SCR-Silicon Controlled Rectifier

Introduction to SCR-
Silicon Controlled Rectifier




As the terminology indicates, The SCR is a
controlled rectifier constructed of a
silicon semiconductor material with a
third terminal for control purposes.
Silicon was chosen because of its high
temperature and power capabilities. The
basic operation of the SCR is different
from that of an ordinary two-layer
semiconductor diode in that a third
terminal called a gate, determines when
the rectifier switches from the open-
circuit to short-circuit state. It is not
enough simply to forward-bias the
anode-to-cathode region of the device. In
the conduction state the dynamic
resistance of the SCR is typically 0.01 to
0.1 ohm and reverse resistance is
typically 100 kilo ohm or more. It is
widely used as a switching device in
power control applications. It can
control loads by switching on and off
upto many thousand times a second. It
can switch on for a variable lengths of
time duration, thereby delivering de­
sired amount of power to the load. Thus,
it possesses the advantage of a rheostat
as well as a switch with none of their
drawback. A schematic diagram and
symbolic representation of an SCR are
shown in figures a & b respectively. As
illustrated in fig-a, SCR is a three-
terminal four-layer semiconductor
device, the layers being alternately of P-
type and N-type. The junctions are
marked Jj, J2 and J3 (junctions Jj and J3
operate in forward direction while
middle junction J2 operates in the
reverse direction) whereas the three
terminals are anode (A), cathode (C) and
gate (G) which is connected to the inner
P-type layer. The function of the gate is
to control the firing of SCR. In normal
operating conditions, anode is positive
with respect to cathode.

Construction of an SCR


            SCR – construction types

From fig a it is clear that SCR is
essentially an ordinary rectifier (PN)
and a junction transistor (N-P-N)
combined in one unit to form PNPN
device. Three terminals are taken: one
from the outer P-type material, known
as anode, second from the outer N-type
material, known as cathode and the
third from the base of transistor section
known as the gate.

The basic material used for fabrication
of an SCR is N-type silicon. It has a
specific resistance of about 6 ohm-mm.
Silicon is the natural choice as base
material because of the following

(i) ability to withstand high junction
temperature of the order of 150° C

(ii) high thermal conductivity;

(iii) less variations in characteristics
with temperature; and

(iv) less leakage current in P-N junction.

It consists, essentially, of a four layer
pellet of P and N type silicon
semiconductor materials. The junctions
are diffused or alloyed. The material
which may be used for P diffusion is
aluminium and for N diffusion is
phosphorous. The contact with anode
can be made with an aluminium foil and
through cathode and gate by metal sheet.
Diffusion must be carried out at a proper
temperature and for necessary duration
to provide correct concentration because
this decides the properties of the device.
Low power SCRs employ the planar
construction shown in fig a. Planar
construction is useful for making a
number of units from a silicon wafer.
Here, all the junctions are diffused. The
other technique is the mesa construction
shown in fig.b. This technique is used
for high power SCRs. In this technique,
the inner junction J2 is obtained by
diffusion, and then the outer two layers
are alloyed to it. The PNPN pellet is
properly braced with tungsten or
molybdenum plates to provide greater
mechanical strength and make it capable
of handling large currents. One of these
plates is hard soldered to a copper or an
aluminium stud, which is threaded for
attachment to a heat sink. This provides
an efficient thermal path for conducting
the internal losses to the surrounding
medium. The uses of hard solder
between the pellet and back-up plates
minimises thermal fatigue, when the
SCRs are subjected to temperature
induced stresses. For medium and low
power SCRs, the pellet is mounted
directly on the copper stud or casing,
using a soft solder which absorbs the
thermal stresses set up by differential
expansion and provides a good thermal
path for heat transfer. For a larger
cooling arrangement, which is required
for high power SCRs, the press-pack or
hockey-puck construction is employed,
which provides for double-sided air for

The salient features to be considered,
while designing an SCR, are the diameter
and thickness of wafer, composition of
the base material, type and amount of
the material to be diffused into the
wafer, shape, position and contact area
of the gate, shape and size of the SCR,
type of heat sink etc.

Fabrication technology determines
various properties of the device. The
voltage rating of a device can be
increased by lightly doping the inner
two layers and increasing their
thickness. But due to this increased
resistance, forward voltage drop
increases and large triggering currents
are required causing greater power
dissipation accompanied by smaller
current ratings. The heat dissipation of
silicon falls from 1.5 W/cm 2 at 25° C to
1.25 W/ cm2 at 125° C. A high voltage
power device can seldom be used
beyond 125° C.

The current carrying capacity and
voltage rating of the device can be
increased by irradiating silicon with
neutrons. The current rating of the
device can also be increased by reducing
the current density at the junction but
this result in a bulky device with large
turn-on time

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