Triac technologies and immunity

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Doc ID 17194 Rev 1

1

Triac technologies and immunity

1.1

Triac silicon structures

Triac devices are by far the most used silicon devices to drive AC loads directly connected

on the mains voltage. Triacs are widely used because they present three major advantages:

●

Low forward voltage drop

●

Turn on by gate current pulse

●

High voltage immunity

The first two advantages come from the active silicon structure of the Triac which is based

on four alternatively doped silicon layers (N-P-N-P). These four layers implement two bipolar

transistors which are coupled to maintain the on state even if the gate current is removed

(see Figure 1).

Figure 1.

Active simplified structure of a Triac

The third advantage comes from the fact that the voltage is withstood by the silicon

thickness. It's then quite easy to reach breakdown levels above 1 kV. The main issue is then

the junction termination. Several low cost technologies are used since the 70’s which used a

glass passivation. For example the “Mesa” technology (see Figure 2) uses glass on both

sides of the die (see References 1, 2, and 3) to passivate both junctions, which hold both

forward and reverse high voltages (referred to as “Cathode” or “A1” or “COM” polarity). The

“Top” glass technology (see References 1, 2, and 3) uses only one glass layer (see

Figure 3). This avoids having some glass at the bottom of the die, and so it is easier to put

such dies in every kind of package.

Figure 2.

“Mesa” glass technology

N3

P2

N2

P1

N1

P2

N2

P1

N4

A2

A1

G

N3

P2

N2

P1

N1

P2

N2

P1

N4

Metalization

N

N

N

P

P

N

N

Anode

Glass

Cathode

Gate