A diode is the most basic semiconductor device. However, it has endless applications. In semiconductor electronics, the p-n junction as the basic diode serves as the basis for all other sophisticated semiconductor components and their design. The design principles that apply to a semiconductor diode are the same as those that apply to transistors and other components.
Interestingly, when we define any electronic component, the definition is usually not based on its construction. Instead, any electronic component is defined by its unique electrical behavior. Following the same convention, we can define a diode as a voltage-controlled two-terminal unidirectional switch. Often the term diode refers to a semiconductor diode. A semiconductor diode is not the only diode device. There are many different types of diodes, and many of them are semiconductor diodes specially designed to have specific physical or electrical properties. There are also diodes that are not simple p-n junction and have different construction and design. In this article, we will take a brief look at the various diodes.
Small Signal Diodes
Small signal diodes are general purpose semiconductor diodes with a small current carrying capacity. These diodes are generally made of silicon or germanium and are designed to serve high frequency and small current applications. Small signal diodes are smaller than typical rectifier diodes and are often encased in glass to protect them from contamination. This is why they are also known as Passivated Glass Diodes . The cathode terminal of the diode is indicated by a red or black stripe on one side. Since these diodes have minimal current carrying capacity, their power is also very low. A small signal diode with a current rating of 150 mA may only have 500 mW of rated power. Small-signal diodes are used in high-frequency or pulsating, low-current applications such as radio, television, digital logic circuits, clipper and clamper circuits, high-speed switching, and parametric amplifiers. Important characteristics to note in the datasheet of a small signal diode are Peak Reverse Voltage, Reverse Current, Peak Forward Current, Peak Forward Voltage, and Reverse Recovery Time.
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Large signal diodes/rectifier diodes
Large signal diodes are different from small signal diodes in the area of their p-n junction. Large signal diodes have a large p-n junction area. This increases the current carrying capacity as well as the peak reverse voltage. They have a very low direct resistance to reverse resistance ratio, with the direct resistance typically being a few ohms, while the reverse resistance is in megohms. This is why these diodes are not suitable for high frequency circuits. They have a large PIV rating, small forward resistance, and large current-carrying capacity. They are generally used for AC to DC voltage rectification or to suppress high peak voltages. In fact, large signal diodes are mainly rectifier diodes.
Small and large signal diodes have the same symbol as a generic diode.
Zener diode
Zener diodes are semiconductor diodes designed with heavy doping to utilize Zener breakdown in their operation. When a normal diode receives a reverse voltage greater than its PIV rating, it becomes permanently damaged and goes open circuit. On the other hand, due to heavy doping, when a Zener diode is applied with reverse voltage higher than its 'Zener voltage', it starts conducting current in reverse direction without damage due to Zener breakdown and avalanche breakdown. A Zener diode has a controlled breakdown in the reverse region. It conducts current above a 'Zener voltage'. It is mainly used as a voltage rectifier in DC applications. Many Zener diodes have Zener voltage in the range of 2V to 200V. These diodes are also used as protection diodes in various semiconductor circuits.
The Zener diode has the following symbol. 
Light emitting diode
Light-emitting diodes are special diodes that emit visible light when forward-biased. In reverse bias, like a normal diode, they are in a non-conducting state and do not emit light. These are semiconductor diodes made from gallium arsenide and similar semiconductor substrates with a high-energy bandgap between their conduction and valence bands. Due to the high-energy bandgap, when electrons and holes combine near the p-n junction, the radiated energy is in the form of visible or infrared light instead of heat.
There are many different types of LEDs. They are usually classified by the light they transmit. For example, IR LEDs are light-emitting diodes that emit light in the infrared region. Different color LEDs have different semiconductor substrates, turn-on voltage, and reverse voltage ratings. LEDs are used in AC and DC applications. It is important to note the maximum forward voltage, PIV rating, and maximum forward current of an LED before using it as an application. LEDs are quite sensitive and can be easily damaged. The PIV ratings of LEDs, like signal diodes, are generally in the tens of volts, while the maximum forward voltage is just a few volts.
The LEDs have the following symbol. 
Schottky diode
Schottky diodes are different from typical pn diodes. A Schottky diode is constructed by forming a junction between N-type semiconductor material and metal such as platinum, chromium or tungsten. Due to the metal-semiconductor junction, these diodes have high current-carrying capacity and fast switching time. The metallic junction also reduces the activation voltage and improves the diode's energy efficiency. Due to all these benefits, Schottky diodes are used for high-frequency rectification and high-frequency switching applications.
A Schottky diode has the following symbol. 
Shockley diode
Just as a generic diode has two layers, the Shockley diode has four. It is also called PNPN diode. It is similar to a thyristor without a gate terminal. It is identified as a diode because it has only two terminals and there are two electrical states of the device – conducting and non-conducting. It can enter into a conducting state only when a direct voltage is applied to it. A PNPN diode is basically a PNP transistor and an NPN transistor coupled together. The other transistor conducts when there is enough voltage to bias the first. Therefore, a PNPN diode requires a sufficient forward voltage to enter a conducting state. If the forward voltage drops or reverse voltage is applied, the Shockley will enter a non-conducting state. In the conducting state, the Shockley diode is considered 'ON', while in the non-conducting state it is considered 'OFF'. The two most common applications of Shockley diode are trigger switches for SCR and relaxation oscillator or sawtooth oscillator. These diodes are used in audio amplifier circuits.
Below is the electrical symbol for the Shockley Diode. 
Tunnel diode
Tunnel diodes are heavily doped semiconductor diodes – 1000 times more than a large signal diode. These diodes use a quantum phenomenon called resonant tunneling. These diodes exhibit a strange negative resistance in their forward characteristics. When forward biased, current increases with voltage and reaches a peak. This is called peak current, and the voltage at this point is called peak voltage. Then, as the voltage increases, the current decreases and drops to a low point called valley current. The voltage at this point is called valley voltage. By increasing the applied voltage beyond the valley voltage, the current increases exponentially without any further drop. These diodes have a very fast switching time on the order of nanoseconds. Its transient response is limited only by the junction capacitance and the parasitic wire capacitance. Tunnel diodes are used as high-speed switches in microwave oscillators and amplifiers. It is possible to tune these diodes both electrically and mechanically.
Below is the electrical symbol of the tunnel diode. 
Varator diode
Varactor diodes work like a variable capacitor, which is why these diodes are also called varicap diodes. They are connected via reverse bias in a constant voltage circuit. Its specialty is that its depletion layer can be increased or decreased by varying the reverse voltage applied. The change in the depletion layer changes the capacitance of the diode. The capacitance of a varactor diode can vary to very high values. These diodes are used in voltage-controlled oscillators, voltage-controlled capacitors, frequency multipliers, parametric amplifiers, phase-locked loops, and FM transmitters.
The Varactor diode has the following electrical symbol.
laser diode
Laser diodes are a type of light-emitting diode. The acronym 'laser' stands for Light Amplification by Stimulated Emission of Radiation. The PN junction of a laser diode has polished ends. When forward biased, the junction emits photons and then the emitted photons are reflected back and forth between the polished ends of the diode. As a result, more electron-hole pairs are generated. Their recombination produces more photons in phase with the previous photon. This leads to the generation of a pencil beam from the semiconductor region of the diode, monochromatic and single phase. The emitted laser beam can be in the visible or infrared region. These diodes are also known as injection, semiconductor, and diode lasers. Laser diodes are used in fiber optic communication, laser printers, optical disk readers, intrusion detection systems, remote control applications, and barcode readers.
A laser diode has the following electrical symbol.
Step recovery diode/ instantaneous diode
Stepped recovery diodes or instantaneous diodes are designed for high frequency operation. They are also called snap-off diodes and charge storage diodes. These diodes are used in higher order multipliers and pulse shaping circuits. When a sinusoidal signal is applied to them, they store charge in the positive pulse and utilize that charge in the negative pulse. The rise time of the current pulse remains the same as the snap time. This is why they are called stepped recovery diodes. The cutoff frequency of these diodes varies between 200 and 300 GHz. The higher the signal frequency, the better its efficiency.
Below is the electrical symbol of the stepped recovery diode.
Gunn diode
Gunn diodes are designed with n-type semiconductor material only. Two n-type materials are joined together to form a depletion region between them. The depletion region of n-type materials is very small. When a direct voltage is applied, the current increases and reaches a peak level. Then, as the forward voltage increases further, the current begins to decrease exponentially. This is called negative differential resistance. Gunn diodes are also called transferred electron devices. In the conducting state, these produce microwave RF signals. Gunn diodes are used in microwave amplifiers.
Below is the electrical symbol for the Gunn Diode.
PIN diode
PIN diodes have a layer of intrinsic material between the p-type and n-type material. In forward bias, electrons and holes are injected into the intrinsic layer of p-type and n-type material, respectively. The presence of an intrinsic layer increases the depletion layer and produces an electric field between the p-type and n-type material. Current flows through the diode due to this electric field. The increased depletion layer decreases the diode's capacitance while increasing the response speed. It also increases the photosensitive region of the diode. PIN diodes are used in high-speed, high-sensitivity applications such as photodetectors, RF switches, and attenuators.
A PIN diode has the following electrical symbol.
Photo-diode
Photodiodes are semiconductor diodes designed to produce an electrical current in response to visible, infrared, and ultraviolet light irradiation. The diode has a thin p-type material and a heavily doped non-type material. There is a narrow depletion region between them, which is exposed to light through p-type material. Due to the design of the diode, exposure to light triggers the formation of a large number of electron-hole pairs in the depletion region. Electrons and holes are diffused into n-type p-type materials due to the built-in electric field that produces an electrical current from the anode (p-type material exposed to light) to the cathode (metallic contact).
A photodiode has the following electrical symbol.
Solar cell
Solar cells are simply photodiodes optimized to supply power to a load. They operate in photovoltaic mode. Voltage across the load resistance causes the solar cell to polarize forward. There are many different types of solar cells based on construction material and design. Silicon solar cells are the most popular. Other materials used to construct solar cells include polycrystalline silicon, cadmium telluride, and cadmium indium gallium diselenide.
A solar cell, also known as a photovoltaic cell, has the following electrical symbol.
IMPATT diode
The impact avalanche transit time diode is called the IMPATT diode. These diodes are used to generate high power radio frequencies in the range of 3 to 100 GHz. These diodes are connected in reverse bias in an oscillator circuit. Due to the avalanche effect, they produce a large amount of current beyond the peak reverse voltage. In an oscillator circuit, the current through the IMPATT diode lags behind the voltage. Due to the negative resistance effect and the resonant circuit, high power radio waves are produced from the diode.
The IMPATT diode has the same electrical symbol as the generic diode.
Constant Current Diode
Constant current diodes are also known as current limiting diodes and current regulating diodes. These are used as current regulators. They have a similar construction to a JFET, but are a two-terminal device. A constant current diode has straight forward characteristics in which initially the current increases exponentially like a normal diode. Then, beyond a current set point, the current becomes saturated. The diode reaches current saturation by dropping more voltage across it. Constant current diodes are used in battery charging, power supply circuits and laser diode circuits.
A constant current diode has the following electrical symbol.
Power diode
Power diodes are large signal diodes. They are specifically used for voltage rectification. The most important parameter of power diodes is the PIV rating. The PIV rating of power diodes typically ranges from 50V to 1000V. The maximum forward current and the ratio of forward resistance to reverse resistance are two other important factors that need to be checked in the datasheet of a power diode.
A power diode has the same symbol as a generic diode.
Point contact diode
Point contact diodes are used to detect high frequency signals. They are produced by creating a PN junction between a gold or tungsten wire and an n-type germanium material. The gold wire allows high current to pass through the junction. The forward characteristics of this diode are similar to those of a normal diode; however, in reverse bias, the diode acts as an insulator. This causes the diode to operate as a capacitor under reverse bias conditions and block DC while passing a high frequency AC signal. The diode body is enclosed in a glass shell.
The point contact diode has the same symbol as the generic diode.
Silicon controlled rectifier
A silicon controlled rectifier (SCR) is similar to a Shockley diode with an additional gate terminal. The forward and reverse characteristics of an SCR are similar to those of a diode, except that it enters a conducting state in the forward region when the gate is activated. They are mainly used in power control applications.
The silicon controlled rectifier has the following electrical symbol.
crystal diode
Crystal diodes are similar to point contact diodes. They are also known as Cat's Mustache. They are designed by pressing a metallic wire against a semiconductor crystal. Its use is limited to microwave detectors and receivers.
The crystal diode has the same electrical symbol as the generic diode.
Transient Voltage Suppression Diode
Transient voltage suppression diodes are similar to Zener diodes. They are used to clamp transient voltages and are designed to offer low impedance in response to a transient voltage, immediately entering the avalanche breakdown region. The diode response time is in picoseconds. These diodes are designed to have a minimum clamping voltage. Voltage suppression diodes are used in a variety of applications, mainly involving signal processing or data communication. 
Voltage suppression diodes have the following electrical symbol.
Super barrier diode
Super barrier diodes are used as rectifier diodes. They are designed to have low forward voltage, like a Schottky diode, and low reverse leakage current, like a normal diode. These diodes have fast switching times and can handle high power with minimal losses.
The super barrier diode has the same electrical symbol as the Schottky diode.
Avalanche diode
Avalanche diodes operate on the principle of avalanche breakdown. They are designed to have accurate reverse breakdown voltage. These diodes are used in radio and microwave applications.
An avalanche diode has the following electrical symbol.
Peltier diode
Peltier diodes are designed to have a junction of two materials. In these diodes, there is heat flow in a single direction along the direction of current flow. These diodes are used as sensors and heat engines in heating and cooling applications.
Gold doped diodes
In these diodes, gold is used as a dopant. These diodes are faster than signal diodes and also have a low reverse leakage current.
Vacuum diodes
This is the simplest vacuum tube with two electrodes – cathode and anode. The cathode emits electrons and the anode collects them. These diodes have a high reverse voltage rating. They are used in audiophiles, amplifiers, field electron emission, fluorescent displays and rectifiers.
The vacuum diode has the following electrical symbol.
