P-i-n DIODE

The p-i-n diode is a refinement of the p-n junction for special applications. After the p-n junction was developed in the late 1940s, the p-i-n diode was first used as a low-frequency, high-power rectifier in 1952 by Hall, and in 1956 by Prince. The presence of an intrinsic layer can substantially increase the breakdown voltage for high-voltage application. This intrinsic layer also provides interesting properties when the device is operated at high frequencies in the microwave and radio-wave range. It was not until 1958 that the device started to be used in microwave applications by Uhlir.

A p-i-n diode consists of an intrinsic layer sandwiched between the opposite types of a p-n junction. The intrinsic layer has a very low concentration of either и-type or p-type in the order of 10¹³ cm^-3, and a resistivity in the order of kfi-cm. The intrinsic-layer thickness ranges between 10 um to 200 um.The outside p- and n-layers are usually heavily doped. The p-i-n diode can be realized as a planar structure or a mesa structure, both fabricated on degenerate substrate material. In the planar structure, an intrinsic epitaxial film is grown and the p⁺-region is introduced by either diffusion or ion implantation. A mesa structure has epitaxially grown layers with dopants incorporated, and is capable of higher-frequency operation because the intrinsic layer can be made thinner with better control. Isolation of the device is achieved by mesa etching and

The special feature of a p-i-n diode is a wide intrinsic layer that provides unique roperties such as low capacitance, high breakdown voltage with reverse bias, and most interestingly, carrier storage for microwave applications with forward bias. Near zero or at low reverse bias, the lightly doped intrinsic layer starts to be fully depleted. Once fully depleted, its capacitance is independent of reverse bias. Since there is little net charge within the intrinsic layer, the electric field is constant. For silicon, the breakdown field is approximately 2xl0⁵ V/cm. These two equations show that the parameter xj controls the trade-off between frequency response (from capacitance) and power (from maximum voltage). When the p-i-n diode is under forward bias, both types of carriers are injected into the intrinsic layer. It is usually assumed that within the intrinsic layer, the electron and hole concentrations are the same, and that they are uniform within the intrinsic layer. The current conduction is through recombination.

For modulation and switching applications, even the mean bias point can vary with time. The upper limit of this modulation frequency is determined by the reverse recovery characteristics. When a p-i-n diode is switched from forward bias to reverse bias abruptly, the stored charges continue to contribute to a large reverse current until they are fully drained away. The reverse current is determined by the series resistance. The transition time is a complicated function of the doping profile and diode geometry. This reverse recovery time is the sum and it puts an upper limit on the rate at which the quiescent bias point can be switched.