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Lecture 12. OUTLINE pn Junction Diodes (cont’d) Deviations from the ideal I-V R-G current series resistance high-level injection Reading : Pierret 6.2. Deviations from the Ideal I-V. R. F. Pierret , Semiconductor Device Fundamentals , Figure 6.10. Reverse-Bias Current - PowerPoint PPT Presentation
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Lecture 12
OUTLINE• pn Junction Diodes (cont’d)– Deviations from the ideal I-V• R-G current• series resistance• high-level injection
Reading: Pierret 6.2
Deviations from the Ideal I-VForward-Bias Current
(log scale)
)10ln(/)log(
log)log()log( /0
qkT
Vconst.ekT
qVconst.
eII
AA
kTqVA
Ideally,
Lecture 12, Slide 2EE130/230A Fall 2013
R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.10
Reverse-Bias Current(linear scale)
Ideally, constant 0 II
Effect of R-G in Depletion Region• The net generation rate is given by
• R-G in the depletion region contributes an additional component of diode current IR-G:
levelenergy state- trap
and where
)(τ)(τ/)(
1/)(
1
11
2
T
kTEEi
kTEEi
np
i
E
enpenn
ppnn
npn
t
n
t
p
TiiT
dxt
pqAI
GR
x
xGR
n
p
Lecture 12, Slide 3EE130/230A Fall 2013
Net Generation in Reverse Bias• For reverse bias greater than several kT/q,
in
ip
iGR n
p
n
nWqAnI 11
00
ττ2
1 τ where
τ2
Lecture 12, Slide 4EE130/230A Fall 2013
R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.15(a)
kTqViGR
AWeqAnI 2/
Net Recombination in Forward Bias• For forward bias:
Lecture 12, Slide 5EE130/230A Fall 2013
R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.15(b)
High-Level Injection (HLI) Effect• As VA increases, the side of the junction which is
more lightly doped will eventually reach HLI:
significant gradient in majority-carrier profile
Majority-carrier diffusion current reduces the diode current from the ideal case.
nn > nno for a p+n junction
or
pp > ppo for a pn+ junction
Lecture 12, Slide 6EE130/230A Fall 2013R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.17(a)
Effect of Series Resistance
Lecture 12, Slide 7EE130/230A Fall 2013
R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.16
Summary: Deviations from Ideal I-V
Lecture 12, Slide 8
C: Excess current under small forward bias is due to net recombination in the depletion region.
B: Excess current under reverse bias is due to net generation in the depletion region.
A: At large reverse biases (high E-field), large reverse current flows due to avalanching and/or tunneling
WI GR kTqV
GRAWeI 2/
EE130/230A Fall 2013R. F. Pierret, Semiconductor Device Fundamentals, Figure E6-9
kTqVAeI 2/
• At large forward biases (high current densities)
D: high-level injection
E: series resistance limit increases in current with increasing forward bias voltage.