Putting Electrons to Work Doping and Semiconductor Devices

What Have We Learned About Optical and Electric Storage?  Laser light is focused through a (circular) lens onto the surface of a CD  The central max of the diffraction pattern must be no larger than one bit if data is to be resolved d sin  = 1.22 tan  = y/D  Capacitors store charge Q in proportion to the voltage V between the plates: C = Q/V =  0 A/d  Capacitors are used in RAM

What Have We Learned About Magnetic Storage?  Two domains magnetized in same direction is a 0  Two domains magnetized in opposite directions is a 1  Direction of magnetization changes at start of new bit.  Magnetic data is written by running a current through a loop of wire near the disk  As magnetic data passes by coil of wire, changing field induces currents according to Faraday’s Law:

What Have We Learned About Magnetoresistance?  Charges traveling through magnetic field experience magnetic force (provided velocity and field are not aligned): F B = qv x B  In a current-carrying wire, this force results in more frequent collisions and thus an increased resistance: Magnetoresistance  Electrons traveling through magnetized material undergo spin-dependent scattering  When magnetic field is present in magnetic superlattice, scattering of electrons is cut dramatically, greatly decreasing resistance: Giant magnetoresistance

What Have We Learned About Atoms?  ENERGY IS QUANTIZED  Electrons can absorb energy and move to a higher level; they can emit light and move to a lower level  In hydrogen the emitted light will have energy E = (13.6 ev)(1/n f 2 – 1/ n i 2 )  The wavelength is given by = hc/E = 1240(nm eV)/E  Energy levels of nearby atoms are slightly shifted from each other, producing bands of allowed energies  Electrons move from the locality of one atom to the next only if an energy state is available within the same band

What have we learned about Resistance?  In many, ohmic, materials, current is proportional to voltage: V = iR  Resistance is proportional to the length of an object and inversely proportional to cross- sectional area: R =  L/A  The constant of proportionality here is called the resistivity. It is a function of material and temperature.

A Good Analogy to Remember

What Have We Learned About Solids?  In conductors, the valence band is only partially-full, so electrons can easily move  In semiconductors and insulators, the valence band is completely full, so electrons must gain extra energy to move  semiconductors have smaller band gap, insulators have larger band gap  Conductors have a partially-filled valence band  The primary effect of higher temperature on resistance is to increase R due to more collisions at higher temperatures  Semiconductors have a completely-filled valence band  The primary effect of temperature on resistance is due to this requirement: the higher the temperature, the more conduction electrons

N-type semiconductors  N-type semiconductor is doped with a material having extra valance electrons  Result is filled energy states in the band gap just below the conduction band  These electrons can easily gain energy to jump to the conduction band and move through the material

P-type semiconductors  P-type semiconductor is doped with a material having fewer valance electrons  Result is “holes”, or empty energy states in the band gap just above the valance band  Since no single electron travels through the material, we describe the charge carrier as a positive hole moving the other way

P-n junction  Originally both p and n sides are electrically neutral  Electrons in n side see holes in p side and combine Second electron needs add’l energy to get over charge barrier – can represent as rise in energy levels of p section

P-n junction  As more electrons from the n-side combine with holes from the p-side, each additional combination adds to the potential difference across junction  This can be envisioned as shifting the energy bands, making it harder for electrons to travel across the barrier

Forward Biasing  Eventually, the potential difference is so large, electrons cannot travel across it without gaining energy  Applying a forward bias decreases the potential difference so current can flow

Reverse Biasing  Applying a reverse bias will increase the barrier rather than decreasing it, so no current flows

Light-emitting Diode  When an electron loses energy to recombine with a hole, it can emit that lost energy in the form of light.  This light always has roughly same E, so LEDs emit small range of wavelengths  This light-emitting property of p-n junctions can be utilized to create a laser  Be sure to come to class to hear Dr. Schowalter say...

Do Today’s Activity  How is an incandescent light bulb different from an LED?  What is the difference between the different colors of LED?  Why might these differences occur?

npn junction  Put another n-type semiconductor on the other side of the p-type semiconductor  No matter which way I apply potential difference, one p-n junction is reverse biased, and electrons entering the p-type region quickly combine with holes, creating more negative charge

MOSFET  If, however, I apply a positive potential to one side of the p-type semiconductor, without allowing another path for electrons to flow out of the device, I will create a channel for e - to get from one n-side to the other. n-type p-type

MOSFET  Now, if I bias the device in either direction, current will flow, electrons toward the positive potential, and conventional positive current toward the negative potential n-type p-type Source Drain Gate

MOSFET  The potential difference between drain and source is continually applied  When the gate potential difference is applied, current flows n-type p-type 1 Source Drain Gate

How do transistors fit in? From How Computers Work, by Ron White

How do transistors fit in?  For now, view transistor as switch:  If switch is “on,” current can pass  If switch is “off,” no current can pass  We can use this simple device to construct complicated functions

NOT Gate - the simplest case  Put an alternate path (output) before a switch.  If the switch is off, the current goes through the alternate path and is output.  If the switch is on, no current goes through the alternate path.  So the gate output is on if the switch is off and off if the switch is on. Output Dump Input Switch

NAND - a variation on a theme  NAND gate returns a signal unless both of its two inputs are on.  Put an extra switch after a NOT device  If both switches are on, current is dumped.  Otherwise the current goes to the output. Output Dump Input Switch

AND - slightly more complicated  AND gate returns a signal only if both of its two inputs are on.  Use the NAND output as input for NOT  If both inputs are on, the NOT input is off, so the AND output is on.  Else the NOT input is on, so the output is off. Dump Input Switch Output