Transcript Transistors and Semiconductors

Transistors and
Semiconductors
Miracle Makers of
Modern Electronics
Research at
http://www.pbs.org/transistor/sc
ience/index.html
And
Graphics courtesy intel.com
http://www.intel.com/education
/transworks/index.htm?iid=sear
ch&
Related links

http://www.pbs.org/transistor/science/i
nfo/transmodern.html
Direct link to field effect animation
 http://www.pbs.org/transistor/quicktim
es/movieclips/fieldeffectVIDEO/fieldeffe
ctVIDEO_56K.mov

State Standard

Student know the properties of
transistors and the role of transistors in
electric circuits.
Microchip vs Transistor vs.
Semiconductor
Semiconductor- element with both
conducting and insulating properties
 Transistor – an electronic device that
uses properties of semiconductors
 Microchip – contains many transistors

Before Transistors
There was the vacuum
tube
Silicon
Element with atomic number 14
 Four electrons in outer shell
 Makes perfect crystals
 No free electrons
available for conduction

Doping – Add Impurities
Add phosphorus or arsenic with 5 outer
electrons – provides free electrons
needed for electric current to flow
 Makes N- type silicon
 Charges that flow are negative –
electrons
 Doped silicon is “semiconductor”

Diode – Simplest
Semiconductor Device
Connected this way, no current flows
 When battery polarity reversed
current can flow

(Holes and electrons meet at
junction, combine; new ones
form)

Diodes conduct only
one way
Graphics courtesy “How Stuff Works”
Transistors – Tiny Switches

Can be On or Off
Microprocessors work by binary flow
of information – ones and zeros
Transistors consist of three terminals;
the source, the gate, and the drain.
This type of
transistor is
called a Field
Effect
Transistor
Text and graphics courtesy intel.com
In the n-type transistor, both the source
and the drain are negatively-charged and
sit on a positively-charged well of psilicon.
When positive voltage is applied to the
gate, electrons in the p-silicon are
attracted to the area under the gate
forming an electron channel between the
source and the drain.
When positive voltage is applied to the
drain, the electrons are pulled from the
source to the drain. In this state the
transistor is on.
If the voltage at the gate is removed,
electrons aren't attracted to the area
between the source and drain. The
pathway is broken and the transistor is
turned off.
P – Type Transistors
Add boron or gallium to silicon
 Have only three outer electrons
 Form “holes” that can accept an
electron from a neighbor
 Also conducts current
 Majority current carriers are holes (+)
rather than electrons (-)
 Less common today

Junction(Bi-polar) Transistors
Terminals are “emitter,” “ base” and
“collector” instead of source, gate and
drain
 Less common today

Bipolar transistors are
essentially "current"
amplifiers while FETS
could be considered
voltage amplifiers.
Amplification in Bi-Polar
Transistor
A small change in
current in the
emitter base
circuit produces a
large change in
current through
the collector.
 link

Closer Look :PNP Junction
Transistor
If a positive voltage is applied to the emitter, current will flow through
the p-n junction with "holes" moving to the right and "electrons
moving to the left. Taken from http://www.electronicstutorials.com/basics/transistors.htm
Field Effect Transistors

weak electrical signal coming in through one
electrode creates an electrical field through
the rest of the transistor. This field flips from
positive to negative when the incoming signal
does, and controls a second current traveling
through the rest of the transistor. The field
modulates the second current to mimic the
first one -- but it can be substantially larger.
From PBS.org http://www.pbs.org/transistor/science/info/transmodern.html
Animation of Junction
Transistor
Explanation at Bell Labs website:
http://www.lucent.com/minds/transistor/tech.html
Microchips
Consist of thousands or millions of transistors and other electrical
elements working together