Home Chapter 5 Silicon semiconductors
Silicon semiconductors

Joe Davis has also works with the very small and working with researchers at MIT he has created a micro hook made of silicon, which was designed to foster his love of fishing. This hook however is designed to hook a paramecium. Joe Davis is a pioneering artist who has done extensive research in bio-informatics and molecular biology and has produced complex genetic databases, which have allowed the production of unique biological art forms.

For this work, Davis collaborated with researchers at MIT, created a machine where humans can fish for the single-celled protozoan, paramecium.

 

 The nano-hook that was fashioned to hook a Paramecium.

 

Davis was able to use a lock out transformer a machine that was able to scale up the pull that a paramecium would exert on the hook. This attaches to a marlin fishing rod, which is a full size surfcasting rod and reel and theoretically allow the artist to fish for the paramecium. 

In fishing for a paramecium, Davis demonstrates that humans can reach into molecular-scale worlds and connect the macro-scale of humans to the scale of single-celled organism with silicon hooks. Here the work also alludes to the issues of telepistemology in probing and “fishing” at the microscale. [4]

So, how are semiconductors made? Are they unique?

Consider a story by the founder and chairman of Intel, a major producer of microchips, Gordon Moore:

We needed a substrate for our chip. So we looked at the substrate of the earth itself. It was mostly sand. So we used that. We needed a metal conductor for the wires and switches on the chip. We looked at all the metals in the earth and found aluminum was the most abundant. So we used that. We needed an insulator and we saw that the silicon in the sand mixed with the oxygen in the air to form silicon dioxide--a kind of glass. The perfect insulator to protect the chip. So we used that. [5]

While silicon is the most prevalent semiconducting material there is a great variety. Sand composes 27% of the earth’s crust, and is the primary ingredient in silicon.[6] However it must be refined to allow it’s atomic structure to be useful and to create that semiconducting state. The atomic structure of silicon gives it the properties of being a good conductor at times and a bad conductor at other times, depending on if valence electrons are available to support current flow.

The reason that silicon in its pure state displays this property is that its atoms have four valence electrons in their outer shell, which allow them to join with their neighbors. In the image below you will see a visual diagram in 2D of a cluster of 12 silicon atoms sharing their 4 valence electrons with atoms next to each other in the crystal lattice. Imagine that the diagram in below is a slice through the material to show you how the atoms and their electrons relate.

If you look at the model of the two center atoms notice how the green electrons circling the nucleus can be a part of these atoms or a part of the other atoms that are surrounding these atoms in the crystal lattice?

 

Cluster of 12 silicon atoms.

 

Adding the elements phosphorus and boron to the mix of silicon can add and subtract available electrons. This is called doping because a small amount of these elements is added to the crystal and this affects the electrical properties of that silicon crystal.

By adding phosphorus, which has five valence electrons, you end up with an extra electron in the mix. As phosphorus atoms have five valence electrons, the extra electron donated by the phosphorus atom supports current flow. Adding phosphorus creates a crystal lattice of the silicon that at times supports conduction and at other times does not depending on the voltages which are applied to the material.

We call this n-type silicon (negative type silicon) because the electrons will now outnumber the protons in the silicon mix. Notice in the image below you have a cluster of 12 atoms, 9 silicon atoms (blue and red nucleus) and 3 phosphorus atoms (blue and yellow nucleus) and you will notice the phosphorus atoms each have an extra electron.

 

Cluster of 12 atoms, 9 silicon atoms and 3 phosphorus atoms with phosphorus adding the extra electrons

 

On the other hand, when you add the element boron to the silicon mix the three electrons in the outer shell create a material that is short on electrons, or electron deficient.

We call this p-type, (positive-type silicon) because the protons in the nucleus now outnumber the electrons. Boron creates a vacant electron, which is called a hole. This electron vacancy makes it easy for another electron to fall into the hole. Many visualize the holes as bubbles in water. You can imagine these bubbles (areas of electron deficiency) propagating through a crystal lattice as bubbles floating through water. [6]

 

Cluster of 9 silicon atoms next to 3 boron atoms, which have an electron deficiency and do not easily support current flow.

 

Now you can visualize what is happening on a molecular level with electrons moving through semiconducting materials. Semiconducting materials are the backbone of our information culture as we know it and this ability to manipulate silicon by doping or adding small amounts of phosphorus and boron has transformed our culture.

An interesting side note is that the beginnings of life and multicellular animals occurred around the same time that phosphorus became available in the environment. Phosphorus plays a vital role in the biological molecules of DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) and functions as a kind of molecular backbone. It is also an important element in the cell protoplasm and for the functioning of nervous tissue. It is speculated that phosphorus came to the earth via meteorites. [7]