5.1 Introduction 5.2 Equilibrium condition Contact potential Equilibrium Fermi level Space charge at a junction 5.

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1 5.1 Itroductio 5.2 Equilibrium coditio Cotact otetial Equilibrium Fermi level Sace charge at a juctio 5.3 Forward- ad Reverse-biased juctios; steady state coditios Qualitative descritio of curret flow at a juctio Carrier ijectio Reverse bias 5.4 Reverse-bias breakdow Zeer breakdow Avalache breakdow Rectifiers The breakdow diode 1

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3 Reverse Biased Breakdow I forward curret br reverse sauratio curret reverse breakdow curret 3

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5 Zeer Breakdow W E C E C e tuelig E C E E d E C E E The Zeer effect: (1) Heavily doed juctio at equilibrium, (2) Reverse bias with electro tuelig from to. 5

6 For Zeer breakdow (tuelig) Exteds oly a very short distace W from each side of the juctio The metallurgical juctio be shar The doig high The tuelig distace d may be too large for areciable tuelig. However, d becomes smaller as the reverse bias is icreased, because the higher electric fields result i steeer sloes for the bad edges This assumes that the trasitio regio width W does ot icrease areciably with reverse bias For low voltages ad heavy doig o each side of the juctio, this is a good assumtio If Zeer breakdow does ot occur with reverse bias of a few volts, avalache breakdow will become domiat 6

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8 E C E e (a) h (b) Avalache Breakdow (c) e e E C E Electro-hole airs created by imact ioizatio. (a) Electro (rimary) gaiig kietic eergy i the field of the deletio regio. (b) A electro-hole air (secodary) is created by imact ioizatio. (c) The rimary electro losig most of its kietic eergy i the rocess. e h e e e h h e e e e A sigle ioizatio collisio evet Primary, secodary, ad tertiary collisios. 8

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10 Avalache breakdow Avalache Multilicatio I lightly doed juctios electro tuelig is egligible, the breakdow mechaism ivolves the imact ioizatio of host atoms by eergetic carriers. A sigle such iteractio results i carrier multilicatio. Aroximate aalysis result: out i (1 P P 2 P 3 ) (Assume o recombiatio) Electro multilicatio M out i 1 P P 2 P P A emirical relatio M 1 3 ~ 6 1 ( ) br 10

11 Probability of Ioizatio P 1 P 2 P 3 P 11

12 For Avalache breakdow Imact ioizatio rather tha field ioizatio (Zeer) Carrier multilicatio The eak electric field withi W icreases with icreased doig o the more lightly doed side of the juctio (from Eq. 5-23b ad Eq. 5-17) 12

13 For Avalache breakdow Imact ioizatio rather tha field ioizatio (Zeer) Carrier multilicatio The eak electric field withi W icreases with icreased doig o the more lightly doed side of the juctio (from Eq. 5-23b ad Eq. 5-17) 13

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15 Examle 5-4 A abrut Si - juctio (A=10-4 cm 2 ) has the followig roerties at 300 K: side side N a s cm 200cm / s N d s 450 cm cm 2 / s The juctio is forward biased by 0.5 v. What is the forward curret? What is the curret at a reverse bias of -0.5? 15

16 5.4.3 Rectifiers f o always I I I o I I I 16

17 Cosideratio for rectifier Juctio diodes desiged for use as rectifiers should have I- characteristics as close as ossible to that of the ideal diode. The reverse curret should be egligible The forward curret should exhibit little voltage deedece (egligible forward resistace R) The reverse breakdow voltage should be large The offset voltage E o i the forward directio should be small. Large badga small i Small reverse saturatio curret Oerable at high temerature Reduced thermal excited EHPs Icreased cotact otetial Icreased offset voltage E o 17

18 Cosideratio of doig cocetratio The doig cocetratio o each side of the juctio iflueces the avalache breakdow voltage, the cotact otetial, adthe series resistace of the diode. For + - juctio, the lightly doed regio determies may of the roerties of the juctio. High-resistivity regio should be used for at least oe side of the juctio to icrease the breakdow voltage br. However, this aroach teds to icrease the forward resistace R Cotribute to resistat heatig Coutermeasure make large area of the lightly doed regio The lightly doed regio of the juctio caot be made arbitrarily short for uch-through. 18

19 Avoid remature breakdow across the edge Bevelig W Guard rig 19

20 Cosideratio of structure I fabricatig a + - or + - juctio, It is commo to termiate the lightly doed regio with a heavily doed layer of the same tye for ohmic cotact to the device The result is a structure with the + - layer servig as the active juctio The lightly doed ceter regio determies the avalache breakdow voltage If this regio is short comared with the miority carrier diffusio legth, the excess carrier ijectio for large forward currets ca icrease the coductivity of the regio sigificatly This tye of coductivity modulatio, which reduces the forward resistace R, ca be very useful for high-curret devices O the other had, a short, lightly doed ceter regio ca also lead to uch-through uder reverse bias. W W 20

21 Cosideratio of Moutig of Rectifier Juctio The moutig of a rectifier juctio is critical to its ability to hadle ower. For diodes used i low-ower circuits, glass or lastic ecasulatio or a simle header moutig is adequate. For high curret devices, secial moutigs to trasfer thermal eergy away from the juctio I s required. A tyical Si ower rectifier is mouted o a molybdeum or tugste disk to match the thermal exasio roerties of the Si. This disk is fasteed to a large stud of coer or other thermally coductive material that ca be bolted to a heat sik. 21

22 5.4.4 The breakdow diode I Breakdow Tur-o Mechaisms for breakdow The Zeer effect (tuelig) breakdow mechaism (field ioizatio) is for abrut juctios with extremely heavy doig. The more commo breakdow is avalache (imact ioizatio), tyically of more lightly doed or graded juctios. Breakdow material destructio There is othig iheretly destructive about reverse breakdow. If curret is ot limited exterally, the juctio ca be damaged by excess reverse curret, through overheat. the destructio of the device is ot ecessarily due to mechaisms uique to reverse breakdow. 22

23 The voltage regulators I I s o t t s o Whe a diode is desiged for a secific breakdow voltage, it is called a breakdow diode. Such diodes are also called Zeer diodes, desite the fact that the actual breakdow mechaism is usually the avalache effect. Breakdow diodes ca be used as voltage regulators i circuits with varyig iuts. 23

24 oltage Dro across the Diode s i D R D R i D s R D s id D o I D s D s Load lie s R s R i D D br i D 24