Low-frequency response to AlGaAs double heterojunction LED's

At low frequencies, the optical response of a light emitting diode (LED) to analog intensity modulation is found to decrease, due to the junction-heating effects. This low-frequency response is analyzed using approximated I-P characteristics, and is compared with the experimental results for AlGaAs double heterojunction LED's. The transfer function of the low-frequency response can be expressed by
1 - (1- \phi)/(1+ j\omega\tau_{\sigma})
, where φ and tau
σ
are the degradation of the fundamental amplitude at lower frequencies normalized to higher frequencies and the thermal time constant with a value of about 10 µs, respectively. It is found that the low-frequency response correlates with the linearity of the I-P characteristics, using the low-frequency degradation factor
\psi
expressed by
-10 \log(1- \phi)
in decibels and the linearity coefficient γ defined as
{P(1.5 I{o}) - P(I_{o})}/{P(I_{o}) - P(0.5 I_{o})}
, where
P(1.5 I_{o})
denotes the power at 1.5 times of the basic current I
o
. Also, the current dependence and modulation degree dependence on both degradation factor
\psi
and second-harmonic distortion are discussed.