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Transcript
SEMICONDUCTORS LECTURE 15 DIODES LECTURE#15 SEMICONDUCTORS ARE CRYSTALS THATHAVE THE PROPERTIES OF BOTH A CONDUCTOR AND A INSULATOR. SILICON (MOSTLY SAND, 27.75 OF EARTH’S CRUST) IS THE MOST COMMON. SILICON HAS 4 ELECTRONS IN ITS OUTER SHELL (VALENCE). IT WOULD LIKE AN ADDITONAL 4 FOR A OVERALL NEUTRAL CHARGE. IT DOES THIS BY SHARING ELECTRONS WITH OTHER SILICON ATOMS. VALENCE ELECTRONS YOU TUBE :Description of the Silicon Atom THIS STRUCTURE IS NOW A CRYSTAL http://www.youtube.com/watch?v=Wvd3hj93Iog PURIFIED SILICON PURE SILICON MUST HAVE TRACT AMOUNTS OF OTHER ELEMENTS ADDED TO IT TO MAKE IT A SEMICONDUCTOR. BORON AND PHOSPHORUS ARE THE MOST COMMON USED, THIS PROCESS IS CALLED DOPING. BORON HAS 3 ELECTRONS IN ITS OUTER SHELL WHEN ADDED TO SILICON THIS LEADS TO A DEFICIENCY OF 1 VALENCE ELECTRON (CALLED A HOLE) THIS GIVES THE SILICON A OVERALL POSITIVE CHARGE. THIS IS CALLED P TYPE SILICON. THESE HOLES CAN MOVE THRU THE CRYSTAL AND CARRY CURRENT. PHOSPHORUS HAS 5 ELECTRONS IN ITS OUTER SHELL WHEN ADDED TO SILICON THIS LEADS TO A EXCESS OF 1 VALENCE ELECTRON THIS GIVES THE SILICON A OVERALL NEGATIVE CHARGE. THIS IS CALLED N TYPE SILICON. THESE ELCTRONS CAN MOVE THRU THE CRYSTAL AND CARRY CURRENT. N TYPE ATOM P TYPE ATOM YOU TUBE : Semiconductors Theory 1 Segment 2A - Doped Silicon Crystal http://www.youtube.com/watch?v=cRvJouvITug Silicon Wafer Processing Animation http://www.youtube.com/watch?feature=endscreen&v=LWfCqpJzJYM&NR=1 THE DIODE IS AN ELECTRONIC ONE WAY VALVE BOTH P AND N TYPE SILICON CAN CONDUCT CURRENT. BY BUTTING P AND N TYPE SILICON AGAINST EACH OTHER CURRENT WILL FLOW IN ONE DIRCTION ONLY.THIS INTERFACE BEWTEEN THE TWO IS CALLED A P-N JUNCTION + - DIRECTION OF CURRENT FLOW (ENCLOSED IN SMALL GLASS CYLINDER) MAKE presents: The Diode http://www.youtube.com/watch?v=AqzYsuTRVRc At the junction, free electrons from the N-type material fill holes from the P-type material. This creates an insulating layer in the middle of the diode called the depletion zone. When the positive end of the circuit is hooked up to the N-type layer and the negative end is hooked up to the P-type layer, free electrons collect on one end of the diode and holes collect on the other. The depletion zone gets bigger. THIS DIODE IS REVERSE BIASED, NO CURRENT FLOWS BY PLACING A POTENTIAL DIFFERENCE (VOLTAGE) ACROSS THE DIODE WITH THE CORRECT POLARITY CURRENT WILL FLOW THRU THE DIODE. HOLES IN ONE DIRECTION,ELECTRONS IN THE OTHER DIRECTION When the negative end of the circuit is hooked up to the N-type layer and the positive end is hooked up to P-type layer, electrons and holes start moving and the depletion zone disappears. THIS DIODE IS FORWARD BIASED, CURRENT FLOWS .Formation and Properties of Junction Diode - Physics . http://www.youtube.com/watch?v=4SlfaocMfdA DIODE OPERATION A DIODE WILL NOT CONDUCT UNTIL THE FOEWARD VOLYAGE REACHES A CERTAIN LEVEL. FOR SICILON DIODES THIS IS ABOUT 0.6V. IF THE FORWARD CURRENT BECOMES TOO GREAT THE DIODE MAY CRACK OR MELT. AT THIS POINT THE DIODE MAY CONDUCT IN BOTH DIRECTIONS AND VAPORIZE THE CHIP. TOO MUCH REVERSE VOLTAGE WILL CAUSE IT TO CONDUCT IN THE WRONG DIRECTION WHICH WILL DESTROY THE CHIP. TYPES OF DIODES MANY TYPES EXIST SMALL SIGNAL: CONVERT AC TO DC, DETECT RADIO SIGNALS, MULTIPLY VOLTAGE, PERFORM LOGIC OPERATIONS. POWER RECTIFIER WORK AT MUCH HIGHER CURRENT LEVELS, CONSTRUCTED IN LARGE METAL PACKAGES TO ABSORBE HEAT AND TRANSFER IT TO HEAT SINKS. USED MAINLY IN POWER SUPPILES ZENER DIODES WORK AT SPECIFIC BREAKDOWN VOLTAGE. FUCTION AS A VOLTAGE SENSITIVE SWITCH. THIS VOLTAGE (VZ) VARY FROM 2 TO 200 VOLTS LIGHT EMITTING DIODE (LED) ALL DIODES EMIT SOME EM RADIATION WHEN FORWARD BIASED. DIODES MADE FROM CERTAIN SEMICODUCTORS EMIT LOTS OF LIGHT. THESE ARE CALLED LEDS LEDS CAN EMIT AND DETECT LIGHT YOU TUBE: MAKE presents: The LED http://www.youtube.com/watch?v=P3PDLsJQcGI (PHOTODIODES) PHOTODIODES ARE DESIGNED TO DETECT LIGHT. ( ALL PN JUNCTIONS ARE LIGHT SENSITIVE) THEY HAVE A WINDOW WHERE LIGHT ENTERS TO A LARGE EXPOSED JUNCTION REGION. SOME COMMON USES INCULDE CAMERAS, ALARMS, LIGHTWAVE DIODES (LASERS). PHOTODIODE SYMBOL PHOTODIODES CAN BE USED TO DETECT FAST PULSES OF INFRARED LIGHT, USED IN LIGHTWAVE COMMUNICATIONS how photodiode works http://www.youtube.com/watch?v=U6Wvmrc3akc HOW PHOTODIODES WORK A INCOMING PHOTON WILL CREATE A HOLE ELECTRON PAIR AT THE PN JUNCTION. A CURRENT WILL FLOW IF BOTH SIDES ARE CONNECTED. INCOMING PHOTONS HOLES + ELECTRONS The Photodiode http://www.youtube.com/watch?v=U6Wvmrc3akc TYPES OF PHOTODIODES SMALL AREA PHOTODIODES THESE HAVE VERY FAST RESPONS TIMES LARGE AREA PHOTODIODES HAVE SLOWER RESPONSE TIMES THEN SMALL AREA PHOTODIODES THESE LARGE SURFACE AREA GIVES HIGH SENSITIVITY. COMPOENTS OF A PHOTODIODE WINDOW METAL CAN LENS CHIP ANODE CATHODE DIFFERENT PACKAGE STYLES FOR PHOTODIODES CHARGE COUPLED DEVICES (CCD) Any CCD HOW DIODES ARE USED anode cathode anode cathode device symbol Silicon rectifier diode + + + _ 0 0 _ _ Input voltage circuit output voltage Input and output of a simple rectifier circuit Fig. 8-23 p.219 A single-phase half-wave rectifier circuit produces one output pulse of dc for each cycle of ac input. Power Diode Rectifier Half Wave Rectifier Circuit Half-wave Rectifier with Smoothing Capacitor Full Wave Rectifier Circuit The Diode Bridge Rectifier The Positive Half-cycle The Negative Half-cycle Full-wave Rectifier with Smoothing Capacitor Full Wave Bridge Rectifier http://www.youtube.com/watch?v=aCfAdIRRw7M VARIOUS TYPES OF POWER RECTIFIERS Various printed circuit board mounted rectifiers and voltage regulators