Lecture #14. Transient Response of pn Diode

Transcription

1 Lecure #14 OUTLNE n Juncions: ransien resonse urn-off urn-on n diode alicaions Reading: Chaers 8 & 9 EE130 Lecure 14, Slide 1 Transien Resonse of n Diode Suose a n-diode is forward biased, hen suddenly urned off a ime = 0. Because of C D, he volage across he n juncion deleion region canno be changed insananeously. (The delay in swiching beween he ON and OFF saes is due o he ime required o change he amoun of excess minoriy carriers sored in he quasi-neural regions.) EE130 Lecure 14, Slide 2

2 Turn-Off Transien n order o urn he diode off, he excess minoriy carriers mus be removed by ne carrier flow ou of he quasi-neural regions and/or recombinaion Carrier flow is limied by he swiching circuiry EE130 Lecure 14, Slide 3 Decay of Sored Charge Consider a + n diode (Q >> Q n ): i() n (x) s x v A () x n For > 0: dn dx = x= x n i qad > 0 s EE130 Lecure 14, Slide 4

3 Examles (qualiaive) ncrease F ncrease R Decrease τ i() i() i() s s s EE130 Lecure 14, Slide 5 Sorage Delay Time s s is he rimary figure of meri used o characerize he ransien resonse of n juncion diodes dq d Q = i τ = By searaion of variables and inegraion from = 0 + o = s, noing ha = Q ( = 0) / τ F R Q + τ + 0 s and making he aroximaion Q ( ) = 0 = s We conclude ha s τ ln 1 + EE130 Lecure 14, Slide 6 F R

4 Turn-On Transien Again, consider a + n diode (Q >> Q n ): i() n (x) x v A () x n For > 0: dn dx = i qad x= x n < 0 EE130 Lecure 14, Slide 7 dq d Q Q = i = F for 0 τ τ By searaion of variables and inegraion, we have Q ( ) = τ f we assume ha he build-u of sored charge occurs quasi-saically so ha Q ( ) = F diffusion τ / τ ( 1 e ) = qva / kt ( e 1) τ 0 + hen v ( ) = A kt q ln 1 + F / τ ( 1 e ) EE130 Lecure 14, Slide 8 0

5 f τ is large, hen he ime required o urn on he diode is aroximaely Q/ F where Q = Q + Q j EE130 Lecure 14, Slide 9 Summary of moran Conces Under forward bias, minoriy carriers are injeced ino he quasi-neural regions of he diode Curren flowing across juncion is comrised of hole and elecron comonens n order for one of hese comonens o be dominan, he juncion mus be asymmerically doed EE130 Lecure 14, Slide 10

6 Summary of moran Conces (con.) The ideal diode equaion siulaes he relaionshi beween J N (-x ) and J P (x n ) f holes are forced o flow across a forward-biased juncion, hen elecrons mus also be injeced across he juncion EE130 Lecure 14, Slide 11 Summary of moran Conces (con.) Under reverse bias, minoriy carriers are colleced ino he quasi-neural regions of he diode Minoriy carriers wihin a diffusion lengh of he deleion region will diffuse ino he deleion region and hen be swe across he juncion by he elecric field. Curren flowing in a reverse-biased diode deends on he rae a which minoriy carriers are sulied in he quasi-neural regions EE130 Lecure 14, Slide 12

7 The n Juncion as a Temeraure Sensor qv kt = 0( e 1) 0 = Aqn 2 i D LN d Dn + L N n a Wha causes he -V curves o shif o lower V a higher T? EE130 Lecure 14, Slide 13 Diode secifically designed o nondesrucively break down a redeermined volage Zener Diodes Commonly referred o as Zener diodes (hough avalanche is ofen he more common mechanism) EE130 Lecure 14, Slide 14

8 Varacor Diodes Volage-conrolled caacior Used in oscillaors and deecors (e.g. FM demodulaion circuis in your radios) Resonse changes by ailoring doing rofile: C for V r j V >> n r V bi 1 n = m+ 2 EE130 Lecure 14, Slide 15 Degeneraely doed such ha E F < E v and E Fn > E c Tunnel Diodes Can achieve negaive differenial resisance useful in high-seed circuis and erhas saic memories EE130 Lecure 14, Slide 16

9 Tunnel Diodes (con.) EE130 Lecure 14, Slide 17 Ooelecronic Diodes = 0 qva ( e kt 1) o o = qag o ( L P + W + L N ) ~ qag o ( L P + L N ) EE130 Lecure 14, Slide 18

10 V oc = V A ln kt L + Ln = + 1 = 0 q g L + L o n n τ n τ n EE130 Lecure 14, Slide 19 -i-n Phoodiodes W W i-region, so mos carriers are generaed in he deleion region faser resonse ime (~10 GHz oeraion) Oerae near avalanche o amlify signal EE130 Lecure 14, Slide 20

11 Ligh Emiing Diodes (LEDs) LEDs are yically made of comound semiconducors (direc bandga) EE130 Lecure 14, Slide 21 Organic LEDs Some organic maerials exhibi semiconducing roeries OLEDs are aracive for low-cos, high-qualiy flaanel dislays EE130 Lecure 14, Slide 22