pin diode

The characteristic qualities of a pin diode are manifested when operating in the strong injection mode, when the i region is filled with charge carriers from heavily doped n + and p + regions to which a direct voltage bias is applied. The pin diode can be functionally compared to a bucket of water with a hole on the side: as soon as the bucket is filled to the level of the hole, it starts to leak. In the same way, the diode begins to pass current as soon as the i- region is filled with charge carriers.

Due to the very low concentration of charge carriers in the i-region , there are practically no recombination processes during injection. But in the forward bias mode, the concentration of charge carriers is several orders of magnitude higher than its own concentration.

At low frequencies, the same equations are valid for the pin diode as for the ordinary one. At high frequencies, the pin diode behaves like an almost perfect resistor - its current-voltage characteristic (CVC) is linear even for a very large voltage value. At high frequencies in the i-region there is a large amount of accumulated charge, which allows the diode to work. At low frequencies, the charge in the i-region recombines and the diode turns off.

The reactance is inversely proportional to the direct current flowing through the pin diode. Thus, it is possible to vary the resistance value over a wide range - from 0.1 Ohm to 10 kOhm - changing the constant component of the current.

The large width of the i-region also means that the pin diode has a small capacitance at reverse bias.

The areas of space charge (SCR) in the pin diode are almost completely located in the i-region . Compared to conventional ones, the pin diode has a much larger SCR, the boundaries of which vary slightly depending on the applied reverse voltage. Thus, the volume of the semiconductor increases, where electron-hole pairs can be formed under the influence of radiation (for example, optical - photon ). Some photodetectors, such as pin photodiodes and phototransistors (in which the base-collector junction is a pin diode), use a pin junction to implement the detection function.

When designing a pin diode, you have to find a compromise: on the one hand, by increasing the i-region (and, accordingly, the amount of accumulated charge), you can achieve the diode's resistive behavior at lower frequencies, but on the other hand, to recombine the charge and transfer to closed state will take longer. Therefore, as a rule, pin diodes are each time designed for a specific application.

pin diodes are typically used as switches in radio and microwave paths, attenuators, modulators, switches and photo detectors.

According to the scope of application, pin diodes are divided into:

• mixing (for example: 2A101 - 2A109);
• detector (for example: 2A201 - 2A203);
• parametric (for example: 1A401 - 1A408);
• switching and restrictive (for example 2A503 - 2A524);
• multiplying and tuning (for example: 2A601 - 2A613);
• generator (3A703, 3A705).

Radio Frequency (RF) and Microwave Switches

At zero or reverse bias, the pin diode has a low capacitance. A small capacitance does not allow a high-frequency signal to pass through. With forward bias and a current of 1 mA, a typical pin diode has a reactance of the order of 1 Ohm, which makes it a good conductor in the RF path. Thus, the pin diode can be used as a good RF and microwave switch.

RF relays are also used as switches, but at a lower speed (switching time ~ 10 ms ), while pin diodes are much faster: tens of nanoseconds, units of microseconds.

The capacity of the switched off discrete pin diode is approximately 1 pF . At a frequency of 320 MHz, the reactance of such a capacity is ~ 500 Ohms. On 50 Ohm systems, signal attenuation will be around 20 dB , which is not enough in some applications. In applications requiring greater isolation, the switches are cascaded: a cascade of three diodes gives attenuation of 60 dB or more (up to 100 dB depending on the frequency).

RF and microwave controlled attenuators

By changing the current through the pin diode, you can quickly change the reactance.

At high frequencies, the pin diode reactance is inversely proportional to the current strength. Accordingly, a pin diode can be used as a controlled attenuator, for example, in amplitude modulator and level shift circuits.

The pin diode can be used, for example, as a bridge or shunt resistor in a T-bridge attenuator circuit.

Limiters

pin diodes are sometimes used to protect input devices during high-frequency measurements. If the input signal is small and is in the range of acceptable values, then the pin diode as a small capacitance introduces minimal distortion. When the signal increases and it goes beyond the permissible limits, the pin diode starts to conduct and becomes a resistor, which shunts the signal to ground.

Photo Detectors

The pin diode can be used in network cards and switches for fiber optic cables. In these applications, the pin diode is used as a photodiode .

As a photo detector, the pin diode works with reverse bias. At the same time, it is closed and does not pass current (with the exception of an insignificant leakage current). A photon enters the i- region, generating the formation of electron-hole pairs. Charge carriers entering the SCR electric field begin to move to highly doped regions, creating an electric current that can be detected by an external circuit. The conductivity of the diode depends on the wavelength, intensity and modulation frequency of the incident radiation.

The magnitude of the reverse voltage can reach large values, while a higher voltage creates a larger field, which pulls carriers from the SCR of the i region more quickly.

Some detectors can use the effect of the avalanche multiplication of charge carriers .

Light-emitting devices

Diamond pin diodes using the phenomenon of super-injection can be used as light emitting devices. ^[one]

• pn junction
• Diode
• Photodiode
• Avalanche Span Diode
• Thyristor