What We’ve Learned So Far A Review of Topics on Test 2

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 magnetoresistanced

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.

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

What Have We Learned About Semiconductors?  Can dope semiconductors to increase conductivity: p-type uses atoms with 3 valence electrons, so empty “hole”, n- type uses atoms with 5 valence electrons, so extra conduction electron  In p-n junction, conduction electrons travel from n side (and holes travel from p side) to p (n) side to combine with holes (electrons)  p-side becomes negatively charged; n-side becomes positively charged: potential difference  If put negative terminal on p-side, increases potential difference and get no current (reverse bias)  If put positive terminal on p-side, lower potential difference and get current

What Have We Learned About Semiconductor Devices?  When electrons and holes combine at p-n junction, excess energy emitted as light (LED)  energy of light depends on bandgap  fairly monochromatic wavelength emitted in all directions  Can pump enough electrons to conduction band to achieve population inversion, and could mirror surfaces to encourage stimulated emission: this produces semiconductor laser see applet for more info about laser operation: 3/F3s5p1.htm