Quantum Physics and Semiconductor Theory

I know you are cringing at the thought of reading about Quantum Physics.  Believe me, I do too.  Just be happy that you never had to crunch through Schrödinger’s equation or attempt to solve it for an H₂ molecule (hint: it is not possible).  HOWEVER, it is one of the fundamental features of nanotechnology.  You see, quantum physics governs the behavior of atoms and their component electrons, protons, and neutrons.  It goes deeper than this, but since nanotechnology deals with many atoms at once, we don’t need to deal with quarks and other fundamental particles.

Quantum Physics

The average diameter of an atom is 1 Angstrom (Å) or 10^-10 meters.  This is one order of magnitude smaller than a nanometer, which would essentially be the length of 10 atoms lined up next to each other.  The thing about quantum theory is that it makes very little sense when it comes to classical theory.  Atoms do not exist as single entities with defined states.  Instead, an atom behaves as a superposition of different possible states.  The traditional analogy is to consider a cat in a box with a vial of poison that is precariously balanced.  Classical thinking would consider that if you don’t look in the box, the cat is still either dead or alive.  Quantum thinking dictates that until you’ve opened the box and looked inside, the cat is BOTH dead and alive in a 50/50 mix of the two states.  That’s a bit funky when you think about cats being half-dead and half-alive, but quantum deals only with atoms and their associated spins (and many other defining properties) so a mix of states doesn’t seem that distasteful.  The moral of the story is that you actually change the state of the things you’re observing just by observing them while you are operating in the nanoscale regime.

While that’s one tenet of Quantum Mechanics, the other one is the fact that all atoms have discrete, or quantized, energy levels (hence the name, Quantum Physics).  This analogy is like walking up a flight of stairs.  You’ll gain potential energy as you rise in height, but you can never be in a position where you’ve stopped halfway between steps.  So when an electron gets excited from, lets say, an external voltage, it will ONLY change energy at specific intervals.

How does this all tie in?  These interesting quantum effects are only felt at the most minute levels.  As you rise in feature size from nanometers to micrometers (1000 nanometers), all of these neat quantum effects disappear and become bulk physical effects.  Nanotechnology focuses on devices with feature sizes below a certain threshold.  In my own studies, I’ve found that this threshold lies below the 20-30 nm range.  That’s when you start seeing values significantly off the bulk value.  By creating devices with these dimensions, you release a whole new set of rules that don’t come into play for devices like a microchip.  These can be both advantageous and disadvantageous.  While power use may go down, you might start losing electrons through barrier leakage, for instance.

Semiconductor Theory

An extension of quantum theory is vital to our understanding of modern electronics.  Any digital device would NOT be possible without proper knowledge of semiconductor materials.

There are three classes of electronic materials.  They are insulators, semiconductors, and conductors.   An insulator does not allow electricity to flow.  They are useful for isolating parts from each other.  A conductor allows electricity to flow easily.  They’re used for device interconnects and the transmission of data.  Semiconductors are somewhere between the two.  In some instances they may conduct, in other instances they won’t.  This element of control in a semiconductor means that it is inherently valuable to designing useful electronics.  Semiconductors allow devices to control the flow of electricity and hence the flow of information.  The height of semiconductor technology right now is the microchip processor. 

While it is certainly impossible to describe semiconductor theory in one paragraph, allow me to make an attempt: a semiconductor is inert in its natural state.  It will not conduct electricity until you’ve given it the right push.  The push comes in the form of applied voltage or doping with other chemicals.  The defining feature of a semiconductor is its ’band gap’, which is the potential difference between the inert state and the conducting state.  By putting several different semiconductors (with different band gaps) in sequence, you can create every known electronic device.

The band gap is an essential ingredient for two reasons: it acts as a barrier for conduction and also plays a key role in the emission of light.  When you send an electron into the conduction state, it takes a specific amount of energy. This puts the electron out of equilibrium with the system, and eventually it must return to its inert state.  On the return trip, it must give up this energy in the form of light.  Lasers (a very important electronic/photonic device) are a prime example of one use of this band gap.

Make no mistake: all semiconductor-based nanotechnology devices are related to the realm of nanoelectronics.  Some materials that only use quantum effects are not in the same class of materials.

While chemistry and materials engineering play a major role in defining the synthesis and physical properties (respectively) of nanotechnology, it’s quantum theory and semiconductor theory that define a device’s electronic properties.