Download of the MOSFET often is connected to the source terminal, making it a

Page 1 of 10
Thyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type
material. They act as bistable switches, conducting when their gate receives a current trigger,
and continue to conduct while they are forward biased.
Symbol
Construction
The thyristor is a four-layered, three terminal semiconductor device, with each layer
consisting of alternately N-type or P-type material, for example P-N-P-N. The main
terminals, labelled anode and cathode, are across all four layers. The control terminal,
called the gate, is attached to p-type material near the cathode
Operation
The operation of a thyristor can be understood in terms of a pair of tightly
coupled bipolar junction , arranged to cause a self-latching action.
The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode)..
When the anode is at a positive potential VAK with respect to the cathode with no
voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is
reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now
if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche
breakdown of J2 takes place and the thyristor starts conducting (On state).If a positive
potential VG is applied at the gate terminal with respect to the cathode, the breakdown
of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value
of VG, the thyristor can be switched into the on state quickly.Once avalanche breakdown
has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until:
(a) the potential VAK is removed or (b) the current through the device (anode−cathode)
Page 2 of 10
is less than the holding current specified by the manufacturer. Hence VG can be a
voltage pulse, such as the voltage output from a UJT relaxation oscillator.
Characteristic curves
Gate turned off (GTO)
A gate turn-off thyristor (GTO) is a special type of thyristor, which is a highpower semiconductor device. It was invented atGeneral Electric.[1] GTOs, as opposed to
normal thyristors, are fully controllable switches which can be turned on and off by their
third lead, the GATE lead.
Symbol
Page 3 of 10
Construction
A distributed buffer gate turn-off thyristor (DB-GTO) is a thyristor with additional PN
layers in the drift region to reshape the field profile and increase the voltage blocked in
the off state. Compared to a typical PNPN structure of a conventional thyristor, this
thyristor would be a PN-PN-PN type structure in here.
Operation
Thyristors can only be turned ON and cannot be turned OFF. Thyristors are switched
ON by a gate signal, but even after the gate signal is de-asserted (removed), the
thyristor remains in the ON-state until any turn-off condition occurs (which can be the
application of a reverse voltage to the terminals, or when the current flowing through
(forward current) falls below a certain threshold value known as the "holding current").
Thus, a thyristor behaves like a normal semiconductor diode after it is turned on or
"fired".
Characteristics curves
Page 4 of 10
Integrated Gate-Commutated Thyristor (IGCT)
The Integrated
Gate-Commutated
Thyristor (IGCT)
is
a power
semiconductor electronic device, used for switching electric current in industrial
equipment.
Symbol
Construction
The structure of an IGCT is very similar to a GTO thyristor. In an IGCT, the gate
turn off current is greater than the anode current. This results in a complete elimination
of minority carrier injection from the lower PN junction and faster turn off times.
Operation
In the conducting state, a GCT is a regenerative thyristor switch like an SCR or GTO as
illustrated in
Page 5 of 10
Fig..1 and as such, is characterized by high current capability and low on-state voltage. In
the blocking state, the gate cathode junction is reverse-biased and is effectively “out of
operation” and the resultant device.
Characteristics curves
The metal–oxide–semiconductor fieldeffect transistor (MOSFET)
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS
FET) is a transistor used for amplifying or switching electronic signals. Although the
MOSFET is a four-terminal device with source (S), gate (G), drain (D), and body (B)
terminals,[1] the body (or substrate) of the MOSFET often is connected to the source
terminal, making it a three-terminal device like other field-effect transistors
Symbol
Page 6 of 10
Construction
The traditional metal–oxide–semiconductor (MOS) structure is obtained by growing a
layer of silicon dioxide (SiO2) on top of a silicon substrate and depositing a layer of
metal or polycrystalline silicon (the latter is commonly used). As the silicon dioxide is
adielectric material, its structure is equivalent to a planar capacitor, with one of the
electrodes replaced by a semiconductor. When a voltage is applied across a MOS
structure, it modifies the distribution of charges in the semiconductor
A metal–oxide–semiconductor field-effect transistor (MOSFET) is based on the
modulation of charge concentration by a MOS capacitance between a body electrode
and a gate electrode located above the body and insulated from all other device regions
by a gate dielectric layer which in the case of a MOSFET is an oxide, such as silicon
dioxide
Operation
The operation of a MOSFET can be separated into three different modes,depending on
the voltages at the terminals. In the following discussion, a simplified algebraic model is
used. Modern MOSFET characteristics are more complex than the algebraic model
presented here. For an enhancement-mode, n-channel MOSFET, the three operational
modes are:
Cutoff, subthreshold, or weak-inversion mode
When VGS < Vth:
where
is gate-to-source bias and
is the threshold voltage of the device.
According to the basic threshold model, the transistor is turned off, and there is
no conduction between drain and source.
When VGS > Vth and VDS < ( VGS – Vth )
Page 7 of 10
The transistor is turned on, and a channel has been created which allows current to flow
between the drain and the source. The MOSFET operates like a resistor, controlled by
the gate voltage relative to both the source and drain voltages. The current from drain to
source is modeled as:
where
is the charge-carrier effective mobility,
is the gate width,
length and
is the gate oxide capacitance per unit area
is the gate
Characteristics curves
Diode for alternating current (DIAC)
The DIAC, or "diode for alternating current", is a diode that conducts current only after
its break over voltage, VBO, has been reached momentarily.
Symbol
Page 8 of 10
ConstructionMost DIACs have a three-layer structure with breakover voltage around 30 V. Their
behavior is somewhat similar to that of a neon lamp, but it is much more precisely
controlled and takes place at a lower voltage.
DIACs have no gate electrode, unlike some other thyristors that they are commonly
used to trigger, such asTRIACs. Some TRIACs, like Quadrac, contain a built-in DIAC in
series with the TRIAC's "gate" terminal for this purpose.
Operation
When this occurs, the diode enters the region of negative dynamic resistance, leading
to a decrease in the voltage drop across the diode and, usually, a sharp increase in
current through the diode. The diode remains "in conduction" until the current through it
drops below a value characteristic for the device, called the holding current, I H. Below
this value, the diode switches back to its high-resistance (non-conducting) state. This
behavior is bidirectional, meaning typically the same for both directions of current.
Characteristics curves
Page 9 of 10
Triode for Alternating Current (TRIAC)
Triode for Alternating Current, is a genericized tradename for an electronic
component that can conduct current in either direction when it is triggered (turned
on), and is formally called a bidirectional triode thyristor or bilateral triode thyristor.
Symbol
Operation
TRIACs are part of the thyristor family and are closely related to silicon-controlled
rectifiers (SCR). However, unlike SCRs, which are unidirectional devices (that is. can
conduct current only in one direction), TRIACs are bidirectional and so current can flow
in either direction. Another difference from SCRs is that TRIAC current flow can be
enabled by either a positive or negative current applied to its gate electrode, whereas
SCRs can be triggered only by current going into the gate. To create a triggering
current, a positive or negative voltage has to be applied to the gate with respect to the
MT1 terminal (otherwise known as A1).
Once triggered, the device continues to conduct until the current drops below a certain
threshold called the holding current
Characteristics curves
Page 10 of 10