Three methods

namely Voltage-Method

Pin-Method and Spectra-Method

for determining the junction temperature of GaN-based white LEDs were studied.In the Voltage-Method

we show that the forward voltage depends on the junction temperature linearly with uniform current.In the Spectra-Method

we show that the ratio

R

=

W/B

is strongly linear with the junction temperature as changing the ambient temperature or changing the forward current

where W and B are the entire and the blue part radiant power of the LED′s emission

respectively.The physical mechanism of Spectra-Method and the factors of controlling the junction temperature were also discussed.Increasing either the ambient temperature or the forward current can lead to the rise of junction temperature and the shift of emission peak wavelength of GaN-based LED chips.These two factors can cause the decrease of excitation and emission efficiency of phosphor much greater than that of LED chips.To lower the junction temperature of white-LEDs

we need to consider the contact and series resistance

the emission efficiency

the thermal conductivity of encapsulation material

the design of reflector cup and pins

and the cooling surface area.