- Diode Definition: A diode is defined as a component that restricts the direction of flow of electric current, mainly allowing current to pass in one direction.
- Symbol and Orientation: The diode symbol represents the direction of conventional current flow, from the anode to the cathode.
- PN Junction Role: The PN junction in diodes helps form a depletion zone that acts as a barrier to current in the reverse direction.
- Forward vs. Reverse Bias: In forward bias, diodes conduct electricity when the external voltage surpasses the barrier potential, while in reverse bias, they block current, creating a wider depletion region.
- Practical Importance: Understanding diode functionality is crucial for various applications in electronics, from basic circuits to complex digital systems.
What is a Diode?
Real diodes do not have zero or infinite resistance. A conducting diode has a forward voltage drop and some dynamic resistance. Under reverse bias, it normally passes only a small leakage current until its breakdown region is reached. The valve analogy is therefore useful for an introductory electrical circuit, but it is an approximation.
Semiconductor diodes are the most common type of diode. A diode is “forward biased” when its anode is at a higher potential than its cathode. The forward current rises gradually at low voltage and then increasingly steeply. The diode is “reverse biased” when the polarity is reversed, so only leakage current normally flows.
The forward current and voltage drop depend on the diode type, current, junction temperature and construction. Values such as 0.7 V for a silicon PN diode are useful circuit estimates, not universal switching thresholds.
A reverse-biased diode blocks substantial current only while it remains within its rated reverse-voltage range. At a high enough reverse voltage, the junction enters breakdown and reverse current rises sharply. The relevant rating is the reverse breakdown voltage.
When the reverse voltage reaches breakdown, the circuit must limit current and power. Zener and avalanche diodes are designed to use controlled breakdown. An ordinary rectifier or signal diode may overheat or fail if its reverse rating is exceeded.
A PN junction forms the basis of many semiconductor diodes. A simple ideal model treats it as a closed switch under forward bias and an open switch under reverse bias. A practical model includes forward voltage, leakage, capacitance, switching time and breakdown limits. These details matter when selecting parts for electronics projects, including best Arduino starter kits.
Diode Symbol
The standard PN-diode symbol is shown below. The vertical bar marks the cathode. Under forward bias, conventional current flows through the symbol from the anode on the p side to the cathode on the n side. Unlike an LED or photodiode symbol, the basic diode symbol does not contain an arrow.
A simple PN junction diode is fabricated by creating adjacent n-type and p-type regions in semiconductor material. Donor doping supplies the n-type region with electrons as majority carriers. Acceptor doping creates the p-type region, where holes are the majority carriers.
The boundary between the p-type semiconductor and n-type semiconductor is the metallurgical junction. Manufacturers normally form these regions within semiconductor material rather than mechanically joining two finished pieces. The p-type terminal is the anode, and the n-type terminal is the cathode.
Working Principle of Diode
A PN diode works through the interaction of its n-type and p-type regions. The n-type region has a high concentration of mobile electrons and a much lower concentration of holes.
Free electrons in the n-type semiconductor are referred to as majority charge carriers, and holes in the n-type semiconductor are referred to as minority charge carriers.
A p-type semiconductor has a high concentration of holes and a low concentration of free electrons. Holes in the p-type semiconductor are majority charge carriers, and free electrons in the p-type semiconductor are minority charge carriers.
The following video gives another introduction to diode operation:
Unbiased Diode
When the n-type and p-type regions form a junction, carrier concentration differences cause diffusion. Holes diffuse from the p-type region towards the n-type region.
Electrons diffuse in the opposite direction, from the n-type region towards the p-type region. Electrons and holes recombine close to the junction.
As mobile electrons leave the n-type side, they expose fixed positive donor ions. As holes leave the p-type side, they expose fixed negative acceptor ions. These ionised dopants remain in the crystal lattice.
The fixed ions create a space-charge region around the junction. It is called the depletion region because it has far fewer mobile electrons and holes than the neutral material on either side. The region is not empty of charge: its fixed ions produce an internal electric field.
The internal electric field opposes further majority-carrier diffusion and produces a drift current in the opposite direction. At thermal equilibrium, diffusion and drift continue microscopically, but their currents balance so the net current is zero.
The positive donor ions on the n-type side and negative acceptor ions on the p-type side establish a built-in potential across the junction. This potential barrier limits the net movement of majority carriers when no external voltage is applied.
Under reverse bias, breakdown can occur through Zener tunnelling, avalanche multiplication or a combination of both, depending on the junction design and breakdown voltage. Avalanche multiplication occurs when accelerated carriers create additional electron-hole pairs by impact ionisation.
Forward Biased Diode
A PN diode is forward biased when the positive terminal of a source connects to the p-type side and the negative terminal connects to the n-type side. Increasing this voltage reduces the junction barrier and narrows the depletion region.
A small forward voltage produces a small current rather than exactly zero current. As the applied voltage increases, more majority carriers cross the junction and the current rises steeply.
The forward current-voltage curve is continuous and approximately exponential over part of its operating range. Silicon PN diodes are often approximated with a 0.7 V drop and germanium diodes with a 0.3 V drop, but the actual value depends on current, temperature and the particular device. A datasheet specifies forward voltage at stated test conditions.
In forward conduction, the diode is not a perfect short circuit. Its voltage drop, internal resistance, junction temperature, current rating and power rating still matter. The surrounding circuit must limit current to keep the junction within the manufacturer’s limits.
Reverse Biased Diode
A PN diode is reverse biased when the negative terminal of the voltage source connects to the p-type side and the positive terminal connects to the n-type side. The applied field pulls holes in the p-type region away from the junction, exposing more fixed acceptor ions.
Electrons in the n-type region also move away from the junction towards the positive terminal, exposing more fixed donor ions.
The depletion region therefore widens and its barrier increases. Majority-carrier current becomes extremely small, so the diode blocks substantial reverse current while the applied voltage remains below breakdown.
Thermal generation leaves a small population of electrons in the p-type region and holes in the n-type region. These are minority carriers.
In reverse bias, the electric field sweeps minority carriers across the depletion region. This process produces a small reverse leakage current.
The leakage is often modelled as reverse saturation current, although practical leakage also depends on device construction, reverse voltage and surface effects. It usually increases strongly as junction temperature rises, so datasheet limits state both voltage and temperature.
At breakdown, reverse current increases sharply. A heavily doped, low-voltage junction can break down mainly through Zener tunnelling. A more lightly doped junction at a higher voltage usually breaks down mainly through avalanche multiplication.
Breakdown itself is not necessarily destructive. The diode can operate there only if it is designed for that mode and the circuit limits current, junction temperature and power. Exceeding those ratings can permanently damage the device.
Types of Diode
The types of diode include:
- Zener diode: designed to regulate or clamp voltage in controlled reverse breakdown.
- PN junction diode: used for general rectification, switching and signal processing.
- Tunnel diode: a heavily doped junction with a negative differential-resistance region.
- Varactor diode: uses voltage-dependent junction capacitance for electronic tuning.
- Schottky diode: uses a metal-semiconductor junction for low forward voltage and fast switching.
- Photodiode: converts incident light into current, commonly under reverse bias.
- PIN diode: includes a wide intrinsic region for RF switching, attenuation and light detection.
- Laser diode: emits coherent light when driven in forward bias.
- Avalanche diode: is designed for controlled avalanche breakdown.
- Light emitting diode: emits light through electron-hole recombination under forward bias.
