Diodes｜Basic

PIN Diodes

table of contents

・Structure and Features
・What is the Diode Capacitance (C_t)

Structure and Features

Structure	Symbol	Applications・Characteristics
		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 (C_t)is very low

A high resistivity I-type semiconductor is utilized to provide significantly lower diode capacitance (C_t). 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

Variable resistance with forward bias	Capacitor with reverse biase

What is the Diode Capacitance (C_t)

The amount of accumulated charge internally when supplying a reverse bias is called the diode capacitance (C_t). 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 C_t.The required Ct will vary depending on the application.

［When supplying a reverse voltage］

Diode 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).

Diode Figure - Capacitance calculation

Key Points

The wider the depletion layer (and greater the distance) the lower the capacitance C_t.

【Download Documents】 Transistors & Diodes Basics and Selection

In this handbook, although there are so many types of power devices using Si semiconductors, the basic points are explained, focusing mainly on diodes and transistors for power supply applications. It also introduces the procedure and decision method for transistor selection when designing circuits, as well as application examples that utilize each characteristic and feature.