University Of Hail
Community College
Electrical Engineering Department
Electronics Engineering and Instrumentation Program
PHYSICS OF APPLIED ELECTRONICS
(PHYS162)
Dr. Fawzy Hashem
Date:
THYRISTORS
A family of devices known thyristors constructed of four semiconductor layers
(pnpn). Thyristor include: the 4-layer diode, the silicon controlled rectifier (SCR),
The diac, the triac, and the silicon controlled switch (SCS). These types of thyristors
share certain common characteristics in addition to their four-layer construction.
4-LAYER DIODE
The 4-layer diode (also known as Shockley diode and USU) is a type of thyristor. The
basic construction of a 4-layer diode and its schematic symbol are shown below:
The upper pnp layers form a transistor Q1 and the lower npn layers form another
transistor Q2. The two middle layers shared by both transistors, as shown below:
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Notice that the base-emitter junction of Q1 corresponds to pn junction (1), the baseemitter junction of Q2 corresponds to pn junction (3), and the base-collector junctions
of both Q1and Q2 corresponds to pn junction (2), as shown in part (a).
When a positive bias voltage is applied to the anode with respect to cathode, as shown
in part (b), the base-emitter junction of Q1 and Q2 (pn junction 1,3) are forwardbiased, and the common base-collector junction (pn junction 2) is reversed-biased.
The currents in a 4-layer diode are in the equivalent circuit below:
At low-bias levels, there is a very little anode current, and thus it is in the off state or
forward-blocking region.
Forward-Breakover Voltage
The operation of the 4-layer diode may seem unusual because when it is forwardbiased, it can act essentially as an open switch. There is a region of forward bias,
called the forward-blocking region, in which the device has very high resistance
(ideally open) and is in the off state.
The forward-blocking region exists from VAK = 0 V up to a value of VAK called the
forward-breakover voltage, VBR(F). This is indicated on the 4-layer diode
characteristic curve below:
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As VAK is increase from 0, the anode current IA, gradually increases. As IA increases a
point is reached where IA = IS, the switch current, at VAK = VBR(F), the internal
transistor structure becomes saturated.
When this happens, the forward voltage drop, VAK, suddenly decreases to a low value,
and the 4-layer diode enters the forward-conduction region.
Now, the device is in the on state and acts as a closed switch. When the anode current
drops back below the holding value, IH, the device turns off.
THE SILICON-CONTROLLED RECTIFIER (SCR)
Like the 4-layer diode, the SCR has two possible states of operation. In the off state, it
acts ideally as open circuit between the anode and the cathode, actually rather than an
open, there is a very high resistance.
In the on state, the SCR acts ideally as short circuit from the anode to the cathode,
actually there is a small on (forward) resistance.
The SCR has three terminals: anode, cathode, and a gate. The basic structure and the
schematic symbol of the SCR are shown below:
SCR Equivalent Circuit
Like the 4-layer diode operation, the SCR operation can be best understood by
thinking of its internal pnpn structure as a two-transistor arrangement, as shown
below:
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The upper pnp layers act as a transistor Q1, and the lower npn layers act as a transistor
Q2. Again, notice that the two middle layers are “shared”.
Turning the SCR On
When a gate current IG is zero, the device acts as a 4-layer diode in the off state. In
this state, the very high resistance between the anode and the cathode can be
approximated by an open switch as indicated below:
When a positive pulse of current (trigger) is applied to the gate, both transistors turn
on, as shown in part (b). IB2 turns on Q2, providing a path for IB1 into the Q2 collector,
thus turning on Q1.
The collector current of Q1 provides additional base current for Q2, so that Q2 “stays”
in conduction after the trigger pulse is “removed” from the gate. By this regenerative
action, Q2 sustains the saturated conduction of Q1 by providing a path for IB1;in turn,
Q1 sustains the saturated conduction of Q2 by providing IB2. Thus, the device stays on
(latches) once it is triggered on, as shown in part (c).
Turning the SCR Off
When the gate returns to 0 V after the trigger pulse is removed, the SCR cannot turns
off, it stays in the forward-conduction region. The anode current must drop below the
value of the holding current IH, in order for turns off occur.
There are two basic methods of turning off an SCR:
Anode current interruption: as shown in the following figure, the series switch in
part (a) simply reduces the anode current to zero and causes the SCR to turn off.
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The parallel switch in part (b) routes part of the total current away from the SCR,
thereby reducing the anode current to a value less than IH.
Forced commutation: this method requires momentarily forcing current through the
SCR in the direction opposite to the forward conduction so that the net forward
current is reduced below the holding value. The basic circuit, shown below, consists
of a switch (normally a transistor switch) and a capacitor.
While the SCR is conducting, the switch is open and Cc is charged to the supply
voltage through Rc, as shown in part (a). To turn off the SCR, the switch is closed,
placing the capacitor across the SCR and forcing current through it in opposite to the
forward current, as shown in part (b).
SCR Characteristics and Ratings
Referring to the following figure
Forward-breakover voltage, VBR(F) :this is the voltage at which the SCR enters the
forward-conduction region. The value of VBR(F) is maximum when IG = 0 and is
designated VBR(F0). When the gate current is increased, VBR(F) decreases and is
designated VBR(F1) , VBR(F2) , and so on, for increasing steps in gate current (IG1, IG2,
and so on).
Holding current, IH: this is the value of anode current below which the SCR switches
from the form forward-conduction region to the forward-blocking region. The value
increases with decreasing values of IG and is maximum for IG = 0.
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Reverse-breakdown voltage VBR(R) : this parameter specifies the value of reverse
voltage from cathode to anode at which the device breaks into the avalanche region
and begins to conduct heavily.
SCR Applications
The SCR is used in many applications, including over-voltage protection, motor
control, relay control, and sawtooth generators.
An Over-Voltage Protection circuit
The figure below shows a simple over-voltage protection circuits, in a dc power
supply:
The dc output voltage from the regulator is monitored by the zener diode (D1) , and
the resistive voltage divider (R1 and R2). The upper limit of the output voltage is set
by the zener voltage. If this voltage is exceeded, the zener conducts and the voltage
divider produces an SCR trigger voltage. The trigger voltage turns on the SCR, which
is connected across the line voltage. The SCR current causes the fuse to blow, thus
disconnecting the line voltage from the power supply.
Sawtooth Generator
The SCR can be used in conjunction with an RC circuit to produce a repetitive
sawtooth waveform. The circuit is shown below:
The time constant is set by R1 and C1, and the voltage at which SCR triggers on is
determined by the variable voltage-divider formed by R2 and R3, when the switch is
closed, the capacitor begins charging and turns on the SCR. When the SCR turns on,
the capacitor quickly discharge through it; the anode current then decreases below the
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holding value IH, causing the SCR to turns off. As soon as the SCR off, the capacitor
starts charging again and the cycle repeated. By adjusting the potentiometer, the
frequency of the sawtooth waveform can be changed.
THE DIAC AND TRIAC
Both the diac and triac are types of thyristors that can conduct current in both
directions (bilateral). The difference between the two devices is that the diac has two
terminals, while the triac has a third terminal, which is the gate for triggering. The
diac functions basically like two parallel 4-layer diodes tuned in opposite directions.
The triac functions basically like two parallel SCR turned in opposite directions with
common gate terminals.
The Diac
A diac is a two-terminal four layer semiconductor device (thyristor) that can conduct
current in either direction when activated. The basic construction and the schematic
symbol are shown below:
Notice that there are two terminals, labeled A1 and A2. The top and the bottom layers
contain both n and p materials. The right side of the stack can regarded as pnpn
structure with the same characteristics as a four-layer diode, while the left side is an
inverted four –layer diode having npnp structure.
Conduction occurs in a diac when the breakover voltage is reached with either
polarity across the two terminals, as indicated below:
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Once breakover occurs, current is in a direction depending on the polarity of the
voltage across the terminals. The device turns off when the current drops below the
holding value IH.
The equivalent circuit of a diac consists of four transistors arranged as shown below:
When the diac is biased, as in part (b), the pnpn structure from A1 to A2 provides the
same operation as was described for the 4-layer diode.
In the equivalent circuit Q1 and Q2 are forward-biased, and Q3 and Q4 are reversedbiased. The device operates on the upper right portion of the characteristic curve
under this bias condition.
When the diac is biased as in part (c), the npnp structure from A2 to A1 is used. In the
equivalent circuit, Q3 and Q4 are forward-biased, and Q1 and Q2 are reverse-biased.
Under this bias condition the device operates on the lower left portion of the
characteristic curve.
The Triac
A triac is like a diac with a gate terminal. A triac can be turned on by a pulse of gate
current and does not require the breakover voltage to initiate conduction, as does the
diac. Basically, a triac can be though of simply as two SCRs connected in parallel and
in opposite directions with a common gate terminal.
Unlike the SCR, the triac can conduct current in either direction when it is triggered
on, depending on the polarity of the voltage across its A1 and A2 terminals. The
following figure shows the basic construction and schematic symbol for the triac.
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The characteristic curve is shown below:
Notice that the breakover potential decreases as the gate current increase, just as with
the SCR. As with other thyristors, the triac ceases to conduct when the anode current
drops below the specified value of the holding current, IH. The only way to turn off
the triac is to reduce the current to sufficiently low level.
Triac Applications
Triacs can be used to control the average power to a load by the method of phase
control. The triac can be triggered such that the ac power is supplied to the load for
controlled portion of each half-cycle.
During each positive half-cycle, the triac is off for a certain interval, called the delay
angle (measured in degrees), and then it is triggered on and conducts current through
the load for the remaining portion of the positive cycle, called the conduction angle.
Similar action occurs on the negative half-cycle except that, of course, current is
conducting in the opposite direction through the load. The following figure illustrates
this action.
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THE UNIJUNCTION TRANSISTOR (UJT)
The unijunction transistor does not belong to the thyristor family because it does not
have a four-layer type of construction. The term unijunction refers to the fact that the
UJT has a single pn junction.
The UJT is a three-terminal device whose basic construction is shown below:
Notice the terminals are labeled Emitter (E), Base1 (B1), and Base2 (B2). The UJT has
only one pn junction, and therefore, the characteristic of this device are different from
those of either the BJT or the FET.
UJT Equivalent Circuit
The equivalent circuit for the UJT is shown below:
The diode represents the pn junction, rB1’ represents the internal dynamic resistance
of the silicon bar between the emitter and base1 (B1), and rB2’ represents the internal
dynamic resistance of the silicon bar between the emitter and base2 (B2). The total
resistance between the base terminals, which is called interbase resistance
rBB’ = rB1’ + rB2’
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The value of rB1’ varies inversely with the emitter current IE. The internal resistance
rB1’ and rB2’ form a voltage divider when the device is biased, as shown in part (b).
The voltage across rB1’ can be expressed by:
VrB1’ = (rB1’ / rBB’) VBB
Standoff Ratio
The ratio rB1’ / rBB’ is a UJT characteristic called the intrinsic standoff ratio and is
designated by η (Greek eta)
η = rB1’ / rBB’
As long as the applied emitter voltage VEB1 is less than VrB1’ + Vpn, there is no
emitter current because the pn junction is not forward-biased (Vpn is the barrier
potential of the pn junction).
The value of the emitter voltage that causes the pn junction to become forward-biased
is called VP (peak-point voltage) and is expressed by:
VP = η VBB + Vpn
When VEB1 reaches VP, the pn junction becomes forward-biased and IE begins. After
turn-on, the UJT operates in a negative resistance region up to certain value of IE, as
illustrated below:
After the peak point ( VE = VP and IE = IP ), VE decreases as IE continues to increase,
thus producing the negative resistance characteristic.
Beyond the valley point (VE = VV and IE = IV), the device is in saturation, and VE
increases very little with an increasing IE.
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A UJT Application
The UJT is useful in certain oscillator applications and as a triggering device in
thyristor circuits. The figure below shows a UJT relaxation oscillator.
The operation is as follows: when dc power is applied, the capacitor charges
exponentially trough R1 until it reaches the peak point voltage VP. At this point, the
pn junction becomes forward-biased, and the emitter characteristic goes into negative
resistance region (VE decreases and IE increases).
The capacitor then quickly discharges through the forward-biased junction, rB’ and
R2. When the capacitor voltage decreases to the valley-pint voltage VV, the UJT turns
off, the capacitor begins to charge again, the cycle repeated, as indicated below:
During the discharge time of the capacitor, the UJT is conducting. Therefore, a
voltage is developed across R2, as shown above.
Conditions for turn-on and turn-off
In the relaxation oscillator shown above, certain conditions must be met for the UJT
to reliably turn on and turn off.
First, to ensure turn on, R1 must not limit IE at the peak point to less than IP. To
ensure this, the voltage drop across R1 at the peak point should be greater than IPR1.
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Thus, the condition for turn-on is:
VBB – VP > IP R1
R1 < (VBB – VP) / IP
or
To ensure turn off of the UJT at the valley point, R1 must be large enough that IE (at
the valley point) can decreases below the specific value of IV. This means that the
voltage across R1 at valley point must be less than IVR1.
Thus, the condition for turn-off is:
VBB – VV < IV R1
or
R1 > (VBB – VV) / IV
Therefore, for proper turn-on and turn-of, R1 must be in the range:
(VBB – VP) / IP > R1 > (VBB – VV) / IV
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