1 Day 3: Finish electric circuits Photocopiers: Bloomfield 10.2 Photoconductors (Blm 12.1 p ). Reminders/Updates: HW 1 due NOW! Phys 1010 notes on website All info is on web (I think) Photocopiers

2 Static electricity and semiconductors Learning goals 1. Explain how you could write on paper with photoconductor, toner, light, high voltage wire. (basic design of copier) 2. Be able to explain why materials are conductors, insulators, or semiconductors in terms of energy levels and electron motion. 3. Explain how heat or light changes resistance of semiconductor. Big picture: 1. Coulomb attraction of positively charged ink particles (toner) to negatively charged surface. 2. Negative charges on surface distributed to match image to be copied. 3. Control charge distribution by shining light on surface, where light hits, negative charges leave, so no ink will stick. The big challenge- how to get charges to leave when light shines on surface?

3 Chester Carlson says: there must be a way to copy things! –Recopying and photography too expensive and time-consuming Chester Carlson produces first xerographic image in his lab in Astoria, Queens ASTORIA. –Materials: Sulfer, handkerchief, bright light, spoors from a club moss: lycopodium powder Wax paper and heat Photocopiers – a brief history Turned down by 20 companies 1959 Launches the Xerox 914, the first automatic, plain-paper office copier--which becomes the top-selling industrial product of all time. – –

4 “Photocopying” - step 1: Overhead Transparencies (insulator) When I drag the copper comb across the transparencies: a.Negative charges flow through the transparencies and into the floor, b.Nothing. c.Negative charges will build up on the transparencies just in the areas where the comb touches d.Negative charges will be everywhere on the transparencies, even the parts I don’t touch with the comb Copper electron comb

5 “Photocopying” step 2: a.Nothing will happen b.The entire plate will lose it’s negative charge. c.Only the areas of the plate that I touch will lose their negative charge What will happen if I draw on the plate with my finger?

6 “Photocopying” step 3: Toner sticking to charged surface Chalk only sticks to places with negative charge because the chalk is a.positively charged, b.negatively charged, c.uncharged (neutral), d.both a and c are correct e.both b and c are correct HI

Photocopying expts - summary We have demonstrated some important steps inside a photocopier: Charging an insulating sheet, discharge certain areas –Make a charge image Apply chalk/toner –Electrostatic attraction produces visible image But where does light (the ‘photo’ bit) come in?

8 Photocopying 1: Making a Charge Image Photoconductor Grounded metal Document Light reflected off document Charge image Corona wire velocity Charge photoconductor Selective discharge

9 Photocopying 2: Transferring Charge Image to Toner and Paper Black image Release toner Light Charge image Positive toner particle toner Roller and brush Attract toner to charge image

10 Straightforward. All just good old physics of attraction between opposite charges, plus photoconductor physics Charge Paper Toner attracted to paper Heat Photocopying 2: Transferring Charge Image to Toner and Paper Copy

V Photoconductor at heart of photocopier: - Behaves like an insulator when in the dark - Where light hits, R low, electrons flow away, - When add toner, sticks only where charges are left. Photoconductor: - Very special material – insulator except when light hits. - One type of semiconductor First have to understand what determines resistance of a material: a) insulators (wood, ceramic, plastic)- very high resistance. b) conductors (metals)- very low resistance c) Semiconductors - in the middle. Resistance depends on temp., light, cleanliness. Semi-conductor physics

What determines resistance of a material - Charged particles (almost always electrons) carry current inside materials - Resistance of material depends on a) Number of charged particles that are free to move and carry current b) Number of obstacles that charge carriers might bump into Can be controlled in a semiconductor At the heart of all modern electronics!

Atomic structure Nucleus - Protons and neutrons - Positively charged - Very small and dense Electron cloud - Negatively charged - MUCH larger than nucleus Quantum Mechanics - Weird physics of very small things (like electrons) - Electrons behave as both particles and waves - As waves, they can only move in certain ways and have certain amounts of energy - Only 2 electrons per energy level (Pauli Exclusion Principle) particle wave

14 Energy Electron energy levels get shifted and shared between all atoms and electrons Atomic structure of solids and energy bands one atom many atoms Discreet energy levels for electrons

15 Levels get shifted and shared for all atoms and electrons So many individual levels, just talk about bands of levels. 2 electrons per level until run out of electrons Lower energy levels – mostly full of electrons Higher energy levels are empty In solid, billions of atoms, electrons, and energy levels!! Electron energy

16 empty full Insulator: Big jump to empties. Can’t move without big boost. Electron like a ball in pit. gap- no levels Conductor: empty levels very close moves easily electron like ball rolling on almost flat ground empty Microscopic look at different materials Semiconductor: Half way in between. Small jump to empties empty full - Electron like ball in shallow pit. - Small boost required to move. Small gap full Conduction rule: For electrons to move (when a voltage is applied) there must be an empty energy level immediately above them

17 empty full Energy x y z w 25 eV 0 Which band structure goes with which material? D=Diamond C=Copper G=Germanium (semi-conductor) a. D=w, C=x, G=y b. D=z, C=w, G=y c. D=z, C=y, G=x d. D=y, C= w, G=y. e. D=w, C=x, G=y

18 empty full Review of semiconductors. Little gap Small gap between filled and empty energy levels. Gap is big enough to stop current (high R). Gap is small enough so humans can find ways to boost electrons up. Concept behind all modern electronics! Use semiconductors to control electric currents. Q: Which will decrease resistance of a semiconductor? add heat, light, shake, add dirt a. T T T T b T T F F c T T F T d F T F F e F T F T Like ball stuck in shallow pit e

19 empty full Photo-conductor: Kick electron to empty levels with light E = energy of photon = h x frequency (h ) = h x c / wavelength (hc/ ) Q: If green light has just enough energy to excite electron into empty level and allow it to move (conduct electricity) a. both blue and red light will also make it a conductor b. blue light will, red light will not c. neither blue nor red light will make it into a conductor Little gap e

Photocopier summary Spread charge on photoconductor Use light to selectively remove some charge and make a charge image of original Electrostatic attraction sticks toner to regions where charge remains Use heat to bond toner to new piece of paper Copying is complete! We will return to semi-conductors and their use in electronics in a couple of weeks