Silicon, Semiconductor, Electrons, Holes, PN Junction, Diode, LED, Photon, 7-Segment LED and Full-Color Flat Panel LED Displays

Posted by PITHOCRATES - May 30th, 2012

Technology 101

Applying a Voltage across a PN junction to Create a Forward Bias Pushes Electrons and Holes towards the Junction

There’s gold in them thar Hills.  And silicon in the valley.  California has been a fountain of wealth.  Much of which they built from two materials located on the periodic table.  Atomic number 79.  Gold.  Or ‘Au’ as it appears on the periodic table.  And atomic number 14.  Silicon.  Or ‘Si’ as it appears on the periodic table.  Both of these metals proved to be valuable.  One by its scarcity.  One by what we could do with it.  For it was anything but scarce.  Silicon is the second most common element behind only oxygen.  But this commonly found material proved to be a greater font of wealth for California.  For it fueled the semiconductor industry.  For when we doped it with impurities we produced negatively (N-type) and positively (P-type) charged material.  Bringing the N and the P together gave us the PN junction.  Giving us the diode, transistor and integrated circuit.

The miracle of semiconductors occurs at the atomic level.  Down to the electrons orbiting the atom’s nucleus.  The nucleus contains an equal number of positively charged protons and neutrally charged neutrons.  The number of protons gives us the atomic number.  Changing the number of neutrons gives us isotopes.  Radioactive material has more protons than neutrons.  Uranium-235 is an isotope.  The stuff that made the atomic bomb dropped on Hiroshima.  Electrons orbit the nucleus.  In discrete energy levels.  The orbits closest to the nucleus have the lowest energy levels.  The orbits father away from the nucleus have higher energy levels.  Most of these orbits are ‘full’ of electrons.  The outer electron shell when ‘full’ is inert.  An outer shell that isn’t ‘full’ or has extra electrons is active.  And can chemically react.  Forming molecules.  When chemicals come into contact with each other and form molecules it is these electrons in the outer orbits (or valence electrons) that move into and out of the orbits of the different chemicals.  That is, the different elements share these valence electrons.

This is what we do when we dope silicon with impurities.  We either remove electrons from the valence shell to create a net positive charge.  Or we add electrons to the valence shell to create a net negative charge.  Giving us P-type and N-type material.  At the PN junction the N-type material loses its excess electrons to the P-type material across the junction as the empty holes in the valence shell attract the excess electrons.  As electrons leave the valence shells in the N-type material they leave holes in the valence shell where they once were.  Or, in the world of electronics, as electrons flow one way holes flow the other.  When we apply a voltage across a PN junction to create a forward bias (negative voltage applied to N-type and positive voltage applied to P-type) we push electrons and holes towards the junction.  If the forward bias is great enough they will continue all the way through the junction and into the material on the far side.  Where electrons will combine with excess holes.  And holes will combine with excess electrons.  Creating an electric current.  If we apply a voltage to create a reverse bias we will pull electrons and holes away from the PN junction.  And there will be no electrical current. We call such a PN device a diode.  A very important and indispensible device in electronics.

Placing Seven LEDs into a Figure-Eight Pattern created the Seven-Segment LED

Now back to those discrete energy levels.  There is another useful property we get when electrons move between these energy levels.  Electrons absorb energy when they move to a higher energy level.  And emit energy when they move to a lower energy level.  We make use of this property in fluorescent lighting.  A charged plasma field in a fluorescent lamp excites a small amount of mercury in the lamp.  As electrons fall into lower orbits in the mercury atoms they release invisible short-wave ultraviolet radiation.  The phosphor coating on the inside of the lamp absorbs this radiation and fluoresces.  Creating visible light.  By using different materials, though, we could see the energy (a photon) emitted by an electron falling into a lower energy level.  We have been able to move the wavelength of this photon into the visible spectrum.  The first commercial application to convert these photons into visible light was a device that gave us a red light.  That device was that important and indispensible PN-junction.  The diode.  And the use of different materials other than silicon moved these photons into the visible spectrum.  Giving us the light-emitting diode.  Or LED.

The first LEDs were only red.  Then we developed other colors using different materials.  Shifting the wavelength of the photon through all colors of the visible spectrum.  Being low-power devices, though, the intensity of light emitted was limited.  So an LED required careful mechanical construction and optics.  To direct the light out of the material forming the PN junction.  With a reflector behind the junction.  And a lens above.  To aim and diffuse the light.  And to prevent it from reflecting back into the material where it may be dissipated as heat.  Early use of LEDs was for indicator lights.  The low power consumption meant little heat was generated as with an incandescent lamp.  Which worked well in the temperature sensitive computer world.  Placing 7 LEDs into a figure-eight pattern created the seven-segment LED display.  With a rectangular shaped piece of translucent plastic above each LED you could see a bar of light for each light emitting diode.  Creating a forward bias on certain bars in the seven-segment display created the numbers we saw on our first calculators and digital watches.

An LED could produce a similar radiation like in the fluorescent lamp.  Using that radiation to fluoresce a phosphor coating inside a lamp to produce white light.  Similar to the fluorescence lamp.  Only while using less power.  Mixing the emitted light from red, green and blue (RGB) LEDs also produced white light.  Further improvements allowed us to emit whiter and brighter lights.  Allowing brighter lamps that consumed less power than the compact fluorescent lamps which were energy saving alternatives to the incandescent lamps.  With the lower power consumption of LEDs creating less heat we expanded the lifespan of lighting sources made from LEDs.  Using them to increase the service life in lamps inconvenient to change.  Like in traffic signal lights over busy intersections.  To the taillights in tractor trailers.  Where anytime spent not hauling freight was lost revenue.

We made Full-Color Flat Panel Displays from LEDs by combining Red, Green and Blue LEDs into Full-Color Pixel Clusters

The market didn’t demand these developments in semiconductors or LEDs.  For the most part the market didn’t even know this technology existed.  But the entrepreneurs gathering in Silicon Valley did.  They had some great ideas of what they could do with this new technology.  All they needed was the capital to bring these ideas to market.  It was risky.  The technology was good.  But could they use it to make useful things at affordable prices?  And would the people be so enamored with the things they built that they would buy them?  There were just too many unknowns for conservative bankers to take a risk.  But thanks to venture capitalists those entrepreneurs got the capital they needed.  Brought their ideas to market.  Created Silicon Valley.  And the modern world we now take for granted.

They continue to advance this technology.  Creating full-color flat panel displays.  By combining red, green and blue LEDs into full-color pixel clusters.  Which, unlike an LCD flat panel display, does not need a backlight as they produce their own light.  So these panels are thinner and use less power than LCD displays.  Making them ideal for the displays in our cellular devices for they allow more battery life between charges.  They also have wide viewing angles.  People looking at these displays from near perpendicular viewing angles see nearly the same quality of picture as those viewing directly in front.  Making them ideal for use in stadiums.  The video replays you see on that mammoth flat panel display in the new Dallas Cowboy stadium is an LED flat panel display.

All of this from joining two differently-charged semiconductor materials together.  Creating that all important and indispensible PN junction.  The foundation for every electronic device.  And of Silicon Valley itself.


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Boolean Algebra, Logic Gates, Flip-Flop, Bit, Byte, Transistor, Integrated Circuit, Microprocessor and Computer Programming

Posted by PITHOCRATES - February 1st, 2012

Technology 101

A Binary System is one where a Bit of Information can only have One of Two States 

Parents can be very logical when it comes to their children.  Children always want dessert.  But they don’t always clean their rooms or do their homework.  So some parents make dessert conditional.  For the children to have their dessert they must clean their rooms AND do their homework.  Both things are required to get dessert.  Or you could say this in another way.  If the children either don’t clean their rooms OR don’t do their homework they will forfeit their dessert.  Stated in this way they only need to do one of two things (not clean their room OR not do their homework) to forfeit their dessert. 

This was an introduction to logic.  George Boole created a mathematical way to express this logic. We call it Boolean algebra.  But relax.  There will be no algebraic equations here.

In the above example things had only one of two states.  Room cleaned.  Room not cleaned.   Homework done.  Homework not done.  This is a binary system.  Where a bit of information can only have one of two states.  We gave these states names.  We could have used anything.  But in our digital age we chose to represent these two states with either a ‘1’ or a ‘0’.  One piece of information is either a ‘1’.  And if it’s not a ‘1’ then it has to be a ‘0’.  In the above example a clean room and complete homework would both be 1s.  And a dirty room and incomplete homework would be 0s.  Where ‘1’ means a condition is ‘true’.  And a ‘0’ means the condition is ‘false’.

Miniaturization allowed us to place more Transistors onto an Integrated Circuit

Logic gates are electrical/electronic devices that process these bits of information to make a decision.  The above was an example of two logic gates.  Can you guess what we call them?  One was an AND gate.  The other was an OR gate.  Because one needed both conditions (the first AND the second) to be true to trigger a true output.  Children get dessert.  The other needed only one condition (the first OR the second) to be true to trigger a true output.  Children forfeit dessert. 

We made early gates with electromechanical relays and vacuum tubes.  Claude Shannon used Boolean algebra to optimize telephone routing switches made of relays.  But these were big and required big spaces, needed lots of wiring, consumed a lot of power and generated a lot of heat.  Especially as we combined more and more of these logic gates together to be able to make more complex decisions.  Think of what happens when you press a button to call an elevator (an input).  Doors close (an action).  When doors are closed (an input) car moves (an action).  Car slows down when near floor.  Car stops on floor.  When car stops doors open.  Etc.  If you were ever in an elevator control room you could hear a symphony of clicks and clacks from the relays as they processed new inputs and issued action commands to safely move people up and down a building.  Some Boolean number crunching, though, could often eliminate a lot of redundant gates while still making the same decisions based on the same input conditions. 

The physical size constraints of putting more and more relays or vacuum tubes together limited these decision-making machines, though.  But new technology solved that problem.  By exchanging relays and vacuum tubes for transistors.  Made from small amounts of semiconductor material.  Such as silicon.  As in Silicon Valley.  These transistors are very small and consume far less power.  Which allowed us to build larger and more complex logic arrays.  Built with latching flip-flops.  Such as the J-K flip-flop.  Logic gates wired together to store a single bit of information.  A ‘1’ or a ‘0’.  Eight of these devices in a row can hold 8 bits of information.  Or a byte.  When a clock was added to these flip-flops they would check the inputs and change their outputs (if necessary) with each pulse of the clock.  Miniaturization allowed us to place more and more of these transistors onto an integrated circuit.  A computer chip.  Which could hold a lot of bytes of information. 

To Program Computers we used Assembly Language and High-Level Programming Languages like FORTRAN

The marriage of latching flip-flops and a clock gave birth to the microprocessor.  A sequential digital logic device.  Where the microprocessor checks inputs in sequence and based on the instructions stored in the computer’s memory (those registers built from flip-flops encoded with bytes of binary instructions) executes output actions.  Like the elevator.  The microprocessor notes the inputs.  It then looks in its memory to see what those inputs mean.  And then executes the instructions for that set of inputs.  The bigger the registers and the faster the clock speed the faster this sequence.

Putting information into these registers can be tedious.  Especially if you’re programming in machine language.  Entering a ‘1’ or a ‘0’ for each bit in a byte.  To help humans program these machines we developed assembly language.  Where we wrote lines of program using words we could better understand.  Then used an assembler to covert that programming into the machine language the machine could understand.  Because the machine only looks at bytes of data full of 1s and 0s and compares it to a stored program for instructions to generate an output.  To improve on this we developed high-level programming languages.  Such as FORTRAN.  FORTRAN, short for formula translation, made more sense to humans and was therefore more powerful for people.  A compiler would then translate the human gibberish into the machine language the computer could understand.

Computing has come a long way from those electromechanical relays and vacuum tubes.  Where once you had to be an engineer or a computer scientist to program and operate a computer.  Through the high-tech revolution of the Eighties and Silicon Valley.  Where chip making changed our world and created an economic boom the likes few have ever seen.  To today where anyone can use a laptop computer or a smartphone to surf the Internet.  And they don’t have to understand any of the technology that makes it work.  Which is why people curse when their device doesn’t do what they want it to do.  It doesn’t help.  But it’s all they can do.  Curse.  Unlike an engineer or computer scientist.  Who don’t curse.  Much.


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