The Diode - Heidelberg...
Transcript of The Diode - Heidelberg...
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The Diode
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 1
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Formula Collection
Field E points from positive to negative charges(electrons flow against field lines)
E = - grad Ψ = - dV(x) / dx
Potential Ψ = - ∫ E(x') dx'
Maxwell-eq.: ∫ E(x) dA= Q / εε0 'Integral of fields = included charge’
Gauss law: div E = ρ / εε0 = differential form
Poisson eq.: ∂2Ψ/ ∂x2 = ρ / εε0 = Gauss law
Laplace eq.: ∂2Ψ/ ∂x2 = 0 = Poisson equation in empty space
Current density: j(x) = - σ E(x) ([j] = A/m2, [E] = V/m, [σ] = A/(Vm) = S/m)
conductivity: σ = q n µ (n: carrier density, q: charge, µ: mobility)([q] = C, [n] = m-3, [µ] = m2/Vs)
resistance ρ[Ωm] = E/j = 1/σ (R= ρ l/A, l=length, A=area)
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A few constants
q 1.602 × 10-19 C elementary charge
k 1.381 × 10-23 J/K Boltzmann constant
4kT 1.657 × 10-20 J Noise Power density @ 300K
UT = kT/q = 25.9 mV Thermal voltage @ 300K
ε0 8.854 × 10-12 F/m vacuum susceptibility(Hint: C = ε0 A/d, 1m x 1m x 1m: ~10pF)
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A few constants for silicon
Eg 1.12 eV band gap at 300K
Natom 5 x 1022 cm-3 atom density
Ni 1.01 x 1010 cm-3 intrinsic carrier density at 300K* (‘old’ value: 1.45)
µe 1400 cm2/Vs electron mobility (@ low fields)
µh 480 cm2/Vs hole mobility (v = µE) µh 480 cm2/Vs hole mobility (v = µE)
Ecit 1 V/µm critical field where mobility starts to drop
εSi 11.9 dielectric constant silicon
εSiO2 3.90 dielectric constant silicon - dioxide
Emax ~ 3 × 107 V/m break through field strength
E 3.6 eV Av. Energy required to generate an e-h pair Eeh 3.6 eV Av. Energy required to generate an e-h pair
ρ 7.87 gcm-3 density
λ 150 W / (m · K) thermal conductivity
α 2.6–3.3 K-1 thermal expansion coefficient(Aluminum: 22-25)
*Sproul AB, Green MA. Improved value for the silicon intrinsic carrier concentration from 275 to 375 K. Journal of Applied Physics [Internet]. 1991;70:846-854. Available from: http://link.aip.org/link/?JAP/70/846/1
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Energy Bands / Doping ⇒ Applets on SuS web site
Silicon-crystal
Creation of bands, Valence & Conduction band
Density of States
Fermi distribution
Intrinsic Carrier density ni Intrinsic Carrier density ni
Hole conduction
N - doping (e.g. phosphorous, arsenic, 1014-20cm-3)
P - doping (e.g. boron)
Location of donator / acceptor Energy levels
Mass action law: n x p = ni2
Fermi – level in doped semiconductor
n - Doping ⇒ EF is moved towards conduction band
p - Doping ⇒ EF is moved towards valence band
Strength of doping is marked with exponent + or –, for instance n-, p++
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The Diode (p-n-junction)
p n
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pn-Diode on wafer
For instance: n-doped Si 'Wafer' is p-doped at the surface
EACH pn junction forms a diode
anodecathode
Aluminium contacts
Cross section of an pn-junction on a wafer
n- doped Wafer
p+
SiO2
anodecathode
n+
300 -
800 µ
m
Cross section of an pn-junction on a wafer
principle
p n
symbol
anode cathode
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Origin of Depletion Layers
We consider an idealized, 'abrupt' transition between n- and p- region (this is smooth in reality)
Due to the concentration gradient, electrons diffuse
from the n → p region (holes from p → n).
The carriers compensate and we get depleted The carriers compensate and we get depleted regions without mobile carriers
The fixed, ionized atoms are positively charged in the n-region (negatively in the p-region)
This leads to an electric field
The field is associated with a electrostatic potential.
This ‘built in' potential depends only on doping.
The field leads to a drift of electrons/holes The field leads to a drift of electrons/holes backwards.
The thickness of the depletion region is determined by the equilibrium between drift- and diffusion currents
In reality, the depletion zone drops more slowly to
zero, but the transition regions is small.
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Build-In Voltage
Derivation steps (p(x)= hole density):
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Derivation using Energy Bands
Fermi Distribution + Density of States
kT/)EE( Fe)E(f
−+=
1
1
e+1
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Derivation using Energy Bands
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Thickness of Depleted Region
Charge on both sides must be equal:
Field at junction:
Potential = Vbi:
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Dominated by low doped side!
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Example
Detector silicon is weakly doped, i.e. high resistivity
For instance: 5 kΩ ⋅ cm, n-doped (very high res!)
N = (q µ ρ)-1 ND = (q µ ρ)-1
= (1.6 × 10-19 As 1400 cm2/Vs 5kΩΩΩΩ cm) -1
~ 1012 cm-3 = 1 per µm3
Assume for instance NA = 1016 cm-3 (ni =1.45 × 1010 cm-3)
Vbi ~ 60mV × [log(NA/ni) + log(ND/ni)] ~ 60mV × (6+2) ~ 480mV
Depletion thickness:
D = ~ 25 µm
D → 2 D for Vext = 3 × Vbi = 1.5 V
D → 10D for Vext = 99 × Vbi = 47 V
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Capacitance
Large detectors are parallel plate capacitors
Depends on Depends on doping profile
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General Numerical Solution
See .pdf document + Demonstration Program
Doping[x] n(x), p(x)Doping[x] n(x), p(x)
ρ(x)
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V(x)
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C(V) or I(V) Measurement Setup
Contacting diode on wafer with probe station (in dark box)
Bias + Current measurement with Source-Monitoring-Unit (SMU) and CV Meter
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Measurement: CV-curve
If C ~ V-1/2, then 1/C2 ~V
Textbook measurement (HLL Munich):
Full depletion reached(Here: d=50µm depleted
with 50V)
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with 50V)
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Similar measurement for a 300µm thick detector
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Direct Measurement of partial depletion
Clever measurement with particle beam (R. Horisberger):
At very shallow beam incidence, the number of hit strips is smaller than the projection if depletion is partial
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Particle track
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Measurement
Some charge from backside gets trapped
(field is weak)
Charge seen also from non-depleted region
Depth decreases as expected
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non-depleted region (diffusion!)
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‘Leakage Current’
Even in a ‚perfect‘ detector, eh pairs are generated by
thermal excitation
This leakage current is This leakage current is
• Proportional to the depleted volume
• Increases with temperature, for instance: IL ∝ T2 Exp(-Eg/2kT)
• There are sometimes ‘surface effects’
Cooling from 20oC to 6oC reduces leakage to ~1/10
Electron/hole emission & capture (→ leakage) are eased by
• Impurity atoms• Impurity atoms
• Crystal defects
• Radiation damage (displaced atoms)
Detector production must be very clean and careful
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Breakdown
At very high reverse voltages, diode can ‘break down’:
E-Field is above critical value
Fields are so high that single (leakage) electrons generate Fields are so high that single (leakage) electrons generate
secondary electrons → avalanches
Breakdown can occur
• at local (point) defects
• at points with high field strengths (strong doping, edges)
The local current heats the detector→ current increases
→ spot gets hotter→ …This situation is called ‘thermal runaway’
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Ideal I-V characteristic
Breakdown
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Depleted volume increases
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Real Large Area Detectors
(ATLAS) Pixel detectors of 9 cm2 area.
Very bad
Problem @ depletion
(bad backside?)
Good‘soft
breakdown’
Perfect, higher I
(bad backside?)
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perfect
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IV-Measurements
Handle wafer side(holes)
n+
backside
p+ diode and guard
Alu
10mm2 diodes of t=50µm
Sensor wafer side(diodes + guards)
backside
Alu
Measured leakage currents are very low:
150 nA/cm3 (~very good strip detector)
No breakdown is observed even at strong
over-depletion
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Diode Forward Current
Diode Forward Current is exponential
No magic ‘0.6V’ forward voltage!
)e(II THD U/U
SD1−=
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
I D[A
]
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0,01
0,1
1
10
100
1000
I D[A
]
SD
-1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0
-0,1
0,0
0,1
VD [V]
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
1E-12
1E-11
1E-10
1E-9
VD [V]
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Diode Summary
Diode is conducting, when p-region is at positive voltage
Forward current ID = IS(exp(VD/UT) – 1).(UT = kT/q ~ 26mV @ 300K). I increases x 10 every 60mV
E-Field is largest at the junction
Potential increases quadratically (in constant doping)
Depletion region grows towards low doped side.
Growth with √ of applied voltage
Capacitance decreases Capacitance decreases
When n-doped regions are depleted, they charge positive!
• E-field can point to one point from all sides!
• Electrons are attracted by depleted n-regions
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Diode as a Detector
Typical thickness ~ 300µm (standard 6’’ wafer material)
Can use
• p implants in n bulk ‘p on n’ ← more common• p implants in n bulk ‘p on n’ ← more common
• n implants in p bulk ‘n in p’
Many reasons to chose one or the other
• Polarity of collected carriers (electrons / holes)
• Availability of material, ease of production (cost)
• Radiation effects
Watch
• Leakage
• Surface leakage
• Light sensitivity!
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Outlook: Double sided detector (n doped bulk)
Strips on the ‘ohmic’ n-side must be isolated by ‘tricks’
• Here: shallow p-implant
Also shown: Guard rings to ‘bring down’ p-side potential
N-doped bulk
- 100V
- 100V- 80V- 60V ‘Diode’ side (p in n)
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‘Ohmic’ side (n in n)
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Full depletion with n+ in n strips
Capacitance of n-strip for increasing bias: At low bias, all
strips are shorted → cap is high
< 100 V > 100 V
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Current (resistivity) between2 n-strips for increasing bias
Kemmer, 1988
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Field Plate Isolation between n+ strips
Can use a MOS-structure (field plate) to isolate the strips:
• Often heard: ‘electron accumulation layer’ – can be considered as an MNOS which is turned on.
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Kemmer, 1988
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DC coupled detectors
Strip / pixel is directly connected to amplifier
Amplifier input defines strip potential
− Amplifier must be on high voltage for double sided detectors. Need level shifters etc.detectors. Need level shifters etc.
− Leakage current flows into amplifier.‘Leakage compensation circuit’ may be required
+Detector is much simpler & cheaper
p+ p+
3 V
1 V
p+ p+
3 V
1 V
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n-
n+
p+
+ 100V
p+
0 Vn-
n+
p+ p+
0 V
n+
99 V
102 V
100 VLevelShift
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AC coupled detectors
Capacitor between strip and amplifier
• capacitor chip
• integrated on sensor (isolator between implant and cond. strip)
Need a ‘bias’ mechanism for strip (to define potential) Need a ‘bias’ mechanism for strip (to define potential)
• Polysilicon resistor (high value)
• ‘punch through bias’ with same-type implant in vicinity ‘pnp’
This can have ‘excess’ noise ifcurrent flows!
+Amplifier does not see leakage
+Amplifier can be at ground+Amplifier can be at ground
− Broken caps are big problem!
• They pull the strip to a ‘far away’potential.
− Detector more complicated and more expensive
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AC coupled detectors
Double sided with polysilicon bias, side view
resistorcapacitor
n-
0 V
3 V
1 V
3 V
resistor
p+
0 V
n+
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 34
0 V
1 V+100 V
+50 / -50, defekte Strips, region defekt
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Problem of Bad Strips in AC Readout
Cross Section of AC coupled Strips:
~ 0 V
Bad strip:
n-
p+
n+
~ - 100 V(via bias)
p+ p+ p+ p+
Short
Punch - ThroughIf potential
Difference too high
Several strips are ‘dead’ because their potential drops
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 35
n-
n+
0 V
Shortcircuit
-100 V-30 V -60 V
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Better Position Resolution with Intermediate Strips
Absorbed particles only deposit charge locally (on one strip)
• → poor position reconstruction (σ = pitch / √12)
Can add unconnected (but biased) intermediate strips
• Signals on these strips share capacitively to neighbours
• → better position reconstruction by interpolation
n-
p+ p+ p+
• → better position reconstruction by interpolation
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 36
n-
p+ p+ p+ p+ p+ p+ p+p+ p+
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RADIATION DAMAGERADIATION DAMAGE
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Reasons
Damage by charged particles (same as signal), depositing
charges (Total Ionizing Dose, TID, given in Mrad)
• No problem in (conducting) bulk (charges are removed)
• Problem in Oxide: e- are mobile & disappear, holes are stuck• Problem in Oxide: e are mobile & disappear, holes are stuck
Nuclear reactions of heavy particles (protons, neutrons):Non-Ionizing Energy Loss, NIEL, given in neq/cm2
• Atomic structure (crystal) is modified
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Effects of TID
Increasing positive oxide charge.
Threshold of any FET structure shifts (NMOS turn on), parasitic FETs can turn onparasitic FETs can turn on
Oxide charges can lead to high field strengths and
breakdown → leakage. Careful design needed. All surface potentials must be well defined.
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Bulk Damage by NIEL
Defects can act as dopants (depending on energy level in
band), as trapping centers, …
• increased ileak → increased noise
• Change in doping → bulk ALWAYS becomes p-type!• Change in doping → bulk ALWAYS becomes p-type!
→ ‘Type inversion’
• Increase in (p-)doping → high depletion voltage, partial depletion
• Trapping Centers → charges do not reach electrodes
10-3
10-2
10-1
A/c
m3]
n-type FZ - 7 to 25 KΩcmn-type FZ - 7 to 25 KΩcm
n-type FZ - 7 KΩcmn-type FZ - 7 KΩcm
n-type FZ - 4 KΩcmn-type FZ - 4 KΩcm
n-type FZ - 3 KΩcmn-type FZ - 3 KΩcm
p-type EPI - 2 and 4 KΩcmp-type EPI - 2 and 4 KΩcm
4
5
6
7
cm-3
]
300
400
00
µm)
neutronsneutrons
neutronsneutrons
pionspions
protonsprotons oxygen rich FZ
standard FZstandard FZNeff
Silicon Detectors - The Diode 40
1011 1012 1013 1014 1015
Φeq [cm-2]
10-6
10-5
10-4
10
∆I
/ V
[
A/
n-type FZ - 780 Ωcmn-type FZ - 780 Ωcm
n-type FZ - 410 Ωcmn-type FZ - 410 Ωcm
n-type FZ - 130 Ωcmn-type FZ - 130 Ωcm
n-type FZ - 110 Ωcmn-type FZ - 110 Ωcm
n-type CZ - 140 Ωcmn-type CZ - 140 Ωcm
p-type EPI - 380 Ωcmp-type EPI - 380 Ωcm
0 0.5 1 1.5 2 2.5 3 3.5
Φeq [1014cm-2]
1
2
3
4
|Nef
f| [1
012 c
100
200
300
Vdep
[V
] (3
0neutronsneutrons
pionspions
protonsprotonsileak
n p
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Choice of N vs. P
For single sided detectors, 4 combinations are possible:
p on n n on n p on p n on p
n-
n+
p+ p+p+
n-
p+
n+ n+n+
p-
n+
p+ p+p+
p-
p+
n+ n+n+
Irradiation → Type inversion+ lower bias voltage
Irradiation immediately increases bulk doping- higher bias voltage
Electron Collection Electron CollectionHole CollectionHole Collection
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 41
Readout
Electron Collection Electron CollectionHole CollectionHole Collection
Initial:Diode
Later:Ohmic
Readout
Initial:Ohmic
Later:Diode
Readout
Initial:Ohmic
Later:Ohmic
Readout
Initial:Diode
Later:Diode
LHC
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Problem of type inversion (n-bulk)
p+ pixels on n- material n+ pixels on n- material
Bulk n- before
HV problem
Bulk n- beforeirradiation
Bulk p- afterirradiation
Silicon Detectors - The Diode 42
irradiation
Need fulldepletion!
Can be operated partially depleted
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Annealing
Some crystal defects (vacancies, atoms at wrong place)
can be ‘repaired’ by heating (operating ‘warm’)This is called ‘annealing’
On the other hand, harmless defects can convert into ‘bad’ On the other hand, harmless defects can convert into ‘bad’ defects: ‘reverse annealing’
Inactive atoms in bulk (Oxygen) can catch away ‘bad’
defects → ‘defect engineering’
Keeping detectors cold (-6oC) reduces effect on leakage
current!
Using ‘oxygenated’ silicon slows down type inversion Using ‘oxygenated’ silicon slows down type inversion
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Performance of irradiated (ATLAS) pixel sensor
Sensors irradiated to
full ATLAS fluence(1015 neq/cm2)
Test beam with Test beam with reference detector to get hit position
Measurement of charge
Homogenous charge collection
Very small loss
collectionalso in pixel corners
Vbias > 600V possible!
Silicon Detectors - The Diode 44
Threshold = 2000 e
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Comparison of two Sensor designs
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Partial Depletion of ATLAS Pixel Detectors
track entrance point
from beam telescope
Track depth 1000
2000 depletion 0.2868
-30*
non irradiated, full depletionNo charge
seen here
not depleted
depleted depth (mm)
00 0.1 0.2 0.3
depth (mm)
0
1000
2000
0 0.1 0.2 0.3
depletion 0.1879
-30*
-600 V 1x1015
1x1015, 600 V, 190µm
Particle track
Silicon Detectors - The Diode 46
depth (mm)
0
1000
2000
0 0.1 0.2 0.3
depletion 0.1047
-30*
-300 V 1x1015
1x1015 , 300 V, 105µm
Depletion depth is 190 µm @ 600 Vafter 1015 cm-2 (full ATLAS dose!)
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Quick look at the JFET
p+ n+n+
n-
p+/-Gate
Drain
Bulk(Backgate)
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 47
Source
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JFET: channel open, linear region
Channel cross section is reduced by two depletion regions
In linear region, channel is still open
n+p+
VS = 0V VD = +1VVG = -1V
n+ n+
n-
p+
Kanal
VB = 0V
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JFET: Saturation
At high Drain-Gate-Voltage, the drain side is pinched off.
Further increase in drain voltage does not increase current
n+p+
VS = 0 VD = +3VVG = -1V
n+ n+
n-
p+
VB = 0V
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JFET: Complete pinch-off
At negative gate voltage, the channel is pinched off completely
There is no current at all drain voltages
n+p+
VS = 0V VD = ...VG = -4V
n+ n+
n-
p+
VB = 0V
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JFET: characteristic
At high drain voltages, channel becomes shorter and current
increases a bit: ‘channel length modulation’ / ‘Early Effect’
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The MOS Transistor
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Cross Section of a NMOS Transistors
Gate
Source DrainBulk
(Body)
n+ n+ p+
Gate-Oxid Feld-Oxid (SiO2)Polysilizium
p- Substrat
Von oben:L
W
Meist W>L:
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 53
Von oben:L
W
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MOS: Accumulation – Depletion – Inversion
Consider isolated gate electrode on p-silicon
Gate sehr negativ:
- reichlich vorhandene Löcher
werden unter d. Gate gezogen
- ‚Akkumulation‘
Gate positiver:
- Löcher werden weggedrückt
-‚Verarmung‘
Gate sehr positiv:
- Elektronen (Minoritätsträger)
werden angesaugt
- ‚Inversion‘- ‚Akkumulation‘
- ‘Kondensatorplatte’ direkt
unter dem Oxid
- Kapazität ist maximal
-‚Verarmung‘
- ‘Kondensatorplatte’ weiter
im Bulk (Dichte negativer
Raumladung durch Dotierung
begrenzt)
- Kapazität sinkt
- ‚Inversion‘
- ‘Kondensatorplatte’ wieder
direkt unter Oxid
- Kapazität wieder maximal
- - - - Gate - - - - + Gate + ++++ Gate++++
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 54
Siehe Skript und Applet http://smile.unibw-hamburg.de/Bauelemente/FET/Mos_struktur.htm
+ + + + + + + +
p-Silizium+
+
++
+
p-Silizium +++
+
+
- - - - - - - - - - - -
p-Silizium +++
+
+
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MOS: Accumulation – Depletion – Inversion
C/C01niedrige Frequenz
C0 = ε0εSiO2 A/tox Bild gilt für
VGate
Akkumulation InversionVerarmung
hohe Frequenz
Bild gilt fürp-Silizium!
VThreshold
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 55
MOS Struktur im Bänderdiagramm:smile.unibw-hamburg.de/Bauelemente/FET/Baender_MOS_struktur.htm
Bei hohen Messfrequenzen können in Inversion die Elektronen nicht schnell genug angesaugt werden. Die
Kapazität bleibt dann klein.
Man definiert die Schwellenspannung (für 'starke' Inversion) oft als die Spannung, bei der die
Elektronendichte in der Inversionsschicht so groß ist wie die Löcherdichte im Bulk.
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MOS in band diagram
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Threshold Voltage
Gate
Drain BulkSource
0V 0V 0V
VT
An den Drain- und Source-Dioden bilden sich Verarmungszonen aus
Drain Bulk
p- Substrat
p+
Verarmungszone
Source
n+ n+
Inversionsschicht
An den Drain- und Source-Dioden bilden sich Verarmungszonen aus
Bei genügend positivem Gate bildet sich unter dem Gate eine (n-leitende)
Inversionsschicht aus, durch die Strom von Drain nach Source fließen kann.
Die Gate-Source-Spannung VGS, ab der starke Inversion vorliegt, ist die
Schwellenspannung VT
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Linear Region
Gate
Drain BulkSource
VS=0V VDS~0.3V 0V
VG>VT
Bei kleinen Drain-Source-Spannungen VDS bleibt der Kanal erhalten
Drain Bulk
p- Substrat
p+
Source
n+ n+
n-Kanal
Bei kleinen Drain-Source-Spannungen VDS bleibt der Kanal erhalten
Dies ist der Fall solange VDS < VGS-VT
Der Transistor verhält sich wie ein Widerstand, der bei VGS < VT unendlich wird: ID= a ×××× VDS
Man spricht vom Linearen Bereich
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Saturation
Gate
Drain BulkSource
VS=0V VDS=VG-VT 0V
VG>VT
Bei sehr positiver Drain-Spannung verschwindet der Kanal an der Drain-Seite.
Drain Bulk
p- Substrat
p+
Source
n+ n+
Kanal verschwindet
Man spricht von 'pinch-off' (Abschnüren)
Diese Sättigung tritt ein, wenn VDS = VDSat = VGS-VT
Der Strom steigt mit steigendem VDS > VGS-VT (fast) nicht weiter an
Genauer: Da mit steigendem VDS die Länge des Kanals abnimmt, steigt der Strom
weiter leicht an. Man spricht von Kanallängenmodulation
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Strom-Spannungs-Formeln in starker Inversion
Formeln in 'starker Inversion', d.h. VGS >> VT
TGSDS
DS
DSTGSNDVVVfür
VV)VV(
L
WKI −<
−−=
2
2
Linearer Bereich:
Sättigung:
TGSDSDSTGS
N
DVVVfür)V()VV(
L
WKI −>λ+−= 1
2
2
OX
NOXNN
tCK
εεµµ 0== Sättigung:
Silicon Detectors - The Diode © P. Fischer, ziti, Uni Heidelberg, Seite 60
OX
NOXNN
tCK µµ ==
Daneben gibt es auch den Bereich schwacher Inversion, 'weak Inversion' oder Subthreshold-
Bereich. Dort ist der Drainstrom klein und hängt exponentiell von VGS ab
tox = Oxid-Dicke