A semiconductor diode is an electronic component that allows current to flow in only one direction. It is a two-terminal device typically made from a single PN junction. Diodes play a strategic role in electrical and electronic systems, with applications ranging from low current signal processing to high current rectifiers and inverters.
This article provides an in-depth overview of the inner workings, characteristics, parameters, types, testing methods and applications for semiconductor diodes. It also covers recent advancements and key manufacturers.
How a PN Junction Diode Works
A PN junction diode consists of a crystal semiconductor with two adjacent regions doped differently, forming a junction:
- P-type region contains excess holes as majority carriers.
- N-type region contains excess electrons as majority carriers.
- At junction boundary, diffusion of carriers creates a depletion region.
- Built-in potential develops across depletion region due to imbalance of charges.
Forward Bias: External voltage applied in forward direction reduces junction barrier potential, allowing current flow.
Reverse Bias: External voltage applied in reverse direction increases junction barrier potential, preventing current flow.
This selective conduction makes the PN junction diode a rectifying device.
Current-Voltage Characteristics
- In forward bias, current increases exponentially as junction barrier voltage reduces.
- In reverse bias, only leakage current flows until breakdown voltage is reached.
- Forward voltage drop remains constant around 0.7V for silicon diodes.
Key parameters from IV curve:
- Peak inverse voltage – Maximum reverse voltage before damage.
- Maximum forward current – Highest allowable forward current.
- Breakdown voltage – Reverse voltage causing surge in reverse current.
Types of Semiconductor Diodes
Various types of diodes optimized for different applications are:
| Diode Type | Description | Application |
|---|---|---|
| Signal Diode | Low power diodes with low junction capacitance. | Wave shaping, demodulation, clamping. |
| Rectifier | Diodes with high PIV rating and current capacity. | AC to DC conversion in power supplies. |
| Zener | Operate in reverse breakdown region for voltage regulation. | Maintain constant voltage. |
| Photodiode | Conduct when illuminated. | Convert light into electric current. |
| Varactor | Junction capacitance varies with reverse voltage. | Voltage controlled capacitors. |
| Tunnel | Made from heavily doped PN junction. | High speed switching. |
| PIN | Sandwiched intrinsic layer storing charge carriers. | Photodetectors, RF switches. |
| LED | Emits light when forward biased. | Lighting, indicators. |
| Schottky | Metal-semiconductor junction. | Switching, clamping. |
Specialized diodes also include transient voltage suppressors, mixer and detector diodes, constant current diodes and microwave diodes.
Key Parameters and Characteristics
Important diode parameters to check when selecting for a design:
- Forward voltage drop – Typically 0.7V for silicon, 0.3V for germanium.
- Maximum repetitive reverse voltage – Must exceed application voltage.
- Maximum forward current – Depends on power handling needs.
- Peak inverse voltage – Rating for maximum transient voltage during reverse bias.
- Reverse recovery time – Time taken to switch junction from forward to reverse.
- Junction capacitance – Inherent capacitance arising from depletion region.
- Breakdown voltage – Minimum reverse voltage to cause surge in reverse current .
- Operating temperature range
Diode Testing and Measurements
Specialized instruments used for testing diodes include:
- Multimeter – Measures forward voltage drop and resistances. Check for short or open fault.
- LCR meter – Measures junction capacitance and inductance.
- Curve tracer – Plots complete IV curve to extract key parameters.
- Oscilloscope – Observe diode switching waveforms.
- EMI meter – Check for electromagnetic interference emitted.
- Spectrum analyzer – Analyze frequency components in diode circuits.
- Network analyzer – Evaluate S-parameters, impedances.
- Transient analyzer – Capture fast switching transients.
Automated test fixtures are used during manufacturing for quality control.
Common Diode Packages and Markings
Diodes are packaged in a variety of through-hole and surface mount formats:
- Through-hole – Axial, DO-4, DO-7, DO-35, DO-41
- Surface Mount – SOD-323, SOD-523, SOD-123, SOD-323F, SOD-128, SOT-23, SOT-143, SC-79, MELF
Markings include:
- Manufacturer logo
- Type number
- Polarity marking – Cathode band for through-hole
Specialized packages used for high power, RF and microwave diodes.
Diode Applications and Circuits
Some major applications of diodes include:
- Rectification – Converting AC to DC, e.g. in power supplies.
- Clamping – Clamping voltage peaks to predetermined levels.
- Voltage Regulation – Maintaining constant voltage, e.g. using Zener diodes.
- Switching – Using fast switching diodes in oscillators, converters.
- Demodulation – Extracting signal from modulated carrier wave like in AM radios.
- Logic Gates – Diode logic operations like AND, OR gates.
- Snubbers – Suppress voltage spikes in inductive circuits.
- Voltage Multipliers – Generating multiplied DC voltage from AC.
Along with discrete diodes, integrated circuits like rectifier bridges and TVS arrays are also used.
Latest Advancements in Diode Technology
Some recent improvements and innovations in diodes include:
- Development of Schottky diodes using graphene and other 2D materials like MoS2 allowing switching speeds up to 100 GHz.
- Gallium Nitride diodes for high efficiency AC-DC conversion and wireless charging with reduced conduction and switching losses.
- Zener diodes with improved power handling capacity and energy efficiency using silicon carbide substrates.
- Photonic-crystal enhanced photodiodes with responsivities exceeding 3000 A/W using nano-resonating structures.
- Single photon avalanche diodes with high gain and low noise for medical imaging and particle detection.
- Microscale tunnel diodes using CMOS manufacturing achieving terahertz bandwidths.
- Microwave Impatt diodes generating high power at millimeter-wave frequencies for 5G, radar and satellite networks.
Continued research and development enables diodes with improved performance across speed, efficiency and power handling.
Major Manufacturers
Some leading global diode manufacturers include:
- NXP Semiconductors
- ON Semiconductor
- Microsemi
- Renesas Electronics
- ROHM Semiconductor
- Semtech
- Infineon
- Toshiba
- Littelfuse
- Micro Commercial Components
These companies offer broad portfolios of diode devices along with application specific products. Custom diodes can also be developed based on specific design requirements.
Advantages of Diodes
Key benefits offered by semiconductor diodes:
- Allow electricity to flow in only one direction.
- Rectify alternating current into direct current.
- Provide voltage regulation and surge protection.
- Offer high speed switching capabilities.
- Exhibit varied capacitance vs voltage characteristics.
- Enable detection and emission of light.
- Simple, low cost two-terminal construction.
- Rugged and reliable with long service life.
These capabilities make diodes indispensable for electrical and electronics engineering.
Conclusion
Semiconductor diodes utilize the unidirectional conductive properties of PN junctions for a wide range of applications, from signal processing to power conversion. Carefully considering diode parameters like peak inverse voltage, maximum current, capacitance and switching speeds allows selecting the optimal device.
Continued research in materials like silicon carbide and manufacturing improvements through techniques like lithography are enhancing diode performance. Innovations such as Schottky and photonic-crystal diodes open up new capabilities.
The versatile diode, over a century after its discovery, continues finding new uses in fields ranging from renewable energy and electric vehicles to 5G wireless networks and AI accelerators.Thanks to its simple working mechanism and reliability, the PN junction diode remains a foundational electronic component.
Frequently Asked Questions
Q1. How is a Zener diode different from normal PN junction diodes?
Unlike regular diodes which block reverse current, Zener diodes are designed to operate in breakdown region allowing precisely controlled reverse current flow for applications like voltage regulation.
Q2. What are tunnel diodes used for?
Tunnel diodes exploit a quantum mechanical effect called tunneling to achieve very fast switching speeds. They are used in high frequency oscillators, amplifiers and fast logic circuits.
Q3. How do LEDs emit light?
LEDs contain a PN junction designed to emit photons when forward biased. Electron-hole recombination near the junction generates photons causing electroluminescence. Doping and materials determine emitted color.
Q4. How can diodes be tested with a multimeter?
Set multimeter to diode checking mode. Connect positive probe to anode, negative probe to cathode. Forward voltage drop should indicate around 0.7V for silicon diode. Reverse connection should show high resistance.
Q5. What is a PIN diode and its applications?
A PIN diode has an intrinsic semiconductor region between P and N regions. It can control microwave signals owing to variable resistivity of the intrinsic layer. Used in RF switches, attenuators, photodetectors.
