Diode comprehensive knowledge analysis

December 16, 2020

Among the family of electronic components, there is an element that allows only current to flow in a single direction and has two electrodes, called a diode. English: Diode

Diodes are the cornerstone of the modern electronics industry.

Early diodes

Early diodes included Cat's Whisker Crystals and Thermionic Valves.

In 1904, the British physicist Fleming invented the world's first electronic diode--vacuum electronic diode based on the "Edison effect."

It relies on the cathode to emit electrons to the anode for conduction.

The positive and negative poles of the power supply do not conduct electricity. It is an electronic device that conducts current in one direction.

Early electronic diodes were bulky, requiring preheating, large power consumption, and fragile problems that prompted the invention of crystal diodes.

Crystal diode

Also known as semiconductor diodes, invented by Americans in 1947.

Inside the semiconductor diode there is a PN junction and two terminals.

This electronic device has unidirectional current conductivity in the direction of the applied voltage. Most of the most common diodes today use semiconductor materials such as silicon or germanium.

Crystal diode structure

About PN junction

The core of the crystal diode is the PN junction. The first thing to know about the PN junction is three concepts:

Intrinsic semiconductor:

Refers to a semiconductor that does not contain any doping elements, such as pure silicon wafers or pure germanium wafers.

P type semiconductor:

A semiconductor containing a hole-generating impurity containing a lower electric charge, such as a semiconductor doped with Al(3+) in Si(4+) in an intrinsic semiconductor.

N type semiconductor:

A semiconductor containing a hole-generating impurity containing a lower electric charge, such as a semiconductor doped with phosphorus P(5+) in silicon Si(4+) in an intrinsic semiconductor.

When a P-type semiconductor and an N-type semiconductor are in contact with each other, a unique PN junction interface is created, and space charge layers are formed on both sides of the interface to constitute a self-built electric field.

When the applied voltage is equal to zero, the diffusion current due to the difference in carrier concentration on both sides of the PN junction is equal to the drift current caused by the self-constructed electric field and is in the electric equilibrium state, which is also the normal state PN junction.

A PN junction is used as a core structure, and leads or pins are used to form a unidirectionally conductive diode.

When the direction of the applied voltage is from the P pole to the N pole, it turns on.

Crystal diode classification

Classified by material

According to PN junction structure

Point contact diode

The point contact diode is formed by pressing a metal pin on a single wafer of germanium or silicon material and then applying an electric current method.

The PN junction has a small electrostatic capacity and is suitable for high frequency circuits. Because of its simple structure, it is cheap. For general purposes such as detection, rectification, modulation, mixing, and limiting of small signals, it is a widely used type. Compared with the junction type, the point contact diode has poor forward and reverse characteristics and therefore cannot be used for high current and rectification.

Production process: One end of the thin aluminum wire is connected to the anode lead, and the other end is pressed on the doped N-type semiconductor. After the voltage is applied, the thin aluminum wire melts at the contact point and penetrates into the melted portion. In this way, the contact point is actually a P-type semiconductor and is attached to the N-type semiconductor to form a PN junction.

Surface Contact Diode

Surface-contact diodes have a large "PN junction" area, allowing large currents (several amps to several tens of amps) to be used in "rectifier" circuits that convert alternating current into direct current. Surface contact type crystal diodes are suitable for high current switching.

Planar diode

Planar diodes are special silicon diodes that get their name from the smoothness of the semiconductor surface. Initially, the semiconductor material used was formed by an epitaxial method, so the planar type is also referred to as an epitaxial planar type.

On semiconductor single crystal wafers (mainly N-type silicon single crystal wafers), P-type impurities are diffused, and a PN junction formed by selectively diffusing only a part of the N-type silicon single crystal wafers is shielded by the oxide film on the surface of the silicon wafer. The PN-bonded surface is covered with an oxide film, resulting in good stability and long life.

It can not only pass a large current, but also has stable and reliable performance. It is mostly used in switching, pulse and high frequency circuits.

The main characteristics of the crystal diode

Diode voltage characteristic curve

PN junction

When the applied voltage Uw direction is P→N, Uw is greater than the starting voltage, and the diode is turned on;

When the applied voltage Uw direction is N→P, Uw is greater than the reverse breakdown voltage, and the diode breaks down;

Crystal diode main performance parameters

Idm maximum rectified current

The continuous operation of the diode allows the maximum forward current to flow through. If the current is too high, the diode will burn due to overheating. High-current rectifier can install heat sinks.

Urm maximum reverse voltage

Urm is generally smaller than the reverse breakdown voltage, and the selection is based on Urm, leaving a margin. Overvoltage can damage the diode.

Reverse saturation current Is

The current value of the diode when reverse voltage is applied. Is very small before the reverse breakdown, the change is also very small. Is will increase with temperature rise, in general, under normal temperature silicon tube Is <1uA, helium tube Is = 30 ~ 300uA.

Maximum operating frequency Fm

Refers to the highest operating frequency for which the diode can maintain good operating characteristics.

Differences in structural properties of diode materials for different applications

Rectifier Diode

Most of the current directionality that diodes have is commonly referred to as "Rectifying".

Transform AC power into DC power.

Surface contact structure, the use of silicon materials, can withstand a larger forward current and a higher reverse voltage, the performance is more stable, but due to a larger junction capacitance, should not work in high-frequency circuits, so can not be used as a detector tube . Metal and plastic package.

Detector diode

Remove the low-frequency signal adjusted on the high-frequency electromagnetic wave.

The detection diode is a device for detecting a low-frequency signal superimposed on a high-frequency carrier. It has a high detection efficiency and a good frequency characteristic.

Tantalum material point contact type, working frequency up to 400MHz, small forward voltage drop, small junction capacitance, high detection efficiency, good frequency characteristics, 2AP type.

Detecting diodes, such as touch type detectors, can be used in circuits such as limiting, clipping, modulation, mixing, and switching, in addition to detection. There are also two diode assemblies that have good characteristic consistency for FM detection.

Mostly used in glass or ceramic package for good high frequency performance

Switching diode

A switching diode is a kind of semiconductor diode and is a kind of diode specially designed and manufactured for "opening" and "closing" on a circuit. The time required for it to turn from turn-on to turn-off or turn-on to turn-on is shorter than that of a normal diode.

The barrier capacitance of the switching diode is generally very small, which is equivalent to blocking the barrier capacitance and achieving a good one-way conductivity at high frequencies.

Switching diode from the cut-off (high resistance state) to turn on (low resistance state) is called the on-time; the time from the conduction to the cut-off is called the reverse recovery time; the sum of the two times is called the switching time. Generally, the reverse recovery time is longer than the on-time, so only the reverse recovery time is given on the use parameter of the switching diode. Switching diode switching speed is quite fast, like silicon switching diode reverse recovery time is only a few nanoseconds, even if it is a switching diode, it is only a few hundred nanoseconds.

Switching diodes are characterized by fast switching speed, small size, long life, and high reliability. They are widely used in switching circuits, detection circuits, high-frequency and pulse rectifying circuits, and automatic control circuits of electronic devices.

Zener diode

A Zener diode is a diode that acts as a voltage stabilizing diode by using a pn junction reverse breakdown state in which the current can be varied over a wide range and the voltage is substantially constant.

The forward characteristic of the voltage-voltage characteristic curve of the zener diode is similar to that of an ordinary diode. The reverse characteristic is that when the reverse voltage is lower than the reverse breakdown voltage, the reverse resistance is large and the reverse leakage current is extremely small. However, when the reverse voltage approaches the critical value of the reverse voltage, the reverse current suddenly increases, which is called breakdown.

Variable capacitance diode

Varactor diodes are also known as "variable reactance diodes." Most of the material is silicon or gallium arsenide single crystal, and the use of epitaxial process technology.

It is a diode that uses a PN junction capacitor (barrier capacitor) and its reverse bias voltage Vr depending on the principle and structure, as shown in the structure diagram.

The role of varactor diode is to use the principle of variable capacitance between PN junction made of semiconductor devices, in the high-frequency tuning, communications and other circuits for the use of variable capacitors.

A varactor diode is a reverse biased diode. Changing the reverse bias on its PN junction changes the PN junction capacitance. The higher the reverse bias, the smaller the junction capacitance, and the relationship between reverse bias and junction capacitance is nonlinear.

The relationship between the capacitance value of the varactor diode and the reverse bias voltage is illustrated:

(a) Increased reverse bias, resulting in reduced capacitance;

(b) The reverse bias is reduced, resulting in increased capacitance.

The larger the reverse bias voltage, the smaller the junction capacitance.

led

Light emitting diodes are simply referred to as LEDs. It is made of compounds containing Ga, As, P, N, etc.

When electrons and holes recombine, they emit visible light and can be used to make light emitting diodes. The gallium arsenide diode emits red light, the gallium phosphide diode emits green light, the silicon carbide diode emits yellow light, and the gallium nitride diode emits blue light. Due to chemical properties, organic light emitting diodes (OLEDs) and inorganic light emitting diodes (LEDs) are also divided.

Damping diode

Damping diodes are similar to high-frequency, high-voltage rectifier diodes, which are characterized by lower voltage drop and higher operating frequency, and can withstand higher reverse breakdown voltages and larger peak currents.