ECEN325: Electronics Spring 2017

Transcription

1 ECEN325: Elecronics Srg 207 Semiconducor n Juncion Diode Sam alermo Analog & Mixed-Signal Cener Texas A&M Universiy

2 Announcemens & eadg HW5 due Mar. 9 azavi Ch2 (oional) Basic semiconducor device hysics, which is useful o undersand how diodes work Covered more deail ECEN 370 azavi Ch3 Diode models and circuis 2

3 Agenda Semiconducor n juncion diodes Diode curren-volage (I-) characerisics Consan volage dro model Solvg circuis wih diodes Diode recifier circuis 3

4 Semiconducors A semiconducor is a maerial whose conduciviy lies somewhere beween an sulaor and a conducor Examle: ure or rsic Silicon (Si) has 4 valence elecrons and is no a very good conducor A semiconducor s conducive roeries can be changed by dog he maerial wih eiher n- ye doans (hoshorous) or -ye doans (Boron) A diode is formed a he boundary or juncion of a and n ye semiconducor 4

5 Semiconducor n Juncion Diode hysical Schemaic Inrsic Si -ye Si dog (Boron) dog (hoshorous) n-ye Si [Sedra/Smih] 5

6 Diffusion, Drif Curren, & Barrier olage [Sedra/Smih] Majoriy-Carrier Diffusion Curren, I D Caused by majoriy carriers diffusg o oher region Near he juncion, holes diffusg o he n-region recombe wih free elecrons, delee he carriers close o he juncion, and form a osiive charged region Similarly, elecrons diffusg o he -region recombe wih free holes, delee he carriers close o he juncion, and form a negaive charged region This charge searaion creaes a Barrier olage which limis he diffusion curren Moriy-Carrier Drif Curren, I S Caused by hermally generaed moriy carriers sweeg across he juncion due o he E-field 6

7 Oeraion w/ Differen Biases [Sedra/Smih] Oen Circui, I D =I S everse-biased, I S >I D, Weak Moriy Carrier Drif Curren Forward-Biased, I D >>I S, Srong Majoriy Carrier Diffusion Curren 7

8 I- Characerisic I d d n I T S e I S Sauraion Curren A [Sedra/Smih] T Thermal olage kt q T 25.9m a T 300K n Idealiy Facor - 2, Assume n if no given 8

9 everse Breakdown For large negaive volages, he revious exonenial equaion redics ha he reverse bias curren should saurae a I S However, wih a large negaive volage - Z he diode breaks down and a large negaive curren exiss Mos diodes should be designed o avoid his reverse-breakdown region [Sedra/Smih] Secial diodes, called Zener diodes, are design o oerae reverse breakdown and used alicaions such as volage regulaors 9

10 Consan-olage-Dro Model Used o simlify analysis If d < consan I d =0 (Oen Circui) If d > consan I d can go o, and d clams a consan (Baery w/ consan volage) We will assume consan = 0.7 [Sedra/Smih] 0

11 Solvg Circuis wih Diodes. A diode will eiher be on or off, resulg 2 ossibiliies for each diode he circui 2. Assume condiion and solve he circui 3. Check soluion for consisency wih he diode model 4. If i is consisen, he soluion is correc and you are done 5. If no consisen, you need o solve he circui wih anoher ossible condiion

12 Diode Circui Examle # Solve for ou and Id Firs assume ha he diode is OFF, i.e. an oen circui Are he diode I- condiions consisen wih he consanvolage-dro model? d =0 and I d =0A This is no consisen wih he diode model! We need o ry anoher diode condiion 2

13 Diode Circui Examle # (con.) Now assume ha he diode is ON, i.e. a 0.7 baery KC a OUT 0mA 5.35, Now, d =0.7 and I d =4.65mA This is consisen wih he diode model! This is he correc soluion OUT OUT 0.7 k I d k OUT 4.65mA 0 3

14 ecifier Circuis [Karsilayan] 4

15 Half-Wave ecifier [azavi] For eak ou s D, on Maximum everse olage 5

16 Half-Wave ecifier Transfer Characerisic [Karsilayan] [Sedra/Smih] Only recifies osiive half of he u signal ose one diode volage dro from he eak value 6

17 Half-Wave ecifier w/ a Filer Ca 7 [azavi] D on C D on ou D on C D on ou e e,, ,, 2 Maximum everse olage,, 3

18 How Much is he ile olage? [azavi] Also C A C e D, on ou 3 ou For a roerly designed filer : D, on T T T C D, on should be T e D, on e T C T C e wheret 3 C eak - o - eak ile olage D, on C f is he u eriod T C D, on T C 8

19 Wha is he eak Diode Curren? and () s cos s Usg cos cos eak value a reach a will This cos s s 0 To simlify his analysis,le's assume ha ,, ou C D D on ou D on C I C I C C I x x x x C I C d d C I I I [azavi]

20 Full-Wave ecifier [Karsilayan] osiive ½ cycle To diode on Negaive ½ cycle Boom diode on [Sedra/Smih] Maximum everse olage 2, D on 20

21 Full-Wave ecifier Transfer Characerisic [Karsilayan] [Sedra/Smih] ecifies all of he u signal ose one diode volage dro from he eak value 2

22 Bridge ecifier s-0.7 [Karsilayan] s-0.7 osiive ½ cycle [Sedra/Smih] D & D3 on Negaive ½ cycle D2 & D4 on Maximum everse olage D, on 22

23 Bridge ecifier Transfer Characerisic [Karsilayan] ecifies all of he u signal ose wo diode volage dros from he eak value 23

24 Full-Wave & Bridge ecifier w/ a Filer Ca [Sedra/Smih] The caacior only discharges for T/2 esuls ½ Ca size for a given rile oughly ½ diode curren due o smaller Ca 2 2 C D, on f I 2 C 24

25 ecifier Trade-Offs Half-Wave ecifier + Simles design wih fewes comonens - equires larges caacior for a given rile Full-Wave ecifier + educes caacior size by ½ relaive o half-wave - equires cener-aed ransformer - Maximum reverse volage almos double ha of half-wave Bridge ecifier + educes caacior size by ½ relaive o half-wave + Save maximum reverse volage as half-wave recifier - ose wo diode volage dros eak value 25