What are Diodes? - Characteristics and Functions

Light emitting diode (LED) is a light source that meets the requirements of green lighting. LEDs are safe, efficient, environmentally friendly, long-lived, responsive, small in size and robust in construction with many features that are unmatched by ordinary light-emitting devices.

Moreover, it is one of the first semiconductor devices and is widely used. Currently, LEDs are widely used as indicators for various electronic products and as light sources for fiber optic communication.

How dose a diode work?


I What is a diode?

II How dose a diode work?

III What is diode characteristics?

3.1 Forward characteristics

3.2 Reverse characteristic

IV What are diode parameters?

4.1 Rated forward working current (IF)

4.2 Forward Voltage(VF)

4.3 Maximum reverse operating voltage (VR)IV What are diode parameters?

4.4 Reverse current IR

4.5 Reverse critical current (IZ)

4.6 Reverse critical voltage (VZ)

4.7 Reverse recovery time (Trr)

V What are the types of diodes?

5.1 Light emitting diode

5.2 Zener diode

5.3 Switching diode 

5.4 Variable capacitance diode( Varactor Diodes )


I What is a diode?

Diode is an electronic device made of semiconductor materials (silicon, selenium, germanium, etc.) . It has a unidirectional conductivity, that is, the diode anode and cathode to add a forward voltage, the diode conducts. When the reverse voltage is added to the anode and cathode, the diode cuts off. Therefore, the on and off of the diode is equivalent to the on and off of the switch.

In almost all electronic circuits, semiconductor diodes are used. The use of semiconductor diodes in the circuit can play a role in protecting the circuit, extending the life of the circuit. The development of semiconductor diodes has made integrated circuits more optimized and has played an active role in various fields. Diodes have many roles in integrated circuits and maintain the proper functioning of the integrated circuits.

Diodes were one of the first semiconductor devices to be created, and their applications are very widespread. Especially in a variety of electronic circuits, the use of diodes and resistors, capacitors, inductors and other components to make a reasonable connection to form a circuit of different functions, you can achieve a variety of functions such as rectification of alternating current, detection of modulated signals, limiting and clamping, and voltage regulation of the supply voltage. Whether in common radio circuits or in other household appliances or industrial control circuits, diodes can be found.

A diode is made of a PN junction with corresponding electrode leads and a tube housing package. The diode has two electrodes, the electrode leading from the P area is the positive electrode, also known as the anode; the electrode leading from the N area is the negative electrode, also known as the cathode.


Diode structure

There are many kinds of diodes:

 - According to the semiconductor materials used, they can be divided into germanium diodes and silicon diodes. 

 - According to their different uses, they can be divided into detector diodes, rectifier diodes, zener diodes, switching diodes, etc. 

 - According to the structure of the tube core, they can be divided into point-contact diodes, surface-contact diodes and planar diodes. 

   --  The point contact diodes are pressed on the smooth surface of the semiconductor wafer with a thin metal wire. With pulse current, one end of the contact wire is sintered firmly with the wafer to form a "PN junction". Due to point contact, only small currents (no more than a few tens of mA) are allowed, which is suitable for high frequency low current circuits, such as radio detection, etc. The area of "PN junction" of surface contact diode is large, which allows large currents passing through and is mainly used in "rectifying" circuits that convert AC to DC. 

  --  Planar diode is a kind of special silicon diode. It not only can pass through large current, but also has stable and reliable performance. It is widely used in switching, pulse and high frequency circuits.

II How does a diode work?

The crystal diode is a p-n junction formed by p-type semiconductor and n-type semiconductor. It forms a space charge layer on both sides of the interface and has a self-built electric field. When there is no applied voltage, the diffusion current caused by the carrier concentration difference on both sides of the p-n junction is equal to the drift current caused by the self-built electric field, so it is in an electric equilibrium state.

When the external positive voltage is biased, the mutual suppression of the external electric field and the self-built electric field results in the increase of the carrier diffusion current, which is shown in the conduction region below.

When the external reverse voltage is biased, the external electric field and the self-built electric field are further strengthened to form a reverse saturation current I0 independent of the reverse bias voltage value within a certain reverse voltage range, which is shown in the cut-off region below.

When the applied reverse voltage is high enough to a certain extent, the electric field intensity in the space charge layer of p-n junction reaches the critical value to produce the multiplying process of the carriers, resulting in a large number of electron hole pairs and a numerical reverse breakdown current is generated, known as the diode breakdown, which is shown in the breakdown region below.

III What is diode characteristics?

The most important characteristic of the diode is the unidirectional conductivity. In the circuit, the current can only flow from the positive electrode of the diode, and flows out from the negative electrode . The forward and reverse characteristics of the diode are illustrated by simple experiments.

3.1 Forward characteristics

In electronic circuits, if the diode is connected to the high potential terminal and the negative electrode to the low potential terminal, the diode will be switched on. This connection is called forward bias. It must be noted that when the forward voltage applied to both ends of the diode is very small, the diode still cannot be switched on, and the forward current flowing through the diode is very weak. Only when the forward voltage reaches a certain value (about 0.6 V of the silicon tube) can the diode be truly switched on. The voltage at both ends of the diode after conduction is called the forward voltage drop of the diode.

3.2 Reverse characteristic

In the electronic circuit, the positive end of the diode is connected to the low potential end, the negative electrode is connected to the high potential terminal, and the diode is in the cutoff state. This mode of connection is called reverse bias. When the diode is in reverse bias, there will still be a weak reverse current flowing through the diode, called leakage current. When the reverse voltage of the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose the single direction conduction characteristic. This state is called the breakdown of diode.

IV What are diode parameters?

The technical specifications used to test the performance of diodes are called diode parameters. Here are some of the main parameters in diode testing:

4.1 Rated forward working current (IF)

Refers to the maximum forward current that is allowed to pass through the diode when it is in continuous operation over a long period of time. When a larger current passes through the diode, the dice is heated and the temperature rises, and when the temperature exceeds the allowable limit, the dice is overheated and damaged. Therefore, it should not exceed the diode rated forward operating current value when the diode is in use.

Eg. The rated forward working current of DFM is 1A

4.2 Forward Voltage(VF)

Refers to the voltage at both ends of the diode when the rated forward working current IF is passed through the diode.

Eg. The voltage at both ends of the diode is about 0.9V when the forward working current of DFM is 1A.

4.3 Maximum reverse operating voltage (VR)

When the reverse voltage at both ends of the diode is raised to a certain value, the diode will be broken down and the unidirectional conductivity will be lost. In order to ensure the safety of operation, the maximum reverse operating voltage is specified.

Eg. The maximum reverse operating voltage of DF10M is 1100V and the breakdown voltage is about 1400V

4.4 Reverse current IR

Refers to the reverse current that flows through the diode when the maximum reverse operating voltage VR is applied to both ends of the diode. The smaller the reverse current, the better the unidirectional conductivity of the diode will be.

Eg. When the reverse voltage of DF10M is 1100V, the VR is about 0.2uA.

4.5 Reverse critical current (IZ)

Refers to the reverse current of the diode increases sharply to close to the breakdown phenomenon.

Eg. Set the IZ of DF10M to 0.1 Ma (Ma)

4.6 Reverse critical voltage (VZ)

Refers to the reverse voltage of the diode when the reverse current is IZ. If the reverse voltage is greater than this value, the reverse current increases dramatically and the unidirectional conductivity of the diode is destroyed, thus causing reverse breakdown.

Eg. The VZ is about 1300V when IZ of DF10M is 0.1mA.

4.7 Reverse recovery time (Trr)

When diodes are in low frequency applications, it generally do not need to consider its conduction to the cut-off, or cut-off to the transition time. But if the diode works in a high-speed switching circuit environment, when diode suddenly turns to reverse bias from the forward biased conduction state, it will take a certain time to become a cut-off state, which is called reverse recovery time.

But if the diode works in a high-speed switching circuit environment, when diode suddenly turns to reverse bias from the forward biased conduction state, it will take a certain time to become a cut-off state, which is called reverse recovery time.

Eg. The maximum Trr of EDF1DM is 50nS.

V What are the types of diodes?

5.1 Light emitting diode

Light emitting diode, also called LED, is a semiconductor diode that converts electrical energy into luminous energy. Like ordinary diodes, LEDs are made up of a PN junction and have unidirectional conductivity. When a forward voltage is applied to a light-emitting diode, Holes injected from P region to N region and electrons injected from N region to P region are recombined with N region electrons and P region holes in several microns near PN junction to produce spontaneous emission fluorescence. 

The energy states of electrons and holes in different semiconductor materials are different. When electrons and holes are combined, the energy released is different. The more energy is released, the shorter the wavelength of light is. The commonly used diodes are red, green or yellow light.The reverse breakdown voltage of a light-emitting diode is greater than 5 volts. Its forward volt-ampere characteristic curve is so steep that it must be used in series to control the current passing through the diode. The current limiting resistance R can be calculated by the following formula: R=(E-UF)/IF . In this formula, E is the power supply voltage, UF is the forward voltage of LED, IF is the running current of LED.

5.2 Zener diode

Zener diode, is also called voltage stabilizing diode. By using the reverse breakdown state of pn junction, the current can be changed in a wide range and the voltage is basically unchanged, thus form a diode which has voltage stabilizing function. This diode is a semiconductor device with high resistance until it reaches the critical reverse breakdown voltage. 

The following picture is a typical Zener diode application circuit diagram:

At this critical breakdown point, the reverse resistance is reduced to a very small value, where the current increases and the voltage remains constant in this low resistance region, and the Zener diode is divided according to the breakdown voltage, because of this characteristic, The regulator is mainly used as a voltage regulator or voltage reference element. Zener diodes can be connected in series for use at higher voltages, and higher stable voltages can be obtained by serializing them.

5.3 Switching diode 

Working principle: The semiconductor diode is equivalent to switch-on when it is turned on (the circuit is turned on), and is equivalent to switch-off when it is turn-off (the circuit is cut off), so the diode can be used as a switch. The common used model is 1N4148. Due to the unidirectional conductivity of semiconductor diodes, the PN junction is on at positive bias, and the resistance is very small at the on-state, which ranges from tens to hundreds of ohs. At reverse bias, it is in a cut-off state, and its resistance is very large. Generally, silicon diodes are above 10 μ Ω and germanium diodes have tens to hundreds of kilos. By using this property, the diode will play the role of controlling the current on or off in the circuit and become an ideal electronic switch.

At high frequency, the barrier capacitance of the diode exhibits extremely low impedance and is parallel to the diode. When the capacitance of the barrier itself reaches a certain level, the switching performance of the diode will be seriously affected.

In extreme conditions, the diode will be short-circuited, and the high-frequency current will no longer pass through the diode, but will pass directly through the barrier capacitance, and the diode will fail to work. The barrier capacitance of the switching diode is generally small, which is equivalent to blocking the barrier capacitance path and achieving the effect of maintaining good unidirectional conductivity at high frequency. 

Classification: General switching diode, high speed switching diodes, ultra-high speed switching diodes, low-power switching diodes, high reverse voltage switching diodes, silicon voltage switching diodes and so on.

5.4 Variable capacitance diode( Varactor Diodes )

Variable capacitance diode, also known as varactor Diodes, are semiconductors that change the junction capacitance according to the voltage supplied. That is, as variable capacitors, they can be used in resonant circuits such as FM tuners and TV tuners and FM modulation circuits.

Working principle: Varactor Diodes is a kind of special diode. When applied forward bias voltage, the depletion region of PN (positive and negative electrode) junction is narrowed and the capacitance becomes larger, which results in diffusive capacitance effect. However, the leakage current will be generated when the forward bias is added, so the reverse bias is supplied in application.

In fact, we can think of it as a PN junction. If a reverse voltage V is added to the PN junction (the varactor diode is used in reverse direction), the electrons in the N-type semiconductor are directed to the positive electrode and holes in P-type semiconductor will be led to the negative electrode. Then forms a depletion layer that has neither electrons nor holes, and the width of the depletion layer is set to d, which changes with the reverse voltage V. In this way, when the reverse voltage V increases, the depletion layer d becomes wider and the diode capacitance C decreases (according to C=kS/d), and the reverse voltage decreases, the depletion layer width d becomes narrower and the diode capacity becomes larger. The change of reverse voltage V leads to the change of depletion layer, which changes the junction capacity of the variable capacitance diode.

- Application: the varactor diode is a semiconductor device based on the principle of variable capacitance between PN junctions. It is used as a variable capacitor in high frequency tuning and communication circuits.

As shown in the following figure, the reverse voltage of the diode is changed by changing the different R2. This will result in a change in the capacitance of the diode, thus changing the resonant frequency in which the varactor diode can pull out the full range of the required capacitance in the parallel resonant band-pass filter.


1. What is diode and its symbol?

Diode, an electrical component that allows the flow of current in only one direction. In circuit diagrams, a diode is represented by a triangle with a line across one vertex.

2. What is special about a diode?

Some semiconductor junctions, composed of special chemical combinations, emit radiant energy within the spectrum of visible light as the electrons change energy levels. Simply put, these junctions glow when forward biased. A diode intentionally designed to glow like a lamp is called a light-emitting diode, or LED.

3. Are diodes AC or DC?

It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction. Diodes are also known as rectifiers because they change alternating current (ac) into pulsating direct current (dc). Diodes are rated according to their type, voltage, and current capacity.

4. Why do we use zener diode?

Zener diodes are used for voltage regulation, as reference elements, surge suppressors, and in switching applications and clipper circuits. The load voltage equals breakdown voltage VZ of the diode. The series resistor limits the current through the diode and drops the excess voltage when the diode is conducting.

5. What is unit of diode?

A diode is not a measurable quantity. Hence,it does not have a unit. Generally,for a diode,we measure characteristics like forward voltage drop,reverse voltage drop and reverse breakdown voltage which are usually measured in Volts.

6. Do diodes have resistance?

Just like a resistor or any other load in a circuit, a diode offers resistance in a circuit. Unlike resistors, though, diodes are not linear devices. This means that the resistance of diodes does not vary directly and proportional to the amount of voltage and current applied to them.

7. Does diode reduce current?

Ideally, diodes will block any and all current flowing the reverse direction, or just act like a short-circuit if current flow is forward. Unfortunately, actual diode behavior isn't quite ideal. Diodes do consume some amount of power when conducting forward current, and they won't block out all reverse current.

8. How are diodes classified?

Diodes are classified according to their characteristics and are offered in a number of different types, including rectifiers, switching diodes, Schottky barrier diodes, Zener (constant voltage) diodes, and diodes designed for high-frequency applications.

9. What is the most common diode?

The most commonly used signal diode is the 1N4148. This diode has a close brother called 1N914 that can be used in its place if you can't find a 1N4148. This diode has a forward-voltage drop of 0.7 and a peak inverse voltage of 100 V, and can carry a maximum of 200 mA of current.

10. What is the difference between a Zener diode and a Schottky diode?

As their switching speed is very high, Schottky diodes recover very fast when the current reverses, resulting in only a very small reverse current overshoot. ... A special type of diode, called the Zener diode, blocks the current through it up to a certain voltage when reverse biased.

11. What is difference between Schottky diode and normal diode?

In the normal rectifier grade PN junction diode, the junction is formed between P type semiconductor to N type semiconductor. Whereas in Schottky diode the junction is in between N type semiconductor to Metal plate. The schottky barrier diode has electrons as majority carriers on both sides of the junction.

12. Why it is called diode?

A diode is called a diode because it has two distinct electrodes (i.e. terminals), called the anode and the cathode. A diode is electrically asymmetric because current can flow freely from the anode to the cathode, but not in the other direction. In this way, it functions as a one-way valve for current.

13. Is a diode the same as a resistor?

Key Difference: A diode is a type of electrical device that allows the current to move through it in only one direction. ... A resistor is an electric component that is used to provide resistance to current in the circuit. They are mostly used to produce heat or light.

14. How much voltage can a diode take?

Silicon diodes have a forward voltage of approximately 0.7 volts. Germanium diodes have a forward voltage of approximately 0.3 volts. The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.

15. Can a resistor replace a diode?

Diodes only conduct in one direction whereas resistors conduct in both directions. Without analyzing the actual circuit the results would be unpredictable but, generally speaking, being that diodes & resistors are designed to do different things, substituting one for the other is something you wouldn't want to do.

Leave A Reply

Your email address will not be published. Required fields are marked*