Frequency doubler diodes include varactor diodes and step recovery diodes. Varactor diodes used for frequency doubling are known as variable reactance diodes. Although they operate on the same principle as varactor diodes used for automatic frequency control (AFC), their construction allows them to handle higher power levels. Step recovery diodes, also known as step recovery diodes, have a very short reverse recovery time (trr) when switching from conduction to blocking, which allows them to switch off rapidly and produce many high-frequency harmonics when subjected to a sine wave input.
 

Zener diodes are designed to stabilize voltage using the reverse breakdown characteristic of the diode. They maintain a constant voltage across them when operating in the breakdown region. Made from silicon in diffused or alloyed forms, they are characterized by a sharp change in the reverse breakdown curve. Zener diodes are used for voltage regulation and are available in a wide range of breakdown voltages from approximately 3V to 150V, and power ratings from 200mW to over 100W. They are categorized based on their temperature coefficient α, which indicates the change in the Zener voltage per degree Celsius. Zener diodes with voltages below 4V have a negative temperature coefficient, while those above 7V have a positive temperature coefficient. Diodes with voltages between 4V and 7V have a near-zero temperature coefficient.
 

A PIN diode has an intrinsic (or low-doping) semiconductor layer between the P and N regions. The "I" in PIN stands for "Intrinsic." When operating above 100MHz, the diode acts as an impedance element due to the storage effect of minority carriers and the transit time effect in the intrinsic layer. In zero-bias or reverse DC bias, the intrinsic layer exhibits high impedance; with forward DC bias, it shows low impedance due to carrier injection. PIN diodes are used as variable resistors in high-frequency switches, phase shifters, modulators, and limiters.
 

Avalanche diodes are designed to generate high-frequency oscillations under applied voltage. The principle of operation involves using avalanche breakdown to inject carriers into the crystal. The current lags behind the voltage due to the time required for carriers to traverse the chip, resulting in a delay time. Proper control of this transit time produces a negative resistance effect, generating high-frequency oscillations. Avalanche diodes are commonly used in microwave oscillators.
 

Tunnel diodes use tunnel effect current as the primary conduction mechanism. Made from materials like gallium arsenide and germanium, they have heavily doped P and N regions. The tunnel effect occurs under conditions where the Fermi level is within the conduction band and valence band, with a very narrow depletion region. Tunnel diodes are two-terminal active devices with parameters like the peak-to-valley current ratio (IP/PV). They are used in low-noise high-frequency amplifiers, high-frequency oscillators (up to millimeter-wave frequencies), and high-speed switching circuits.
 

Step recovery diodes have a PN junction with a steep impurity profile, creating a "self-help field" at the junction. This structure allows for rapid recovery of reverse current, producing rich harmonic components. These diodes are used in pulse circuits and harmonic generation circuits due to their fast recovery time and ability to generate comb-filtered frequency spectra.
 

Schottky diodes feature a "metal-semiconductor junction" and have a low forward voltage drop. They are made with metals like gold, molybdenum, nickel, and titanium, and use silicon or gallium arsenide as the semiconductor material. Schottky diodes are ideal for high-frequency and fast-switching applications due to their low storage effect and fast switching speed, with operating frequencies up to 100GHz. They can be used in power rectifiers, as flyback diodes, and in photovoltaic cells or light-emitting diodes (LEDs).
 

Damping diodes are used in high-frequency voltage circuits, featuring high reverse working voltage and peak current capabilities, with low forward voltage drop. They are typically employed in TV line scan circuits for damping and boosting rectification. Common damping diodes include 2CN1, 2CN2, and BSBS44.
 

TVS diodes provide rapid over-voltage protection for circuits. They come in bipolar and unipolar types, with classifications based on peak power (500W to 5000W) and voltage (8.2V to 200V).
 

The Shockley diode has two base terminals and one emitter terminal, used in relaxation oscillators and timing voltage circuits. It features adjustable frequency and good temperature stability.
 

LEDs are made from materials like gallium phosphide and gallium arsenide. They emit light when forward biased and are known for their low operating voltage, low current consumption, uniform light emission, long lifespan, and availability in various colors including red, yellow, green, and blue. Advances in technology have led to the development of high-brightness white LEDs, contributing to the growth of LED lighting as a new industry. LEDs are also used in VCD, DVD players, and calculators.
 

Silicon power switching diodes are designed for high-speed switching and are used in high-power switching circuits, voltage regulation circuits, DC converters, high-speed motor control, and high-frequency rectification. They are characterized by their soft recovery properties and strong overload capability, making them suitable for applications in computers, radar power supplies, and stepper motor speed control.