Making A Transistor

I recently finished the book CODE - a book on how to build a computer from scratch. The book was awesome and I wanted to learn more about how a physical computer was built

In this article I will be going over how a transistor is made on a high level. I wanted it to feel like a "how's it made" episode. This originally came from my notes when learning myself, I am not an expert in this and if you see anything wrong, please send me a message.

This article is split into three parts

  1. Theory - What should happen
  2. Implementating - Work with solid materials to build something useful
  3. Resources - Useful links

Theory

Silicon is great, being a semiconductor it has unique properties we can take advantage of including tuning how much energy it takes to release an electron from its orbit and how much energy it takes for an atom to accept an electron.

This difference between an electron being released and being captured is important and has the name band gap

When electrons can move we call that electricity. Some material like metal have no band gap meaning that electrons flow easily through them. Insulators there is a band gap and it is difficult.

As seen in the diagram below semicondutors are between conductors and insulators

https://upload.wikimedia.org/wikipedia/commons/thumb/9/9d/Band_filling_diagram.svg

You can shift this band gap by doping silicon

  • Silicon with boron atoms is called p-type silicon, where there is a lack of electrons called holes
  • Silicon with phosphorus atoms is called n-type silicon, where free electrons are added
  • http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/imgsol/psem2.gif and http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/imgsol/nsem.gif

    Lets take a block of p-type silicon and a block of n-type silicon and put them together.

    Where they touch electrons will move from n-type material to p-type material. This is because the n-type atoms have less "hold" on there valence electrons and the p-type is "pulling" these electron

    Since the n-type lost an electron, it is not balanced and has to many protons. Making the area positively charged. The equilent and opposite for the p-type. In the diagram below, the lighter yellow has a positive charge.

    This charge causes a magnetic effect due unbalanced charges. The magnetic effect is the opposing effect to the movement of electrons causing an equilibrium at a certain distance

    Since we are displacing electrons from there position, this magnetic effect can also be seen as an electrical potential, aka voltage, within the material

    https://upload.wikimedia.org/wikipedia/commons/thumb/f/fa/Pn-junction-equilibrium-graphs.png

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    What happens if we try to put electricity though it.

    First, focus on voltage. Don't worry about current. When you put + on the p-type side it will attract the electrons back into the n-type and if you put electrons on the p-type it will push away the electrons / attached the holes.

    This causes the number of electrons that are in the other region. Making the voltage to cross the region less. This is what happens when it allows the current to flow. If we change the direction and put the electrons on the n-type.

    It will push away the electrons towards the holes and putting + on the p-type will attract the electrons farther in. This increases the number of electrons in the other region increasing the voltage needed to overcome in the materials. Now it is very difficult for the current to move, it would have to overcome the entire region voltage.

    Try out the slider below, if you see electricity can only go one direction. We just made an electrical diode!

    0.1V

    When you try to push electrons to go backwards, the NP junction pushes back at the same but opposite potential.

    ------------------

    Okay, now the MOSFET part.

    If you put another n-type material infront of the diode you get a material that doesn't flow any electrity and isn't useful. 0.1V

    Put a connector on the base. But not directly on the base but close enough that the magnetic field effects the material but electrons moving between is not possible. Usally an Oxide.

    Aside: this is how the MOSFET = Metal-Oxide-Silicon Field-Effect Transistor gets it's name

    Now when you apply a voltage to the base connected the the p-type it attracts the electrons close to the connection to the gate. And when enough electrons are near the top.

    Status: ON

    This breaks the np junction. Now that there is this base most the voltage potential is from the top to the bottom and not left to right. The electrons can move with little trouble

    In channel is formed that allows electrons to easily pass. There is no - and + difference to pass. Electrons can pass through without a change from - and +. This is called an n-channel.

    Quartz Rock

    Our base raw material is quartz rock, a form of silicon dioxide

    Silicon makes up around 28% by mass of the earth's crust, the second after oxygen and technically we could use any silicon dioxide, but there is already an ultrapure silicon dioxide called quartzite, a type of quartz rock

    Quartizite was originally sandstone, but it has been deeply buried in the Earth's crust and cooked at high temperatures and pressures that many impurities have been distilled out and the sand grains completely welded together.

    https://commons.wikimedia.org/wiki/File:Quartzite_Solli%C3%A8res.jpg

    Metallugical Silicon

    The next step is to remove all of the oxygen present in silicon dioxide. This can be done though the following reaction

    2SiO2 + 2C -> Si + 2CO

    The process occurs in an electric arc furnance to a temperature of 2,000C and the quartz is mixed with highly pure coke. Coke is a highly pure carbon fuel made by heating coal in the absensce of air.

    This processs is know as a carbothermal reduction of silicon dioxide.

    With the reaction above, there is a good chance of silicon carbide, SiC, forming. Excess SiO2 is used to help prevent this forming a secondary reaction.

    2 SiC + SiO2 -> 3Si + 2CO

    Now we have silicon with up to 99% purity but we are looking up to have "eleven nines" purity or 99.999999999%.

    Silicon wikipedia

    Ultra Pure Silicon (poly-silicon)

    The next step is to remove the contaniments to get an ultra pure silicon we are looking for.

    The silicon is reacted with hydrogen chloride gas to form triclorosilane (SiHCl2)

    Si + 3 HCl → HCl3Si + H2

    Reduced to very pure solution by reacting it with hydrogen at high temperatures

    HSiCl3 → Si + HCl + Cl2

    Or triclorosilane can be converted to silicon tetrachloride, SiCl4, in a hydrogenation reactor and reduced by vapor phase epitaxy with hydrogen

    SiCl4 + 2H2 -> Si + 4HCl

    Depending on specifics of the cost, both process are used.

    http://images-of-elements.com/silicon.jpg

    Single crystal silicon (mono-silicon)

    In a process called the Czochralski process the poly-silicon which contains mulitple crystals of silicon will be transformed into a single crystal

    https://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Schematic_of_allotropic_forms_of_silcon.svg/800px-Schematic_of_allotropic_forms_of_silcon.svg.png

    A seed crystal is dipped into liquid pure silicon and slowly raised and rotated built the single cystal ingot of pure silicon.

    As the crystal is being formed some containimates are pushed out if the contanimates aren't able to fit in that structure. This called zone refining

    Wafer

    The ingot is sliced into wafer by a diamonded edged saw and cleaned to specifications.

    https://upload.wikimedia.org/wikipedia/commons/thumb/f/f0/Siliziumwafer.JPG/1024px-Siliziumwafer.JPG

    Transistor

    There are many types of transistors because of space, efficency and cost the metal-oxide-semiconductor FET (MOSFET) is currently the most used.

    In this part we are going to look how it is built. Under each slide there are details on how it completed.

    Start with the P-Type Si Wafer

    Side View Top View P-Type Silicon

    The start of the process is at the begining of the other. Usaully this is done in another facility called a foundary.

    Oxide Grown

    Side View Top View P-Type Silicon Silicon Dioxide

    The first step is generate an SiO2 layer(0.5 -1 um thick) by thermal oxidation. Usally within the range of 900 to 1200 degrees C and a gas flow rate of 1cm/s

    Photoresist Applied

    Side View Top View P-Type Silicon Silicon Dioxide Photoresist

    Next step is first mask used by photolithography and is developed with raditation and the excess is removed

    Photoresist Developed

    Side View Top View P-Type Silicon Silicon Dioxide Photoresist

    The next step is to etch the material that is showing. A chemical is used to attack the oxide layer but not the photoresist or the silicon

    Oxide Etched

    Side View Top View P-Type Silicon Silicon Dioxide Photoresist

    The next step is to etch the material that is showing. A chemical is used to attack the oxide layer but not the photoresist or the silicon

    Photoresist Removed

    Side View Top View P-Type Silicon Silicon Dioxide Photoresist

    The photoresist is removed using a solvent or plasma oxidation

    Phosphorus Diffused

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    Phosphorus is diffused to make it an n-type region. This is done by Constant Surance Concentration Condition and followed by a drive-in diffusion under a Constant-Total-Dopant Conition

    Oxide Grown

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    An oxide layer is grown again. The phosphorus spreads out a little due to diffusion but remains at a high concentration

    PR Applied

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    The second photolithography is allplied (PR Drop -> Spinning -> Pre-Baking -> Mask Alignment-> UV Exposure -> PR Developing -> Rinsing and Drying -> Post-Baking -> Oxide Etching) as in Lithography #1 is use

    Photoresit Developed

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    The second photolithography is allplied (PR Drop ->Spinning ->Pre-Baking ->Mask Alignment->UV Exposure -> PR Developing -> Rinsing and Drying -> Post-Baking -> Oxide Etching) as in Lithography #1 is use

    Oxide Etched

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    PR Stripped

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    Gate Oxide Grown

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    Photoresist Added

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    Photoresist Developed

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    Oxide Etched

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    Photoresist Removed

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

    Aluminium Film Deposited

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

    Now a metal such as Aluminum is then evoporated and deposited on the surface to form a film to create the contacts

    Photoresist Applied

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

    Photoresist Applied

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

    Aluminum Etched

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

    Photoresist Removed

    Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

    Thank you for reading. I enjoyed making this summary and learnt so much.