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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