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What is a diode?

High-Frequency Diodes (PIN Diodes):Structure and Features

Structure Symbol Applications・Characteristics
Figure - High-Frequency Diode Structure Figure - High-Frequency Diode Symbol
  • High-frequency switching→Suitable for mobile phones and the like
  • Variable resistor element for AGC(※) and attenuator circuits
    ※AGC: Automatic Gain Control
  • The diode (internal) capacitance (Ct)is very low

A high resistivity I-type semiconductor is utilized to provide significantly lower diode capacitance (Ct). As a result, PIN diodes act as a variable resistor with forward bias, and behave as a capacitor with reverse bias. High frequency characteristics (low capacitance ensures minimal effect of signal lines) make them suitable for use as variable resistor elements in a wide variety of applications, including attenuators, high-frequency signal switching (i.e. mobile phones requiring an antenna), and AGC circuits.

Forward Voltage Reverse Voltage
Figure - Forward Voltage:Charge Accumulation→Decreased Resistivity Figure - Reverse Voltage: An electrically neutral depletion layer is formed by filling the intrinsic layer - created between P and N layers - with charge carriers (holes and electrons).
Variable resistance with forward biasCircuit Diagram Symbol - Variable Resistor Capacitor with reverse biase Circuit Diagram Symbol - Capacitor
What is the diode capacitance (Ct)?

The amount of accumulated charge internally when supplying a reverse bias is called the diode capacitance (Ct). An electrically neutral depletion layer is formed by filling the intrinsic layer - created between P and N layers - with charge carriers (holes and electrons). The depletion layer acts as a parasitic capacitor, with a capacitance proportional to the PN junction area and inversely proportional to the distance d. The distance is determined by the concentration of the P and N layers. Supplying a voltage to the diode will increase the depletion layer and decrease Ct.The required Ct will vary depending on the application.

[When supplying a reverse voltage]

Figure - Reverse bias: An electrically neutral depletion layer is formed by filling the intrinsic layer (created between P and N layers) with charge carriers (holes and electrons).
Figure - Capacitance calculation
Key Points
  • The wider the depletion layer (and greater the distance) the lower the capacitance Ct.
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