes):Structure and Features

Structure Symbol Applications・Characteristics
Figure - High-Frequency Diode Structure Figure - High-Frequency Diode Symbol
  • High-frequency switching→Suitable for mobile phones and the like
  • Variable resistor element for AGC(※) and attenuator circuits
    ※AGC: Automatic Gain Control
  • The diode (internal) capacitance (Ct)is very low

A high resistivity I-type semiconductor is utilized to provide significantly lower diode capacitance (Ct). As a result, PIN diodes act as a variable resistor with forward bias, and behave as a capacitor with reverse bias. High frequency characteristics (low capacitance ensures minimal effect of signal lines) make them suitable for use as variable resistor elements in a wide variety of applications, including attenuators, high-frequency signal switching (i.e. mobile phones requiring an antenna), and AGC circuits.

Forward Voltage Reverse Voltage
Figure - Forward Voltage:Charge Accumulation→Decreased Resistivity Figure - Reverse Voltage: An electrically neutral depletion layer is formed by filling the intrinsic layer - created between P and N layers - with charge carriers (holes and electrons).
Variable resistance with forward biasCircuit Diagram Symbol - Variable Resistor Capacitor with reverse biase Circuit Diagram Symbol - Capacitor

What is the Diode Capacitance (Ct)

The amount of accumulated charge internally when supplying a reverse bias is called the diode capacitance (Ct). An electrically neutral depletion layer is formed by filling the intrinsic layer - created between P and N layers - with charge carriers (holes and electrons). The depletion layer acts as a parasitic capacitor, with a capacitance proportional to the PN junction area and inversely proportional to the distance d. The distance is determined by the concentration of the P and N layers. Supplying a voltage to the diode will increase the depletion layer and decrease Ct.The required Ct will vary depending on the application.

[When supplying a reverse voltage]

Figure - Reverse bias: An electrically neutral depletion layer is formed by filling the intrinsic layer (created between P and N layers) with charge carriers (holes and electrons).
Figure - Capacitance calculation
Key Points
  • The wider the depletion layer (and greater the distance) the lower the capacitance Ct.