Qualitative Picture of the Ideal Diode. G.R. Tynan UC San Diego MAE 119 Lecture Notes
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1 Qualitative Picture of the Ideal Diode G.R. Tynan UC San Diego MAE 119 Lecture Notes
2 Band Theory of Solids: From Single Attoms to Solid Crystals Isolated Li atom (conducting metal) Has well-defined, isolated allowable electron energy levels N isolated atoms N x isolated atom levels Strongly interacting Li atoms Interaction shifts (or splits) individual energy bands into isolated regions separated by forbidden bands
3 Next, consider N interacting sodium atoms at 0 deg K Electrons in config 1s 2 2s 2 2p 6 3s 1 Band Theory of Solids: Shells filled to 3s, which has 1 electron Conductors
4 Band Theory of Solids: Conductors Next, consider N interacting sodium atoms w/ T>0 Electrons in config 1s 2 2s 2 2p 6 3s 1 Shells filled to 3s, which has 1 electron This Valence electron is weakly bound =>> if T High enough can move to mobile state è conductor! T=0 K T > 0 K
5 Band Theory of Solids: Insulators Lower levels: VALENCE BAND Upper levels: CONDUCTION BAND Carbon in Diamond Form Electrons in 1s 2 2s 2 2p 2 State 2p band has 2N electrons, but 6N states BUT crystal structure splits 2p into two distinct bands BAND GAP is ~6 ev >> Temperature (~ ev) Thus Diamond is An Insulator
6 Band Theory of Solids: Some crystalline materials have smaller band-gap energy At low temperatures behave like insulators E bg ~1eV >> Temperature With an electric field Electrons gain energy Can move into upper (conduction) band Semiconductors
7 Si as a Semiconductor Material 7
8 N-type Semiconductor Materials N-type Si has an extra electron for each dopant atom, This electron is mobile 8
9 P-type Semiconductor Materials P-type Si has a hole (i.e a missing electron) that acts like A mobile positive charge 9
10 Dopants create allowed energy states between the pure material valence and conduction bands Pure semiconductor matl s conduction and valence bands separated by E gap In pure materials this gap has no allowed states -> no particles in these energy ranges IF ADD donor or acceptor impurities then this creates allowed states between the pure-material conduction & valence bands
11 Basics of Solar PV Cells Key Concepts Un-illuminated p-n junction diode Photon Energy Spectrum Charge Carrier Generation Via Photon Absorption Charge Carrier Loss Mechanisms Illuminated p-n junction diode: The Solar PV Cell Solar PV Cell s as an Electricity Source
12 Basics of Solar PV Cells Key Concepts Un-illuminated p-n junction diode Photon Energy Spectrum Charge Carrier Generation Via Photon Absorption Charge Carrier Loss Mechanisms Illuminated p-n junction diode: The Solar PV Cell Solar PV Cell s as an Electricity Source
13 A Solar PV Cell is just a p-n junction ( diode ) illuminated by light. Photon Flux with E>Egap Examine This p-n Junction P-type n-type
14 Simplified Model of PV Cell: No Light
15 Conduction Band & Valence Band Energy Levels in SEPARATE p-type and n-type materials E F E F
16 Conduction Band & Valence Band Energy Levels in p-n junction THERE CAN ONLY BE ONE FERMI ENERGY IN A SYSTEM AT EQUIL (!) Result:
17 P-N Junction formed by joining p & n type materials Concentration gradient of n and p densities leads to diffusion of mobile electrons into the p-region, and mobile holes into the n-type region. This leaves behind immobile positive donor ions in n- region (N D+ ) and negative acceptor (N A- ) ions in p- region P r o c e s s s t o p s w h e n sufficiently large potential gradient develops 17
18 Equilbrium potential develops across the p-n junction Get an electric field naturally occuring across junction 18
19 Now connect p and n regions via external circuit 19
20 Connecting w/ext. circuit and adding an external bias that ADDS to natural bias ( Reverse bias) widens depletion zone 20
21 Connecting w/ext. circuit and adding an external bias that ADDS to natural bias ( Reverse bias) widens depletion zone Uncover more un-neutralized N D+ ions Uncover more un-neutralized N A- ions 21
22 Current-voltage response for reverse bias 22
23 Current-voltage response for reverse bias Hard for current to flow 23
24 Canceling out natural bias (i.e. Forward Bias ) Causes current to flow! Forward bias reduces width of depletion zone & injects minority carriers (i.e. holes in N-region, Electrons in P-region) which can then diffuse thru that zone 24
25 Current-voltage response forward bias When forward bias voltage reaches or exceeds the Natural bias of the p-n junction, large current can Begin to flow 25
26 Current-voltage response forward bias EASY for current to flow When forward bias voltage reaches or exceeds the natural bias of the p-n junction, large current begins to flow 26
27 Diode current-voltage characteristics 27
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