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AN1001

Fundamental Characteristics of Thyristors

Introduction

The thyristor family of semiconductors consists of several very

useful devices. The most widely used of this family are silicon

controlled rectifiers (SCRs), triacs, sidacs, and diacs. In many

applications these devices perform key functions and are real

assets in meeting environmental, speed, and reliability specifica-

tions which their electro-mechanical counterparts cannot fulfill.

This application note presents the basic fundamentals of SCR,

triac, sidac, and diac thyristors so the user understands how they

differ in characteristics and parameters from their electro-

mechanical counterparts. Also, thyristor terminology is defined.

SCR

Basic Operation

Figure AN1001.1 shows the simple block construction of an SCR.

Figure AN1001.1

SCR Block Construction

The operation of a PNPN device can best be visualized as a spe-

cially coupled pair of transistors as shown in Figure AN1001.2.

Figure AN1001.2

Coupled Pair of Transistors as a SCR

The connections between the two transistors trigger the occur-

rence of regenerative action when a proper gate signal is applied

to the base of the NPN transistor. Normal leakage current is so

low that the combined hFE of the specially coupled two-transistor

feedback amplifier is less than unity, thus keeping the circuit in

an off-state condition. A momentary positive pulse applied to the

gate biases the NPN transistor into conduction which, in turn,

biases the PNP transistor into conduction. The effective hFE

momentarily becomes greater than unity so that the specially

coupled transistors saturate. Once saturated, current through the

transistors is enough to keep the combined hFE greater than

unity. The circuit remains “on” until it is “turned off” by reducing

the anode-to-cathode current (IT) so that the combined hFE is less

than unity and regeneration ceases. This threshold anode current

is the holding current of the SCR.

Geometric Construction

Figure AN1001.3 shows cross-sectional views of an SCR chip

and illustrations of current flow and junction biasing in both the

blocking and triggering modes.

Figure AN1001.3

Cross-sectional View of SCR Chip

Gate

Schematic Symbol

Block Construction

Cathode

Anode

Cathode

Anode

Gate

Cathode

Anode

Cathode

Gate

Anode

Load

Two-transistor

Schematic

Two-transistor Block

Construction Equivalent

Gate

Cathode

(-)

(+)

IGT

(+)

Anode

Forward Bias and Current Flow

Gate

Cathode

(-)

Anode

Reverse Bias

Reverse Biased

Junction

(-)

Anode

Equivalent Diode

Relationship

Forward

Blocking

Junction

Cathode

(-)

(+)

Anode

Equivalent Diode

Relationship

Cathode

(+)

Reverse Biased

Gate Junction

AN1001

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