Transcript Lecture 12

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
(linear scale)
Ideally, I   I 0  constant
Forward-Bias Current
(log scale)
Ideally,

log( I )  log( I 0 )  log e qVA / kT

 qVA 

kT 
qVA 


 const.  
 log( e)  const. 
kT


ln( 10)
EE130/230A Fall 2013
Lecture 12, Slide 2
Effect of R-G in Depletion Region
• The net generation rate is given by
ni  np
p n


t t τ p (n  n1 )  τ n ( p  p1 )
2
where n1  ni e ( ET  Ei ) / kT and p1  ni e ( Ei  ET ) / kT
ET  trap - state energy level
• R-G in the depletion region contributes an additional
component of diode current IR-G:
I R G
EE130/230A Fall 2013
p
 qA
dx
 x p t
R G
xn
Lecture 12, Slide 3
Net Generation in Reverse Bias
R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.15(a)
• For reverse bias greater than several kT/q,
I R G
qAniW
1  n1
p1 

where τ 0   τ p  τ n 
2τ 0
2  ni
ni 
EE130/230A Fall 2013
Lecture 12, Slide 4
Net Recombination in Forward Bias
R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.15(b)
• For forward bias:
I R G  qAniWe
EE130/230A Fall 2013
qVA / 2 kT
Lecture 12, Slide 5
High-Level Injection (HLI) Effect
• As VA increases, the side of the junction which is
more lightly doped will eventually reach HLI:
nn > nno for a p+n junction
or
pp > ppo for a pn+ junction
 significant gradient in majority-carrier profile
Majority-carrier diffusion current reduces the diode
current from the ideal case.
EE130/230A Fall 2013
Lecture 12, Slide 6
R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.17(a)
Effect of Series Resistance
R. F. Pierret, Semiconductor Device Fundamentals, Figure 6.16
EE130/230A Fall 2013
Lecture 12, Slide 7
Summary: Deviations from Ideal I-V
• At large forward biases
(high current densities)
D: high-level injection
 I  e qVA / 2 kT
E: series resistance
limit increases in current
with increasing forward
bias voltage.
B: Excess current under reverse bias
is due to net generation in the
depletion region.
I RG  W
A: At large reverse biases (high E-field),
large reverse current flows due to
avalanching and/or tunneling
EE130/230A Fall 2013
C: Excess current under small forward
bias is due to net recombination in
the depletion region.
I R G  We qVA / 2 kT
Lecture 12, Slide 8
R. F. Pierret, Semiconductor Device Fundamentals, Figure E6-9