654_ch11 - ECE 654 Prof S Mohammadi Solid State Devices II...

ECE 654 Solid State Devices II
Prof. S. Mohammadi
- 158 -
Chapter 11
Light Emitting Diodes
In this chapter we discuss theory and implementations of light emitting diodes (LEDs). LEDs are
the basis for the development of lasers. In a junction LED photons of near bandgap energy are
generated by the process of spontaneous emission, in which a large population of electrons,
injected into a normally empty conduction band by a small forward bias, recombine with the
holes in the valence band. Spontaneous emission is different from stimulated emission that is
used in junction lasers. Spontaneous emission occurs at low forward bias voltages as opposed to
the stimulated emission that occurs at much higher forward bias and much higher current
densities.
LEDs do not require an optical cavity and mirror facets to provide feedback of photons. The
emitted photons have random phases, and therefore an LED is an incoherent light source. The
linewidth of the spontaneous emission is approximately equal to the photoluminescence
linewidth which is a few kT (typically 30-50nm at room temperature). Thus many optical modes
are supported, thus LED is a multimode optical source suitable for use with multimode fibers.
Advantages of LEDs as a light source are simpler fabrication process, lower cost, higher lifetime
and reliability and simple drive circuitry when compared to laser diodes. Ideally, the LED
exhibits a linear output light – current characteristics thus is suitable for analog modulations.
The light –current characteristics is less sensitive to temperature than that in a laser diode. Main
disadvantages of LEDs compared to laser diodes are lower output light power (makes them only
suitable for short distance telecommunications), smaller modulation bandwidth (less data
capacity), and harmonic distortion due to multimodal output. Superradiant LEDs have output
light power comparable to that of lasers.
Basic Principles of LEDs:
In a forward biased PN junction of a direct bandgap material which has high doping
concentration on both P and N side, excess minority carriers (electrons in the P-side and holes in

ECE 654 Solid State Devices II
Prof. S. Mohammadi
- 159 -
the N-side) recombine with the majority carriers through a radiative recombination process. The
resulting photons emitted through the radiative recombination process has an energy of about the
direct bandgap of the material
g
h E ν = (11.1)
Transitions of band to band, impurity level to band, donor to acceptor and excitonic transitions
all contribute to the spontaneous emission. The rate of the radiative recombination is normally
proportional to the forward bias injection rate, and hence to the diode current given by
0 0
1
Forward
qV
p n n p
kT
h e
D p D n
I qA e
L L
 
 
= + -
   
  
(11.2)
where A is the area of the LED junction, p
n0
and n
p0
are, respectively, the concentration of the
minority carriers at the edge of the depletion region in N-type and P-type materials, and L
h
and
L
e
are the Debye length of holes and electrons as the minority carriers.