DJM2032 Lecture 2

DJM2032 ELECTRONICS SYSTEM
CHAPTER 2 : SEMICONDUCTOR DEVICES

(DIODE, TRANSISTOR, TRIAC)
LECTUERER : EN. TAN POH CHUAR

JKM/PUO
Outline
Diodes
What are diodes?
P-N Junction Diode
Real vs. Ideal
Types of Diodes & Applications
Triacs
What are thyristors?
What are triacs?
Applications
What is a Diode?
A diode is a two terminal electric component which

conducts current more easily in one direction than in the
opposite direction.
The most common usage of a diode is as an electronic
valve which allows current to flow in one direction but not
the opposite direction.
4
A bit of history
Diodes were known as rectifiers until
1919, when a physicist by the name of
William Eccles coined the term diode,
which from its Greek roots means
through-path.
In 1873 Fredrick Guthrie discovered
thermionic diodes (vacuum tube diodes)
. Heating the cathode in forward bias
permitted electrons to be transmitted into
the vacuum, but in reverse bias the
electrons were not easily release from
the unheated anode.
5
A bit of history
In 1874 Karl Braun discovered the first
solid state diode (crystal diode). It
consists of using Galena crystals as
the semiconducting material.
In 1939 Russell Ohl discovered the
first P-N junction at Bell Labs.
Today, the majority of diodes are
made of semiconductor silicon P-N
junctions.
6
P-N Junction Diode

A P-N junction diode consists of a p-
Majority carriers
type semiconductor (silicon) joined
with an n-type semiconductor.
p n
P-type A semiconductor doped with
impurities to create positive charge
carriers (holes). Depletion Region
N-type A semiconductor doped with
impurities to create negative charged
carriers.
A depletion region is created when
negative charge carriers from the N-
type region diffuse into the P-type
region, and vice versa.
P-N Junction Diode
Depletion Region
The behavior of a diode depends upon

p n
the polarity of the supply voltage. if
Under forward bias the depletion region
is reduced in size and less energy is
required for the charged majority carriers
to cross the depletion region.
Forward Biased
This decrease in energy requirement
results in more charged majority carriers
to cross the depletion region which
induces a current.
P-N Junction Diode
Under reverse bias the depletion p nDepletion Region

region is greatly increased in size
ir
and requires significantly more
energy from the majority carriers in
V
order to cross. Reverse Biased
Most majority carriers wont be able
to cross the depletion region and
thus are unable to induce a current.
The current-versus-voltage characteristics
of a typical p-n junction diode
Real vs. Ideal
Ideal P-N Diode no resistance to current in I

forward bias and infinite resistance in reverse
bias. (Similar to a switch) conduction
In reality there is resistance to current flow in region
forward bias. It requires a certain voltage to V
non-conduction
be reached before the depletion region is
region
eliminated and full current flow is permitted.
Likewise, in reverse bias there is a small
Ideal Curve
reverse (leakage) current induced by the flow
of minority carriers. At a certain voltage (break
down voltage) the reverse current will
increase significantly. This is called the
Avalanche current.
Schottky Diode
Unlike P-N junction diodes, Schottky diodes are
based on a metal and semiconductor junction.
An advantage of Schottky diodes over P-N
junction diodes is that Schottky diodes have no
recovery time when switching from conducting to
non-conducting state and vice versa.
The main disadvantage of Schottky diodes are
that they operate in low voltage compare to P-N
junction diodes (up to 50V). Metal N-Type
Another significant difference is that the on-
voltage for a Schottky diode is around .3V while
it is .7V for a P-N junction diode.
Flyback Diode
Schottky diodes are often used as Flyback
diodes due to their quick recovery and low
forward voltage drop.
A Flyback diode is a diode used to eliminate the
sudden voltage spike that occurs across an
indicutive load when voltage is abruptly reduced
or removed.
Lenzs law - if the current through an inductance
changes, this inductance induces a voltage so
the current will go on flowing as long as there is
energy in the magnetic field.
Flyback diodes are important in mechatronics
applications where one may want to vary the
voltage of an inductive load to control its
operation.
Other Types of Diodes
Light Emitting Diodes (LEDs) - A diode formed
from a semiconductor such as gallium arsenide,
carriers that cross the junction emit photons
when they recombine with the majority carrier on
the other side.
Photodiode Exploits the fact that all
semiconductors are subject to charged carrier
generation when they are exposed to light.
Photodiodes are often used to sense light such
as in an Opto-isolator.
Zener Diode Allows current in forward bias like
a regular diode, but also in reverse bias if the
voltage is larger than designed voltage, called the
Breakdown voltage.
14
What are Thyristors?
Class of semiconductor components that can only go in

1 direction.
Wide range of devices, SCR (silicon controlled rectifier),
SCS (silicon controlled switch), Diacs, Triacs, and
Shockley diodes
Used in high power switching applications
i.e. hundreds of amps / thousands of watts
How do Thyristors work?
PNPN (4-layer) device:
PNP and NPN transistor back-to-back.
With forward voltage, small gate current pulse turns on
device.
once on, each transistor supplies gate current for the other, so no
need for gate input
only way to turn it off is to stop current (i.e. bring voltage to zero)
16
Thyristors contd.
now then, what are TRIACS?
A TRIAC (TRIode for Alternating Current)
is a 3-terminal AC semiconductor switch.
Composed of 2 thyristors facing opposite
directions such that it can conduct
current in either direction.
MT1 and MT2 are current carrying
terminals while the Gate terminal is used
for triggering by applying a small voltage
signal.
Once triggered, it continues to conduct
current until the current falls below a
threshold value.
Triac Operation
5 layer device
Region between MT1 and MT2 are parallel switches (PNPN and NPNP)
Allows for positive or negative gate triggering
Triac Characteristic Curve
Triac Characteristic Curve
o 1st quadrant - MT2 is (+) with respect to MT1
o VDRM is the break-over voltage of the Triac and the
highest voltage that can be blocked
o IRDM is the leakage current of the Triac when VDRM is
applied to MT1 and MT2
o IRDM is several orders of magnitude smaller than the on
rating
Triacs
Pros:
Better than a transistor as it has much better current
surge rating it can handle more current as it simply
turns on more
Inexpensive compared to relays
Cons:
Can't manually control turn-off with the gate; must turn
off by stopping current through the device via the
terminals.
Specs to buy one:
Gate signal requirements
Voltage drop
Steady-state/holding current (continuously handle)
Peak current (maximum amount to handle surge)
22
Wye-Chi Chok
Triac Applications
High Power TRIACS
Switching for AC circuits, allowing the control of very large
power flows with milliampere-scale control currents
Can eliminate mechanical wear in a relay
Low Power TRIACS

Light bulb dimmers (done by applying power later in the
AC cycle aka PWM of AC wave)
Motor speed controls for electric fans and other AC
motors, and heaters
Modern computerized control circuits in household
appliances
Triac Applications
Simple Triac Switch

Small control
current/voltage
Eliminates
Mechanical wear in a
Relay
Much Cheaper
Real World Triacs
Come in various shapes

and sizes
Essentially all the same
operationally
Different mounting
schemes

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DJM2032 ELECTRONICS SYSTEM

CHAPTER 2 : SEMICONDUCTOR DEVICES

LECTUERER : EN. TAN POH CHUAR

A diode is a two terminal electric component which

P-N Junction Diode

The behavior of a diode depends upon

Under reverse bias the depletion p nDepletion Region

Ideal P-N Diode no resistance to current in I

What are Thyristors?

Class of semiconductor components that can only go in

Low Power TRIACS

Simple Triac Switch

Come in various shapes

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