Transcript class15
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 q = 1.22 l tan q = y/D Capacitors store charge Q in proportion to the voltage V between the plates: C = Q/V = e0 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 a1 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: e d B dB iR A dt dt What Have We Learned About Magnetoresistance? Charges traveling through magnetic field experience magnetic force (provided velocity and field are not aligned): FB = 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/nf2 – 1/ ni2) The wavelength is given by l = 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 crosssectional area: R = rL/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 Conductors have a partially-filled valence band semiconductors have smaller band gap, insulators have larger band gap 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 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 Gate Drain Source n-type n-type p-type MOSFET The potential difference between drain and source is continually applied When the gate potential difference is applied, current flows Gate Drain Source 1 n-type p-type n-type 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. Output Input Switch Dump 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. 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 Output Input Switch Input Switch Dump If both switches are on, current is dumped. Otherwise the current goes to the output. 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 Output Switch Input Switch Input Switch Dump 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.