Semiconductors

✓ Semiconductors are crystalline or amorphous solids that can conduct

electricity under specific circumstances, making it a good medium for the
control of electrical current.
✓ Semiconductors are made from materials that have free electrons in their
structure that can move easily between atoms, which aids the flow of
electricity.

Development of Semiconductors

Back in 1874, the invention of rectifiers was introduced, and this was the
birth history of semiconductors. In 1947, decades later, inventors named
Bardeen and Brattain at Bell Laboratories in US invented the first point
contact transistor, that were used in radios. And the next year, in 1948,
another inventor name Shockley invented a junction transistor. This heralded
the arrival of transistor era.

The semiconductor industry grew rapidly following the invention of

the transistor. In 1957, it already exceeded the scale of 100 million dollars. In
1959, the bipolar integrated circuit (ICs) was invented by Kilby of Texas
Instruments and Noyce of Fairchild Semiconductor in the US. This invention
had a major impact on the history of semiconductors, and it marked the dawn
of the IC era. Being small in size and light in weight, the IC was widely used
in a variety of electric appliances.

The image shows the advance developments of semiconductors as years

goes by. From a normal silicon transistor that was used in radios, it became
a chip type semiconductor that were used in appliances and gadgets that we
are using today.
Properties of Semiconductors

A substance that conducts electricity is called a conductor, and a

substance that does not conduct electricity is called an insulator.
Semiconductor are substance with properties somewhere between them.

Electrical properties can be indicated by resistivity. Conductors such

as Copper, Silver, Iron, Aluminum and Gold has low resistance and conduct
electricity easily. While insulators like rubber, glass, ceramic and mica has
high resistance preventing electricity to pass through. But semiconductors
have properties somewhere between them. Their resistivity might change
according to the temperature for example.

Silicon Battery

LED Bulb

(Image A) A simple circuit with (Image B) The same circuit with heated
unheated silicon metal. The LED silicon metal. The LED lights up.
did not light up.

At a low temperature, almost no electricity passes through them. But when

the temperature rises, electricity pass through easily.

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Types of Semiconductor

Semiconductors falls into two categories:

1. Intrinsic Semiconductors or pure semiconductors contains only one kind

of material, like Silicon or Germanium. They are also called “undoped
semiconductors” or “i-type semiconductors”.

2. Extrinsic Semiconductors or impure semiconductors are intrinsic

semiconductors with other substance added to their properties through
the process of doping.

Important Notes

The conductivities of silicon can be drastically increased and

controlled by the addition of impurities to the intrinsic (pure)
semiconductive material. This process, called doping, increases
the number of current carriers (electrons or holes).

Types of Semiconductors

1. N-Type is one that carries current in the form of negatively-charged

electrons. This is very similar to conduction of current in a wire.

2. P-Type is one that carries current predominantly as electron

deficiencies called holes. A hole has a positive electric charge. This
charge is equal and opposite to the charge on an electron. These holes
flow in the opposite direction of electrons.

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Function of Semiconductor

A semiconductor can help controlled flow of electricity. The basic

function of such a device is to switch ON and OFF the flow of electricity as
and when required. A semiconductor device can perform the function of a
vacuum tube with hundreds of times its volume. A single integrated
circuit (IC), such as a microprocessor chip, can do the work of a set of vacuum
tubes.

Some electronic semiconductors are,

1. Op-amp - Operational amplifiers (op amp) are linear devices that have
all the properties required for nearly ideal DC amplification and are
therefore used extensively in signal conditioning or filtering or to
perform mathematical operations such as adding, subtracting,
integration, and differentiation.

2. Resistors - In electronic circuits, resistors are used to reduce current flow,

adjust signal levels, to divide voltages, bias active elements, and terminate
transmission lines, among other uses.

3. Integrated Circuits (IC’s) - is small chip that can function as an amplifier,

oscillator, timer, microprocessor, or even computer memory. An IC is a
small wafer, usually made of silicon, that can hold anywhere from
hundreds to millions of transistors, resistors, and capacitors.

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 4. Capacitors - is an electronic component that stores and releases electricity
in a circuit. It also passes alternating current without passing direct
current.

5. Transistors - is a semiconductor device used to amplify or switch

electronic signals and electrical power. It is composed of semiconductor
material usually with at least three terminals for connection to an external
circuit.

6. Diodes - a diode allows an electric current to pass in one direction (called

the diode's forward direction), while blocking it in the opposite direction
(the reverse direction). As such, the diode can be viewed as an electronic
version of a check valve.

Diodes
Diode is a semiconductor device that plays an important role in electronics
circuits. They are used mainly in uncontrolled rectifiers to convert AC to fixed
DC voltages and as freewheeling diodes to provide a path for the current flow
in inductive loads. The basic circuit symbol used for diode is,

The anode is the electrode where electricity moves into. The cathode is the
electrode where electricity is given out or flows out of. The anode is usually
the positive side. A cathode is a negative side. It acts as an electron donor.

Various Types of Diodes and their Functions

1. PN Junction Diode - It consists of two layers of semiconductors. One layer

is doped with P-type material and the other layer with N-type material. The
combination of these both P and N-type layers form a junction known as
the P-N junction. Hence the name P-N junction diode.

2. Rectifier Diode - A rectifier diode is a type of P-N junction diode, whose

P-N junction area is very large. This results in high capacitance in reverse
direction. It has low switching speed.

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3. Schottky Diode – A type of diode which consists of a small junction
between an N-type semiconductor and a metal. It has no P-N
junction. The plus point of the Schottky diode is that it has very low
forward voltage drop and fast switching. As there is no capacitive
junction (P-N junction), the Schottky diode switching speed is very fast.

4. Light Emitting Diode (LED) - The light emitting diode is also a type of P-
N junction diode that emits light in the forward bias configuration. LED is
made up of a direct-band semiconductor. When the charge carriers
(electrons) cross the barrier and recombine with electron holes on the other
side, they emit photon particles (light). While the color of the light depends
on the energy gap of the semiconductor.

5. Photo Diode - The photodiode is a type of P-N junction diode that converts
the light energy into electrical current. Every semiconductor diode is
affected by optical charge carriers. It is why they are packaged in a light
blocking material.

6. Laser Diode - A laser diode is similar to LED because it converts electrical

energy into light energy. But unlike LED, Laser diode produces coherent
light. Laser diodes are used in optical communication, laser pointer, CD
drives and laser printer etc.

7. Zener Diode – It is a type of diode, which not only allows the flow of current
in the forward direction but also in reverse direction. when the reverse
voltage reaches the breakdown voltage known as Zener voltage it allows

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 the current flow. A Zener diode is mainly used in reverse bias
configuration. It provides a stabilized voltage for protection of circuits from
overvoltage.

8. Varactor Diode - Varactor diode also known as Vericap diode are voltage-
controlled capacitors. They have a P-N junction with variable junction
capacitance. The varactor diode operates under reverse bias conditions.
The depletion layer between the P and N-type material is varied by
changing the reverse voltage.

9. PIN Diode - PIN diode is a three-layer diode i.e. P-layer, I-layer & N-layer.
The ‘I‘ intrinsic semiconductor layer is placed between heavily doped P and
an N-type semiconductor. The electron and holes from N and P-type region
respectively flow to the intrinsic region (I). Once the “I” region fills
completely with electron-holes, the diode starts conduction.

10. Avalanche Diode - The Avalanche diode is a P-N junction diode that is
specifically designed to operate in the avalanche breakdown region.
Avalanche breakdown is a phenomenon where sufficient reverse voltage is
applied to the P-N junction. Due to which, the minority carrier ionizes &
starts a heavy current flow in reverse direction.

11. Tunnel Diode - A tunnel diode is a heavily doped P-N junction diode. It
works on the principle of the tunneling effect. Due to heavy doping
concentration, the junction barrier becomes very thin. This allows the

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electron to easily escape through the barrier. This phenomenon is known
as tunneling effect.