Diodes are essential components in electronics devices, used to control the current flow direction; their unidirectional current flow nature, is a fundamental feature that support different functions in circuits ranging from power rectification, limiting voltages, clamping to radio frequency signal detection and protection, just to name a few.
There are various diodes types, and our discussion in this article will focus on some of the commonly used diode types such as the standard/rectifier diode, Schottky diode, LED, Photodiode, TVS diode, among others.
Each diode type is designed to accomplish specific purposes in a circuit as per the specifications. Learning about the different types of diodes, will equip you with the relevant knowledge you may need to enable you choose the right diode for your projects and avoid common design errors; this will go a long way in helping you build reliable electronic systems that can stay to their full life.
Diode Basic Principles
A diode is a device that allows current to flow only in one direction. A semiconductor diode is made from P-type and N-type semiconductor materials joined together. It has a depletion layer/P-N junction, also referred to as potential barrier.
Semiconductor diode i.e. P-N junction device conduction depends strongly on its bias.
- Forward bias – a positive voltage is applied to the anode while the cathode is connected to the negative terminal. This reduces the depletion region. The diode conducts once the forward voltage overcomes the potential barrier approximately ≈0.6 to 0.7 V in silicon and 0.2 – 0.3 V in germanium semiconductor materials. The current then rises exponentially with voltage – a practical device behavior that is set by forward voltage and a series resistance.
- Reverse bias – In this case, the anode is connected to negative terminal while the cathode is connected to positive terminal of the power supply. The depletion layer is widened. Ideally there is no current flow, until breakdown; real diodes usually have leakage current and a defined breakdown behavior, case in point is the Zener/avalanche diodes that are engineered for controlled breakdown.
Diode Parameters
There are various diode parameters important to circuit design and component selection. Semiconductor manufacturers usually provide the detailed specifications for their products, diodes included in datasheets.
A typical diode datasheet will contain figures for the following parameters:
- Forward voltage (VF) – the voltage across the diode when it is conducting at a specified forward current under normal conditions.
- Peak inverse voltage (PIV) also called PRV in datasheets – the maximum reverse (inverse) voltage that a diode can withstand without breaking down when it is reverse-biased.
- Maximum repetitive reverse voltage (VRRM) – the maximum amount of voltage the diode can withstand in reverse-bias mode in repeated pulses.
- Maximum DC reverse voltage (VR or VDC) – the maximum amount of voltage the diode can withstand in reverse-bias mode on continual basis.
- Maximum (average) forward current (IF(AV)) – the maximum average amount of current the diode is able to conduct in forward bias mode. This is basically a thermal limitation: how much heat the P-N junction can handle, given that dissipation power is equal to current (I) multiplied by voltage (V or E) and forward voltage is dependent on both current and junction temperature.
- Maximum total dissipation (PD) – this is the amount of power (in watts) allowable for the diode to dissipate, given the dissipation (P = IE) of diode current multiplied by diode voltage drop, and also the dissipation can be given as (P = I2R) of diode current squared multiplied by bulk resistance. Essentially this is limited by the diode’s thermal capacity i.e. the ability to tolerate high temperatures.
- Operating junction temperature (TJ) – the maximum allowable temperature for the diode’s PN junction, usually given in degrees Celsius (°C).
- Thermal resistance R(Ɵ) – this is the temperature difference between junction and the outside air – R(Ɵ)JA or between junction and leads – R(Ɵ)JL for a given power dissipation. It is normally expressed in units of degrees Celsius per watt (°C/W).
- Reverse recovery time (trr) – the amount of time it takes for a diode to “turn off” when the voltage across it alternates from forward-bias to reverse-bias polarity.
- Maximum reverse current (IR) – the amount of current through the diode in reverse-bias operation, with the maximum rated inverse voltage applied (VDC), sometimes referred to as leakage current.
- Typical junction capacitance (Cj) – the typical amount of capacitance intrinsic to the junction, due to the depletion region acting as a dielectric separating the anode and cathode connections.
Common Diode Types: Characteristics and Applications
Utilizing the different phenomena in semiconductors, it is possible to create various types of diodes with specific characteristics. Each type of diode is designed to accomplish certain purposes in electronic circuits according to specifications. It is important to consider the diode’s parameters per the manufacturer’s datasheet when selecting a given diode say for a specific PCB project. This ensures that your projects meet the design specifications and requirements.
There are many diode types, we can’t cover each and every one of them here, but we will look at some of the common types for our discussion herein.
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Semiconductor Diode/P-N Junction Diode
This diode is a p-n junction packaged as an electronic component. This is a two-terminal device: One terminal is known as the anode (A) and the other Cathode (K). The symbol for this diode is given below:
It acts as one-way gate to current flow, from anode (A) to cathode (K).
They are created either from silicon or germanium semiconductor material.
Both require a forward-bias voltage to conduct: typically ≈0.6 to 1.7 V for silicon type, and ≈0.2 to 0.4 V for germanium type.
Applications of Semiconductor Diodes
Semiconductor diodes have extensive application in modern electronic devices; you will find them in circuits that include: rectification, transient suppression, voltage multiplication, analog logic, RF demodulation, fast switches, voltage regulation, clamps, and so on,
Schottky Diode
It has similar operation to p-n junction diode, but it is designed with special metal semiconductor instead of a p-n junction. This provides for extremely low junction capacitance that stores less charge.
As the junction is between a metal and a semiconductor the charges can move fast when polarities are changed in this mode. Additionally since the junction capacitance of these diodes is small; it allows a high switching speed required for fast clamping and high-frequency applications approaching the gigahertz range.
Furthermore, Schottky diode has a lower forward-bias voltage, of around ≈0.4 V but can be from ≈0.15 to 0.9 V or more.
Schottky Diode Applications
Schottky diodes are used in similar applications as p-n junction diode, but offers low-signal level detection, speed, and low-power loss in rectification due to low forward threshold.
Typical applications include: high-frequency rectifiers, low-drop rectification, RF circuits and solar power systems where they are used to prevent reverse current.
Related article: Schottky Diode – Features & Applications
Zener Diode
Zener diodes are designed in such a way to conduct current under reverse bias. They are fabricated to work in the breakdown region under a special tunneling breakdown condition called Zener breakdown. This breakdown occurs at a specifically defined value of voltage. As this breakdown voltage is maintained as the reverse current increases, this diode can be used as a voltage reference.
The breakdown voltage is kept constant even if the current changes without damaging the diode.
With reference to the figure above, the Zener diode will conduct from A to C just like p-n junction diode, but will also conduct from C to A, if the applied reverse voltage is greater than the Zener’s breakdown voltage rating Vz.
The Zener diode comes with various breakdown voltages: 1.2 V, 3.0 V, 5.1 V, 6.3 V, 9 V, 12 V, etc., and power ratings.
Zener Diode Applications
Typical applications of Zener diodes include: voltage regulation, waveform clipping, voltage shifting, and transient suppression.
Light-Emitting Diode (LED)
Light-emitting diode (LED) is a diode that gives off visible light when forward-biased. LEDs are not made from silicon or germanium but are made using elements like gallium, phosphorus, and arsenic. By varying the quantity of these elements, it is possible to produce light of different wavelength with colors that include: red, green, yellow and blue. A case in point is when a LED is fabricated using gallium arsenide, it produces a red light. If the LED is manufactured using gallium phosphide, it will produce a green light.
The operation of light-emitting diode (LED) can be described as follows:
When a LED is forward-biased, the electrons from the n-type material cross the p-n junction and recombine with the holes in the p-type material. When recombination takes place, the recombining electrons release energy in the form of heat and light. In germanium and silicon materials, almost the entire energy is given up in the form of heat and emitted light is insignificant. But in materials such as gallium arsenide, the number of photons of light energy is sufficient to produce quite intense visible light.
Light-emitting diode (LED) emits a near constant wavelength of light when forward-biased (A > C) by a voltage of about 1.7 V.
LEDs come in various wavelengths (IR though visible), sizes, power ratings, and so forth.
LEDs Applications
LEDs are used as indicators and emitting source in IR and light-wave communications.
Transient Voltage Suppression (TVS) Diode
The TVS diodes are built to protect sensitive electronics from voltage spikes by clamping the excess voltage to a safe level, thus preventing damage to the device. They are the main defense against electrostatic discharge (ESD) events, surge transients and voltage spikes.
TVS Applications
Transient voltage suppression diodes are used in power supplies, communication lines, and automotive electronics to protect against transient surges from lightning or power line faults.
Photodiode
Photodiode generates a current when exposed to light, or can be used to alter current flow passing through it when the light intensity changes.
The photodiode operates in reverse-bias direction, i.e. the current flows from (C to A).
When a photodiode is exposed to light, the current through it increases with light intensity.
Photodiode Applications
Photodiodes are use in a number of applications that include:
- Alarm circuits
- Automatic switching systems
- Counting
- Fiber optic networks – to change light energy to electric energy
Tunnel Diode
A tunnel diode is a p-n junction that exhibits negative resistance between two values of forward voltage, that is, between peak-point voltage and valley-point voltage.
A conventional diode exhibits positive resistance when it is forward-biased or reverse-biased. But if a semiconductor junction diode is heavily doped with impurities, it exhibits negative resistance, that is, current decreases as the voltage is increased in certain regions in the forward direction. This type of diode is termed to as tunnel diode. They are also known as Esaki diodes.
Tunnel Diodes Applications
Tunnel diodes are used in oscillators e.g. tuning circuits and in fast switches.
PIN Diode
A PIN diode is a fast low capacitance switching diode. It is manufactured like a silicon switching diode with an intrinsic region added between the PN junction layers yielding a thicker depletion region, the insulating layer at the junction of a reverse-biased diode. This results in lower capacitance than a reverse-biased switching diode.
PIN Diode Applications
PIN diodes are used as RF and microwave switches. To high-frequency signals, the PIN diode acts like a variable resistor whose value is controlled by an applied dc forward-bias current.
A common application for PIN diodes is their use as transmit/receive switches in transceivers operating from 100 MHz and up. They are also used as photo detectors in fiber optic systems.
Related article: PIN Diode – Features & Applications
Diode Selection Guide: Key Factors to Consider When Selecting a Diode for Your Electronic Circuit
Key specifications to consider when selecting a diode are peak inverse voltage – PIV; forward current handling capacity – IO(max), response speed – tR (time for diode to switch on and off), reverse-leakage current – IR(max), and maximum forward-voltage drop – VF(max).
For example, in rectification applications (e.g. power supplies, transient protection); the most important specification to consider is PIV and current rating. The peak negative voltages that are stopped by the diode must be smaller in magnitude than the PIV, and the peak current through the diode must be less than IO(max). In fast and low-voltage applications, tR and VF are important characteristics to consider.
To avoid design errors and unforeseen breakdowns of the electronic devices in the field, you need to have a clear checklist of application requirements to enable you make an informed decision on what kind of diode is the right choice for that specific application.
- Define electrical requirements: voltage, current, switching frequency, power dissipation and ambient temperature range. For example here you can define the maximum reverse voltage the diode can handle, that includes transients.
- Identify the functional role of the diode: do you intend to use it for rectification, switching, regulation, protection, signal processing, sensing, optical, RF function, etc.?
- Prioritize parameters: VF vs. leakage current vs. speed vs. capacitance vs. power. For power delivery – prioritize IF, PD, VF, thermal path. For protection – prioritize VBR/clamping voltage, peak pulse power, response time. For high-speed circuits – prioritize trr and Cj.
- Choose device family that matches the role e.g. Schottky for low VF and fast switching; TVS for surge protection; Zener diode for reference; PIN diode for RF, and so forth.
- Select package with sufficient thermal performance and mechanical robustness.
- Verify derating and safety margins; ensure that surge ratings and thermal design meet worst-case scenarios.
- Confirm availability, cost, and manufacturer support (datasheets, models).
- Prototype and validate (measure VF, leakage, temperature rise, switching behavior, etc.) under real operating conditions.
Conclusion
Having a good understanding of the various diode types and their features, is essential to making the right choice of diode for a specific application and thus being able to meet the circuit design requirements. For instance if you are working on fast switching application, a strong background knowledge about the various types of diodes and their capabilities will enable you to make an appropriate choice of what kind of diode can meet the fast switching requirements of you design.
Selecting the right diode boils down to striking a balance of electrical performance, thermal management, cost, and reliability. Match the diode type to the device role, prioritize the most critical parameters for your application, and always validate on hardware with realistic operating conditions.
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