AG312: Designing with PIN Diodes

[sukishan]

From M/A-COM

The PIN diode finds wide usage in RF, UHF and microwave circuits. It is fundamentally a device whose impedance, at these frequencies, is controlled by DC excitation. A unique feature of the PIN diode is its ability to control large amounts of RF power with much lower levels of DC.

The PIN diode is a current controlled resistor at radio and microwave frequencies. It is a silicon semiconductor diode in which a high resistivity intrinsic I-region is sandwiched between a P-type and N-type region. When the PIN diode is forward biased, holes and electrons are injected into the I-region.

These charges do not immediately annihilate each other. Instead, they stay alive for an average time called the carrier lifetime. This results in an average stored charge, Q, which lowers the effective resistance of the I-region to a value of RS. When the PIN diode is at zero or reverse bias there is no stored charge in the I-region and the diode appears as a capacitor, CT, shunted by a parallel resistance Rp.