P-N Junction in equilibrium. Diode. Diode bias. Forward and reverse bias. Diode current-voltage characteristics. Models. Applications.

Transcription

1 Unit 12: Semiconducto devices. Diode. P-N Junction in equilibium. Diode. Diode bias. Fowad and evese bias. Diode cuent-voltage chaacteistics. Models. Applications.

2 PN junction in equilibium Electons Holes P N At oom tempeatue, holes on p aea go by diffusion towadsn aea, and e - fomnaea coss to p aea (majoity caies cuents) K K J pdif J ndif X p 0 E Diffusion Cuents of majoity caies X n On junction aea, holes and e - ae ecombined, appeaing a naow depletion aea (without e - and holes) with a chage density due to the ions of the impuities, negative on p aea and positive onnaea. J pdift J ndift Dift Cuents of minoity caies So, an electic field appeas, flowing dift cuents of minoity caies (e - fom p to n aea, and holes fom n to p aea), canceling diffusion cuents.

3 PN junction in equilibium. Not biased junction. p 0 E n x p x n J dift J dift + J dif = 0 J dif On not biased p-n junction (o in equilibium), diffusion cuents of majoity caies ae cancelled by dift cuents of minoity caies. A P-N junction is a DODE

4 PN junction in equilibium. Featues. p p p0 N A n p0 X p -qn Ā ρ 0 Xp Xn n E 0 qn D + n n0 N D X n p n0 Chage caies density distibution. Chage distibution

5 PN junction in equilibium. Featues. X p Xp E 0 E Xn X n 0 Electic field on pn junction Dop of potential known as Built-in potencial X p X n 0 = 0.3 fo Ge diodes 0 = 0.7 fo Si diodes at 20 ºC

6 Diode bias. Fowad bias p E n J J Jdift des J J dif D D ceates an electic field opposite to the field on the depletion aea, being lowe E total and the dop of potential on the junction: = 0 - D. So, diffusion majoity cuent is inceased, and dift minoity cuent is deceased.

7 Diode bias. Fowad bias. p E n J dift J dif J D > 0 f D > 0, diffusion and dift cuents have same diection and cuent can be highe. Thee isn t opposition fo flowing of cuent.

8 Diode bias. Revese bias. p E n J dift J des J J J dif dif R 0 <<<< R ceates an electic field einfocing the field in the depletion aea, inceasing the dop of potential: = 0 + R. Depletion aea enlages. So, the diffusion cuent of majoitay caies deceases (holes fom p to n aea and e- fom n to p), and the dift cuent of minoitay caies should incease (e - fom p to n aea and holes fom n to p). But thee ae only few minoitay caies availables (geneated by themic geneation), and this cuent 0 is vey small, and it s called REERSE SATURATON CURRENT.

9 Diode cuent-voltage chaacteistic. 0 : Revese satuation cuent u : Diode fowad voltage dop 0,15 m Symbol fo diode: (ma) 0,05 p aea Anode n aea Cathode o 0 < µa u -0, (m) nvese of the slope (m) on high voltage egion is the intenal esistance of the diode ( d =1/m)

10 Models of diode Behaviou of diode can be modeled with thee appoachings: 1 st appoaching. deal diode: ε Not taken in account neithe u no d. Fowad biased, the diode is a shot-cicuit. Revese biased, the diode is an open cicuit. R R ε = ε R R R voltage-cuent chaacteistic fo a diode in 1 st appoaching ε ε =0

11 Models of diode 2 nd appoaching. Simplified model: Only taken in account diode fowad voltage dop: u = 0.3 fo Ge diode u = 0.7 fo Si diode 0 = 6 R=1kΩ 0 = 6 R=1kΩ u =0.7 u = R k 0 u = = 5.3mA voltage-cuent chaacteistic fo a diode in 2 nd appoaching

12 Models of diode 3 d appoaching. Linea diode: Taken in account both diode fowad voltage dop as diode intenal esistance. 1/ d ε= 6 R=1kΩ u =0,7 d =25 Ω u voltage-cuent chaacteistic fo a diode in 3 d appoaching R=1kΩ ε= 6 d = 25Ω u =0.7 ε u = = = R mA

13 Models of diode Thee models fo junction diode deal diode (1 st appoaching) Simplified model (2 nd appoaching) u u Linea model (3 d appoaching) u d d u

14 Revese ecovey time of diode Should be polaización Fowad biased diecta polaización Revese biased invesa t s.. t t i Revese tiempo de ecovey ecupeación time inveso

15 Application: Diode as ectifie Half-wave ectifie: output U t ~ input U t Full-wave ectifie: ~ input U output output Routput output ~ input t

16 Applications: logician cicuits AND and OR logic gates =10 1 Logic = 0 0 Logic AND gate with diodes OR gate with diodes A A R s s B s B R s R R >>>R s R 10 A B S 0 (0) 0 (0) 0,7 (0) 0 (0) 10 (1) 0,7 (0) 10 (1) 0 (0) 0,7 (0) 10 (1) 10 (1) 10 (1) A B S 0 (0) 0 (0) 0 (0) 10 (1) 0 (0) 9,3 (1) 0 (0) 10 (1) 9,3 (1) 10 (1) 10 (1) 9,3 (1)