Archive for August, 2016

Science Makes Sense: Week 42-Metalloids and Semi-conductors

August 29, 2016

Even before visiting the west coast of the United States, many of us have heard of Silicon Valley which is a little south of San Francisco.  This is where the electronics/Computer industry boom occurred.  Why is Silicon attached to the name?   This element, besides being famous for creating glass centuries ago is the 20th and 21st century metalloid used in the semi-conductor industry.

Metalloids are considered to be neither metals nor non-metals.  ( As you already know, metals are willing to give up electrons to become cations, while non-metals are eager to take in electrons to become anions.)  Or they could be considered to be both a metal and a non-metal.  The metalloids are on the right side of the Periodic Table, closer to where the non-metals are positioned.  The metalloids  form a stairs-like configuration and include, Boron, B, silcon, Si, germanium,Ge, arsenic,As, antimony, Sb, tellurium,Te, and polonium, Po.(Ref.1)

These metalloids are usually brittle, shiny and behave like electrical insulators at room temperature.  When heated , they behave like conductors.  They also behave like conductors when small quantities of impurities are introduced or ‘doped’ into the crystalline lattice structure of metals.  Metalloids have the electronic structure in between that of the nearly empty outer shell of typical metals and the nearly filled shells of non-metals.  They have enough empty electronic orbitals into which electrons can be moved to conduct an electric current.  Their chemical properties  are also in between electropositive and electronegative atoms.  In physics one would call these elements semi-conductors.(Ref.2)  Semiconductors  have electrical resistance in between that of a conductor and an insulator. (Ref.3)  The electro negativity and ionization energies of metalloids lie between those of metals and non-metals.(Ref.4)

To understand semi-conductors, let us step back a little to look at the history of radios and transistors.  A radio  is a device that needs electricity and needs to be plugged while a transistor runs on batteries and could be carried around, the’boom box’, as it was called.  The latter relies on semi-conductor technology.  The ‘transistor’ is short for ‘transfer resistance’, and is made up of semiconductors and is a part/component used to regulate the amount of current/voltage  used to amplify/modulate/switch on or off an electronic signal.(Ref. 3)  This is the primary building block for an electronic chip, including the CPO ( central processing unit) in every computer we use.

Let us elaborate.  Silicon changes its behavior to a conductor when small amounts of impurities are added to it.

N-type: small quantities of phosphorus, P, or arsenic, As, are added to pure crystals of Si.  P and As have 5 outermost shell electrons, whereas Si has only 4.  The extra electron is free to move around and causes the silicon to turn into a conductor.( Ref.5)

P- type:when boron,B, or gallium, Ga, is added to Si,these atoms have only 3 electrons in their outermost shell. This results in’holes’or positive charges and once again causes a flow of current in the silicon crystal.   A minute amount or P or N- type doping in silicon causes an insulator into a viable conductor; hence the name semi-conductor.(Ref.5)

One can see that the P or N- type doping, evolved to the P-N-P and N-P-N sandwiches leading to transistors and electronic chips through micro-processors.( See Nuggets)  After the invention of transistors in 1954, the field of electronics has evolved dramatically to where we are now, using smart phones and watches like we always had them, handling all our needs from these devices.  Metalloids have been responsible for these amazing changes in our electronic lives.  Yet there are other significant uses for these metalloids that need to be mentioned as well.  Here are some selected few uses that is by no means complete.

Boron has been used to make high resistance glass, especially to thermal shock, control rods in nuclear reactors, also in strong magnets, in CD and DVD players and MRI machines.  Silicon has been used to make high temperature waxes, alloys of Si have been used to make car parts, also in breast implants and contact lenses.  Germanium has a high refractive index and has been used in wide-angle camera lenses and as a catalyst in polymerization reactions, fiber optics as well as in the treatment of AIDS.  Arsenic as an isotope has been used in locating tumors.  It is also used as an insecticide, fungicide and in the treatment of cancer.  Alloys of antimony have been used to make bullets, in acid batteries. In addition Sb has been used as a catalyst in polymer production.  Tellurium has been used to tint glass, cast in alloys to regulate temperature and making solar cell panels as a semiconductor.  Finally polonium has been used in thermoelectric cells because it releases a large amount of energy.

So these elements in a stair-like formation next to the non-metals in the Periodic Table have made quite a splash in our lives!

Activities for Middle School Teachers:

The first computers were automatic arithmetic engines.(Ref.6)  Logic has something to do with it and George Boole started it all.(See Nuggets)  Students can create and study Truth Tables.  The True or False will correspond to 1 or 0 (zero),switch on or off.  This naturally leads students to review/study binary numbers and compare with the decimal system used in arithmetic.  Instructors could help in letting students see the direct connection between binary numbers and computer arithmetic.

Students can look at the history of the digital age from 1947 onwards.  How do the innovations of say, Apple, social media like Twitter,Facebook etc and on-line ordering services affect our daily lives?  How has it affected small stores using traditional/ old modes of business practices?  Let students compare the seismic shifts in our lives today versus the onset of the printing press or the invention of the telephone.

Nuggets of Information:

John Bardeen, Walter Brattain and William Shockley at Bell Labs developed the transistor on December 23, 1947.(Ref.3)

Transistors replaced vacuum tubes in computers in 1954.(Ref.3)

A diode is the simplest possible semi-conductor device, also called a one- way turnstile for electrons.  The transistor is caused by using three layers of N-P-N or P-N-P sandwich; the transistor could be a switch or an amplifier.  A silicon chip is a piece of silicon with thousands of transistors!(Ref.5)

Boolean logic was developed by George Boole in the mid-1800s.  It allows a few unexpected things to be mapped into bits and bytes ( little pathways of logical information).(Ref.7)

References:

1.http://www.chemicalelements.com/groups/metalloids.html

2.https://www.britannica.com/science/metalloid

3.http://www.computerhope.com/jargon/s/semicond.htm

4.schmoop.com/periodic-table/metals-metalloids.htm

5.electronics.howstuffworks.com.diode/.htm

6.i-programmer.info/babbages-bag/235-logic-ev

7.computerhowstuffworks.com/Boolean.htm

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