Schottky diodes: I-V characteristics

Transcription

1 chottky diodes: - characteristics The geeral shape of the - curve i the M (-type) diode are very similar to that i the p + diode. However the domiat curret compoets are decidedly differet i the two diodes. p + M Forward-biased Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-1

2 p + p + juctio diode v.s. chottky diode Forward-biased Uder small, the domiat curret compoets arise from recombiatio i the depletio regio. Uder large, the domiat compoet is the hole ijectio (miority carrier ijectio by diffusio) from p + to -side. Uder large, the electro ijectio from to p + -side is egligible due to the light dopig. Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-

3 p + juctio diode v.s. chottky diode Forward-based M lthough the recombiatio ad hole ijectio currets still eist, because of the relatively low potetial barrier see by electros i the -side, the domiat compoet is the electro ijectio from to M uder the forward bias. Reverse-biased Uder reverse bias, electro flow from M to totally domiates the observed curret. o, the M diode is ofte said to be a majority carrier device. Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-3

4 Thermioic emissio theory The curret resultig from majority carrier ijectio over the potetial barrier i a M diode is referred to as the thermioic emissio curret. f a electro eterig the depletio regio from the semicoductor bulk has a velocity v directed toward the iterface, the kietic eergy i the - directio is give by 1 * KE m v Whe KE q( bi ), the electro ca surmout the surface barrier ad cross ito the metal. 1 * v m q( Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-4 bi )

5 Thermioic emissio theory 1 * v m q( The velocity required for surmoutig the barrier is v v mi q * m Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-5 bi ( bi ) ssumig there are (v ) electros/cm 3 i the semicoductor bulk with a velocity, v, which is sufficiet to surmout the barrier. The curret associated with this set of electros with v will be qv ( v ) M, v ummig over all electros with sufficiet eergies, M q v mi v ( v ) ) dv

6 Thermioic emissio theory M q For a o-degeerate semicoductor, (v ) is give by 4kTm 3 h * v mi v ( v ) dv * ( EF EC )/ kt ( m / kt) v ( v ) e e ubstitutig, itegratig, ad simplifyig the results, M * T e e q where * m m * ad 4πqm k 3 h 1 amps/cm K The costat ad * are called Richardso costat ad effective Richardso costat, respectively. Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-6

7 Thermioic emissio theory Electros crossig the iterface from M to always see the chottky barrier height ( ). Cosequetly, M ( ) ( ) M Moreover, uder equilibrium, the M ad M currets across the barrier must precisely balace. M ( ) M ( ) * T e The total curret at a arbitrary is give by M Combiig the equatios, M M s s M ( e q * T ( ) e 1) Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-7

8 p + juctio diode v.s. chottky diode D q L N i D L p p N i / kt ( e q 1) D s * T s ( e q e 1) > a few kt/q < mius a few kt/q s e q s s Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-8

9 Deviatios from the ideal i chottky diodes Forward-biased reverse-biased series resistace breakdow (avalache) thermal geeratio?? Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-9

10 chottky barrier lowerig Differig from the p juctio diode, the o-saturatig reverse curret is primarily attributed to a pheomeo kow as chottky barrier lowig. Eve though it is assumed that is bias-idepedet i the ideal theory, the chottky barrier is rather lowered uder E-field (which is also called image force-iduced lowerig). where is the barrier height whe E = ad q Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-1 q E 4K M The electric field at the semicoductor surface (E ) ca be computed from qnd E( ) W whe =. K ε

11 chottky barrier lowerig s ( e q 1) s * T e q q E 4K ice s epoetially varies with, eve a small decrease i gives rise to a oticeable icrease i the reverse-bias curret. chottky barrier lowerig Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-11

12 v a a.c. respose i chottky diode bi small a.c. sigal superimposed o a d.c. reverse bias gives rise to a charge fluctuatio iside the M diode. variatio i the depletio width associated chage i the depletio capacitace C K W K ecause W bi, qnd ε C K Kε qn D bi The larger reverse bias, the smaller C. Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-1

13 a.c. respose i chottky diode C Kε qn Takig the reciprocal ad the squared, K D bi 1 C qndkε bi The 1/C plot agaist gives a straight lie. The slope of the straight lie gives the dopig cocetratio. The etrapolated itercept at 1/C = gives the built-i potetial. Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-13

14 Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec a.c. respose i chottky diode bi D ε K qn C 1 bi D qn ε K K C

15 Practical cosideratios: rectifyig cotact M cotact is defied to be ideal if the M ad are i itimate cotact o a atomic scale, there is o itermiig of compoets, ad there is o adsorbed impurities or surface charges at the M iterface. practically it s ot the case. E) ccordig to the chottky-mott model, ut.. Regardless of the metal employed i a Ge M diode, early all metals form a sigificat chottky cotact to -type Ge ad a ohmic cotact to p-type Ge. M The E of Ge is strogly pied to the E FM. Fermi level piig due to surface states Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-15

16 Fermi level piig itrisic -type before equil. -type after equil. E C E C E i E F E i E C E i = E F E E E door-like or accepter-like surface states. ρ ρ ρ o space charge o space charge + - Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-16

17 Practical cosideratios: ohmic cotact ideal cases, for ohmic cotact, M < (-type) M > (p-type) However, due to the Fermi level piig, the depositio of ay metal o -type Gas forms a barrier type cotact. How are ohmic cotacts achieved i practice? by heavily dopig the surface regio The depletio depth decreases with dopig!! Tuelig through the arrow W. Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-17

18 oucemets Net lecture: p. 563 ~ 575 Homework: 14.; 14.3; 14.9 Prof. Yo-ep Mi Electroic Materials: emicoductor Physics & Devices Chapt Lec 15-18