A diode that exhibits an impedance at high frequency that is almost entirely resistive, and can be controlled over a wide range by the applied d.c. bias. These devices are used as voltage-controlled attenuators and modulators, and as switches in signal control circuits, such as phase shifters.
The PIN diode structure consists of a layer of lightly doped high-resistivity semiconductor (the ‘I’ or intrinsic layer), sandwiched between heavily doped (highly conductive) p-type and n-type regions. With zero or negative applied voltage bias, the I-layer is depleted of charge carriers and exhibits a very high resistance. In forward bias, carriers are injected from the P and N regions into the I-layer, increasing its conductivity rapidly. In these conditions, the diode behaves essentially as a pure resistor at radio frequencies. The resistance ratio from the fully ‘on’ state to the ‘off’ state can be as large as 5000 to 1.
The capacitance associated with the diode can be thought of as that of a parallel-plate capacitor, with the P and N regions as the two plates. This capacitance is usually very low as a result of the thickness of the I-layer. A major parameter of the PIN diode is the carrier lifetime τ, which is of the order of a few nanoseconds to a few microseconds. It determines the effective lower frequency limit of operation, f0 = 1/(2πτ). At frequencies below this limit the PIN diode behaves like a p-n junction diode, but with higher distortion. Above this frequency the diode resistance is the dominant feature.