Transcript Week 5: Semiconductors

SMV ELECTRIC TUTORIALS
Aditya Kuroodi
2016
Relevant Course(s): EE2, EE121B, EE112, EE115A
SEMICONDUCTOR
THEORY & APPLICATIONS
Conductors and Insulators

Electrons of different types of atoms have different degrees of freedom to
move around.


Metals: electrons are loose, and they form a sea of free electrons
Electrical conductivity: the relative mobility of electrons within a material.

Conductors: high electron mobility (many free electrons)

Insulators: low electron mobility (few or no free electrons)
Why are Semiconductors important?

Semiconductors revolutionized electrical engineering, began in Bell Labs (now
AT&T) ~1950s

Basis of all semiconductor devices: The PN Junction

PN junction allowed discovery of diodes + transistors, which led to integrated
circuits (ICs)

First came Bi-Polar Junction Transistors (BJT) and then field effect (FETs)

Integrated circuits built with tiny transistors are the foundation for all
modern electronic devices (computers, smart phones, TVs, radios, etc.)
What is a Semiconductor?

Just as it sounds, a material that has semi-conductive properties

Widely used semiconductor elements: Silicon (Si), Germanium (Ge), Arsenic
(As), Gallium (Ga) and more

Nowadays the long list has been refined to Silicon or GaAs devices

Important chemical/quantum property of semiconductors: small bandgap
energy
Semiconductor and Energy Bands

Electrons in elements reside in many discrete levels that appear as continous
energy bands

The two bands of interest for us are the valence band and conduction band

Free electrons in the conduction band facilitate current flow, while electrons
in the valence band do not (think of orbitals, highest orbital = valence band)

Since current is flow of valence electrons, we are interested in the band gap
(energy level jump) needed for electrons to go from valence to conduction
band
Semiconductors, Holes, and Doping

Two main charge carriers in semiconductors: electrons (-) and holes (+)

A “hole” is simply a lack of an electron at a particular space

Electrons flowing to right means holes flowing left and vice versa (and
conventional current is opposite that of electron motion)

Small band gap energy in semiconductors means we have some measure of
control over their conductivity

This control comes from doping, a process that adds impurities (ions) to
influence chemical structure of semiconductors so they behave more like
metals
Semiconductors, Holes, and Doping
Acceptor  P-Type
Semiconductor
Donor  N-Type
Semiconductor

Suppose we start with intrinsic (undoped, natural) Silicon.

N-Doping means adding donor impurities, results in an N-Type

P-Doping means adding acceptor impurities, results in P-Type
Semiconductor Doping and Energy Bands

Doping changes the concentration of charge carriers in the semiconductor

N-Doping increases electron mobility (negative carriers), P-Doping increases hole
mobility (positive carriers)

Fermi Level: An energy boundary where all electrons residing below it are not
able to contribute to current flow (or the state that has 50% chance of being
filled)

Doping shifts the Fermi Level either up ( N-Doped) or down (P-Doped)
The PN Junction: Depletion Zone

Joining P and N type semiconductors results in the very useful PN junction

Due to charge stabilization, the middle of a PN junction develops a potential
barrier (“depletion zone”)

Now, we have to apply an appropriate voltage to overcome depletion zone
and make the junction conduct
The PN Junction: Forward and Reverse
Bias

If you hook up + terminal of battery to P-Type, and – terminal to N-Type you
will forward bias the junction

Forward repels majority carriers back into depletion zone, causing depletion
zone to shrink (due to recombination) and that lets current flow easily

If you hook up – terminal of battery to P-Type, and + terminal to N-Type, you
will reverse bias the junction

Reverse bias attracts majority carriers to terminals, expanding the depletion
zone and impeding current flow
Diodes

A diode is a device that only allows current to flow in one direction, it is
achieved through a PN junction

The blue arrow represents conventional current flow (opposite of electron
flow)
Alternating Current: Why Do We Need It?

AC has 3 main uses: Power distribution, motors, signal transmission

Power from a generator is sent to households in AC to minimize losses

The use of transformers with AC allows for higher voltage (lower current)
transmission  hence less IR loss on transmission line

The sinusoidal nature of AC makes it easy to work with rotary mechanics

Communication signals (radio, satellite, phone, etc.) are always AC

Note: a pulsed DC wave is rather similar to AC, making AC even more
widepsread
Transformers and Power Distribution

Transformers use mutual inductance to
step up or step down voltages/currents

The transformer below steps DOWN
voltage from primary to secondary
depening on the ratio of # turns in each
side’s inductor coil

To keep power constant it therefore
must step current UP by same factor

Another common type of transformer
used in U.S. power distribution is the
center-tapped transformer
Center tap allows better load applicaton + safety
for consumers
Single and Split-Phase Power Systems

Single phase system composed of 1 AC source with multiple loads in parallel

Necessitates really large current (expensive)

We can increase voltage source to lower current, but that increases ESD
dangers

Placing loads in series (reduces voltage on each individual load) not practical

Thus, American power distributors have mostly setlled on split-phase power
systems giving rise to the 120/240 VAC label
AC-DC Conversion: Half-Wave Rectifier

We can use the unidirectional property of diodes to convert AC to DC

The simplest is called a half-wave rectifier: place diode in series with source

Now you’re load gets half the power of the source

Useful for a lamp that has high and low brightness settings

But this is still AC right?
AC-DC Conversion: Half-Wave Rectifier

Almost all (bulbs aside) electronic devices require DC power to operate

Just adding a diode to the AC source doesn’t cut it, so…
The capacitor stores the converted
power supply voltage so that the load
it is connected to sees *near* DC
voltage
AC-DC Conversion: Full-Wave Rectifier

The half-wave is simple, but isn’t nearly as DC as we would want

The full-wave does not lose 50% of mains power in the conversion and is more
uniform

2 common methods: full-wave bridge rectifier + center tap transformer
Since current flowing through load is in
same direction during even/odd cycles,
you get the full wave output