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The Chemistry of Semiconductors.

The Symbol for a Diode

The Symbol for a Diode

It is time to start moving from passive to active. No I am not talking politics here, I am talking about our study of electronics. Passive devices are devices that do not require external power.  Active devices require external power.  To me a better set of terms to use is linear for the type of devices we have been dealing with so far and non-linear for the ones we are about to talk about.  To me a diode is a passive device, but it is listed in electronics catalogs as an active device because all semiconductors are grouped as active devices.

The simple model for a diode is that it acts as a check valve.  It allows current to flow in one direction but not in the other direction.   But it turns out that even in the case of check valves that statement is not true.  A check valve requires a small amount of pressure to unseat the valve element and the same is true for a semiconductor diode.  The direction of allowable flow in a diode is shown by the arrow assuming conventional current.   If you prefer to use electron current the direction is opposing the arrow.

The Periodic Table

The Periodic Table

To really understand a diode and all semiconductors it is necessary to talk a little about chemistry.  The base unit in matter (at least in this description) is an atom.  An atom consists of positive charged particles in a tight center mass called the nucleus.  Floating around that is electrons.  The location of the electrons is defined by a probability function in areas that are called “clouds”.  This is not the pretty circular or even elliptical orbits like those on the NASA emblem.   The number of electrons in an atom have to be equal to the number of positive charged particles in the nucleus for an atom to remain neutral charged.   This number is called the atomic number and defines the element.   It is the number on each of the blocks in the periodic table.  The letters in the block are the symbols for each element.

The key to chemistry is that each of these electrons is in a “shell” and each shell would like to be completed.  This is shown on the periodic table by the location of each element in the rows of the table.   For example on the top row the first shell wants to contain 2 electrons.  The right most element Helium (He) contains two electrons so it is perfectly happy all by itself.  However the left most element Hydrogen (H) only has one electron but it would really like to have 2 to complete the shell.  Hydrogen as a gas normally consists of two hydrogen atoms and this is written as H2 because each of the hydrogen atoms shares its electron so both shells are happy and both atoms remain neutral electrically.

The 2nd and 3rd shells each would like to have 8 electrons.  Elements on the left side of the chart often combine with those on the right side to complete the shell.  The elements on the far right side have complete shells and again prefer to remain alone.  These elements are called the noble gases because they do not want to combine.  For one more example before we get into semiconductors, we will use the common chemical water.  (As a side note… you are chemically dependent… we all are!)   Oxygen (O) is in group (column) 16.  This is two short of the right most group of 18, so oxygen really wants two more electrons.  It often combines with two hydrogen atoms to form H2O or water. Please note that the column numbers on this table conform to an international standard.  I prefer the old numbering system that would call the oxygen column VI which clearly shows that shell needs 2 more electrons.

The 4th shell consists of several sub-shells, but the important thing to remember is the outer part of that shell also wants 8 electrons.

The elements used to form the base semiconductors are found in group 14.  Each of these wants 4 more electrons to fill the shell.  One form of these elements is to partner up with 4 of its closest neighbors.  Each of these also partners with 3 more close neighbors and together this forms a crystal.   In the case of carbon (C) this is called a diamond.  However, silicon (Si) is the most used of the semiconductors, but it still is used in the crystal form and the base element of the crystal is called the diamond cubic shape.  Germanium (Ge) is also used as a semiconductor.  Several compounds also form crystals for semiconductors.  I got curious if carbon, diamond, was ever attempted to be used and found an interesting article about it.   Why Diamond is Better than Any Thing Else.  This is a typical case of it looks good on paper but we are not there yet.

The first step in making a semiconductor is to grow a crystal.  This is not an easy process and requires a highly controlled environment because the crystal must be highly pure and form a perfect crystal. Now, so far we have “found the magic crystal” (sounds like a computer adventure game doesn’t it?) but we have yet to get it to do any magic.   This crystal is called a semiconductor, because it is only semi-good at conducting and only semi-good as an insulator.  In other words it is pretty worthless in this condition.

The key to making it useful is to “dope” it with impurities.  (In my vernacular, when it gets on dope it starts acting weird.)  If they precisely implant some of the group 17 15 elements in the crystal then the crystal is still neutral charged but some of the atoms have one more electron than the shell wants.  This electron is easily dislodged and the semiconductor has become a conductor.  This is called an N type semiconductor.  A similar situation occurs when the pure crystal is implanted with some group 15 13 elements.  Again, the crystal is still neutral but some shells would really like to have another electron.  These “holes” can easily drift from atom to atom and the crystal will now become a conductor.  This would be called a P type semiconductor and the current through it is called hole current.  Actually the electrons are filling those holes but the holes drift toward the negative connection of the crystal.

The real magic happens with a small chunk or chip of this crystal is doped with half of it doped as N type and the other half doped as P type.   This is called a PN junction.  This post is starting to become fairly long so that will be the next post.

This post was not an easy one for me to write, because it really is not the kind of details I am interested in and I had to do some review.  The reason for that is it is impossible for someone to have in their basement the necessary equipment to fabricate these devices.  However, I felt it is necessary to discuss the internal workings of these devices so we can understand the strange way they work when viewed from the outside.   The next post should quickly get to discussing an actual datasheet and from there we can go back to discussing the components we already know combined in some very useful circuits.



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The picture of the Periodic Table is from Wikipedia. The copyright holder has placed it in the public domain.


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