Basic Electrical Engineering
“The” current view, Earth, western science year 2005
Current is a flow of electrical charges. Electrical charges can be electrons, “holes” (i.e.
absences of electrons), ions (electrically charged atoms), plasma (electrons and ions).
How current happens: microscopic (i.e. inside-out) view
An atom consists of one nucleus in the middle (charged positive), and a bunch of
electrons (charged negative) in a cloud circling the nucleus. Inside the nucleus are
protons (charged positive) and neutrons (no charge). Usually, the number of protons and
electrons is equal, so that an atom is electrically neutral. “Positively charged protons
attract negatively charged electrons and hold them in orbit around the nucleus of the
atom.” However, the further from nucleus an electron is, the weaker this attraction is and
the easier it is to get it to leave that atom and start wandering and “jumping over” to other
atoms. Especially if the farthest bands are not filled up (there is a prescribed number of
electrons that can exist at any given band), electrons in the farthest bands are free to join
with other atoms. Electrons seek to fill in any absence of electrons in other atoms (i.e.
they seek the “positive” charge to join with and make neutral - think of it as miniature
magnets – whichever magnet is stronger, will attract).
High-level view: positively charged
nucleus surrounded by a cloud of
negatively charged electrons;
electrons have different levels of
energy (valence bands). Total charge:
0.
Close up, using example of
silicon lattice: nucleus with
four positively charged
protons, and orbits with 4
negatively charged
electrons. Total charge: 0.
Conductors are materials which have a lot of free electrons, therefore the free electrons
can be easily moved and thus produce current. All metals are good conductors. Electrical
wires are made out of conductors.
Semiconductors are materials which are insulators below room temperature but conduct
at room temperature or when excited in some other way (optically, etc.) Diodes and
transistors are made out of semiconductors.
Insulators are materials which do not conduct electrical current because it is too hard to
pull any electrons out of completely filled valence bands and get them moving. Plastic is
an insulator. Capacitor filling is made of out insulator. Electrical cables (made out of
conductors) and shielded with insulators.
How current happens: macroscopic (i.e. outside-in) view:
Current is analogous through water flowing. It needs a path, and it always seeks the path
of least resistance. Current can continue flowing only if the path (called electric circuit)
is closed, i.e. the path forms a loop, and there is something that keeps on moving the
charges along. A power supply is a device that coaxes charges to keep on moving, i.e. it
adds enough energy to atoms to make the electrons leave the atoms and move.
Voltage is the difference between electric potential of two points. The reference is
totally arbitrary, it is sufficient that one point has more positive charge than the other.
The point with more positive charge is called higher potential. Electrons want to move
from low potential to high potential.
As electrons are moving in one direction, they leave “empty spaces,” i.e. positive
charges, behind them, so we can say that the positive charges move in the opposite
direction than electrons. Think of electrons as cars on the parking lot, or people sitting in
a row. If there is one empty space, then the closest car will move into it; then the next car
will move in the newly created empty space, etc. So, cars are moving in one direction,
and empty spaces are moving in the opposite direction. Conventional current flow is a
convention that states that the current flows in the direction of positive charges.
Inside a battery, the + side is on high potential, the – side is on the low potential.
Electrons cannot keep on moving from – to + inside the battery, because it is not a closed
loop. So, if we connect the battery into a circuit, e.g. a resistor, then current can flow. It
will flow from the battery -, through the circuit, and then through the battery + into the
battery -. (The battery operates on chemical reaction and is physically constructed so that
electrons will find it easier to go from the battery – through the circuit, instead of from
the battery – through the battery and to the battery +). So, battery does not produce any
electrons, it just “pushes them along.” It is the same electrons moving around.
e
e- shows the flow of negative charges,
i.e. electrons: from battery – to battery
+, and then through battery
-
E
e+
e+ shows the flow of positive charges,
i.e. absence of electrons: from battery
+ to battery -, and then through
battery
Semiconductors, p-n junction, Diodes and Transistors
Semiconductors are materials which are insulators below room temperature but conduct
at room temperature. Semiconductors’ properties can be enhanced by doping. Doping is
the process by which impurities are introduced into a semiconductor material. An
impurity is a material which is suitable for creating excess electrons or excess holes in
the semiconductor, so that the semiconductor can conduct well. If the impurities have
more electrons than the doped semiconductor, then the doping results in an n-type
semiconductor. If the impurities have less electrons, then a p-type semiconductor is
obtained. For example, silicon is a semiconductor that has 4 protons and 4 electrons, and
is usually doped with phosphorus or arsenic (which have 5 electrons) to obtain an n-type
semiconductor, or with boron (which has 3 electrons) to obtain a p-type semiconductor.
hole
Example of p-type
semiconductor: silicon doped
with boron
extra
electron
Example of n-type
semiconductor: silicon doped
with arsenic or phosphorus
In summary:
1. n-type semiconductors have an excess of electrons (they are doped with negative
charges, i.e. extra electrons are added). For n-type semiconductors, the majority
carriers are electrons, and minority carriers are holes.
2. p-type semiconductors have a lack of electrons, i.e. an excess of “holes” where those
electrons should have been, i.e. some electrons are missing. For p-type
semiconductors, the majority carriers are holes, and minority carriers are electrons.
A p-n junction is obtained when a p-type semiconductor is joined with an n-type
semiconductor.
A diode is an implementation of the p-n junction where the p-type semiconductor is the
anode, and the n-type is the cathode.
A transistor is a p-n-p junction or an n-p-n junction. It can be made as a bipolar junction
transistor (BJT) or a field effect transistor (FET), which can be MOSFET, JFET, etc. The
kind of transistor is obtained by physically arranging p and n portions on the chip in
certain ways. http://en.wikipedia.org/wiki/Bipolar_junction_transistor
“A p-n junction may be created by doping adjacent regions of a semiconductor with ptype and n-type dopants. If a positive bias voltage is placed on the p-type side, the
dominant positive carriers (holes) are pushed toward the junction. At the same time, the
dominant negative carriers (electrons) in the n-type material are attracted toward the
junction. Since there is an abundance of carriers at the junction, current can flow through
the junction from a power supply, such as a battery. However, if the bias is reversed, the
holes and electrons are pulled away from the junction, leaving a region of relatively nonconducting silicon which inhibits current flow. The p-n junction is the basis of an
electronic device called a diode, which allows electric current to flow in only one
direction. Similarly, a third region can be doped n-type or p-type to form a three-terminal
device, such as the bipolar junction transistor (which can be either p-n-p or n-p-n).”
[Wikipedia]
p
n
e-
e+
Forward biased p-n junction:
the diode conducts current due
to majority carriers
http://www.tpub.com/content/
neets/14179/css/14179_32.ht
m
p
e+
e-
Reverse biased p-n junction: the
diode does not fully conduct current;
but there is a tiny reverse current due
to minority carriers
http://www.tpub.com/content/neets/
14179/css/14179_33.htm
n
Schematic symbol
for a diode (with p
and n parts labeled)
p
n
p
C
E
C
e+
B
B
p-n-p bipolar junction transistor (BJT) : microscopic view and schematic
symbol
http://www.infodotinc.com/neets/book7/25b.htm
E
n
p
n
IC
C
E
C
IB
e+
B
B
IE
E
IE = IB + IC
n-p-n bipolar junction transistor (BJT) : microscopic view and schematic
symbol
http://www.infodotinc.com/neets/book7/25a.htm
http://www.tpub.com/content/neets/14179/css/14179_70.htm
Joke of the day:
A hydrogen atom and a helium atom go into a bar. The hydrogen atom is clearly upset and
moans, "I've lost my electron, my only electron." The concerned helium atom says, "Just calm
down now ... are you sure you've lost it?" The hydrogen atom replies, "Yes, I'm positive!"
(Later, a neutron walks in to the same bar. He sits down and says to the bar tender, "Hey, how
much for a beer?" The bar tender looks at him and says, "For you, no charge!")
Quiz: why is this joke funny?
http://en.wikipedia.org/wiki/Semiconductor
http://www.tpub.com/content/neets/14179/
http://en.wikipedia.org/wiki/Electrical_conduction
http://www.dl.ket.org/physics/companion/ThePC/compan/Current/
http://www.infodotinc.com/neets/book7/25b.htm
Transistors
A common way to use an n-p-n BJT is to connect it in the common emitter mode. Use
Kirchoff’s laws to calculate the currents.
Input voltage is considered to be VBE, and output voltage can be taken as VE or VCE,
depending on what we want. Input current is IB, and output current is either IC or IE.
A transistor can work either as an amplifier or as a switch. Amplifiers are used mostly for
electronics, and switches are the basis for gates used to implement Boolean algebra.
Used for switching: shaded yellow (cutoff mode) and shaded
red (saturation mode).
Used for amplification: no shade (forward active mode).
http://www.mitedu.freeserve.co.uk/Design/bjtsw.htm
IC
C
IB
B
IE
E
Always: IE = IB + IC
If amplifier:
IC = βIB
VBE = 0.7V
If in saturation mode:
VCE = 0.7V
IC = Isaturation
If you don’t have transistor characteristics, then you have to determine
on your own if the transistor is saturated or not. Calculate IC as
minimum between Isaturation and βIB
1. Transistor as amplifier:
When the input voltage is low enough, then the transistor’s p-n junctions behave as we
discussed already (BE is forward biased, CB is reverse biased), and the entire transistor
behaves as an amplifier. It is said that the transistor works in the active mode.
In the active mode, BE junction is a forward biased diode, and thus VBE = 0.7 Volts
(some people take it as low as 0.5V or 0.6V) when the transistor is ON. IB should be very
small (usually μA). Usually, β=100, so IC is on the order of mA.
Example: run a motor using a logic gate as the input.
http://www2.ics.hawaii.edu/%7Epagerg/331/7
http://www2.ics.hawaii.edu/%7Epagerg/331/9
2. Transistor as switch:
When the input voltage VBE is too high and thus produces high input current, BE
overpowers BC, i.e. BE is higher than CB. BE is forward biased and because it
overpowered BC, BC is forward biased too. The transistor works in the saturation mode.
In the saturation mode, VCE is almost 0, usually VCE = 0.2V.
When VBE is low, the transistor cannot work because the BE diode is not on, therefore the
entire transistor is off, i.e. not working. When the transistor is off, it means that it acts as
an open circuit, which means that the output voltage VCE is high, i.e. almost straight from
the power supply.
If we take the output voltage to be VCE and run the transistor between cutoff mode and
saturation mode, then the transistor behaves as an inverter. When VBE is high, the
transistor is in saturation mode and so VCE is low; when VBE is low, the transistor is in
cutoff mode and so VCE is high.
Example: turn a light on and off.
http://www.mitedu.freeserve.co.uk/Design/bjtsw.htm