ch1 5 pn iv non ideal
TRANSCRIPT
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 1/20
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 2/20
Current-Voltage Characteristics: Non-Ideal Effects
Breakdown Phenomenal
- “Breakdown“ is referred to event when larger reverse current
flows at a reverse voltage exceeds a certain value.
- Breakdown is a completely reversibleprocess. That means breakdown does
not damage the diode.
- The absolute value of the reverse
voltage where the current goes off to infinity is known as breakdown
voltage (VBR)
- Practically VBR
is measured where
current exceeds a certain value, for
example 1uA or 1mA.
- Normally, two kinds of mechanisms exist: Avalanche
breakdown and Zener breakdown
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 3/20
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 4/20
Current-Voltage Characteristics: Non-Ideal Effects
Breakdown Phenomenal: Avalanche Breakdown
- Approaching the breakdown voltage, the
energy transferred per collision becomes
sufficient to ionize a semiconductor atom.
- Ionize means that the collision frees a valence
electron from the atom, or causes an electron
from the valence band to jump into the
conduction band, thereby creating an electron-hole pair. (called impact ionization)
- The added carriers are immediately
accelerated by the electric collisions and
create even more carriers.
- The result is a snowballing creation of carriersvery similar to an avalanche of snow on a
mountain side.
- At the breakdown voltage, the carrier creation
and reverse current effectively goes to infinity.
Large Reverse Voltage
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 5/20
Current-Voltage Characteristics: Non-Ideal Effects
Breakdown Phenomenal: Avalanche Breakdown
Dopant concentration dependence
- For avalanche breakdown, considering a step junction, the
breakdown voltage is related with doping concentrations as follow
D A
D A BR
N N
N N V
- For asymmetrically doped junctions
B
BR N V
1
where NB is doping concentration of low side.
- Physical view:
lower doping leads to a wider depletionwidth, which can support more voltage and
hence a higher breakdown voltage.
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 6/20
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 7/20
Current-Voltage Characteristics: Non-Ideal Effects
Breakdown Phenomenal: Zener Breakdown
Zener breakdown – a result of “tunneling”
- Zener process in the name given to the occurrence of
“tunneling” in a revsrse-biased diode.
- Tunneling is a phenomenon we have encountered that is of
a purely quantum-mechanical nature. It has no classic
analog.
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 8/20
Current-Voltage Characteristics: Non-Ideal Effects
Breakdown Phenomenal: Zener Breakdown
There are two major requirements for
tunneling to occur and be significant
(1) There must be filled states on one
side of the barrier and empty states
on the other side of the barrier at the
same energy. Tunneling cannot takeplace into a region void of allowed
states
(2) The width of potential energy barrier
d (or depletion width, W) must be verythin. QM tunneling becomes significant
only if <10nm.
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 9/20
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 10/20
– Zener process always dominates in diodes where VBR<4Eg/q (~4.5
V in Si at 300K)
– Avalanche always dominates in diodes where breakdown voltage is
higher.
– The breakdown voltage associated with avalanching increasing with
increasing temperature, whereas the VBR associated with Zener
process decreasing with increasing temperature.
– The breakdown characteristics associated with the Zener process are
very “soft” compared to that associated with avalanching.
Current-Voltage Characteristics: Non-Ideal Effects
Breakdown Phenomenal: Comparison of Avalanche and Zener Breakdown
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 11/20
Application of diode breakdown
- clamping devices for voltage stabilization and
protection
- question: can you use the forward bias region as
clamping devices?
Current-Voltage Characteristics: Non-Ideal Effects
Breakdown Phenomenal: Example of Application
pads
to circuit
clamping/protection
VV
time time
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 12/20
Current-Voltage Characteristics: Non-Ideal Effects
Recombination-Generation
R-G current:
Refer to current arises fromthermal carrier recombination-
generation in the depletion region
that was assumed to be negligible
in the derivation of ideal diodeequation.
(1) Non-saturation reverse current
(2) Different slope in small forward
bias region
Different slope
Non-saturation
reverse current
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 13/20
Current-Voltage Characteristics: Non-Ideal Effects
Recombination-Generation
- In reverse bias, the carrier concentrations
in the depletion region are reduced below
their equilibrium values, leading to the
thermal generation of electrons and holes
throughout the region.
- The large electric field in the depletion
region rapidly sweeps the generated
carriers into the quasi-neutral regions,
thereby adding to reverse current.
Reverse Bias
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 14/20
Current-Voltage Characteristics: Non-Ideal Effects
Recombination-Generation
- In forward bias, the carrier concentrations
increase in the depletion region above
their equilibrium values, which giving rise
to carrier recombination in the region.
- Thus, one can view that the resulting
added forward current as arising from the
carriers that cannot make it over the
potential hill being partially eliminated via
recombination at R-G centers in thedepletion region.
Forward Bias
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 15/20
Current-Voltage Characteristics: Non-Ideal Effects
Recombination-Generation
Calculation of R-G Currents
Step 1: Summing either electrons or holes created/destroyed in
depletion region
dxt
nqA I
G R
X
X
G R
N
P
Step 2: recall
)()( 11
2
p pnn
nnp
t
n
n p
i
G R
dx p pnn
nnpqA I
N
P
X
X n p
iG R
)()( 11
2
Step 3: the R-G current can be given as below
Note: the net R-G rate is the same for electrons and
holes under steady state conditions
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 16/20
Current-Voltage Characteristics: Non-Ideal Effects
Recombination-Generation
Calculation of R-G Currents
Step 4: in reverse, the electron and hole concentration inside
depletion can be ignored, so R-G current can be easily
calculated as (W: depletion width)
)(2
1)(
2
1
,
2
/)(/)(110
0
kT E E
n
kT E E
p
i
n
i
p
iG R
T iiT een
p
n
n
Where
W qAn
I
Reverse Bias
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 17/20
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 18/20
Current-Voltage Characteristics: Non-Ideal Effects
High Current Phenomenal: High Level Injection
- The low-level injection begins to fail when the
minority carrier conc at the depletion region
edge on the lightly doped side approaches the
doping conc.
- In Si at room temperature this typically happens
at applied voltage a few tenths of a volt below
Vbi and further increased voltage leads to high-
level injection.- Under high-level injection, both the minority and
majority carrier conc adjacent to the depletion
region are perturbed.
- The majority carrier conc must increaseto maintain approximate chare neutrality in the
quasi-neutral regions.
- Detail analysis of high-level injection leads to a
predicted current varying roughly as exp(q/2kT).
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 19/20
Current-Voltage Characteristics: Non-Ideal Effects
High Current Phenomenal: Series Resistance
- The quasi-neutral regions have an
inherent resistance determined bythe doping and dimensions of the
regions.
- These combines to form a resistance
RS, in series with the current flowacross the junction.
- At low current levels, the voltage
drop across the series resistance,
IRS, is totally negligible compared tothe applied voltage across the
depletion region (or we call junction
voltage, VJ). Thus, VJ=V A.
8/11/2019 Ch1 5 PN IV Non Ideal
http://slidepdf.com/reader/full/ch1-5-pn-iv-non-ideal 20/20
- At current levels where IRS
becomes comparable to V A, theapplied voltage dropping appearing
across the depletion region is
reduced to
VJ=V A-IRS
- Effectively, part of the applied
voltage is wasted.
- To correct for the series resistance,
one merely replaces V A by
VJ=V A-IRS
(or V A IRS)
in previously derived I-V A
relationships.
Current-Voltage Characteristics: Non-Ideal Effects
High Current Phenomenal: Series Resistance