How does a transistor work?

Transcription

1 Semiconductor devices Semiconductors (review) Doping Diodes (pn junctions) Transistors Day 12: Diodes Transistors TV Reminders/Updates: Useful reading : Audio amps: 12.1, 12.2 TV: (all sections EXCEPT laptop screens) Labs this week HW due tomorrow (Friday) at 5pm Cookies Doped semiconductors Doping and conduction. Pure semiconductor e.g. Si semiconductor Si tiny fraction of B semiconductor Si tiny fraction of P Pure semiconductor e.g. Si semiconductor Si tiny fraction of B semiconductor Si tiny fraction of P Si: 4 e Just fill valance band B: 3 e 1 unfilled level in VB Tiny fraction of total (part in a million) a. all about same. b. pure best, P and N less, P: 5 e c. pure no, P and N about same conduct OK but not great 4 electrons fill VB d. only N conducts 1 e goes up to CB e. only P cond. Tiny fraction of total 4 3 How will they conduct? Pure Doping and conduction. Doped semiconductors Add or subtract some electrons from the solid to allow charges to move and conduct electricity NOTE: Doping also adds or subtracts protons, leaving material electrically neutral. No e can move e in VB can move (often think of holes moving) e in CB can move extra free electrons move easily in nearly conduction band e e e e lower level Pure no flow when tilted water in bottom flows, See bubble (hole) moving water in top flows missing free electrons or holes allow electrons to move in valance band Upper level 6 1

2 The diode: P N junction electrons and holes diffuse across border and annihilate depletion layer No moveable charges Remaining fixed charges create E field E field prevents further diffusion across border a. electrons flow from left to right. Group Q Hint: apply two principles: 1. are moveable charges (not excess). Where do they move? 2. Need to have movable charges through entire region to conduct. 8 Battery has to be bigger than 0.6 V to overcome 9 Battery has to be bigger than 0.6 V to overcome 10 Turn on opposite, what happens? Battery has to be bigger than 0.6 V to overcome 11 a. electrons flow from left to right. Hint: apply two principles: 1. are moveable charges (not excess). Where do they move? 2. Need to have movable charges through entire region to conduct. 12 2

3 Turn on opposite, what happens? A useful simulation for diodes and doping I Pulled away from junction Depletion region (no moveable charges) grows Current can t flow through depletion region Reverse biasing Diode summary Diodes create a oneway street for current flow V > 0.6 V in one direction, big current. V in opposite direction, no current. Diodes, or PN junctions are used in a whole load of devices: Light Emitting Diodes LED Rectifying (converting AC to DC current) in power cord for all battery powered devices LEDs Laser diodes Photodiodes make electronic image in your digital camera movement sensors Solar (or photovoltaic) cells Photon I have a diode (PN junction) made from Silicon that emits invisible IR photons. I would like to make a green LED for my Christmas tree. What do I do? I have a diode (PN junction) made from Silicon that emits invisible IR photons. I would like to make a green LED for my Christmas tree. What do I do? d) Build an LED using a different SC material that has a bigger bandgap a) Connect a battery with a higher across the Si diode b) Connect a battery with a smaller across the Si diode c) Operate the Si diode at a higher temperature d) Build an LED using a different SC material that has a bigger bandgap e) Build an LED using a different SC material that has a smaller bandgap Silicon LED Small Gallium Phosphide LED Larger IR Small band gap Low energy (IR) photons Larger band gap Higher energy green photons 17 Photon energy (color) = Electron energy jump = Band gap of SC 3

4 transistor Electron pipe Semiconductors (review) Doping Diodes (pn junctions) Transistors The transistor is a valve for current! Changing signal at Gate like turning knob on faucet changes current through transistor Advantages of LED bulbs: High efficiency/low power consumption Much less heat generated Long lifetime Robust Can be made very small A few LED uses: Bike lights Remote controls (IR) Traffic lights and signs Decorative and functional lighting 19 Gate Gate ( knob is the amount of ve charge at the ) e s Different designs and alphabet soup of names. We will consider the nchannel MOSFET Made of N and Pdoped semiconductors Consists of 2 diodes (PN junctions) backtoback Depletion region controls current flow A separate signal (e.g. CD signal) controls size of this depletion region. We can adjust it up and down, opening and closing the current valve and thus vary how much current flows through the transistor Metal Oxide Semiconductor N P N Depletion region 22 0V V N P N N P N Channel Source Zero V on Large depletion zone fills ptype region, prevents current flow Valve closed, no current flows 23 Positive V on Mobile electrons attracted to positive charge on, creating a conducting path or channel between source and drain Valve partially open, some current flows Width and hence resistance of conducting channel determined by. 24 4

5 N P N V More positive V on Valve open further (conducting channel is wider) so more current flows 25 5