p n junction! Junction diode consisting of! p-doped silicon! n-doped silicon! A p-n junction where the p- and n-material meet!

Transcription

1 juctio! Juctio diode cosistig of! -doed silico! -doed silico! A - juctio where the - ad -material meet! v material cotais mobile holes! juctio! material cotais mobile electros! 1!

2 Formatio of deletio regio" also called sace charge layer! At the juctio, the cocetratios of holes ad electros chages abrutly! The holes ad electros diffuse i the directio of reducig cocetratio! Electro diffusio! v o Hole diffusio! These holes ad electros leave behid charged atoms a deletio regio! A electric field forms i the viciity of the juctio! This electric field costitutes a eergy barrier that ooses diffusio! The device comes to equilibrium whe the voltage v o across the deletio regio is eough to sto further diffusio of charges across the juctio! E 2!

3 The diode uder reverse bias coditios! Alicatio of a exteral reverse voltage to the diode causes the deletio regio to icrease! The exteral voltage is blocked by the deletio regio! Icreasig the reverse voltage requires that charge is added to the deletio regio! v o E Juctio caacitace : deletio regio charge vs. voltage characteristic! 3!

4 The diode uder forward bias coditios! Whe the diode voltage is ositive, the deletio regio voltage is ot large eough to revet diffusio of charge across the juctio! Holes from the -regio diffuse across the juctio, ad become miority carriers i the -regio, whose eergy state is high eough to eable them to coduct! Similarly, electros from -regio diffuse across the juctio ad become miority carriers i the -regio! v o E 4!

5 Miority-carrier stored charge i forward-biased diode! Uder forward-biased coditios, a hole eters the -material from the exteral circuit. It the either (a) diffuses across juctio, the recombies with a electro i the -regio, or (b) recombies i the -regio with a miority-carrier electro! v o Electro cocetratio Hole cocetratio The forward curret of the diode cosists etirely of recombiatio, either i the - or -regio. The forward curret cotiues as log as there is miority charge. To tur off the diode, the miority charge must be elimiated.! 5!

6 Charge-cotrolled behavior of the diode! The diode equatio:! q(t)=q 0 e v(t) 1 Charge cotrol equatio:! dq(t) dt with:! = i(t) q(t) L = 1/(26 mv) at 300 K! L = miority carrier lifetime! (above equatios do t iclude curret that charges deletio regio caacitace)! (lumed-elemet charge cotrol model with 1 lum)! i Electro cocetratio I equilibrium: dq/dt = 0, ad hece! v i(t)= q(t) L = Q 0 L e v(t) 1 = I 0 e v(t) 1 Hole cocetratio Area = total stored miority charge q 6!

7 Removal of stored charge durig reverse recovery! Distributio of miority charge o oe side of - juctio durig reverse recovery! v(t) t 0 t 1 t 2 t 3 t 4 Miority charge t = t 0 V off t t 1 t 2 i(t) I o 0 x 0 x 3 t = t 3 Sloe determies diffusio rate ad hece curret! x 0 7!

8 Charge-cotrol i the diode:" Discussio! The familiar iv curve of the diode is a equilibrium relatioshi that ca be violated durig trasiet coditios! Durig the tur-o ad tur-off switchig trasiets, the curret deviates substatially from the equilibrium iv curve, because of chage i the stored charge ad chage i the charge withi the reverse-bias deletio regio! The reverse-recovery time t r is the time required to remove the stored charge i the diode ad eable it to block the full alied egative voltage. The area of the egative diode curret durig reverse recovery is the recovered charge Q r! 8!