Hardware Basics - diode Basics

The most detailed diode foundation

Recently I saw a strange circuit symbol , Familiar with , But I can't remember what it is for . The advance and retreat of knowledge are vividly displayed in my mind . Just the following symbol , This is the symbol of a typical diode . But what can diodes do ？ In fact, the sign says it clearly ： Unidirectional conduction current .
This one-way conduction function , It lays a very important application foundation for diodes . It's like a one-way road in a city , Direct the current to the place you want . Although I hate one-way roads . But I like it in the circuit , Because we want to control everything in the circuit .

An ideal diode is like a one-way door , Only allow the current to flow in more than one direction , This direction of flow is called positive , And prevent the current from flowing in the other direction , This direction is reverse .

Types of diodes

According to the characteristics of diode , Diodes can be divided into several types .

1. Zener diode —— Zener diode is a kind of diode , It allows current to flow in the positive direction , It can also work in reverse breakdown but in breakdown state , Zener diodes have voltage stabilizing applications , Zener diode uses pn Knot in reverse offset mode , Give the zener effect .

2. Tunnel diode —— Tunnel diodes are used in microwave applications .

3. Schottky diode —— Schottky diode is a kind of metal - Semiconductor junction diode , Also called hot carrier diode 、 Low voltage diode or Schottky barrier diode . Schottky diodes are formed by the junction of semiconductors and metals . Schottky diode provides fast switching action and low forward voltage drop . As we know , stay PN Junction diode ,p The type and n Type connected together to form PN junction . In Schottky diodes , Use metals such as platinum or aluminum instead P Type semiconductor .

4. Light-emitting diode —— It is the most useful kind of diode , When the diode is connected with a forward bias , The current flowing through the junction produces light .

5. Variable capacitance diode —— This diode is also called VARICAP diode , Although the output of the variable capacitor can show general pn Junction diode , But because they are different types of diodes , This diode is approved to provide preferred capacitance variation .

6. photodiode —— The diode that generates current with a certain amount of light energy falls on it , In the forward bias current from p Transferred to the n Under the circumstances , When the reverse photocurrent flows in the opposite direction , There are two types of photodiodes, namely PN Photodiodes and PIN photodiode .

Diode construction

We know that there are two types of semiconductor materials ： Intrinsic and extrinsic semiconductors . In the intrinsic semiconductor , The electron number and hole concentration are equal at room temperature . In extrinsic semiconductors , Impurities are added to semiconductors to increase the number of electrons or holes . These impurities are pentavalent （ arsenic 、 antimony 、 phosphorus ） Or trivalent （ boron 、 indium 、 aluminum ）.

Semiconductor diodes have two layers . The first floor is p type , The other is n Type semiconductor .

If we are in semiconductor （ Silicon and germanium ） Add trivalent impurities , Then more holes will appear and it is positively charged . Therefore, this type of layer is called p Type layer .

If we are in semiconductor （ Silicon or germanium ） Pentavalent impurity added to , Because there are too many electrons , A negative charge will be generated . Therefore, this type of layer is called n Type layer .

stay N Type area , Most charge carriers are electrons , A few charge carriers are holes . And in the P Type area , Most charge carriers are holes , Negative charge carriers are electrons . Due to concentration differences , Diffusion occurs in most charge carriers , They recombine with opposite charges . It produces positive or negative ions . They gathered near the intersection . This region is called the depletion region .

When the positive pole of the diode or p Connect the negative pole ,n Type or when the negative pole is connected to the positive pole of the battery , This diode is connected with reverse bias .

When the positive pole or p Type a terminal is connected to the positive pole of the battery ,n Type or when the negative pole is connected to the negative pole of the battery , This diode is connected to a forward bias .

Forward offset

The bias semiconductor is connected to an external source . When p Type a semiconductor is connected to the positive terminal of the source or battery and the negative terminal is connected to n When it's type , This type of junction is said to be forward biased . In forward bias , The direction of the built-in electric field near the junction is opposite to that of the applied electric field . This means that the synthetic electric field is smaller than the built-in electric field . therefore , Low resistivity , Therefore, the depletion zone is thin . In Silicon , Voltage is 0.6 V when , The resistance in the depletion region is completely negligible .

Reverse bias

In reverse bias ,n Type A is connected to the positive pole of the battery ,p Type A is connected to the negative pole of the battery . under these circumstances , The applied electric field and the built-in electric field have the same direction , And the result of electric field is larger than that of built-in electric field , So as to produce greater resistance , Therefore, the depletion zone is thicker . If the applied voltage becomes larger , The depletion region becomes more resistive and thicker .

Unbiased diode

When no external source is applied to semiconductors , It is called unbiased diode . The electric field is built on p The type and n On the depletion layer between type a materials . This happens because of imbalance . Electrons and holes produced by doping . At room temperature , For silicon diodes ,0.7V Is the barrier potential .

The characteristics of the diode

diode IV curve

Please note that , For typical diodes ,y The current on the shaft is 0.7 How to flow through the diode under a forward voltage of V . The breakdown voltage is where the current begins to flow in the opposite direction , For a typical diode , This is a -50 Volt . All real diodes also have leakage current , The current will flow from the cathode to the anode without forward bias . Sometimes , You may need to understand other features , For example, diode resistance . For many circuits , This is not a real factor . However , For more sensitive circuits , One way to calculate diode resistance in forward bias mode is that you can use classic resistors = voltage / Current equation . under these circumstances , You can measure the diode voltage drop across the diode for different circuit modes , These circuit modes are related to the current through the diode .

Diode equation

Is = Dark saturation current

q = Electron charge value

Vd = The voltage across the diode

n = Ideal factor , For ideal diodes ,n = 1, For actual diodes ,n = 1 To 2

k = Boltzmann constant ,1.38064852E-23 Joules / kelvin

T = temperature （ kelvin ）

Common diode circuits

Rectifier diode

Half-wave rectifier

If we only want the upper half of a sinusoidal signal , You can add a diode directly into the circuit , Using the unidirectional conduction of diode , Filter out the bottom half of the signal . As shown in the figure below .

Full-wave rectifier

One disadvantage of half wave rectification is that half of the signal energy is consumed , Sometimes we need full wave rectification . At this time, the following diode circuit can be used . As shown in the figure below （ Animation is a little slow , Read it patiently ）

Flyback diode

This is sometimes called buffer diode 、 Freewheeling diode and suppression diode . Flyback diodes are a convenient way to use diodes to reduce sudden voltage spikes , This spike occurs when the current passing through the inductive load suddenly changes . As we discussed in our article on inductors , Whenever the inductor sees a change in the current flowing through it , It will produce a EMF Voltage spikes in an attempt to stabilize current changes .

In many circuits , This produces EMF Usually not needed , Sometimes it will damage other parts of the circuit . In order to eliminate damage , A diode can be placed , In order to appear EMF Voltage spikes encourage current to flow through the diode , Instead of flowing through other circuit components that may be damaged . A useful common circuit is the control of small fans or relay inductors . Usually , Most digital pins can provide less than 20 Milliampere current , Therefore, a current amplifier is needed . See the following example diode schematic .

NPN Transistors work well here , Because digital pins can provide 10 Milliampere current to open NPN The transistor , And the transistor can handle the current of about Ampere required by the fan or relay inductor . Whenever the transistor is turned off , The inductor will see the current drop sharply and produce a peak of back EMF .

If there is no diode , The spike will flow through the transistor , Usually it will be damaged . By connecting the diode in parallel with the inductor ,EMF The voltage spike turns on the diode , And allow the current to flow through the diode and return to the inductor and dissipate . This current flowing back to the inductor is the source of the name of this diode .

Zener diode

Zener diodes are designed to operate in breakdown voltage mode . One way to take advantage of this is to use a zener regulator . We only need a resistor and a properly selected Zener , It can provide us with the required voltage output . An example zener diode circuit can be seen below .

The zener diode will reduce the input voltage to the breakdown voltage of the diode in the circuit for output . To do this , It must allow current to flow through the diode , This will dissipate as heat , But only when the input voltage is higher than the breakdown voltage . The required output voltage will determine the zener diode , Because you will choose the diode according to its breakdown voltage to match the output voltage . You have to get a diode that can handle the power that must be dissipated .

Blocking diode

This use of diode is only the name for the case that diode is used to guide current to flow in only one direction .

A good example is solar panels and battery charger circuits . When the sun comes out and the solar panel generates electricity , Their voltage is usually higher than the battery the circuit is charging , Therefore, the current will flow from the battery board to the battery .

However , At night , There is no sunlight on the solar panel , So they don't generate electricity . At this time, the battery will be at a higher voltage and there will be no blocking diode , Current will flow from the battery to the panel , Waste energy . When placing a diode between the solar panel and the battery , It will allow current to flow from the panel to the battery , However, current is not allowed to flow from the battery to the battery board . therefore , it “ prevent ” The current flows in an unwanted way .

Another useful place is the battery in the circuit . At any time, someone may install the battery upside down , Or plug the DC power supply upside down , A good way to protect circuits is to use blocking diodes .

The diode ensures that only the correct voltage polarity allows current to flow into the circuit , Protect them from negative voltage .

Clamp diode

Clamp diode is just a method of using capacitors and diodes to control the DC level of the signal . In the following example circuit , Capacitors and diodes produce a DC offset on the input AC signal .

If we want to move in the other direction DC The offset , Then we just need to reverse the direction of the diode , As shown below .

If you place a voltage source between the diode and the ground , You can go further , In this way, you can add more DC bias in the desired direction .

CLIPPING DIODE

The opposite of clamping is clipping . ad locum , You can use series resistors and diodes to intercept unwanted parts of the input signal . For positive limiting , The arrangement of the diode makes it turn on when the signal is higher than the forward voltage , Therefore, the diode conducts current , Limit the upper limit voltage to 0.7 Lie about . An example can be seen below .R2 Just an example resistor , Not required .

In the example above , Please note how the maximum up voltage is limited to 0.7 Volt , This is the forward voltage of the diode .

If negative clipping is required , You can simply flip the diode . under these circumstances , As long as the signal input is negative and exceeds the forward voltage , The diode will turn on and conduct current , Limit the negative signal to -0.7 Volt . Here is an example . Again , Unwanted R2. Notice how the negative part of the signal is clipped -0.7 v .

Reference material

https://byjus.com/physics/photodiodes-applications/

https://www.circuitbasics.com/what-is-a-diode/

https://byjus.com/physics/diodes/

https://www.fluke.com/en-us/learn/blog/electrical/what-is-a-diode

https://www.geeksforgeeks.org/what-is-a-diode-definition-working-types-applications/

https://www.electronicsdesignhq.com/diodes/

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Reference article ：《 The most detailed diode foundation 》