Electrons, Units, and Semiconductors

CS 441/641 Lecture, Dr. Lawlor

A Brief History of Computing, starting in 150 BC

Folks have been using physical devices to perform computations for a long time. 

Mechancial Devices
Of course, there are serious limitations to mechanical devices: it's hard to even turn a corner with a rotating axle, just like it's hard to make a leak-free joint with liquids.  One huge advantage of electronics is that wires are very easy to route, bend, and join.

Electromechancial Devices
Fully Electronic: Vacuum Tubes and Transistors
Integrated Circuits

Electrical Units

OK, so we're going to use electricity.  That means we need to know the units used.

Voltage (volts)
Current (amps)
Pressure (psi)
Flow rate (gpm) Accumulator Tank

Electron: it's a fundamental particle with negative charge (drawn as e-).  As far as we know, there's nothing inside an electron (other than... electron!).  Electrons are the charge carriers in most solid-state conductors.  For example, metals conduct electricity because they don't mind trading their spare "valence" electrons with their neighbors.

Current: one amp is a coulomb of moving electrons per second (6.24x1018 e-/sec) .  Typical microcontroller output signals are measured in milliamps; on-chip logic signals might only be a few picoamps.  Typical wall plug currents are up to a few dozen amps, and typical arc welding current is about a hundred amps.  A desktop PC CPU also typically uses about a hundred amps!

Voltage: measures how much electrons want to be somewhere, in volts.  Electrons will try to flow from a lower voltage to a higher voltage region.  We define the planet Earth's voltage as "ground", or zero volts, and connect to it with a metal rod stuck into the dirt (usually outside your electrical pole!).  Back in the 1980's, it was common to use 5 volts direct current (5vdc) to represent "true", and 0v as false; in the 1990's they switched to 3.3v == true; now most desktop CPUs use only about 1v == true internally, to save power.  For example:
Resistance: V = I R (volts = amps * ohm).  A 1 ohm resistor will lose 1 volt when conducting 1 amp of current; a 10 ohm resistor will drop 10 volts when conducting one amp.  Ohm's law (V=I R or volts = amps * ohms) is more of a guideline, an assumption of linearity that is only valid for "resistive" materials (OK for most metals; but poor for most insulators, liquids, or semiconductors).  The beautiful part about semiconductors is their resistance can be varied electronically (typically depending on nearby voltages).  For example:
Power: P = I V (watts = volts * amps, at least for DC circuits).  A 0.1 ohm piece of wire will drop 1 volt if you push 10 amps through it, which takes 10 watts: the wire might get fairly warm, but will still be there.  The same wire taking 100 amps will drop 10 volts, so must dissipate 1000 watts: the wire is going to feel some serious heat.  Generally:
For example, a current-signaling network might represent individual byte values as groups of 0 to 255 electrons.  255 electrons per sample at 100 million samples per second is 25.5x109 Ge-/s.  One Coulomb of charge is 6.24x1018 e-, so that's a current of 4x10-9 C/s, or 4 nano-amps.  At 1V signal strength, a one-ohm wire will lose 4 nano-volts.  Not much!  Of course, in practice we usually use voltage-signaled networks, and single electrons have a bad habit of obeying only the funky quantum laws instead of ordinary classical dynamics, which makes things much more complicated.

Semiconductors and the "Depletion Region"

Silicon doesn't conduct well: it's not a conductor like copper, or an insulator like plastic, it's a semiconductor.  Silicon doped with a few extra electrons ("n-type") conducts current because the electrons move.  Silicon doped with a few electron holes ("p-type") conducts current because the holes between electrons move.  Silicon without extra electrons or holes is a reasonably good insulator because it's "depleted" of electrons.

If you put p-type silicon next to n-type silicon, and apply a charge across the two types, something very interesting happens: the electrons and holes can move in opposite directions.  With the charge in one direction, both electrons and holes leave the boundary making an insulating "depletion region".  If you put the charge the other direction, the electrons and holes both converge on the boundary and cancel each other out, conducting current.  This is a semiconductor diode!   An analogy might be a static front in a war zone, like world war I: as long as soldiers from both sides keep flowing toward the front, they kill each other off and the war continues; if the soldiers of both sides start moving *away* from the front, the front is deserted, and the war's over!

Modern transistors are FET transistors: you charge up a small channel called the "gate", and that electrostatic charge pulls carriers into the depletion region, allowing current to conduct between two terminals.  With the gate uncharged, the depletion region insulates the two terminals, so no current flows.
MOSFET, showing big insulating depletion region

In an "n-channel FET", you attract electrons to the gate with a positive voltage, to narrow the depletion region and allow it to conduct.    In a "p-channel FET", you open the gate with a negative voltage.  I like Wikipedia's pictures for these: n-channel is a positive logic input, and p-channel is an inverting input with an inverting circle.