1 Introduction As a key component of semiconductor illumination, power LEDs affect the luminous flux, light extraction efficiency, wavelength red shift, forward voltage drop and other luminosity, chromaticity and electrical parameters, which affect the life and reliability of the device [1. 2], therefore, theoretical analysis and experimental measurement of the junction temperature and thermal resistance of the power LED chip are particularly important. At present, the main methods for measuring junction temperature and thermal resistance of semiconductor devices are [3]: infrared thermal imager method, electrical parameter method, spectroscopy method, photothermal resistance scanning method and optical power type method. Among them, the electrical method uses the relationship between the LED forward voltage drop and temperature, and measures the average temperature of the LEDpn junction and the thermal resistance of the device. It is the most commonly used method for measuring the thermal parameters of LED devices. The forward voltage method measures the average temperature of the pn junction of the LED chip [4]. Although the specific temperature field distribution of the chip pn junction reflects the actual working state of the chip, the average junction temperature is equivalent to the thermal performance of the LED. Significance. This paper first introduces the principle of measuring the thermal resistance by the forward voltage method, the device for measuring the temperature coefficient K and the junction temperature of the LED, and the specific test process. The temperature coefficient and thermal resistance of the sapphire substrate-mounted LED and the silicon-substrate flip-chip LED were measured separately. Finally, by comparing the actual thermal resistance values ​​and measured values ​​of the LED samples, the error is controlled at around 5%, confirming the feasibility of this measurement method. 2 Measurement principle and process 2.1 The principle of measuring the thermal resistance by the forward voltage method For semiconductor devices, including LEDs, the relationship between the current through the voltage at both ends is and The calculation of junction temperature is based on the characteristic that the forward voltage drop VF of the LED is linear with the temperature of the PN junction. The junction temperature of the LED is obtained by measuring the forward voltage drop at different temperatures. In the test, the pn junction is the object to be measured, and it is regarded as a temperature sensor. The temperature change of the pn junction is output through the temperature sensitive parameter, that is, the change of the forward voltage drop of the pn junction, thus eliminating the introduction of the temperature sensor. Measurement error. 
When the constant current is driven, the influence of the temperature rise on the voltage is determined by the first item, and the voltage change caused by the internal resistance is negligible. 
As the ambient temperature increases, the forward voltage drop across the LED decreases monotonically, which can be approximated as 
