Detector Diode
The main function of a detector diode is to extract the low-frequency signal from a high-frequency signal. They are constructed with a point-contact structure, which means they have a small junction capacitance and can operate at high frequencies. Typically, they are made of germanium material. In principle, the process of extracting the modulating signal from the input signal is called detection. With a rectification current of 100 mA as the boundary, those with an output current less than 100 mA are referred to as detector diodes. Germanium point-contact type diodes can operate at frequencies up to 400 MHz, with low forward voltage drop, small junction capacitance, high detection efficiency, and good frequency characteristics, such as the 2AP type. In addition to detection, these diodes can also be used in circuits for limiting, clipping, modulating, mixing, switching, and other functions. There are also special combinations of two diodes with consistent characteristics for FM detection.
Rectifier Diode
In principle, the process of obtaining a direct current (DC) output from an alternating current (AC) input is called rectification. With a rectification current of 100 mA as the boundary, those with an output current greater than 100 mA are referred to as rectifier diodes. They have a planar structure, resulting in a larger junction capacitance and are generally used below 3 kHz. The maximum reverse voltage ranges from 25 volts to 3000 volts, divided into 22 grades from A to X. They are classified as follows: ① Silicon semiconductor rectifier diodes (2CZ type), ② Silicon bridge rectifiers (QL type), and ③ High-voltage silicon stacks used in television sets with a working frequency close to 100 kHz (2CLG type).
Clipping Diode
When a diode is forward-biased and conducts, its forward voltage drop remains essentially constant (0.7 V for silicon diodes and 0.3 V for germanium diodes). This characteristic is utilized in circuits as a clipping element to limit the signal amplitude within a certain range. Most diodes can be used for clipping. There are also specialized clipping diodes, such as those for protecting instruments and high-frequency Zener diodes. To enhance the ability to limit sharp amplitude spikes, silicon diodes are commonly used. Components are also available that consist of several rectifier diodes connected in series to form a single unit based on the required clipping voltage.
Modulating Diode
This typically refers to diodes specifically used for ring modulation, which are combinations of four diodes with consistent forward characteristics. Although other varactor diodes also have modulation applications, they are usually used directly for frequency modulation.
Mixing Diode
When using diode mixing methods, in the frequency range of 500 Hz to 10,000 Hz, Schottky and point-contact diodes are commonly used.
Amplifying Diode
Amplification using diodes generally involves negative resistance devices such as tunnel diodes and bulk-effect diodes, as well as parametric amplification using varactor diodes. Therefore, amplifying diodes usually refer to tunnel diodes, bulk-effect diodes, and varactor diodes.
Switching Diode
A diode has a very low resistance when forward-biased and is in a conducting state, similar to a closed switch; when reverse-biased, it has a very high resistance and is in a cutoff state, similar to an open switch. The switching characteristics of diodes can be used to form various logic circuits. There are switching diodes used for logic operations with small currents (around 10 mA) and those used for core excitation with currents in the hundreds of milliamperes. Small-current switching diodes are typically point-contact and key-type diodes, while silicon diffusion, mesa, and planar diodes are also available for use in high-temperature environments. The main advantage of switching diodes is their fast switching speed. Schottky diodes have particularly short switching times and are ideal switching diodes. The 2AK type point-contact diodes are used for medium-speed switching circuits, while the 2CK type planar diodes are used for high-speed switching circuits. They are also used in switching, limiting, clamping, or detection circuits. Schottky (SBD) silicon high-current switching diodes have low forward voltage drop, fast speed, and high efficiency.
Varactor Diode
Varactor diodes are small-power diodes used for automatic frequency control (AFC) and tuning. They are also known by various names among Japanese manufacturers. By applying a reverse voltage, the junction capacitance of the diode changes. Therefore, they are used in applications such as automatic frequency control, scanning oscillation, frequency modulation, and tuning. Typically, they are made of silicon using a diffusion process, but other special types such as alloy diffusion, epitaxial junction, and double-diffusion diodes are also used because these diodes exhibit a particularly large change in capacitance with voltage. The junction capacitance varies with the reverse voltage VR, replacing variable capacitors and used in tuning circuits, oscillator circuits, and phase-locked loops. They are commonly used in television high-frequency heads for channel conversion and tuning circuits and are mostly made of silicon.
Frequency Multiplier Diode
For frequency multiplication using diodes, there are two types: frequency multiplication using varactor diodes and frequency multiplication using step (or abrupt) recovery diodes. Varactor diodes used for frequency multiplication are called variable reactance diodes. Although the working principle of variable reactance diodes is the same as that of varactor diodes used for automatic frequency control, the construction of reactance diodes can handle higher power. Step recovery diodes, also known as step recovery diodes, have a short reverse recovery time trr when switching from conduction to cutoff. Therefore, their main advantage is the significantly short transfer time when rapidly switching to the cutoff state. If a sinusoidal wave is applied to a step recovery diode, due to the short transfer time tt, the output waveform is sharply truncated, resulting in the generation of many high-frequency harmonics.
Zener Diode
This type of diode is made using the reverse breakdown characteristic of a diode, maintaining a nearly constant voltage across its terminals to stabilize the voltage in a circuit. It is a product that replaces voltage stabilizing electronic diodes. It is a diode with a steep reverse breakdown characteristic curve. It is used as a control voltage and reference voltage. The diode's operating voltage (also known as Zener voltage) ranges from around 3 V to 150 V, with many grades divided by 10%. In terms of power, products are available from 200 mW to over 100 W. Operating in the reverse breakdown state, made of silicon material, with a very small dynamic resistance RZ, typically 2CW, 2CW56, etc.; two complementary diodes connected in reverse series to reduce the temperature coefficient are 2DW type.
The temperature coefficient α of a Zener diode: α represents the change in the stabilization voltage per 1°C temperature change. Diodes with a stabilization voltage less than 4 V have a negative temperature coefficient (due to Zener breakdown), meaning the stabilization voltage decreases with increasing temperature (temperature causes valence electrons to move to higher energy levels); diodes with a stabilization voltage greater than 7 V have a positive temperature coefficient (due to avalanche breakdown), meaning the stabilization voltage increases with increasing temperature (temperature increases atomic vibration amplitude, hindering carrier movement); diodes with a stabilization voltage between 4 and 7 V have a very small temperature coefficient, nearly zero (both Zener and avalanche breakdowns occur).
PIN Diode
This is a crystal diode constructed with an intrinsic semiconductor (or a semiconductor with low impurity concentration) sandwiched between the P and N regions. The "I" in PIN stands for "intrinsic." When operating at frequencies above 100 MHz, due to the storage effect of minority carriers and the transit time effect in the "intrinsic" layer, the diode loses its rectifying action and becomes an impedance element, with its impedance value changing with the bias voltage. In zero bias or reverse bias, the impedance of the "intrinsic" region is very high; in forward bias, due to the injection of carriers into the "intrinsic" region, the "intrinsic" region exhibits a low impedance state. Therefore, a PIN diode can be used as a variable impedance element. It is commonly used in high-frequency switching (microwave switching), phase shifting, modulation, limiting, and other circuits.
Avalanche Diode
This is a transistor that can generate high-frequency oscillations under the influence of an applied voltage. The working principle of generating high-frequency oscillations is as follows: using avalanche breakdown to inject carriers into the crystal, since the transit time of carriers across the wafer is finite, the current lags behind the voltage, resulting in a delay time. If the transit time is appropriately controlled
Tunnel Diode
The tunnel diode is a crystal diode whose primary current component is the tunneling current. Its base materials are gallium arsenide and germanium. The P-type and N-type regions are highly doped (i.e., with high concentrations of impurities). The tunneling current is generated by the quantum mechanical effects of these degenerate semiconductors. The occurrence of the tunneling effect requires the following three conditions: ① The Fermi level is located within the conduction band and the valence band; ② The space charge layer must be very narrow (less than 0.01 micrometers); there is a possibility of overlap between the energy levels of holes in the P-type region and electrons in the N-type region of the degenerate semiconductor. The tunnel diode is a two-terminal active device. Its main parameter is the peak-to-valley current ratio (IP/PV), where the subscript "P" represents "peak" and the subscript "V" represents "valley." The tunnel diode can be used in low-noise high-frequency amplifiers and high-frequency oscillators (with operating frequencies reaching the millimeter wave band) and can also be used in high-speed switching circuits.
Step Recovery Diode
This is also a diode with a PN junction. Its structural feature is a steep impurity distribution at the PN junction boundary, forming a "self-sustaining electric field." Since the PN junction conducts with minority carriers under forward bias and has a charge storage effect near the PN junction, the reverse current requires a "storage time" to drop to its minimum value (the reverse saturation current value). The "self-sustaining electric field" of the step recovery diode shortens the storage time, allowing the reverse current to cut off quickly and produce rich harmonic components. These harmonic components can be used to design comb spectrum generation circuits. The step recovery diode is used in pulse and higher-order harmonic circuits.
Schottky Barrier Diode
This is a diode with a "metal-semiconductor junction" that exhibits Schottky characteristics. It has a low forward voltage drop. The metal layer can be made of materials such as gold, molybdenum, nickel, and titanium. The semiconductor material is typically silicon or gallium arsenide, usually in the form of N-type semiconductors. This device conducts with majority carriers, so its reverse saturation current is much larger than that of a PN junction, which conducts with minority carriers. Since the storage effect of minority carriers in Schottky diodes is minimal, its frequency response is limited only by the RC time constant, making it an ideal device for high-frequency and fast switching applications. Its operating frequency can reach up to 100 GHz. Additionally, MIS (metal-insulator-semiconductor) Schottky diodes can be used to make solar cells or light-emitting diodes. It can also serve as a freewheeling diode, providing freewheeling function in inductive loads such as inductors in switch-mode power supplies and relays.
Damping Diode
Damping diodes are commonly used in high-frequency voltage circuits. They have a high reverse working voltage and peak current, with a low forward voltage drop. These high-frequency, high-voltage rectifier diodes are used for damping and voltage boosting in the horizontal scanning circuit of televisions. Common damping diodes include 2CN1, 2CN2, and BSBS44.
Transient Voltage Suppression Diode
The TVP diode provides rapid overvoltage protection for circuits. It is available in both bipolar and unipolar types, classified by peak power (500W - 5000W) and voltage (8.2V - 200V).
Double-Base Diode (Single-Crystal Transistor)
This is a three-terminal negative resistance device with two bases and one emitter. It is used in relaxation oscillator circuits and timing voltage readout circuits. It has the advantages of easy frequency adjustment and good temperature stability.
Light-Emitting Diode (LED)
Made of gallium phosphide and gallium arsenide phosphide materials, these LEDs are small in size and emit light when forward-biased. They operate at low voltage and current, with uniform light emission, long life, and the ability to emit red, yellow, green, and blue monochromatic light. With technological advancements, white high-brightness LEDs have been developed, giving rise to the emerging LED lighting industry. They are also used in displays for VCDs, DVDs, calculators, and other devices.
Silicon Power Switching Diode
The silicon power switching diode has the capability for high-speed conduction and cutoff. It is primarily used in high-power switching or voltage stabilizing circuits, DC converters, high-speed motor speed control, and as a high-frequency rectifier and freewheeling clamp in drive circuits. It has the advantages of soft recovery characteristics and strong overload capacity, and is widely used in computer power supplies, radar power supplies, and step motor speed control applications.
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