A Numerical Study On Thermal Performance Of PHEMT Based On The Non-isothermal Energy Balance Model

The GaAs and other compound semiconductor devices have wide range of applications for theircharacteristics of high-frequency, high-bandwidth and high-power. But with the development of theintegrated electronic component and miniaturization, their powers are also greatly improved, whichwill cause an exponential growth of the local heat flux for a semiconductor chip. If the effectivecooling methods are not taken, the temperature of the device will increase rapidly. As a result, the lifeof the electronic components will be greatly reduced, leading to a serious failure of functionality.Therefore, we need reliable modeling techniques for semiconductor components to study theirelectrical and thermal behavior and understand the mechanisms of heat generation and heat transfer.Based on it, the location of the hot spots can be determined and the design and parameter optimizationof a device can be guided effectively.In this thesis, we analyzed the mechanism of heat generation in GaAs PHEMT devices firstly. Inconditions of high electrical field and high heat flux, the electrons mostly scatter with the opticalphonons, which phonons are slow and they make no contribution to heat transport. Consequently, theenergy will not be carried away from the hottest region (hot spot) to other regions until the opticalphonons decay into the faster acounstic modes. This process is known as the sub-continuous heattransport process. In this process, the temperature of the carriers is much higher than that of the lattice,and the non-isothermal energy balance model can be used to simulate the electrical and thermalcharacteristics of the GaAs PHEMT devices.Several numerical simulation models for InGaAs/AlGaAs PHEMT devices are discussed,including mobility model, carrier generation composite model, band narrowing model, Augerrecombination model, collision ionization model and thermal conductivity model. Moreover, and thework function is calculated. With the help of commercial ATLAS TCAD software, the numericalsimulation for InGaAs/AlGaAs PHEMT are performed. The influences of the electrical and thermalcondtions as well as the finger number on the electrical current, voltage, the distributions of the hot spotand the temperature are discussed. The numerical results show that as the gate voltage increases, thehot spot temperature increases, together with a corresponding expansion of the hot region. Moreover,the power of the device increases with the increase of the finger number, but the position of hot spotand the temperature distribution of the device do not be affected by the finger number. In addition, asthe convective heat transfer coefficient of the substrate increases, the temperature of the hot spot andthe average temperature of the device are decresed, where there is a linear relationship between the temperature of the hot spot and the convective heat transfer coefficient.A comparison between the results of the numerical simulation and the experiment shows that thehighest temperatures for different operation conditions obtained by the experiment have the same trendwith those obtained by the numerical simulation. Nevertheless, the latter are slightly larger than theformer for the reason that in the experiment, the highest temperature that can be measured is the surfacetemperature of PHEMT device, while the highest temperature obtained by the numerical simulation islocated in the channel of PHEMT. It verifies that the numerical results are comparable with theexperimental ones.Recently, a new concept of entransy was introduced based on analogy between electricalconduction and heat conduction. It is defined as half of the product of internal energy and temperatureof an object, with the physical meaning of the ability to transfer heat over a period of time. In heattransfer processes, heat is conserved but entransy is not conserved. The entransy dissipation is ameasure of the heat transfer irreversibility. The thermal resistance can be defined as the ratio of theentransy dissipation rate to the square of the heat transfer rate, which can be used to describe themulti-dimensional heat conduction problem. In this thesis, we apply the entransy-theory-based thermalresistance to the analysis of the thermal performance of PHEMT device, and explore the relationshipbetween the thermal resistance and the performance. It is shown that the average temperature of thedevice increases with the increase of the substrate temperature, leading to a decrease of the thermalconductivity of the semiconductors. The decrease of the thermal conductivity ability results in anincrease of the entransy-theory-based thermal resistance. Moreover, when the ambient temperature isconstant, the larger the convective heat transfer coefficient of the substrate, the smaller theentransy-theory-based thermal resistance, and the better the cooling effect. Therefore, theentransy-theory-based thermal resistance is applicable to describe the heat transfer performance of thePHEMT devices.