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.