Transcript Lecture 29, 24 Mar 14 - Michigan State University

ECE 875:
Electronic Devices
Prof. Virginia Ayres
Electrical & Computer Engineering
Michigan State University
[email protected]
Lecture 29, 24 Mar 14
Chp 04: metal-insulator-semiconductor junction: GATES
Q, E , V/y: WD , Vi
Capacitances
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Lec 26:
p-type Si
Use energy band diagram to find:
Electron concentration in channel
V requirements: battery = $
E –field/Vi across the insulator: breakdown not good
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2
Or nn0
Or pp0/nn0
1. Know substrate doping NA or ND
Or nn0
3: @ ys for known condition
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From Lec 26-27 example: Known condition: n-channel in p-substrate:
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From Lec 27 example: Known condition: p-channel in n-substrate:
Have: values for ys and Qs :
Therefore have potential drop across the dielectric: Vi = |Qs| d
ei
Makes sense to evaluate this in strong inversion/accumulation
Therefore have battery V
Example: is this an n-channel or p-channel device?
Answer: is this an n-channel in p-substrate (NA)
Lec 26:
p-type Si
Use energy band diagram to find:
Electron concentration in channel
V requirements: battery = $
E –field/Vi across the insulator: breakdown not good
VM Ayres, ECE875, S14
Qs = Qn + Qdepletion = Qn + q (NA or ND) WD
Qs = Qn + Qdepletion = Qn + q (NA or ND) WD
Lecture 29, 24 Mar 14
Chp 04: metal-insulator-semiconductor junction: GATES
Q, E , V/y: WD , Vi
Capacitances
VM Ayres, ECE875, S14
ON to OFF: n-channel in a p-substrate:
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OFF to ON: n-channel in a p-substrate:
ON: Strong inversion
OFF: Strong accumulation
Going OFF: intrinsic
Going OFF: Flatband
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Fig 07: n-channel in p-substrate:
ON
C / Ci
OFF
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Fig. 07 shows all behaviors.
Complication: C-V curves depend on frequency of voltage sweep.
Gate voltage:
Sweeping Vgate for example ± 4 Volts over and over to turn the
channel OFF and ON: binary logic
Low: 1- 1 kHz
Intermediate: 1 kHz - 1 MHz
High: > 1 MHz
VM Ayres, ECE875, S14
Start:
Low frequency C-V curves
C-V curve for an n-channel in a p-type Si block
(low frequency curve)
OFF
ON
C (F/cm2)
C-V curve for an p-channel in a n-type Si block
(low frequency curve)
ON
OFF
C (F/cm2)
VM Ayres, ECE875, S14
Example: what values are easiest to determine experimentally?
C-V curve for an n-channel in a p-type Si block
(low frequency curve)
OFF
ON
C (F/cm2)
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Answer: @OFF, @ON and minimum
C-V curve for an n-channel in a p-type Si block
Example: Why so?
(low frequency curve)
Answer: compare to slide 18, works
for now
OFF
ON
C (F/cm2)
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Note that Qn can be
big number
Units = C/cm2
E (x) = constant
|| plate capacitor Ci
E (x) = distributed capacitor
CD = dQs/dys
Total C: Ci and CD are in series
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VM Ayres, ECE875, S14
CD = dQs/dys = large
neglect
C ≈ Ci
Also true in strong inversion
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Can normalize curve to Ci
I will continue with experimental readings in F/cm2. Fancy C means Farads/Area.
C / Ci
OFF
ON
1.0
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OFF
C (x 10-7 F/cm2)
2.0
ON
1.73
1.0
0.5
C-V curve for an n-channel in a p-type Si block
Low frequency curve: 1 Hz
Ci = eie0
d
Units: F/cm2
VM Ayres, ECE875, S14
Important: Use C-V reading to experimentally solve for the gate
oxide thickness d:
Ci = eie0 => d = eie0
d
Ci
OFF
C (x 10-7 F/cm2)
2.0
ON
1.73
1.0
0.5
C-V curve for an n-channel in a p-type Si block
Low frequency curve: 1 Hz
VM Ayres, ECE875, S14
Next: Cmin is easy to identify on the experimental curve:
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