PN Junctions and Diodes
Transcript of PN Junctions and Diodes
Lecture 17a
PN Junctions and Diodes
18-322 Fall 2003Reading: Section 3.2
Lecture 17a
Crystalline Silicon
Si
Si
Si Si
Si
Si
Si
Si
Si
SiSi Si SiSi
SiSi Si SiSi
SiSi Si SiSi
Si
Covalent bond filled with an electron
NOTE:2D representation
of 3D reality
Lecture 17a
Generation and Recombination
Si
Si
Si Si
Si
Si
Si
Si
Si
SiSi Si SiSi
SiSi Si SiSi
SiSi Si SiSi
Si
Hole
Free electron(-q)
(+q)Electron-hole pairs
appear:Generation
Pairs disappear:Recombination
Lecture 17a
Holes and Electrons
Called Carriers
They’re both mobileHoles move by “swapping” missing electronWith an electric field:Holes move in positive directionElectrons move in negative direction
E
Lecture 17a
Intrinsic Semiconductors
Electron concentration: nHole concentration: p
n = pIntrinsic concentration: ni (T)
ni = 1010 cm-3 at 300Kn0 p0 = ni
2
Only 1 in 20 trillion bonds broken!
Lecture 17a
Doping with Impurities
Donor impurities:Elements that “donate” electronsHave 5 valence electrons in outer orbitExample: Phosphorus
Acceptor impurities:Elements that “accept” electronsHave 3 valence electronsExample: Boron
Lecture 17a
Donors
Si
Si
Si Si
Si
Si
Si
Si
Si
PSi Si SiSi
SiSi Si SiSi
SiSi Si SiSi
Si
Free electron(-q)
(+q)
Donor Ions:Positive charge
Immobile
Donated electronmobile
N-type silicon
Lecture 17a
Acceptor
Si
Si
Si Si
Si
Si
Si
Si
Si
BSi Si SiSi
SiSi Si SiSi
SiSi Si SiSi
Si
(-q)
(+q) Hole
Acceptor Ions:Negative charge
Immobile
Donated holemobile
P-type silicon
Lecture 17a
N-type Semiconductors
Donor concentration: Nd
Dominates over intrinsic: Nd >> ni
n0 = Nd
Still holds: n0 p0 = ni2
p0 = ni2 / Nd
Majority carriers: electronsMinority carriers: holes
Lecture 17a
P-type Semiconductor
Acceptor concentration: Na
p0 = Na
Still holds: n0 p0 = ni2
n0 = ni2 / Na
Majority carriers: holesMinority carriers: electrons
Lecture 17a
Carrier Transport
E
Carriers “drift”
Apply an electric field: EAverage electron velocity “drift velocity”
Vdn = - µn E
Lecture 17a
Carrier Mobility
Electron Mobility: µn
Function of temperature: T ⇑ -> µn ⇓Function of total doping: (Na + Nd) ⇑ -> µn ⇓
Hole Mobility: µp
µp ≈ µn / 2.5
Holes don’t move as fast as electrons
Lecture 17a
N-type Resistance
Lt
W
Current: Coulombs per secondCharge: ND * q * tWLRate: Vdn / L = µn E / L = µn V / L2
Resistance: V/I = L / (ND * q * µn * tW)
Lecture 17a
Resistance
R = Constant * L/WTypical Measure: Ω per (per square)
Six Squares
Lecture 17a
The Junction
ppo
npo x
nno
pno
1018 cm-31016 cm-3
100 cm-3 104 cm-3
log n, pMajority Carriers (holes)NA
P - type N - type
Majority Carriers (electrons)ND
Lecture 17a
At Absolute Zero:
ppo
npo x
nno
pno
N - type
Majority Carriers (electrons)ND
1018 cm-31016 cm-3
100 cm-3 104 cm-3
Majority Carriers (holes)NA
P - type
Lecture 17a
Diffusion
P-type
Hole Diffusion
N-type
Electron Diffusion
Lecture 17a
Depletion Region
Few Majority Carriers(only fixed ions)
P-type N-type
Lecture 17a
Depletion Region
N-type
-+
P-type
xp xn
qND Charge Density
qNA
xnxp
NAND
=
Lecture 17a
Built In Potential
P-type N-type
-Charge Density+
Electric Field
Potential
Lecture 17a
Built In Potential
φ = φT ln [NA ND / ni2 ]
Charge without applied voltageNot really measurableField creates current opposite to diffusion
Drift!
Lecture 17a
In Equilibrium
Hole Diffusion
Electron Diffusion
Hole DriftElectron Drift
P-type N-type
Lecture 17a
Reverse BiasReverse Bias: Make N-type Higher voltage
Increases field.Increases DRIFT current!
Hole Diffusion
BUT:There aren’t many carriers
available for drift this direction
(free holes in N,free electrons in P)
Hole DriftElectron DriftP-type N-type
Electron Diffusion
Lecture 17a
Forward BiasForward Bias: Make P-type Higher voltage
Decreases fieldDecreases DRIFT current!
Hole Diffusion
Hole DriftElectron DriftP-type N-type
Depletion region shrinksMore carriers to diffuse
increased diffusion current
Electron Diffusion
Lecture 17a
Ideal Diode Behavior
-Is
ID ID = IS [ e (V/VT) -1 ]
VT = kT/q = 26mV @ 300K
V
Lecture 17a
Diode Models
Physical ModelsIdeal Diode Equations“On” voltage: VD
Determine a voltage when current is “large”Typical value: 0.6V - 0.7V
+
V
-
+- VD
V < VD V > VD