The emission intensity of LEDs decreases with increasing temperature. This decrease of the emission intensity is due to several temperature-dependent factors including (i) non-radiative recombination via deep levels, (ii) surface recombination, and (iii) carrier loss over heterostructure barriers.

Near room temperature, the temperature dependence of the LED emission intensity is frequently described by the phenomenological equation

T - 300K

(5.38)

Ti

where T1 is the characteristic temperature. A high characteristic temperature, implying a weak temperature dependence, is desirable.

It is interesting to note that both LEDs as well as semiconductor lasers have a distinct temperature dependence of the emission intensity. In LEDs, the decrease is expressed in terms of the “T1 equation”. In semiconductor lasers, the threshold current, i. e. the electrical current required for the onset of lasing, increases. In lasers the increase in threshold current is expressed in terms of the well-known T0 equation. This equation is given by

(5.39)

I

=I

th

T0

th 300K exp

T - 300K

where Ith is the threshold current of the laser. Note the formal similarity of the “T1 equation” (Eq. 5.38) and the “T0 equation” (Eq. 5.39). Both equations are purely phenomenological equations intended to describe the experimental results without a strong theoretical framework allowing the derivation of the equations from basic principles.

500 400 300 200

Г, = 295 К GalnN/GaN green LEDs (525 nm)

Slope = - 1/7Ї

Г, = 1600 К GalnN/GaN blue LEDs (470 nm)

LED light output vs. T:

7-, = 95 К AlGalnP/GaAs red LEDs (625 nm) ~~і I

с 3 с II

100 80 60 50 40 30 20

о ‘є и >  к аг

,= l cxp T-300 К 1 ;|300К exP T,

X

X

80

100

-20 0 20 40 60 Ambient temperature T (°С)

X

Fig. 5.7. Characteristic tem­perature T of GalnN/GaN blue, GalnN/GaN green, and AlGalnP/GaAs red LEDs near room temperature (after data from Toyoda Gosei Corp., 2000). More recent data (Toyoda Gosei Corp., 2004) show the following values for 7*i: Blue GalnN LED, 460 nm, T = 1600 K; Cyan GalnN LED, 505 nm, T = 832 K; Green GalnN LED, 525 nm, T = 341 K; Red AlGaInP LED, 625 nm, 7*1 = 199 K.

Experimental results of the temperature dependence of the emission intensity are shown in Fig. 5.7 (Toyoda Gosei Corporation, 2000). The figure shows the temperature dependence of the

emission intensity at a constant current for a blue GaInN/GaN, a green GaInN/GaN, and a red AlGaInP/GaAs LED. Inspection of Fig. 5.7 reveals that the blue LED has the highest T1 and the red LED has the lowest T1. III-V nitride LEDs have deeper wells so that carrier confinement is more effective in III-V nitride structures than in the III-V phosphide structures.