Page 110 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

104 Photonic Devices

6.2 Recombination of Excess Carriers—Direct
Generation of Light

LEDs made from semiconductors generate light via the same mecha-
nism as do LEDs made from polymers: by recombination of excess
concentrations of electrons and holes. The wavelength of the emitted
light results from conservation of energy; that is, the energy differ-
ence between the hole state and the electron state before recombina-
tion is the energy of the photon emitted. The usual conversion, =
1240 nm/Energy (eV), gives the corresponding wavelength. In this
chapter, we will describe light emission principles for semiconductor
LEDs, since they are commercially available, whereas polymer LEDs
are still in the laboratory stage.
The forward bias voltage on a p-n junction creates excess minority
carrier concentrations near the edge of the depletion region. The ener-
gy versus distance diagram is shown in Fig. 6.1.
The excess carrier density is maintained at a constant value by a
balance between the carrier generation caused by the bias voltage and
recombination of the excess minority carriers and majority carriers in
the same spatial region. The balance means that the generation rate
and recombination rates must be equal. The current in the diode is

n p (x = 0) = n p0 (e qV A /kT – 1)

E f
qV A
Energy E f

p n (x = 0) = p n0 (e qV A /kT – 1)

Figure 6.1. Energy–distance diagram of a p-n junction in forward bias. The applied
voltage, V A , induces excess concentrations of holes and electrons and also reduces the
spatial separation between holes and electrons. The steady-state is maintained by re-
combination of electrons and holes in proportion to the excess carrier densities. Direct
electron–hole recombination causes light emission.

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