ECEN Microelectronics. Semiconductor Physics and P/N junctions 2/05/19

Transcription

1 ECEN 3250 Microelectroics Semicoductor Physics ad P/N juctios 2/05/19 Professor J. Gopiath Professor J. Gopiath Uiversity of Colorado at Boulder Microelectroics Sprig 2014

2 Overview Eergy bads Atomic eergy levels Atoms to molecules to solids Metals, isulators, ad semicoductors Bad structure Curret flow mechaism Carrier trasport ad cocetratio Drift ad diffusio Ohm s law

3 Periodic Table

4

5 Double slit experimet

6 Hydroge eergy levels

7 Hydroge atom lies Professor J. Gopiath Uiversity of Colorado at Boulder Microelectroics Sprig 2014

8 Formatio of Molecule Whe two hydroge atoms are brought close together, Stadig wave for two electros Molecular orbitals Two differet ways to form stadig waves Symmetric combiatio Bodig orbital Lower eergy Atisymmetric combiatio Atibodig orbital Higher eergy Kasap, Fig. 4.1

9 Formatio of Molecule Electros rearrage themselves i the ewly formed molecular eergy levels If the total eergy of electros i the molecule is lower tha the sum of atomic eergies Stable molecule For example, H 2 If ot, molecules are ustable ad the species ted to stay i atoms For example, He Kasap, Fig. 4.3 & 4.5

10 Formatio of Solid - Lithium Kasap, Fig. 4.8 As more ad more atoms are brought together, formatio of bodig ad ati-bodig orbitals cotiue. Evetually eergy bads are formed. I the case of Li, oe partially filled eergy bad is formed.

11 Formatio of Solid - Silico Neame, Fig. 3.4 Silico has 4 valece electros participatig i the bodig sp 3 hybridizatio Splittig of 3s ad 3p atomic orbitals overlap ad produce Two disjoit eergy bads separated by a gap.

12 Silico Crystal ad Eergy Bad Structures From Priciples of Electroic Materials ad Devices, Third Editio, S.O. Kasap ( McGraw-Hill, 2005)

13 Metals, Isulators & Semicoductors I metals, electros fill up to the middle of a bad. Partially filled bad. Good electrical coductors. I isulators, electros fill up to the bad gap. Completely full or completely empty bads. Poor electrical coductors.

14 Metals, Isulators & Semicoductors Rigorous defiitio of semicoductor: The solids that are isulators at T = 0 K but whose eergy badgap is of such a size that thermal excitatio leads to observable coductivity at temperatures below its meltig poit are called the semicoductors. E c T > 0K E v

15 Mobility Determied by collisios with lattice ad impurities

16 Coductivity Now use the ohm s law, J = σe, to fid the coductivity ( µ p ) σ = q + µ I extrisic semicoductors, oly oe of the two currets, J ad J p are importat, because of the large differece betwee ad p. p

17 ( T ) = Equilibrium Carrier Cocetratios 3/ 2 1/ 2 ( E EC ) [( E E )/ k T ] * 1 2m de E 2 2 c 2π exp + F B 1 See also Semicoductor Applet Service: 3.3. Carrier cocetratios Kasap, Fig. 5.7

18 Impurity Eergy Level Impurity with oe extra electro: door Impurity with oe less electro: acceptor Treat the impurity as additioal charge (+e for doors ad e for acceptors) distributed i a perfect crystal. Hydroge atom model Bidig eergy is drastically modified. Cosider, for example, a As door i Ge crystal. I free space, the bid eergy would have bee the first ioizatio eergy of As atom, 9.81 ev! I Ge crystal, the bidig eergy becomes ev Because of dielectric costat ad effective mass.

19 Doors ad Acceptors If the dopig desity is much greater tha itrisic carrier cocetratio, the total carrier cocetratio is roughly equal to the dopig desity. Coductivity is egieered by dopig.

20 Door ad Acceptor Eergy Levels Sze, p.21

21 P-N juctio formatio

22 P-N juctio at equilibrium

23 P-N juctio uder bias

24 Forward bias

25 E C Avalache 1 2 Reverse bias Zeer E V 2 1. Electro acceleratio 2. Impact ioizatio

26 Carrier Distributio uder Forward Bias Uder forward bias, carriers are ijected across the juctio Miority carrier cocetratio icreases expoetially. Ijected miority carriers the diffuse further ito the semicoductor Creatig a expoetially decayig fuctio = = T a A i T a po p p V V N V V x exp exp ) ( 2 = = T a D i T a o V V N V V p x p exp exp ) ( 2 + = p T a o o L x x V V p p x p exp 1 exp ) ( + + = p T a po po p L x x V V x exp 1 exp ) (

27 Curret i p Juctio Expoetially decayig carrier profile leads to expoetial diffusio curret. Curret i p juctio is miority carrier diffusio curret. = = p T a D i p p p p L x x V V N L qd dx dp qd x J exp 1 exp ) ( 2 + = p T a A i L x x V V N L qd x J exp 1 exp ) ( 2 = + = + = 1 exp 1 exp ) ( ) ( 2 T a o T a A D p p i p p t V V J V V N L D N L D q x J x J J total curret

28 Avalache Breakdow At large reverse bias, curret begis to icrease rapidly: breakdow No-destructive, reversible process Avalache breakdow & Zeer breakdow Avalache breakdow occurs whe electros gai high eough kietic eergy to kock electros out of valece bad impact ioizatio. E C E V Cosider a sigle electro eterig the depletio regio. Goes through oe impact ioizatio evet, geeratig a electro-hole pair. The two electros exit but the geerated hole travel i the opposite directio, 1. Electro acceleratio 2. Impact ioizatio Ad goes through aother impact ioizatio, This process repeats idefiitely breakdow.

29 Zeer Breakdow A differet kid of breakdow occurs whe the carriers gai eough eergy to tuel through the eergy barrier of the juctio. - Zeer breakdow. Zeer breakdow occurs whe the electric field is high eough to rip a covalet electro from a bodig, creatig a electro-hole pair. I eergy bad diagram, this process may be described as a electro i the valece bad i the p-side tuels through the eergy barrier at the juctio ad reaches the coductio bad i the -side. Whe dopat cocetratio is high, the depletio width is small ad the critical field required for avalache breakdow is high. I this case, Zeer breakdow domiates. 29

30 Juctio Breakdow Breakdow mechaism is distiguished by their temperature depedece. With icreasig temperature, tuelig curret icreases due to the icrease of iflux of valece bad electros. The impact ioizatio rate, however, decreases with icreasig temperature. This is due to the decrease of λ, the mea free path for phoo scatterig. The electros are more likely to lose eergy by phoo scatterig ad cosequetly less likely to acquire eough eergy to iitiate impact ioizatio. 30

31 Other P-N juctio devices LEDs Forward biased radiative recombiatio Use blue LEDs ad phosphor to make white

32 Light Emittig Diode (LED) Some semicoductor has very strog radiative recombiatio of carriers Uder forward bias, curret is coverted to light. Light emittig diode

33 Semicoductor Laser Whe LED is drive very hard, populatio iversio may be reached. Laser facets act as mirrors ad lead to laser oscillatio. Ijectio level is high populatio iversio stimulated emissio laser oscillatio. Vertical cavity surface emittig laser (VCSEL) Edge emittig LD 33

34 Semicoductor Laser Populatio iversio reached at very high ijectio level Output power icreases rapidly ad liewidth collapses. Fig 6.63 Fig 6.65 From Priciples of Electroic Materials ad Devices, Third Editio, S.O. Kasap ( McGraw-Hill, 2005)

35 Photodiode Photodiode: a reverse-biased p diode. (1) light absorptio e-h pair geeratio (2) E-field i the depletio regio sweeps carriers out of the depletio regio. geeratio curret Compact, solid-state detector High quatum efficiecy ad large badwidth Speed is determied by Carrier diffusio Juctio capacitace 35

36 Avalache Photodiode Whe reverse bias is large, the photodiode may experiece impact ioizatio ad avalache breakdow. Sigle electro geerated by a photo ca produce may electros ad holes through avalache breakdow process Produces gai Effective low light level detector

37 Solar Cell Solar cell is also a p- diode. Operatig mechaism is the same as the photodiode. Photogeerated curret (reverse curret) is used to power a load. Fig 6.49 From Priciples of Electroic Materials ad Devices, Third Editio, S.O. Kasap ( McGraw-Hill, 2005) 37

38 Curret i Solar Cell Short circuit curret = photogeerated curret Whe there is a load, photocurret produces a voltage drop Which is cosequetly imposed o the diode Ad therefore produces diode curret Total curret = diode curret - photocurret Fig 6.52 From Priciples of Electroic Materials ad Devices, Third Editio, S.O. Kasap ( McGraw-Hill, 2005)

39 Operatio of Solar Cell Solar cell I-V characteristic: Delivered power: P = V I Fig 6.53 From Priciples of Electroic Materials ad Devices, Third Editio, S.O. Kasap ( McGraw-Hill, 2005)

40 BJT Trasistor Actio Uder reverse bias, p- juctio exhibits little curret because the curret arises from the flow of miority carriers. Ca be ehaced if the miority carrier supply is somehow icreased. e.g photos or forward-biased p- juctio Cotrol of reverse-biased p- juctio curret with a earby forward-biased p- juctio: trasistor actio. Cosider a structure cosistig of two back-to-back p- juctios. Bipolar juctio trasistor (BJT) p bipolar juctio trasistor back-to-back p ad p juctios The, p, regios are called emitter, base ad collector. Applied bias V BE, V BC

41 Carrier Cocetratio equilibrium cut-off saturatio active mode

42 Trasistor Switchig Forward active bias: V BE > 0 ad V BC < 0. Reverse active bias: V BE < 0 ad V BC > 0. I a prototype device which is perfectly symmetrical, reverse active bias is idetical to forward active bias, except the reversal of curret. I real trasistors where the base is doped ouiformly ad emitter regio is much arrower tha collector, the curret behaviors uder the two bias coditios could be markedly differet. I the saturatio regio, both juctios are forward biased. This state represets the trasistor switched o. Uder this coditio, both juctios iject miority carriers ito the base regio ad at the same time both juctios are collectig electros as well.

43 Extra slides

44 Survey of Light Emittig Diode (LED)

45 Blue LED Why Blue? LED is a excellet light source foe its high efficiecy, fast respose time ad log device life. Compared to fluorescet ad icadescet lamps Solid-state light source Applicatios iclude: Traffic lights, color displays, lightig, etc. Efficiet AlGaAs red LEDs are available. Reasoably efficiet GaP gree LEDs are also available. But full color displays ad white light sources were uable to achieve due to the lack of blue LED. That was util the emergece of GaN blue LEDs. Furthermore, GaN-based blue-violet (405m) laser diodes ca dramatically improve the data storage capacity of CD ad DVD sice data desity is primarily limited the operatig wavelegth. Covetioal CD players use laser diodes operatig at 780 m. Blu-ray DVD

46 Applicatios of LED

47 LED Spectral Characteristics Emitted light color is determied by the badgap eergy, E g. Spectral width is determied by the broadeig of electro-hole distributio, which is typically ~3k B T. From Priciples of Electroic Materials ad Devices, Third Editio, S.O. Kasap ( McGraw-Hill, 2005) Fig 6.47

48 LED I-V, L-V Characteristics Light output is proportioal to the curret. I-V characteristics follow the typical p juctio behavior. Fig From6.48 Priciples of Electroic Materials ad Devices, Third Editio, S.O. Kasap ( McGraw-Hill, 2005)

49 Phosphor-Coverted White LED Yellow phosphors are coated o the surface of blue LED. - Yellow emissio by phosphors plus blue LED emissio produces white. Color ca be improved by havig two color phosphors excited with blue LED or three color phosphors excited by UV LED. IEEE Spectrum Aug. 2009, p.22