Optoelectronic Semiconductor Devices Principals and Characteristics

4.2 BASIC CHARACTERISTICS OF LED 
The output light of the LEDs originates from the spontaneous emission in the active layer, which in turn 
is a product of the radiative recombination of the injected electrons and holes. 
The band structure and the distribution of injected carrier density under a forward bias are shown 
schematically in Figure 
27.
for a p-type/p-type/n-type DH composed of direct band-gap semiconductors. 
Figure 27.: Energy band diagram and injected carrier distribution for a double heterostructure under a 
forward bias. 
[4]
 
Injected carriers distribute uniformly in the active layer because its thickness is less than the carrier 
diffusion length (a few µm). The thickness of the active layer in LEDs is ordinarily between 0.5 and 2 
µm. 
The electron and hole densities in the active layer under the injection excitation can be expressed with the 
equations:

---

(29) 
0
n n
n
=
+ ∆
(30) 
0
p
p
p
=
+ ∆
and
(31) 
n
∆ = ∆p
where
∆n and ∆p - are the injected electron and hole densities. 
n
0
and p
0
- are the electron and hole densities in the active layer in a thermal equilibrium state and 
. 
2
0 0
i
n p
n
=
Current-light output power and current-voltage characteristics for 1550 nm-band InGaAsP/InP surface-
emitting LED are shown in Figure 
28.
Figure 28.: Typical device characteristics of a surface-emitting 1550 nm-band InGaAsP/InP LED. 
[4]
 
The light power emitted in the active layer is given by the product of the number of photons emitted and 
the photon energy, hν. 
Consequently, the light output power of the active layer is proportional to the injected current density in 
the low-excitation range, while in the high-excitation rate it is proportional to the square of the injected 
current density if the lifetimes are constant.