What Is a Pin Diode?by Kim Blakesley
A PIN diode is a very small semiconductor that works as a variable resistor for both microwave and RF frequencies. A forward biased direct current determines the resistance value. The PIN diode regulates the RF frequencies without introducing distortion. Small levels of direct current are used while still controlling a large volume of RF frequencies.
How the PIN Diode is Made
The PIN diode begins as a pure silicon wafer. One surface of the wafer is infused with a P-region while the other is infused with a N-region. An I-region thickness is the function of the size of the original silicon wafer. The area is how small each chip is sectioned.
The semiconductor material within the finish coat and the geometry of the chip directly affects the performance of the PIN diode. This is especially true in the I-region. A properly constructed PIN diode will have a long carrier life and high resistivity. The better made chip enhances the controllability of the RF signal with little, if any, distortion.
Forward Biased Pin Diode
A PIN diode can be forward biased. In this case, electrons and holes are injected into the I-region of the chip through the P and N regions. This particular charge does not recombine instantaneously. The chip becomes lower in the I-region resistivity because a finite amount of charge remains stored within the diode chip.
Reverse Biased PIN Diode
A parallel plate capacitor is created when a reverse biased PIN diode is used at a high RF frequency and the PIN diode is at zero. Zero biased is equivalent to 100 MHz and a dielectric relaxation frequency will occur when below 20 MHz. Additional data is seen in the form of the typical curve. The curve shows the capacitance variation.
The PIN diode is used to provide electronic switching. It is very useful in any RF designed application to provide the attenuating element or switching for RF attenuators and RF switches. PIN diodes supply a high level of reliability. RF relays are not as reliable and are the only replacement as an alternative for a PIN diode.
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