DO PHYSICS ONLINE
9.4
FROM IDEAS TO IMPLEMENTATION
#3.4 ATOMS TO TRANSISTORS
SEMICONDUCTOR DEVICES
Without the understanding of our atomic world through the theory of quantum
physics and the ability to control the doping of semiconductor materials such as silicon
we would not have the computers, mobile phones, internet, LED lighting, etc, that our
modern society now revolves about. The doping of semiconductors (adding impurity
atoms to the crystal structure) makes it possible to construct circuits on a single silicon
chip (integrated circuit) made up of millions of components such as resistors,
capacitors, pn junctions and transistors.
pn junction diodes
A pn junction diode is made by joining a piece of p-type semiconductor to an n-type
semiconductor. They are vital electronic component in the control of the direction of
current and used where it is necessary to convert an alternating current to a direct
current (or an alternating voltage to a direct voltage).
pn diode (forward bias):
low resistance
p
+
n
pn diode (reverse bias):
high resistance
p
n
I
I~0
+
Fig. 1.
A diode’s resistance depends upon the polarity of the bias voltage
connected to it.
rectification: AC to DC
Fig. 2. Diodes are used to convert an AC signal into a DC signal in a process known
as rectification.
A LED (light-emitting diode) is a semiconductor pn junction
diode light source. When a LED is switched on, electrons are
able to recombine with holes within the device, releasing
photons and the colour of the light emitted depends upon the frequency of the
emitted radiation and this is determined by the energy band gap of the semiconductor.
Appearing as practical electronic components in 1962, early LEDs emitted low-intensity
red light, but modern versions are available across the visible, ultraviolet, and infrared
wavelengths, with very high brightness. White light LEDS are now
becoming available to general purpose lighting in the home. LEDs
present many advantages over incandescent light sources
including lower energy consumption, longer lifetime, improved physical robustness,
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smaller size, and faster switching. Light-emitting diodes are used in applications as
diverse as aviation lighting, digital microscopes, automotive lighting, advertising,
general lighting, and traffic signals. Infrared LEDs are also used in the remote control
units of many commercial products including televisions, DVD players and other
domestic appliances.
Transistors
The simplest types of transistors are a combination of three doped semiconductor
materials to give either a pnp or a npn transistor. The three structures within a
transistor are called the emitter, base and collector.
npn transistor
n
p
n
pnp transistor
emitter
collector
p
base
n
p
collector
base
collector
collector
base
base
emitter
emitter
Fig. 3. Structure and circuit symbols for transistors.
Transistors are used as switches and as amplifiers.
current through transistor
can be switched on an off by
a small voltage pulse
connected to BASE
I
small voltage pulse
supplied to BASE
off on off
Fig. 4. Transistor as a switch.
small ac voltage
connected to the
BASE
VE
the variation in the voltage
connected to the BASE is
amplified by the transistor action
so that a much large variation in
voltage is measured across the
EMITTER resistor
Fig. 5. Transistor as an amplifier.
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The transistor was invented in 1948 by J. Bardeen and W. H. Brattain of the Bell
Telephone Laboratories (U.S.A.). Before the extensive use of semiconductor materials,
electronic circuits and devices were made from individual components. Valves were
used for rectification and amplification. Valves are just a type of cathode ray tube
where a gas at low pressure is enclosed in a glass container. At hot cathode is used for
the effective emission of the electrons from the cathode in a process known as
thermionic emission. Diode valves were used in rectification, where the current (flow
of positive charges) was from the anode to cathode. Multi-electrode valves were used
for amplification. The current from anode to cathode was controlled by a very small
voltage applied to grid electrodes positioned between the cathode and anode. Valves
were fragile, bulky and became quite hot.
One advantage of the transistor is its extremely small size and now millions of them of
them can be arranged in circuits on a micros-chip that is smaller than a ten cent piece.
which has more
computing power?
1963
2013
a micro-chip may
contain millions of
circuit components
Fig. 6. Comparison of computers from 1963 to 2013.
Transistor are mechanically rugged, a valve is not. Transistors and micro-chips have a
long service life because there is no heated element like in a valve. Operation is
instantaneous with transistors and there is no warm-up time unlike circuits with valves.
Transistors and micro-chips operate at low voltages where as high voltages are
required to operate valves. Therefore, the power used by transistors is much less than
the power consumption of valves.
transistors
electronic valve
artistic picture for the
structure of a transistor
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Solar Cells
photon E = h f
A solar cell (photocell or
photovoltaic cell) is an
electrical device that converts
the energy of light directly into
electrical energy by the
photovoltaic effect
(photoelectric effect).
an electron absorbs a photon 
electron – hole pair produced 
current in external circuit
Photoelectric effect: Light energy absorbed (i.e. energy of a photon absorbed) by an
electron to produce a photoelectron.
Photovoltaic effect: Theses photoelectrons can generate and support an electric
current without being attached to any external voltage source.
A photovoltaic cell is a pn junction. When photons with energy greater than the band
gap energy are absorbed, they will create electron-hole pairs. The holes and electrons
produce a current when connected to an external circuit, hence, the solar cell becomes
a source of emf and energy, just like a normal battery.
Light travels in packets of energy called photons. These photons can release electrons
from atoms – photoelectric effect. The generation of electric current happens inside
the depletion zone of the PN junction. The depletion region is the area around the PN
junction where the electrons from the N-type silicon, have diffused into the holes of
the P-type material creating a potential barrier to prevent further electrons moving
from the N-region to the P-region. When a photon of light is absorbed by an atom in
the depletion region it will dislodge an electron, creating a free electron and a hole.
The free electron and hole has sufficient energy to jump out of the depletion zone. The
electrons move to the N-region and the holes to the P-region, the potential barrier
causes by the diffusion of electrons prevents the electrons moving into the P-region
where they would recombine with the holes. If an external circuit is connected from
the cathode (N-region) to the anode (P-region) electrons will flow through the external
circuit. The electron is attracted to the positive charge of the P-region and travels
through the external load creating a flow of electrons, that is, a current is established.
The hole created by the dislodged electron is attracted to the negative charge of Nregion and migrates to the back electrical contact. As the electron enters the P-type
silicon from the back electrical contact it combines with the hole restoring the
electrical neutrality. The N-type layer is usually made quite thin so that the light can
pass through into the depletion region.
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