Diodes can be classified in several ways. According to semiconductor material, they are divided into germanium (Ge) and silicon (Si) diodes. According to application, common categories include detector, rectifier, voltage-regulator, and switching diodes. According to internal die structure, diodes can be classified as point-contact, junction-contact, and planar diodes.
A point-contact diode is made by pressing a very fine metal wire against the polished surface of a semiconductor wafer and applying a pulse current. One end of the wire is then firmly bonded to the wafer, forming a PN junction. Because the contact area is extremely small, the device can conduct only a relatively small current, normally no more than several tens of milliamperes. It is therefore suitable for high-frequency, low-current circuits such as radio detectors.
A junction-contact diode has a relatively large PN-junction area and can conduct much higher current, from several amperes to several tens of amperes. It is mainly used in rectifier circuits that convert AC into DC. A planar diode is a specially fabricated silicon diode. It can conduct relatively large current while providing stable and reliable performance, and is widely used in switching, pulse, and high-frequency circuits.
1. Classification by Internal Construction
Semiconductor diodes primarily rely on PN junctions to achieve their electrical characteristics. Point-contact diodes, which contain very small PN junctions, and Schottky diodes, which use metal-semiconductor junctions, are both generally classified as common semiconductor diodes.
Based on the structure of the internal junction, semiconductor diodes can be divided into the following types.
1.1 Point-Contact Diodes
A point-contact diode is one of the earliest semiconductor devices and is historically regarded as a predecessor of the transistor. It consists of two semiconductor regions made from different materials: an N-type semiconductor doped primarily with electrons and a P-type semiconductor doped primarily with holes. The two regions are connected through an extremely small point contact.
During operation, a forward voltage is applied across the P-N contact, causing current to flow from the P-type region into the N-type region. Under forward bias, electrons are injected from the N-type region into the P-type region, while holes are injected from the P-type region into the N-type region. Current can therefore flow on both sides of the PN junction, placing the diode in the conducting state.
Point-contact diodes have several disadvantages, including high temperature sensitivity, relatively high manufacturing cost, and limited stability. They were gradually replaced by transistors, particularly as transistor structures were improved. Transistors provide higher stability, reliability, and overall performance and became fundamental components in modern electronics. Although point-contact diodes have largely lost their former technical importance, they remain historically significant as early semiconductor devices.
A point-contact diode is fabricated by pressing a metal needle against a single crystal of germanium or silicon and then passing current through the contact. Its PN-junction capacitance is very small, making the device suitable for high-frequency circuits. Compared with a junction diode, however, its forward and reverse characteristics are relatively poor. It is therefore unsuitable for high-current operation and power rectification. Its simple construction makes it inexpensive, and it has been used for small-signal detection, rectification, modulation, mixing, and amplitude limiting.
1.2 Junction-Contact Diodes
The junction-contact diode, also described in the source material as a surface-barrier diode, is a special diode type used in high-frequency applications. Its main characteristics include short reverse-recovery time and low capacitance. These properties make it useful in RF amplifiers, mixers, detectors, and other high-frequency circuits.
Junction-contact diodes are generally formed by bringing a metal layer into direct contact with semiconductor material. Because carriers travel a short distance across the interface, the reverse-recovery process can be rapid. The device structure also provides relatively low capacitance, which helps preserve high-frequency performance.
These diodes are widely used where fast switching and low capacitance are required. Typical application areas include wireless communications, radar systems, and radio equipment.
1.3 Planar Diodes
A planar diode is fabricated on a semiconductor single crystal, typically an N-type silicon wafer. A P-type impurity is selectively diffused into part of the wafer through the masking effect of an oxide layer on the silicon surface, thereby forming a PN junction. Because the junction area can be defined through selective processing, chemical etching is not required to adjust the junction area.
The name "planar" comes from the flat semiconductor surface produced during fabrication. The PN-junction surface is covered by an oxide layer, which provides good stability and contributes to long service life. Early planar devices were fabricated using epitaxial processes and were therefore also called epitaxial planar diodes.
Only a limited number of planar diode models are intended for high-current rectification, whereas many are designed for low-current switching. Planar structures are valued for their stable electrical characteristics and suitability for switching and pulse applications.
1.4 Bonded-Wire Diodes
A bonded-wire diode is fabricated by fusing a fine gold or silver wire to a single-crystal germanium or silicon wafer. Its characteristics fall between those of a point-contact diode and an alloy-junction diode.
Compared with a point-contact diode, the bonded-wire diode has somewhat higher PN-junction capacitance, but its forward characteristics are particularly good. It is mainly used as a switching device and may also be used for detection or power rectification at currents no greater than approximately 50 mA. Devices made with gold wire are sometimes called gold-bonded diodes, while those made with silver wire are known as silver-bonded diodes.
The principal internal elements are as follows:
- Metal layer, or anode: One terminal is normally a metal layer that serves as the anode. Possible metals include platinum, molybdenum, chromium, and aluminum.
- Semiconductor layer, or cathode: The other terminal is the semiconductor layer, which serves as the cathode. It is commonly made from N-type or P-type silicon, silicon carbide, or another semiconductor material.
- Schottky contact interface: A metal-semiconductor contact is formed between the metal and semiconductor layers. This interface is different from the clearly defined P-type and N-type regions of a PN-junction diode.
- Surface structure: The metal-semiconductor interface may contain microscopic depressions or irregularities. These features can increase the effective contact area and reduce contact resistance.
Because of the characteristics of the metal-semiconductor interface, bonded-wire diodes can provide fast switching, low forward voltage drop, and good high-temperature performance. They have been used in high-frequency circuits, power electronics, mixers, and detectors.
1.5 Alloy-Junction Diodes
An alloy-junction diode is made by forming a PN junction on an N-type germanium or silicon single crystal using metals such as indium or aluminum in an alloying process. It has a low forward voltage drop and is suitable for high-current rectification.
Because its PN junction has relatively high capacitance under reverse bias, it is not well suited to high-frequency detection or high-frequency rectification.
1.6 Diffused Diodes
A diffused diode is fabricated by heating an N-type germanium or silicon single crystal in a high-temperature atmosphere containing a P-type dopant. Part of the crystal surface is converted to P-type material, forming a PN junction.
Because the PN junction has a low forward voltage drop, diffused diodes are suitable for high-current rectification. In high-current rectifier applications, silicon diffused structures have largely replaced silicon alloy-junction structures.
1.7 Mesa Diodes
The PN junction of a mesa diode is produced using a process similar to that of a diffused diode. However, only the PN junction and the portions required for operation are retained; unnecessary material is removed through chemical etching. The remaining structure has a raised mesa-like profile, which gives the device its name.
Early mesa diodes were made by diffusion and were therefore also called diffused mesa diodes. Relatively few models are intended for high-current rectification, while many are designed for low-current switching.
1.8 Alloy-Diffused Diodes
An alloy-diffused diode is a type of alloy-junction diode that uses an alloy material capable of facilitating diffusion. Through careful impurity mixing, materials that are otherwise difficult to process can diffuse together with the alloy. This makes it possible to obtain a suitable impurity concentration profile in the already formed PN junction.
This process is suitable for manufacturing high-sensitivity varactor diodes.
1.9 Epitaxial Diodes
An epitaxial diode is formed by fabricating the PN junction through an epitaxial growth process. This process requires highly controlled manufacturing technology. Because the concentration and distribution of impurities can be controlled over a wide range, epitaxial structures are suitable for high-sensitivity varactor diodes.
1.10 Schottky Diodes
A Schottky diode uses a metal-semiconductor contact, such as a contact between a metal and an N-type silicon wafer, to provide rectifying behavior. Its operating principle is fundamentally different from that of a PN-junction diode. The voltage rating described for this type in the source material is approximately 40 V.
Schottky diodes are characterized by very fast switching and extremely short reverse-recovery time, or trr. They are therefore suitable for switching diodes and low-voltage, high-current rectifiers.
The term "Schottky" refers to the German physicist Walter H. Schottky, who lived from 1886 to 1976. He made important contributions to electronics and semiconductor physics, particularly in the theory and design of semiconductor devices. One of his best-known contributions is the Schottky diode, which uses a metal-semiconductor junction and has been applied in radio, microwave, and other electronic systems.
Schottky's work also covered vacuum tubes, transistors, and other electronic devices and had a substantial influence on the development of modern electronics.
The operating principle of a Schottky diode is based on the potential barrier formed at the metal-semiconductor interface. This barrier is lower than the barrier associated with a conventional PN junction. Its operation can be summarized as follows:
- Carrier injection and depletion: When metal and semiconductor materials come into contact, charge redistribution occurs at the interface. A space-charge region with reduced free-carrier concentration is formed, producing a potential barrier without requiring an externally applied voltage.
- High barrier conductivity: Because the barrier is relatively low, electrons can cross the interface comparatively easily. This contributes to a low forward voltage drop and rapid switching.
- Fast switching: The short switching time makes Schottky diodes useful in RF circuits and high-speed switching power supplies.
- Reverse leakage: The reverse leakage behavior differs from that of a conventional PN-junction diode and must be considered when selecting a device for a particular circuit.
2. Classification by Application
2.1 Detector Diodes
Detection is the process of extracting a modulated signal from an input waveform. Using 100 mA of rectified current as a general boundary, diodes with output current below 100 mA are commonly described as detector diodes.
Germanium point-contact diodes can operate at frequencies up to approximately 400 MHz. Their low forward voltage drop, small junction capacitance, high detection efficiency, and favorable frequency response make them suitable for detection. The 2AP series is a representative example.
Detector diodes with point-contact-like characteristics can also be used for limiting, clipping, modulation, mixing, and switching. Matched pairs of diodes with closely controlled characteristics are available for dedicated frequency-modulation detection.
2.2 Rectifier Diodes
Rectification is the process of obtaining a DC output from an AC input. Devices with rectified output current above approximately 100 mA are commonly classified as rectifier diodes.
Junction-type rectifier diodes generally operate below 3 kHz in the classification described here. Their maximum reverse voltage ranges from 25 V to 3,000 V and is divided into 22 grades designated A through X. Representative categories include:
- Silicon semiconductor rectifier diodes, such as the 2CZ series;
- Silicon bridge rectifiers, such as the QL series;
- High-voltage silicon stacks for television circuits, such as the 2CLG series, with an operating frequency close to 100 kHz.
2.3 Limiter Diodes
Most diodes can be used in limiter circuits. Dedicated limiter diodes are also available for applications such as instrument protection and high-frequency voltage regulation. Silicon diodes are commonly selected when strong limitation of sharp-amplitude peaks is required.
Some limiter assemblies are made by connecting the required number of rectifier diodes in series to obtain a specified limiting voltage.
2.4 Modulator Diodes
Modulator diodes commonly refer to diode assemblies intended for ring modulators. Such an assembly contains four diodes with closely matched forward characteristics. Varactor diodes can also be used for modulation, but they are generally used directly for frequency modulation.
2.5 Mixer Diodes
When diode mixing is used, Schottky and point-contact diodes are commonly selected in the frequency range from 500 to 10,000 Hz, according to the classification provided here. In practical circuit design, the appropriate diode type is determined by the required frequency, signal level, capacitance, and switching characteristics.
2.6 Amplifier Diodes
Diode-based amplification is generally implemented in two ways. The first uses negative-resistance devices such as tunnel diodes and bulk-effect diodes. The second uses varactor diodes for parametric amplification.
Accordingly, amplifier diodes commonly include tunnel diodes, bulk-effect diodes, and varactor diodes.
2.7 Switching Diodes
Switching diodes are used in low-current logic circuits at approximately 10 mA and in magnetic-core drive circuits operating at several hundred milliamperes. Low-current switching diodes commonly use point-contact or bonded-wire structures. Silicon diffused, mesa, and planar diodes can also operate at elevated temperatures.
The primary advantage of a switching diode is its short switching time. Schottky diodes have particularly short switching times and are therefore well suited to high-speed switching. A 2AK point-contact diode is intended for medium-speed switching circuits, while a 2CK planar-contact diode is intended for high-speed switching circuits.
Switching diodes are also used in limiting, clamping, and detection circuits. Silicon Schottky barrier diodes are suitable for high-current switching because they combine low forward voltage drop, high speed, and good efficiency.
2.8 Varactor Diodes
Low-power diodes used for automatic frequency control (AFC) and electronic tuning are called varactor diodes. They are also known by various names in different manufacturers' product classifications.
When reverse voltage is applied, the capacitance of the PN junction changes. Varactor diodes are therefore used in automatic frequency control, sweep oscillators, frequency modulation, and tuning circuits. A silicon diffused structure is commonly used, although alloy-diffused, epitaxial-junction, and double-diffused structures are also available. These special structures can provide a relatively large change in junction capacitance as the applied voltage changes.
The junction capacitance varies with reverse voltage VR, allowing the diode to replace a variable capacitor in tuning circuits, oscillator circuits, and phase-locked loops. Varactor diodes are frequently used for channel selection and tuning in television tuners and are commonly fabricated from silicon.
2.9 Frequency-Multiplier Diodes
Diode frequency multiplication can be achieved using varactor diodes or step-recovery diodes. Varactor diodes used for frequency multiplication are often called variable reactance devices. Although their operating principle is similar to that of varactor diodes used for AFC, their construction is designed to withstand higher power.
A step-recovery diode is also called a snap-off diode. Its reverse-recovery time, trr, is very short when the device switches from conduction to cutoff. The transition to the off-state is therefore abrupt.
When a sinusoidal signal is applied to a step-recovery diode, the short transition time sharply clips the output waveform and produces many high-frequency harmonics. These harmonics can be used to design comb-spectrum generator circuits.
2.10 Voltage-Regulator Diodes
A voltage-regulator diode is designed to operate in the reverse-breakdown region and provide a relatively stable voltage. It is generally fabricated using a silicon diffused or alloy-junction process and has a reverse-breakdown characteristic that changes sharply.
These diodes are designed for voltage control and voltage-reference applications. The operating voltage, also called the Zener voltage, ranges from approximately 3 V to 150 V in the classification described here, with numerous voltage grades available. Power ratings range from approximately 200 mW to more than 100 W.
Silicon voltage-regulator diodes operate in reverse breakdown and have a low dynamic resistance, RZ. The 2CW series is a representative type. Two complementary diodes connected in reverse series can be used to reduce the temperature coefficient; this configuration is designated as the 2DW type.
2.11 PIN Diodes
A PIN diode contains an intrinsic semiconductor layer, or a very lightly doped semiconductor layer, between the P and N regions. The "I" in PIN is an abbreviation for "intrinsic."
At frequencies above approximately 100 MHz, minority-carrier storage and carrier transit through the intrinsic layer cause the diode to lose its conventional rectifying behavior and operate as a controllable impedance element. Its impedance changes with bias voltage.
Under zero bias or DC reverse bias, the intrinsic region has high impedance. Under DC forward bias, carriers are injected into the intrinsic region, causing its impedance to become low. A PIN diode can therefore be used as a voltage-controlled impedance element.
Compared with a conventional PN-junction diode, a PIN diode has a wider intrinsic region. This gives it high resistance under reverse bias and allows it to handle relatively high power, high frequency, and high reverse voltage.
PIN diodes are used in both low-frequency and high-frequency circuits. At low frequencies, they can function as variable resistors or current-control elements. At high frequencies, they can be used as switches, modulators, mixers, and limiters.
PIN diodes are particularly common in microwave and RF applications because of their high-speed operation and low loss. Typical applications include RF switches, modulators, and limiters. They are also used as photodetectors because they can convert optical signals into electrical signals, and they are widely used as optical receivers in fiber-optic communication systems.
2.12 Avalanche Diodes
An avalanche diode is a special diode that uses avalanche breakdown. Under forward bias, its current-voltage behavior is similar to that of a conventional diode. Under reverse bias, however, it exhibits a distinctive breakdown characteristic.
When a sufficiently high reverse voltage is applied, carriers in the depletion region gain enough energy to accelerate to high energy levels. When these carriers collide with lattice atoms, they generate additional electron-hole pairs. The newly generated carriers can be accelerated and collide with additional atoms, creating a chain reaction similar to an avalanche.
The main characteristics of an avalanche diode include the following:
- Reverse breakdown voltage: Before avalanche breakdown begins, the diode can withstand a high reverse voltage. When the reverse voltage reaches a certain level, avalanche multiplication begins and the diode enters breakdown. This voltage is called the reverse breakdown voltage or avalanche voltage.
- High reverse-current capability: In the breakdown state, the diode can conduct substantial reverse current, allowing it to be used in voltage regulators and overvoltage-protection circuits.
- Voltage stability: Within a specified operating range, the voltage across the diode remains relatively stable during breakdown, enabling voltage-reference and regulation functions.
- Temperature behavior: The breakdown voltage is comparatively insensitive to temperature over the intended operating range, providing useful temperature stability.
Avalanche diodes are used where a controlled reverse voltage is required, including voltage references, regulators, and overvoltage-protection circuits.
2.13 Esaki or Tunnel Diodes
An Esaki diode, also called a tunnel diode, is a special semiconductor diode that operates using quantum-mechanical tunneling. It was first developed by the Japanese physicist Leo Esaki. The device uses tunneling current as its primary current component and is commonly fabricated from gallium arsenide or germanium. Both the P-type and N-type regions are heavily doped.
A key characteristic of a tunnel diode is its negative-resistance region. In this region, increasing the voltage causes the current to decrease. This property makes tunnel diodes useful in specialized circuits.
The operating principle is based on tunneling through a narrow energy barrier. When the energy bands of two heavily doped semiconductor regions overlap, electrons can transition to the other side without overcoming the barrier through ordinary thermal excitation. Under a suitable bias condition, electrons can tunnel from the valence band to the conduction band, producing a rapid change in current.
Tunnel diodes have been used in high-frequency and microwave circuits, oscillators, switches, and logic gates. Although later devices replaced them in many digital applications, tunnel diodes retain value in certain analog and microwave circuits.
2.14 Fast Turn-Off or Step-Recovery Diodes
A fast turn-off diode, also called a step-recovery diode, is a PN-junction device with a sharply varying impurity profile near the PN-junction boundary. This profile produces a built-in electric field.
Under forward bias, the diode conducts through minority carriers, and charge is stored near the junction. When reverse bias is applied, the reverse current normally requires a storage interval before it falls to a minimum value. The built-in field of a step-recovery diode shortens this storage time and causes the reverse current to fall rapidly.
The abrupt turn-off produces rich harmonic content. These harmonics can be used in comb-spectrum generator circuits. Step-recovery diodes are therefore used in pulse circuits and high-order harmonic-generation circuits.
2.15 Damping Diodes
A damping diode has a relatively high reverse working voltage and peak-current capability, together with a low forward voltage drop. It is commonly used as a high-frequency, high-voltage rectifier, including in television scanning circuits for damping and boost rectification.
Damping diodes are also used in protection circuits for inductive components such as coils. When the current through an inductive component is interrupted, the inductor produces a reverse-voltage pulse as a result of its self-inductance. The pulse can reach a high amplitude and damage other circuit components.
The damping diode provides a low-impedance path for the inductive current, allowing the stored energy to be dissipated and limiting the resulting voltage pulse. It is normally connected in parallel with the inductive component. During normal operation, the diode is reverse biased and does not conduct. When the current is interrupted, the diode becomes forward biased and conducts the inductor current.
Fast-recovery diodes or Schottky diodes are often selected for this function because their short recovery times allow them to respond rapidly and dissipate inductive energy effectively.
2.16 Transient-Voltage-Suppression Diodes
A transient-voltage-suppression diode, or TVS diode, protects circuits against transient or excessive voltage. It limits the propagation of overvoltage and transient voltage and diverts the associated energy away from sensitive components.
The EOS waveform on the left represents a sharp transient pulse caused by an electrical overstress or overcurrent event. When the EOS pulse arrives, the TVS diode changes from a high-impedance state to a low-impedance state within the nanosecond range. It diverts the transient current to ground or another safe return path while clamping the voltage to a safer level. The waveform on the right shows the voltage after TVS protection.
When the applied voltage exceeds the device's specified working voltage, a TVS diode rapidly conducts and provides a low-impedance path. The excess voltage or transient energy is discharged to ground or another safe location, limiting the circuit voltage to a safe level.
The main characteristics of a TVS diode are:
- Fast response: The diode turns on rapidly and can protect sensitive components from transient damage.
- Low dynamic resistance: In the conducting state, its relatively low dynamic resistance helps absorb and divert transient energy.
- High energy absorption capability: The device can absorb substantial transient energy and is useful for protecting circuits against high-power voltage surges.
- Repeatable operation: Once the transient disappears, the diode returns to its nonconducting state and can protect the circuit against subsequent transient events.
TVS diodes are widely used to protect integrated circuits, sensors, communication equipment, and other sensitive electronics against lightning-induced surges, voltage spikes, and other transient events.
2.17 Unijunction Transistors and Dual-Base Diodes
A dual-base diode, also known as a unijunction transistor, is a three-terminal negative-resistance device with two bases and one emitter. It is used in relaxation oscillators and timing-voltage readout circuits. Its advantages include convenient frequency adjustment and good temperature stability.
To fabricate the device, two aluminum electrodes are attached to opposite ends of a high-resistivity N-type semiconductor substrate. These electrodes are designated base 1, B1, and base 2, B2. A P-type semiconductor region is then embedded into one side of the N-type substrate. A PN junction forms at the interface, and an electrode connected to the P-type region serves as the emitter, E.
Between B1 and B2 is the high-resistivity N-type semiconductor, so the interbase resistance, RBB, is relatively high, approximately 4 to 12 kΩ. The PN junction divides the N-type semiconductor into two parts. The resistance between the PN junction and B1 is represented by RB1, and the resistance between the junction and B2 is represented by RB2:
RBB = RB1 + RB2
The PN junction between the emitter and the N-type semiconductor can be represented by an equivalent diode.
2.18 Light-Emitting Diodes
A light-emitting diode, or LED, is a semiconductor device that converts electrical energy directly into light. It is commonly fabricated from materials such as gallium phosphide and gallium arsenide phosphide. Under forward bias, carrier recombination produces light. LEDs are compact, operate at relatively low voltage and current, provide uniform illumination, and have a long service life. They can emit monochromatic red, yellow, green, and other colors.
An LED contains N-type and P-type semiconductor regions that form a PN junction. When forward voltage is applied, electrons move from the N-type region toward the P-type region, while holes move in the opposite direction. Near the PN junction, electrons and holes recombine and release energy in the form of photons. The semiconductor material determines the wavelength and therefore the emitted color.
The main advantages of LEDs include:
- High efficiency: LEDs convert a large portion of electrical energy into light and generally use energy more efficiently than traditional incandescent sources.
- Long service life: LEDs can operate for tens of thousands of hours and, in some applications, even longer.
- Fast startup: An LED can reach full output almost immediately without a warm-up period.
- Low energy consumption: Their relatively low power consumption can reduce energy use.
- Small size: LEDs can be manufactured in many shapes and sizes, making them suitable for a wide range of applications.
Because of these characteristics, LEDs are widely used in lighting, displays, indicators, backlights, automotive lamps, and electronic equipment.
3. Classification by Electrical Characteristics
Point-contact diodes can also be classified according to their forward and reverse characteristics.
3.1 General-Purpose Point-Contact Diodes
As the name suggests, these diodes are commonly used in detection and rectification circuits. Their forward and reverse characteristics are neither particularly strong nor particularly weak, placing them between specialized diode types. Examples include SD34, SD46, and 1N34A.
3.2 High-Reverse-Voltage Point-Contact Diodes
These devices have high maximum peak reverse voltage and maximum DC reverse voltage ratings. They are used for detection and rectification in high-voltage circuits. Their forward characteristics are generally average or relatively weak.
Examples of germanium point-contact diodes in this category include SD38, 1N38A, and OA81. The reverse-voltage capability of germanium devices is limited. When a higher reverse-voltage rating is required, silicon alloy-junction or silicon diffused diodes are used instead.
3.3 High-Reverse-Resistance Point-Contact Diodes
The forward-voltage characteristics of these diodes are similar to those of general-purpose devices. Their reverse-voltage capability is also high, but their reverse current is particularly low, giving them high reverse resistance.
They are used in circuits with high input resistance or high-resistance load resistors. Examples of germanium high-reverse-resistance point-contact diodes include SD54 and 1N54A.
3.4 High-Conductance Point-Contact Diodes
High-conductance point-contact diodes have characteristics opposite to those of high-reverse-resistance devices. Their reverse characteristics are relatively poor, but their forward resistance is sufficiently low. Examples include SD56 and 1N56A.
High-conductance bonded-wire diodes can provide even better forward characteristics. When the load resistance is particularly low, this type of diode can provide relatively high rectification efficiency.
