Operational princle of SCR

SCR working principle
The SCR is a four-layer, three-junction
and a three-terminal device and is
shown in fig.a. The end P-region is the
anode, the end N-region is the
cathode and the inner P-region is the
gate. The anode to cathode is
connected in series with the load
circuit. Essentially the device is a
switch. Ideally it remains off (voltage
blocking state), or appears to have an
infinite impedance until both the
anode and gate terminals have
suitable positive voltages with respect
to the cathode terminal. The thyristor
then switches on and current flows
and continues to conduct without
further gate signals. Ideally the
thyristor has zero impedance in
conduction state. For switching off or
reverting to the blocking state, there
must be no gate signal and the anode
current must be reduced to zero.
Current can flow only in one direction.
In absence of external bias voltages,
the majority carrier in each layer
diffuses until there is a built-in voltage
that retards further diffusion. Some
majority carriers have enough energy
to cross the barrier caused by the
retarding electric field at each
junction. These carriers then become
minority carriers and can recombine
with majority carriers. Minority carriers
in each layer can be accelerated
across each junction by the fixed field,
but because of absence of external
circuit in this case the sum of majority
and minority carrier currents must be
zero.
A voltage bias, as shown in figure, and
an external circuit to carry current
allow internal currents which include
the following terms:

The current I_x is due to

• Majority carriers (holes) crossing
  junction J₁
  
• Minority carriers crossing junction
  J₁
  
• Holes injected at junction J₂
  diffusing through the N-region and
  crossing junction J₁ and
  

Minority carriers from junction J₂
diffusing through the N-region and
crossing junction J₁ .
Similarly I₂ is due to six terms and I₃
is due to four terms.
The two simple analogues to explain
the basic action for the thyristor are
those of the diode and the two
transistor models.
1. Diode Model. The thyristor is
similar to three diodes in series as
there are three P-N junctions.
Without gate bias, there is always
at least one reverse biased junction
to prevent conduction irrespective
of the polarity of an applied voltage
between anode and cathode. If the
anode is made positive and the
gate is also biased positively with
respect to cathode, the P-layer at
the gate is flooded by the electrons
from the cathode and loses its
identity as a P-layer. Accordingly
the thyristor becomes equivalent
to a conducting diode.

Scr working
2. Two Transistor Model.
Imagine the SCR cut along the
dotted line, as shown in fig. a.
Then we can have two devices, as
shown in fig.b. These two devices
can be recognized as two
transistors. The upper left one is P-
N-P transistor and the lower right
N-P-N type. Further it can be
recognized that the base of the P-
N-P transistor is joined to the
collector of the N-P-N transistor
while the collector of P-N-P is
joined to the base of N-P-N
transistor, as illustrated in fig. c.
The gate terminal is brought out
from the base of the N-P-N
material. This construction has
been conceived merely to explain
the working of SCR, otherwise in
physical shape the SCR has four
solid layers of P-N-P-N type only.
Now we can see that the two
transistors are connected in such a
manner that the collector of Q₁ is
connected to the base of Q₂ i.e. the
output collector current of Q_t
becomes the base current for Q₂ . In
the similar way the collector of Q₂ is
joined to the base of Q₁ which shows
that the output collector current of Q₂
is fed to Q₁ as input base current.
These are back to back connections of
transistors in such a way that the
output of one goes into as input of
other transistor and vice-versa. This
gives net gain of loop circuit as β₁x
β₂ where β₁ and β₂ are current gains
of two transistors respectively.
When the gate current is zero or the
gate terminal is open, the only current
in circulation is the leakage current,
which is very small in case of silicon
device specially and the total current
is a little higher than sum of individual
leakage currents. Under these
conditions P-N-P-N device is said to
be in its forward blocking or high
impedance ‘off state. As soon as a
small amount of gate current is given
to the base of transistor Q₂ by
applying forward bias to its base-
emitter junction, it generates the
collector current as β₂ times the base
current. This collector current of Q₂ is
fed as input base current to Q : which
is further multiplied by β₁ times as I_Cl
which forms input base current of Q₂
and undergoes further amplification.
In this way both transistors feedback
each other and the collector current
of each goes on multiplying. This
process is very quick and soon both
the transistors drive each other to
saturation. Now the device is said to
be in.on-state. The current through
the on-state SCR is controlled by
external impedance only.
Souce: circuitstoday