OE_50200: Compound Semiconductors

1
OE_50200 Compound Semiconductors Lecture 6 LED
Basics Electrical Properties ???????? Nationa
l Dong-hwa University, Hualian, Taiwan 2008.
03.21
2

• Points to be covered in Lecture 6
• Derivation of ideal Diode I-V characteristics
• Non-ideal (practical) I-V chracteristics

3
Outlines

• Electrical Characteristics
• Ideal I-V
• Nonideal I-V
• - Evaluation of parasitic resistance
• Heterostructures

4
Shockley Equation for Ideal p-n Diodes (1)
n ni exp(Efn-Ei)/kT
p ni exp(Ei-Efp)/kT
npni2exp(Efn Efp)/kT
V (Efn Efp)/e
npni2exp(eV/kT)
5
Shockley Equation for Ideal p-n Diodes (2)
6
Shockley Equation for Ideal p-n Diodes (3)
?
7
Shockley Equation for Ideal p-n Diodes (4)
8
Threshold Voltage of p-n Junction Diode
Shockley eq.
The threshold voltage Vth ? VD
9
I-V Characteristics of Different Semiconductors
10
Forward Voltage for Diodes of Different
Semiconductors

• In general, Vf ? Eg except the III-nitrides
  because
• 1.large bandgap discontinuities, 2.high ohmic
  contact resistance 3. low p-type conductivity
  4. parasitic voltage drop across n-buffer
• Large Vf will cause excess ohmic heating and
  degrades the performance

11
Non-ideal Diode I-V Characteristics
The non ideal (real) I-V usually written as
ndeal ? 1.1 1.5 typical As high as 2 has been
seen for III-V asenides and phosphides . As high
as 6 for GaN/GaInN
The parallel resistance can be evaluated by the
slope near the origin
12
Evaluation of Diode Series Resistance
The series resistance can be evaluated at high
current
The slope of IdV/dI gives Rs,the intercept can be
used to deduced the nideal
13
Stabilization of Diode Voltage Against
Temperature

• Under constant voltage
• Due to the exponential dependence on voltage, a
  small voltage change in the supply voltage or the
  threshold voltage change with temperature, the
  current will change significantly. The series
  resistance can stabilize the current.
• Under constant current,
• Independent of Vth and temperature

14
Basic PN Junction Diode Structures

• Advantage of Hetero-junctions
• Higher Carrier confinement, higher recombination
  rate.
• RBnp
• tltltdiffusion length,
• No light re-absorption
• Waveguiding for edge emitting lasers

15
Hetrostructure and Band Alignment type I
Straddle Lineup
GaAs
AlxGa1-xAs
In.53Al.47As
In.53Ga.47As
6248
4357
In.53Ga.47As
InP
GaSb
AlSb
4357
16
Hetrostructure and Band Alignment type II
Staggered Lineup
InP
In.53Al.57As
AlSb
InAs
InAs
GaSb
Broken bandgap
17
Band Structure Lineup
18
Heterostructure Effects in Diodes

• Heterostructure is preferable for confinement due
  to
• The generated light is not reabsorbed
• Increased current injection by DEc,,and ,DEv for
  electrons and holes,
• In/Ip exp(DE/kBT),
• Increased n, p concentration with thickness will
  increase the carrier recombination

SH- single heterostructure
DH- double heterostructure
19
Resistance of Hetero-interface

• The barrier at the interface will inhibit charge
  transport and cause a resistance which can have a
  strong deleterious effect on device performances.
  
• Parabolical grading of the chemical composition
  can completely eliminate the band discontinuity.

20
Electron Blocking Layers
Carriers tend to escape from the active region
into the confinement layers. High temperature
promotes the carrier escape. The electron
leakage is larger than the hole leakage, a layer
of higher bandgap energy is often used to solve
this problem. Doping may screen the barrier in
the valence band result in no barriers for the
flow of holes.
21
Exercises for Lecture 6