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The diode

This is the basic element. It all started with her: she was the key element of the very first logic circuits!


Symbol :


Feature :

Characteristics of a diode

Principle :

If V <0 <=> V (A) 0 <=> V (A)> V (K) then the diode is on: the current flows from A to K and its voltage V = 0.6 Volts. The diode is said to be forward biased.

Consequently, a diode only lets current flow in one direction (A to K) when V> 0. Thanks to this property, the diode is used in many applications: circuit protection, filtering, rectification, etc.


A diode is a so-called active component, which is part (like the transistor) of the semiconductor family. By definition, a diode refers to any electronic component with two electrodes. It is a polarized component which therefore has two electrodes, an anode and a cathode. The cathode (sometimes called K, for Kathode) is located by a locator ring (there may be several rings, in this case the locator ring is the one closest to the edge of the diode).

Diode 00b

A diode does not allow current to flow in the same way depending on whether it is plugged in one direction or the other (the Anode to Cathode current can be greater than the Cathode to Anode current, at a given voltage, for example ). This characteristic allows the diode to be used to rectify an alternating current, ie to let pass only the positive half-wave or only the negative half-wave (depending on the orientation of the diode). The material most used to manufacture “standard” diodes is to date silicon, germanium now being used much less than in the past.

Some germanium diodes:

germanium diodes
germanium diodes
germanium diodes

Main characteristics of a diode

There are a multitude of diodes, see following paragraphs. The type of diode to be used depends on the application: detecting RF signals in a radio receiver, rectifying in a linear power supply, improving the switching speed of power transistors in a switching power supply, overvoltage protection, for example .

Threshold voltage

Corresponds to the voltage from which the diode begins to conduct when it is biased in the forward direction. In other words and probably more correctly: a diode only conducts when its anode is brought to a more positive potential than its cathode, and when the potential difference between anode and cathode reaches the threshold voltage. The two diagrams which follow use two different diodes which are traversed by a different current, and show the voltage drops introduced by these diodes. We can therefore say that the threshold voltage corresponds to the minimum operating voltage below which the diode cannot be used. This threshold voltage also corresponds to the dropout voltage, that is to say the voltage which will be lost because of the very use of the diode (voltage which remains at its terminals when it is conducting). The greater the current flowing through the diode, the greater the voltage drop. The evolution of the voltage drop however remains quite low compared to the evolution of the current.

The diode used here is a conventional so-called switching diode, model 1N4148. The voltage drop it introduces is a few tenths of a volts, when the current flowing through it is a few mA.

The diode used here is a classic so-called rectifier diode, model 1N4007. The voltage drop that it introduces is of the order of one volt, when the current flowing through it is a few hundred mA.

Note: the resistors placed on the preceding diagrams represent the load, that is to say the circuit which is supplied through the diodes. In the first case, the value of the resistance is higher because a switching diode is generally not used for large currents, while a rectifier diode can be crossed by currents of several hundred milli-amps. or several amps.

Reverse voltage

Also called breakdown voltage. This is the maximum voltage that can be applied to the diode in the blocking direction (ie diode connected upside down), before it conducts. A “normal” diode may burn out when this maximum reverse voltage is exceeded, but some diodes support this operation and have even been designed to work that way. Zener diodes, for example, behave like normal diodes when plugged in in the forward direction, and exhibit a voltage aptly called zener voltage, when plugged in the reverse direction. This reverse voltage (zener) is used as a voltage reference, for power supply regulation for example.

Direct current

This is the maximum intensity that can travel through the component continuously when it is polarized in the passing direction, without the latter burning. In general, the diode can withstand a (very brief) current surge much greater than the maximum current.


Some diodes are said to be fast. We can therefore imagine that there are nits. And it is indeed the case. So what do we mean by fast? Well, as in any context of use, everything is relative. The same diode can be considered as fast in a given application, and be considered as slow in another application. A diode of type 1N4007 for example, is suitable for rectifying at the secondary output of a power supply transformer (50 Hz), but is not suitable for use as a cover diode in a switching power supply (cover diode: diode connected to the power transistor which operates in switching at a high frequency – for example 150 KHz, to accelerate the switching times and thus limit heating). Schottky diodes are said to be fast, and can sometimes even be preferred for “slow” applications (linear power supplies to high quality audio circuits, for example).

Component notation

Just like with resistors and capacitors, there is a color code for the diode, as well as a clear notation. The component’s letter and number notation is based on standardization.

Signal or switching diode

Example : AA119, BAX13, OA95, 1N4150, 1N914, 1N4148
This type of diode is used above all in logic, where in assemblies where little power is involved. They are often called general purpose diode, even if paradoxically there are also other diodes also called general purpose diode .

Rectifier diode

Example: 1N400x (1N4003, 1N4007, …), 1N5408, BY255
Used for half-wave (one diode) or two-wave (two or four diodes) rectification in mains power supplies. Note that there are power diodes mounted in pairs in the same box, with a common leg (two diodes with common anode in the TO220 box for example), which give them the appearance of a three-pole component (see photo at the top of the page). There are also boxes incorporating four rectifier diodes, wired as a bridge (see below).
Note: Power rectifier diodes often exhibit a large voltage drop when the current flowing through them is large. For example, diode1N4007, well known to electronics engineers, exhibits a voltage drop of the order of 0.7V for a current of 50 mA, and a voltage drop of 1.1V for a current of 1A. The 1N5818 diode is a Schottky type diode which can be used for rectification, and which has a voltage drop half of the 1N4007, or 0.55V for a current of 1A. On the other hand, it only supports a reverse voltage of 30V, against 1000V for the 1N4007, which is important to know for power supplies that exceed 24V … For greater current requirements, the Schottky MBR20100CT diode does not shows a voltage drop of 0.7V under 10A, and the 43CTQ100, 0.67V under 20A. Not bad is not it ?

Power diodes

Large power diodes are bulkier than small diodes (I got this information from La Palice). Some of them have a tubular shape like low power diodes, and others are in a TO220 type package, such as those used by certain power transistors (IRFZ44N for example) or by common triacs (TIC226D for example ).

Power diodes
Power diodes

This type of box has a metal sole with a hole, which allows the diode to be fixed on a radiator, thus facilitating the evacuation of the heat given off by the component. Two diodes can be found in the same box, the latter then comprises three legs, one of which is common to the two diodes (in general it is the cathodes which are put in common).

Bridge diodes (diode bridge)

Ponts diodes
Bridge diodes (diode bridge)

Example : PR1, BY164, 110B6, B40C, KBP02M, KBL04
It is neither more nor less an assembly of four identical rectifier diodes in the same box, wired together “in the round”, each common point corresponding to a connection. Diode bridges are mainly used for rectifying AC voltages in a mains supply. Having only four legs instead of eight simplifies PCB mounting.

diode bridge

Old diode bridge

There are also boxes with three legs (in the TO220 box for example) which contain two identical diodes mounted head to tail (common cathodes or common anodes). Hey, I already said it just before …

THT diodes

THT diodes are diodes capable of working at very high voltages (THT), from several KV to several tens of KV. It is quite rare to find a diode that does this on its own. Most of the time, these are several diodes mounted in series, in the same box. This type of component is sometimes called a diode bar. The representation of this type of diode in an electronic assembly is often made by several diodes glued together, in series:

Diodes THT
THT diodes

If for a particular assembly (small THT generator for example), you need high voltage diodes, go and take a look at the microwave oven spare parts. There are indeed 12 KV or 16 KV THT diodes for a few euros. You can also, if the high voltage comes down to a few 1000 V, connect in series traditional diodes whose maximum reverse voltage is 1000 V, such as the famous 1N4007. But be careful, in order to distribute the voltages on each diode when they are blocked, it is necessary to mount “balancing” resistors in parallel with them (an example is given on the page Power supplies with transformer).

Detection diode

Sense diodes are (were?) Used for RF detection (demodulation) in radio receiver input stages. This type of diode, generally made of germanium, had a lower conduction threshold (0.2V to 0.3V) than those of silicon diodes (0.6V to 0.7V) and therefore had the advantage of being more sensitive and therefore work better with low reception levels. I do not know if this type of diode is still used, because there are now more efficient detection techniques. But it seems that they are endangered if we judge the difficulty of obtaining them …

Networked diodes (diode array)

Several diodes are mounted in the same box.

Reseau 8 diodes

Either they are completely isolated from each other, or they have one of their leg pooled (common cathode or common anode network). When we speak of networked diodes, it is generally a case comprising four, seven or eight diodes. But there are also boxes comprising only two diodes, mounted head to tail, which are found in particular in the field of power rectification (in the TO220 box), or in the HF field with varicap diodes.

Zener diode

Zener diodes are mainly used for supply voltage regulation.

Voltage reference diodes

These diodes have similar characteristics to zener diodes, but are characterized by a much lower temperature coefficient. The voltage at their terminals, for a given current, therefore varies less as a function of the ambient temperature. They are mainly used in the field of instrumentation, notably in the measurement of temperature or more simply still of voltages (voltmeters).

Varicap Diode (Varactor)

Examples : BA102, BA104, BB105, BB112, BB142, BB204, BB405B
Also called Varactor (acronym for Variable Reactor) or tuning diode, the Varicap diode has the particularity of behaving like a capacitor whose value depends on the direct voltage applied to its terminals, when it is reverse biased. The main characteristic of a varicap diode is its capacitance variation range (a few pF or a few tens of pF) for a given reverse voltage range (a few volts to a few tens of volts). It can therefore be considered as a variable capacitor programmable by a voltage. This type of diode is frequently used in RF assemblies to effect frequency modulation, or to ensure tuning (in the RF input stage of a TV receiver, or to stabilize with great precision the output frequency of a VCO, for example). But there are also power varicap diodes used to produce low loss frequency multipliers, and varicap diodes where gallium arsenide is preferred to silicon for very high frequency applications. In certain applications (television receiver for example), it is necessary to use two varicap diodes mounted head to tail, which must have similar characteristics. Some manufacturers thus offer double diodes (two in the same box) or paired, and where the difference in capacitance between the two diodes does not exceed 2.5% (BB204, BB112 or BB405B diodes for example).

Protection diode

Example : Transil 1N6286
The Transil diode is the name given by Thomson to the Transzorb diode. This type of diode is of the “avalanche” type. It runs in parallel with the power supply, and absorbs any overvoltage. The major drawback of this type of diode is that a very strong overvoltage puts it in a short circuit. There, it is sure that it protects even better the equipment which follows, but the fuses and circuit breakers which precede do not like that, and play their role well.

Electro-Luminescent Diode (LED or LED)

An LED is a diode that emits visible radiation (light, whatever).

Led ronde 001
Led rectangulaire 001
Led CMS 001

Photo diode

Examples : OAP12, BP104
This type of diode is similar to photoresist cells, in the sense that it is more or less conductive depending on the intensity of the light it receives. It can be in the form of a discrete component with two legs, or it can be integrated in an optocoupler.


The PIN photo-diode is an ultra-fast photo-diode mainly used in the field of optics (infra-red in particular).

Tunnel Diode

Also called Esaki diode. The Tunnel diode takes its name from the “Tunnel” effect, which consists in the way in which an electronic particle can cross a potential barrier too high to be crossed in a normal way (barrier which prevents reverse conduction in the case of normal diodes). In a tunnel diode, the current first begins to increase substantially proportionally with the applied voltage. Then this voltage always increasing, from a certain point, the current reverses and decreases. We are then in the presence of a “negative resistance”, which makes the diode ideal for producing oscillators or amplifiers, especially in the frequency domain of microwaves.

Backward diode

Also called Unitunnel diode. This diode is similar to the Tunnel diode in its overall behavior, except that the “negative resistance” portion does not exist in the latter. This type of diode is particularly used for the rectification of signals of low amplitude and very high frequency (microwave)

Schottky diode

Examples : MBD102, FH1100, HP2800
Rectifier diode made on the basis of a junction between a metal and a semiconductor. The Schottky effect gives this diode the ability to switch at very high speeds. It is therefore naturally employed where high switching and recovery speeds are required, such as for example for the protection of transistors in power control circuits or in switching power supplies. This diode also has the advantage of having a lower voltage drop at its terminals than that of conventional diodes. Another argument of choice to prefer it in power supplies where the least tenth of a volt is counted …

Gunn diode

Negative resistance microwave oscillator operating on the principle of the Gunn effect. It is a diode composed of gallium arsenide producing coherent microwave oscillations when a strong electric field is applied to it.

Diode Pin

This type of diode can be used to ensure the detection of optical modulation (PIN photo-diode at the input of a receiver of an optical fiber link for example). It can also be used as a controlled attenuator element in an HF stage, or as a switching element to route HF signals.

Current regulator diode

Field effect diode whose particularity is to generate a constant current, the value of which is independent of the voltage applied to its terminals (a few hundred uA to a few mA).

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