Review for Exam 2 Spring, 2002

Charges in Conductors  Electric fields are created when positive charges and negative charges are separated  A uniform electric field existing over a region sets up a potential difference between points in that region:  V=E  x, where  x is the distance along a field line.  If I apply a potential difference across a conducting object (including semiconductors), charges experience a force, and charge carriers will flow until the potential difference is removed.

What Have We Learned About Electrical Storage The electric force F E on a charge q 0 can be considered due to an electric field which is produced by other charges in the area F E = q 0 E If moving a charge between two points requires work (or does work), the charge gains (or loses) potential energy:  V = –  E  dx = (for a constant field) E  x Capacitors store charge Q in proportion to the voltage V between the plates: C = Q/V = C =  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 = (if v perpendicular to B) qvB 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

Stuff to remember about GMR Electrons (and other elementary “particles”) have intrinsic magnetic fields, identified by spin The scattering of electrons in a ferromagnetic material depends on the spin of the electrons Layers of ferromagnetic material with alternating directions of magnetization exhibit maximum resistance In presence of magnetic field, all layers align and resistance is minimized

What Have We Learned About Spectra? ENERGY LEVELS ARE QUANTIZED Different elements have different allowed energies (since different numbers of protons and electrons provide different structure of attraction Light emitted when electrons move from a high energy level to a lower energy level in an atom will have only certain, QUANTIZED, allowed energies and wavelengths. Those wavelengths depend solely on the element emitting the light and compose the characteristic emission spectrum for that element

Our Model of the Atom If the atom is in the “ground state” of lowest energy, electrons fill the states in the lowest available energy levels. The first shell has two possible states, and the second shell has eight possible states. Higher shells have more states, but we’ll represent them with the eight states in the first two sub-shells. Electrons in the outermost shell are called “valence” electrons. We’ll make them green to distinguish from e - in filled shells E=0 (unbound) n=1 n=2 n=3 n=4 Really eight distinct states with closely spaced energies, since two electrons cannot occupy the same state.

Electrons in Solids The shifted energies in adjacent atoms combine to create a continuous “band” of allowed energies for each original energy level; each band, however, has a finite number of states equal to the number in original atoms Electrons can move from the locality of one atom to the next only if an energy state is available within the same band

Conductors & Semiconductors In conductors, the valence band is only partially- full, so electrons can easily move from being near one atom to being near anotherconductors In semiconductors and insulators, the valence band is completely full, so electrons must gain extra energy to movesemiconductors In semiconductors, extra electrons (or holes) can be introduced in a “controlled” way.

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 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

p-n junction n-type p-type Energy depleted region (electric field) VoVo

Wide Bandgap Semiconductors What is a wide bandgap semiconductor? Larger energy gap allows higher power and temperature operation and the generation of more energetic (i.e. blue) photons Traditional wide bandgap semiconductors include AlAs/GaAs, SiC The III-nitrides (AlN, GaN and InN) have recently become feasible.

Impact Automotive industry Avionics and defense Traffic lights Solid state lighting Electric power industry Health care Information technology (data storage) Wireless communications Displays

Heterojunctions