ELEC425/6261
Assignment 3
1. GaAs has an effective density of states at the conduction CB Nc of 4.71017 cm-3 and an
effective density of states at the VB edge Nv of 1018 cm-3. Given its bandgap Eg of 1.42 eV
calculate the intrinsic concentration and the intrinsic resistivity at room temperature (take as 300
K). Where is the Fermi level? Assuming that Nc and Nv scale as T3/2, what would be the intrinsic
concentration at 100 C? If this GaAs crystal is doped with 1018 donors cm-3 (such as Te), where
is the new Fermi level and what is the resistivity of the sample? The drift mobilities in GaAs are
shown in the following table.
Dopant concentration (cm-3)
0
1015
1016
1017
1018
e (cm2 V-1 s-1)
8500
8000
7000
4000
2400
h (cm V s )
400
380
310
220
160
2
-1 -1
2.
AlGaAs LED emitter An AlGaAs LED emitter for use in a local optical fiber network
has the output spectrum shown in the following Figure. It is designed for peak emission at 820
nm at 25C.
(a)
Why does the peak emission wavelength increase with temperature?
(b)
Why does the peak intensity decrease with temperature?
(c)
What is the bandgap of AlGaAs in this LED?
(d)
The bandgap, Eg, of the ternary alloys AlxGa1-xAs follows the empirical expression,
Eg(eV) = 1.424 + 1.266x + 0.266x2.
What is the composition of the AlGaAs in this LED?
3. (a) It is found that approximately 200 W is coupled into a multimode step index fiber from a
surface emitting LED when the current is 75 mA and the voltage across the LED is about 1.5 V.
What is the overall efficiency of operation?
(b) For a particular AlGaAs LED emitting at 850 nm it is found that r = 50 ns and nr = 100
ns. What is the internal optical power generated at a current of 100 mA?
ELEC425/6261
4. Show that the change in the emitted wavelength  with temperature T from a LED is
approximately given by
dE 
1
d
hc
 2 k  g 

2
dT
dT 
( E g  12 kT ) 
where Eg is the band gap. Consider a GaAs LED. The band gap of GaAs at 300 K is 1.42 eV
which changes (decreases) with temperature as dEg/dT = - 4.5  10-4 eV K-1. What is the change
in the emitted wavelength if the temperature change is 10 C?
5.
LED luminous flux
Consider a particular green LED based on InGaN MQW active region. The emission wavelength
is 528 nm. At an LED current of 350 mA, the forward voltage is 3.4 V. The emitted luminous
flux is 92 lm. Find the power conversion efficiency, external quanmtum efficiency, luminous
efficacy and the emitted optical power (radiant flux)? (Data for Osram LT CPDP)