Transistors and Semiconductors

Miracle Makers of Modern Electronics

                                               

Graphics courtesy intel.com

Research at

http://www.pbs.org/transistor/science/index.html

And

http://www.intel.com/education/transworks/index.htm?iid=search&

Related links

http://www.pbs.org/transistor/science/info/transmodern.html

Direct link to field effect animation http://www.pbs.org/transistor/quick

times/movieclips/fieldeffectVIDEO/fieldeffectVIDEO_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

Diodes conduct only one way

Graphics courtesy “How Stuff Works”

(Holes and electrons meet at junction, combine; new ones form)

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.

                                               

Text and graphics courtesy intel.com

This type of transistor is called a Field Effect Transistor

In the n-type transistor, both the source and the drain are negatively-charged and sit on a positively-charged well of p-silicon.

                                               

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 todayBipolar 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.electronics-tutorials.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