Diode in electronic circuits. (+) (-) i D

Transcription

1 iode i electroic circuits Symbolic reresetatio of a iode i circuits ode Cathode () (-) i ideal diode coducts the curret oly i oe directio rrow shows directio of the curret i circuit Positive olarity of the voltage at a aode ad egative oe at a cathode corresod to a forward bias coditio Mius at the aode ad lus at the cathode corresod to reverse biasig

2 iode as a o-liear resistor Curret-Voltage (I-V) Characteristics I-V of a LIER RESISTOR i() Rv/i v(v) I-V of a IELIZE IOE i() R 0 R v(v)

3 I-V Characteristic of a Ideal iode qv / kt J J ( e ) S i() ~ e(qv/kt) v(v) t room temerature T300K the thermal voltage kt/q 6 mev For a Si diode the tyical value of the saturatio curret J S ~ 0-0 /cm For a high forward bias V >> 6 mv: J J e S qv / kt The forward curret shows a eoetial deedece For a high reverse bias V <0, V< < -6 mv: J J S Reverse curret of diodes is quite small

4 I-V Characteristic of a iode i semilogarithmic scale J(/cm ) 0 o-e icrease saturatio regio J S sloe q/kt~ 0-40 V qv/kt The slo i logarithmic scale ca be used to defie the o-ideality factor The itersectio of lgi-v characteristic with vertical ais gives the value of saturatio curret J S The curret icrease differs from eoetial at high forward bias voltage

5 I-V Characteristic of a Real iode Silico (Si) iode i() 0.7 V v(v) The tyical voltage dro across a Si diode at forward bias is 0.7V Germaium (Ge) iode i() 0.4 V v(v)

6 iode at High irect Curret t high curret I-V characteristic become to be liear i() sloe R S dv/di v > 0.7 V Series Resistace R S describes the eerimetal I-V characteristic Ideal iode R S

7 Breakdow of a iode at Reverse Voltage The reverse curret starts to icrease raidly at some high voltage V B i() V B sloe R diff dv/di v(v) This caused by avalache or tuel breakdow The breakdow voltage V B ca be as high as 00 th Volts or ca be urosely made a small dow to 3-5V Zeer diode ca be used as a voltage referece source ifferetial resistace R is a imortat device arameter

8 Poit-by-oit measuremets of I-V characteristics usig MM R s < 0.0 Ω C source 0 5V V Floatig Source (o groud) R 300 Ω V MM R it ~ 0.0 Ω ccurate measuremets of curret ad voltage usig two-dislay MM require a floatig voltage source Make sure that the groud cli o the ower suly is discoected!

9 Metal-semicoductor juctio Work fuctio Φ characterizes miimum eergy that has to be trasferred to the electro i order to remove it from the material Metal VCUUM LEVEL Semicoductor q Φ METL q Φ SEMICOUC TOR q χ - electro affiity _SEMICOUCTOR _METL Φ 4.8, l ev Φ ( E - E ) C F 4.0 q Si, ev fter Metal ad Semicoductor are brought i cotact the Fermi eergy should be costat throughout the system i thermal equilibrium

10 chottky barrier ad built-i otetial barrier Whe Φ METL > Φ SEMICOUCTOR electros from semicoductor will flow ito lower eergy states i metal q Φ M q χ q ΦS q Φ B0 q Vbi E C Metal Semicoductor Whe electros move from metal to semicoductor they are stoed by Schottky barrier Φ Φ χ B0 M Whe electros i CB move from semicoductor to metal they are stoed by built-i otetial barrier V bi Φ M Φ S Φ B0 ( E E ) C F

11 eletio regio q Φ B0 - q Vbi Whe electros are moved away from the metalsemicoductor iterface they leave charge of the ioized doors ucomesated. Metal Semicoductor egative charge of the etra electros Positive charge of the ioized doors W width of deletio regio Electric Field eletio regio charge is *W. W W ε ε0 V q bi

12 Reverse bias Lecture 07/08 Positive voltage is alied to -semicoductor side q ( V bi V) q Φ B0 eletio regio width icreases Metal W ε ε0 q ( V V) bi Semicoductor Barrier for electro motio from metal to semicoductor remais uchaged ad if high eough o curret icrease will flow through the structure Reversely biased Schottky diode ca be see as a arallel late caacitor with late searatio equal to deletio regio width. C q ε ε ( V V) bi 0

13 Forward bias egative voltage is alied to -semicoductor side q Φ B0 Metal Barrier for electro motio from semicoductor to metal decreases q ( V V) bi Semicoductor Electros with kietic eergies E > q ( V V) bi ca overcome barrier ad flow to metal, hece, coduct curret through the structure Rectifier I J * * T 4 π q m 3 h q Φ e k T * k B0 q V e k T - Richardso costat V

14 Lecture 09/03 - ad -semicoductors at distace -Semicoductor -Semicoductor VCUUM LEVEL q Φ S q Φ S q χ - electro affiity ue to differece i work fuctios betwee - ad -tye semicoductors the electros ca lower their eergy if they move from - to -semicoductor.

15 Gedake eerimet -Semicoductor 0 0 i -Semicoductor 0 0 i Large gradiets of electros ad holes at the -iterface (metallurgical juctio) will drive holes from - to -semicoductor ad electros from - to -semicoductor. Electros comig from - to - semicoductor will ecouter large umber of holes ad recombie with them (same with holes from - semicoductor) The ucomesated charge of oors (-side) ad ccetors (side) will build iteral electric field that will sto diffusio of electros ad holes.

16 -juctio ad built-i otetial -Semicoductor -Semicoductor 0 0 Built-i otetial 0 i - q V bi i V bi k T q l i The built-i otetial maitais equilibrium (comesates for diffusio currets) so o et curret is roduced by V bi. q V bi

17 - q E - q - eletio regio Equatios: de d ( ) ρ q, -, ρ( ) ε ε q, 0 0 < < < W? < 0 -Semicoductor Semicoductor Solutio: W ε ε q 0 V bi

18 -juctio reverse bias Zero bias Reverse bias ( to, - to ) q V bi q V bi ( V V ) q bi ( V V ) q bi - ( d ) - ( d ) W ε ε q 0 V bi W ε ε 0 ( V V ) q bi Uder reverse bias both the deletio regio width ad electric field at metallurgical juctio icrease

19 Reverse bias Zero bias Reverse bias ( to, - to ) q V bi q V bi ( V V ) q bi ( V V ) q bi ε ε0 W q V bi Uder reverse bias both the deletio regio width ad electric field at metallurgical juctio icrease. - ( d ) ε ε W C' 0 - ( d ) ( V V ) eletio layer caacitace decrease. ( Vbi V ) ( ) q q ε ε 0 bi

20 Forward bias q ( V V) bi Forward bias ( - to, to ) q ( V V) bi Eergy barrier (built-i otetial) is lowered ad diffusio currets of miority electro ad holes will flow. J J J S S q V e k T q L J 0 q L 0 Rectifier 0 ( ) ( ) i L - L 0 i JS V