Characteristics: Triac - nptel

A
A
p
n -
G
K
p
n + n +
(a)
G
(b)
K
(c)
Fig. 4.1: Constructional features of a thysistor
(a) Circuit Symbol, (b) Schematic Construction, (c) Photograph
As shown in Fig 4.1 (b) the primary crystal is of lightly doped n - type on either side of
which two p type layers with doping levels higher by two orders of magnitude are grown. As in
the case of power diodes and transistors depletion layer spreads mainly into the lightly doped n -
region. The thickness of this layer is therefore determined by the required blocking voltage of the
device. However, due to conductivity modulation by carriers from the heavily doped p regions
on both side during ON condition the “ON state” voltage drop is less. The outer n + layers are
formed with doping levels higher then both the p type layers. The top p layer acls as the “Anode”
terminal while the bottom n + layers acts as the “Cathode”. The “Gate” terminal connections are
made to the bottom p layer.
As it will be shown later, that for better switching performance it is required to maximize
the peripheral contact area of the gate and the cathode regions. Therefore, the cathode regions are
finely distributed between gate contacts of the p type layer. An “Involute” structure for both the
gate and the cathode regions is a preferred design structure.
4.3 Basic operating principle of a thyristor
The underlying operating principle of a thyristor is best understood in terms of the “two
transistor analogy” as explained below.
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