Although diodes with a basic PN junction are by far the most popular type of diode in use, other types of diode may be used in a number of applications. One type that is used for a variety of circuits is the PIN diode. This form of diode is used in a number of areas. The PIN diode is very good for RF switching, and the PIN structure is also very useful in photodiodes.
The PIN diode found its first applications in 1952 as a low frequency high power rectifier. It was also used in a number of microwave applications, although it took until around 1960 before its use became more popular in this application. A further use of the PIN diode is as a photo-detector (photodetector or photo-diode) where its structure is particularly suited to absorbing light.
PIN diode structure
The PIN diode receives its name from the fact that is has three main layers. Rather than just having a P type and an N type layer, the PIN diode has three layers:
- P-type layer
- Intrinsic layer
- N-type layer
The instrinic layer of the PIN diode is the one that provides the change in properties when compared to a normal PN junction diode. The intrinsic region comprises of the undoped, or virtually undoped semiconductor, and in most PIN diodes it is very thin - of the order of between 10 and 200 microns.
There are a two main structures that can be used, but the one which is referred to as a planar structure is shown in the diagram. In the diagram, the intrinsic layer is shown much larger than if it were drawn to scale. This has been done to better show the overall structure of the PIN diode.
PIN diode with a planar construction
PIN diodes are widely made of silicon, and this was the semiconductor material that was used exclusively until the 1980s when gallium arsenide started to be used.
PIN diode characteristics
It is found that at low levels of reverse bias the depletion layer become fully depleted. Once fully depleted the PIN diode capacitance is independent of the level of bias because there is little net charge in the intrinsic layer. However the level of capacitance is typically lower than other forms of diode and this means that any leakage of RF signals across the diode is lower.
When the PIN diode is forward biased both types of current carrier are injected into the intrinsic layer where they combine. It is this process that enables the current to flow across the layer.
The particularly useful aspect of the PIN diode occurs when it is used with high frequency signals, the diode appears as a resistor rather than a non linear device, and it produces no rectification or distortion. Its resistance is governed by the DC bias applied. In this way it is possible to use the device as an effective RF switch or variable resistor producing far less distortion than ordinary PN junction diodes.
PIN diode applications
The PIN diode is used in a variety of different applications from low frequencies up to high radio frequencies. The properties introduced by the intrinsic layer make it suitable for a number of applications where ordinary PN junction diodes are less suitable.
In the first instance the diode can be used as a power rectifier. Here the intrinsic layer gives it a high reverse breakdown voltage, and this can be used to good effect in many applications.
Although the PIN diode finds many applications in the high voltage arena, it is probably for radio frequency applications where it is best known. The fact that when it is forward biased, the diode is linear, behaving like a resistor, can be put to good use in a variety of applications. It can be used as a variable resistor in a variable attenuator, a function that few other components can achieve as effectively. The PIN diode can also be used as an RF switch. In the forward direction it can be biased sufficiently to ensure it has a low resistance to the RF that needs to be passed, and when a reverse bias is applied it acts as an open circuit The fact that the PIN diode has a low level of capacitance because of the additional intrinsic layer in the diode, means that it can switch more effectively than other forms of diode.
Another useful application of the PIN diode is for use in RF protection circuits. When used with RF, the diode normally behaves like a resistor when a small bias is applied. However this is only true for RF levels below a certain level. Above this the resistance drops considerably. Thus it can be used to protect a sensitive receiver from the effects of a large transmitter if it is placed across the receiver input.
PIN diode attenuator and switch circuit
The circuit above can be used as either a switch or an attenuator. This particular circuit is current driven, although by placing a resistor in series wit the inductor to the switched "+" line, the circuit becomes voltage driven with the resistor limiting the maximum current.
Although not as widely used as normal PN junction diodes, PIN diodes are nevertheless used in large quantities. PIN diodes are particularly used in RF applications where there low levels of capacitance and also their switching and variable resistance properties make them very good in switching and variable attenuator applications.