What Is a Diode?

A diode is a two-terminal semiconductor device with an anode and a cathode.

Its basic electrical characteristic allows current to flow relatively easily when the diode is forward biased, while current is greatly restricted when the diode is reverse biased.

• Anode → positive side in forward bias
• Cathode → negative side in forward bias

The cathode is commonly identified by a stripe or band printed on the body of many through-hole diodes.

How a Diode Works

A conventional semiconductor diode is formed from a PN junction.

The junction creates a depletion region that influences the movement of charge carriers across the device.

Forward Bias

When the diode is forward biased, the applied voltage reduces the effective barrier of the junction and allows significant current to flow.

Reverse Bias

When the diode is reverse biased, the depletion region becomes wider and only a small leakage current normally flows until the reverse breakdown region is reached.

Diode Terminals

The cathode marking is especially useful when identifying the orientation of a diode on a circuit board.

Diode Symbol

The standard diode symbol indicates the direction of conventional current flow during forward operation.

Anode                    Cathode
  A                         K
  |                         |
  ────────────|>|───────────

The vertical line in the symbol represents the cathode side.

Forward Voltage

A forward-biased diode requires a voltage across its junction before significant current flows. The value depends on the semiconductor material, diode construction, current, and temperature.

These values are approximate. The actual forward voltage should be obtained from the manufacturer's datasheet for the specific diode.

Reverse Voltage

A diode must also be able to withstand the voltage applied in reverse direction.

The maximum specified reverse voltage is an important parameter when selecting a diode for a circuit.

Exceeding the diode's rated reverse voltage can cause breakdown and may damage the component if the resulting current is not limited.

Main Types of Diodes

• Rectifier Diodes → used for converting AC into DC
• Signal Diodes → used for low-current switching and signal processing
• Schottky Diodes → low forward voltage and fast switching
• Zener Diodes → voltage regulation and reference circuits
• LEDs → convert electrical energy into light
• Photodiodes → convert light into electrical current
• Varactor Diodes → provide voltage-controlled capacitance
• TVS Diodes → protect circuits against voltage transients

Common Diode Applications

Diodes in Rectifier Circuits

One of the most important uses of diodes is rectification.

A diode allows one half of an alternating waveform to pass while blocking the opposite direction. Multiple diodes can be arranged into bridge rectifiers to convert both halves of an AC waveform into pulsating DC.

Rectifier diodes are commonly found in:

• Linear power supplies
• Battery chargers
• Amplifier power supplies
• AC-to-DC converters

Diode Reverse-Recovery

When a diode switches from forward conduction to reverse blocking, some diodes require a finite amount of time to recover their blocking ability. This is called reverse-recovery time.

Reverse-recovery characteristics are particularly important in high-frequency switching circuits.

• Ordinary rectifier → suitable for many low-frequency applications
• Fast-recovery diode → suitable for faster switching
• Schottky diode → very fast switching with no conventional PN-junction minority-carrier storage

Important Diode Ratings

When selecting a diode, several electrical characteristics must be considered.

• Forward current → maximum continuous current capability
• Reverse voltage → maximum specified reverse voltage
• Forward voltage → voltage drop during conduction
• Reverse-recovery time → important in switching applications
• Power dissipation → determines allowable heat generation
• Junction temperature → maximum safe operating temperature

Testing a Diode

A digital multimeter with a diode-test function can be used to perform a basic diode check.

Forward Test

• Place the red probe on the anode.
• Place the black probe on the cathode.
• A forward-voltage reading should normally be displayed.

Reverse Test

• Reverse the probes.
• A healthy conventional diode should normally show an open or very high resistance condition.

The exact reading depends on the diode type and the multimeter.

Common Diode Faults

Advantages of Diodes

• Simple two-terminal device
• Small physical size
• Fast switching versions are available
• Available for very low to very high currents
• Many specialized types exist
• Low cost in many applications

Limitations

• Forward voltage causes power loss
• Reverse voltage rating must not be exceeded
• Current rating must not be exceeded
• Real diodes have leakage current
• Switching characteristics vary between diode types

Key Points

• A diode is a two-terminal semiconductor device.
• It has an anode and a cathode.
• It normally conducts in forward bias and blocks in reverse bias.
• Different diode types are designed for different applications.
• Forward current and reverse-voltage ratings are critical selection parameters.
• A multimeter's diode-test mode is useful for basic testing.