利用温变电容特性测量发光二极管结温的研究

结区的温度, 简称结温, 是发光二极管(LED) 的重要参数之一, 它对LED 器件的出光效率、光色、器件可靠性和寿命均有很大影响, 准确测量LED 器件的结温对制备LED 芯片、器件封装和应用有着重要的意义. 本文利用反向偏压下的LED的势垒电容随温度变化的特性, 提出了一种LED结温测量的新方法. 论文首先测量和分析了LED在室温下反向偏压时的电容-电压(C-V)曲线和不同反向偏压下的电容-温度(C-T)曲线, 结果表明, 在合适的偏压下, LED的电容随温度的增大而显著增加, 并呈现良好的线性关系. 在LED工作中监测其电容的变化, 并与C-T曲线进行对比, 实现了LED结温的测量, 其测量结果和传统的正向电压法的结果相对比, 两者符合较好. 最后, 利用上述方法测量了LED 在恒流和恒压条件下的结温的实时变化过程. 较传统的结温测量方法, 本方法的优点在于只须要一次定标测量, 且可实现LED在任意电压和电流下的结温测量.

Junction temperature measurement of light-emitting diodes using temperature-dependent capacitance

Junction temperature, as one of the most important properties of light-emitting diodes (LEDs), has great impact on LEDs’ power efficiency, luminosity, reliability, life-time, and so on. Precise measurement of junction temperature for LED device is quite important in the research of chip’s fabrication, device packaging and related applications. In this paper, we propose a new approach to measure the junction temperature of LEDs by using temperature-dependent capacitance. The capacitance of white LEDs at room temperature is measured and found to be decreased first and then increased with an increasing reverse bias. Equivalent model using vertical and horizontal capacitances connected in parallel is proposed to qualitatively explain the variation of capacitance under different reverse bias. Result obtained from the model fitting agrees well with the experimental result. The capacitance-temperature (C-T) curve of white LEDs under different reverse bias is measured and analysed. Results show that the capacitance of LEDs is sensitive to temperature at all biases. Under a reverse voltage of 0.5 V, the capacitance has the maximal response of 1.971 pF/℃ and a good linear temperature-dependent property. The C-T curve is used as the calibration for the measurement of junction temperature. By monitoring the change of capacitance of the working LEDs and comparing it with the C-T curve, the junction temperature of the LED device is successfully measured. The junction temperature of a white LED obtained by the proposed C-T method is compared with that by tranditional forward voltage method, and they are in good agreement. The C-T method is also used to measure the real-time junction temperatures of white LEDs under a constant current of 350 mA and a constant voltage of 3.2 V, respectively. In both conditions, the junction temperature of an LED needs approximately 110 sec to rise from room temperature to a steady value, and subsequently needs approximately 500 sec to fall back to room temperature after the LED is turned off. Compared with traditional methods, C-T method only needs to measure one calibration and this calibration can be applied to LEDs working at any current and voltage. Therefore, C-T method is a simple and flexible alternative to the existing technique of temperature measurement in electronic device.