pin-Diode

The PIN diode ( positive intrinsic negative diode English ) is an electrical component. The structure is similar to a pn junction diode, with the crucial difference that there is an additional weak or undoped layer between the p- and n- doped layer. This layer is thus only intrinsic ( intrinsic ) and is therefore called the i-layer. The p-and n-layer are thus not in direct contact, and wherein applying a reverse bias leads to the formation of a larger space charge region than in the conventional pn diode. Since the i-layer contains few free charge carriers, it is high impedance.

The PIN diode is also called psn diode ( s for lightly doped ) or power diode (due to the application in the power electronics ).

Construction

A PIN diode consists essentially of a lightly n -type silicon base material (substrate), which is provided on one side with a strong p-type and on the other side with a strong n-type doping ( also possible to use weak p- doped substrates, but n-type material is usually available in higher purity ).

The doping may be optionally accomplished by diffusion processes, epitaxy or ion implantation. Metal layers are applied to contacting on two highly doped regions and causes a so-called ohmic contact. As a metallization material often found using aluminum.

Function

The PIN diode is positively biased, electrons are injected into the i-type layer of the p-type layer and holes from the n- layer. The lifetime of the charge carriers in the undoped i-layer is particularly high ( ≈ 0.05 ... 5 microseconds for silicon). Therefore, the PIN diode will remain conductive when only a short voltage pulse with a pulse duration of rest. If you operate the PIN diode in the reverse direction, arises between the p- and i- zone depleted in charge carriers space charge zone. The depth of this zone is given for a given blocking voltage by the following equation (see also pn junction ):

Here, ε0 = 8.85 × 10-12 F / m is the permittivity of free space, εr the dielectric constant and e is the elementary charge. The approximation on the right side applies to the case of the PIN diode as the acceptor concentration in the p -type impurity is very much larger than the donor concentration in the n- doping the i-type layer ( the substrate is typically at 1012 to 1014 cm - 3 and the p -type impurity at 1018 to 1020 cm -3).

In DC mode, the pin diode works like a normal semiconductor diode only during switching operations, the high number of records in the i-layer charge carriers is noticeable. For alternating current, the PIN diode having up to about 10 MHz ( depending upon the thickness of the i-type layer ) rectifying characteristics. Above 10 MHz, it behaves as an ohmic resistance which is inversely proportional to the average current through the diode.

, The capacity of the negatively biased pin diode, a similar functional capacity depending on the volume as the plate of the capacitors, which depends on the surface and the plate spacing.

The area corresponds to a PIN photodiode of active area of ​​the detector, and the distance between the plates corresponds to the depth of the space charge zone. In a fully depleted pin photodiode is approximately equal to the thickness of the chip and a detector surface of 5 mm ² and a chip thickness of 0.5 mm to obtain a capacitance of 1 pF.

Wherein the pin - photodiode by incident radiation measurable voltage change is, the charge of the electrons and holes, respectively, and the detector capacitance. The radiation generated by the electron-hole pairs are separated by the electric field, the electrons and the holes drift to the most positive to the most negative potential. The voltage difference should be as large as possible, so that the signal -to-noise ratio becomes large. To this end, the capacity should be as small as possible, for which either minimizes the sensitive area in conventionally constructed pin photodiodes or increases the sensitive thickness. On the other hand, the largest possible area is often desirable. However, it should not be too large, lest an extremely high reverse voltage must be applied.

Application

Pin diodes are used mainly in high-frequency technology as the same current-controlled resistors ( attenuators or amplitude control) or the same voltage controlled RF switch using. Based on the existing i-layer are obtained in power electronics at voltages above 1 kV and a better passage behavior and by the wide space charge region is higher by a factor of 5 voltage resistance than pn diodes, which is why they as a rectifier and freewheel diodes for high voltages and currents be used. As photodiodes are used for radiation measurement and as a receiver in Fiber Optic (FO) transmission technology.

DC -controlled resistance

By the behavior of ohmic resistance at high frequencies, ie, one can use a pin diode as DC- AC voltage controlled resistor. This is superimposed on the high frequency alternating current with a direct current, thereby to control the resistance of the i-region.

In high frequency circuits with most attenuators are used with three pin diodes. This allows one to make a signal attenuation at constant adjustment to the characteristic impedance (usually 50).

In addition, have pin diodes due to the relatively thick i-region a low junction capacitance. This allows you this, use the circuit of π - attenuator in the short - Series Short - operation, also known as high-frequency switch, which arises in a strong stop band attenuation.

Circuit of a π - attenuator.

Photodiode

The pin photodiode and the avalanche photodiode are mainly used in optoelectronics for optical signal transmission in communications technology. The pin photodiode represents the main detector for fiber optic applications dar. pin photodiodes are due to the thick i-layer of thermally stable and cost-effective, but because of the lack of internal gain less sensitive than the avalanche photodiodes. Peak values ​​for the sensitivity of Si pin diodes are in the maximum at 850 nm is between -40 dBm (25 Mib s -1) and -55 dBm ( 2 Mib s -1). For wavelengths above 1000 nm materials are such as germanium (Ge), indium gallium arsenide (InGaAs ) and indium gallium arsenide phosphide ( InGaAsP ) are used, with InGaAs has the largest cut-off wavelength of 1600 nm.

A position sensitive device is the use of a two-dimensional lateral effect photo - diode having a plurality of pin electrodes to locate a light spot on the diode.