Research On Large Signal Model Of Mirowave And Millimeter-wave GaN HEMT

Gallium Nitride(GaN) high electron-mobility transistors(HEMTs) have been recently arising as a forefront in semiconductor devices due to its high frequency, high power density, and high efficiency properties. The characterization and large signal(or nonlinear) modeling of GaN HEMT would be an important technique in many aspects, including the device processing and structure optimization, the design of electronic circuit, and the improvement of circuit performance. Conventional Si and GaAs based field effect transistor models and modeling methods would fail to accurately characterize some unique physical properties of GaN HEMT devices, such as trapping effect, self-heating effect, harmonics and ambient temperature properties. Accurate large signal model of microwave and millimeter-wave GaN HEMT is significant for improved device performance, reduced design-to-production cycle of circuits and systems, yield enhancement, cost saving, and the development of massive system integration and applications, and so on. In view of the characteristic dimension scaled down, frequency and output power enhanced, and the further demands on high yield, characterization and modeling of microwave and millimeter-wave GaN HEMT would be significant for the development of GaN devices and circuits. As a result, based on the device principle, the large signal model of microwave and millimeter-wave GaN HEMT fabricated by domestic technology has been comprehensively researched by using empirical equivalent circuit modeling approach in this dissertation. The main content researched are as follows:1. Research on small signal equivalent circuit modeling of ultra wideband GaN HEMT. Due to the parasitic properties in microwave and millimeter-wave GaN HEMT devices, the small signal equivalent model with wide band characteristic has been developed by adding the parasitic resistance Rpgd between gate and drain terminal and the T type parasitic inductances for both gate and drain terminals in conventional small signal model. As a result, the fitting precision of small signal S-parameters has been improved at low and high frequency band. Combined with the direct extraction of element parameters, an optimization method for the values of element parameters has been used, based on the multi-dimensional objective error function related to bias voltages. This method largely overcomes the drawback of local minimum and non-physical element values resulted from conventional optimization method and the optimization under a single bias voltage. Based on the proposed modeling approach, the predicted S-parameters by the small signal models of three GaN HEMTs with different structures show that accurate results and effectiveness have been achieved over the wide frequency band of 40 GHz.2. Research on electrothermal large signal model of microwave and millimeter-wave GaN HEMT. Due to the nonlinear thermal conductivity of device materials, the model of nonlinear thermal sub-circuit model as a function of power dissipation has been developed by using the finite element method thermal simulation. Due to the asymmetric transconductance characteristics of GaN HEMT, the electrothermal large signal model including self-heating and trapping effects, based on the conventional Angelov empirical large signal model, has been developed using by improving and modifying the nonlinear drain-source current model and gate capacitance model which are dependent on gate voltages. The measurement of dynamic pulsed I-V has been carried out for the characterization of trapping effect of GaN HEMT. The proposed large signal model shows more accurate predictions of DC I-V, output power and efficiency properties as compared with the traditional large signal model. Moreover, based on the proposed modeling approach, the models of field plate GaN HEMTs with different structures have been developed, with the investigation of the large signal characteristics, which further verifies the accuracy and modeling effectiveness of the large signal model.3. Research on large signal model of high/low temperature GaN HEMT. Since GaN HEMT devices are widely used in high/low temperature environment, its electrical performance is not only sensitive to self-heating effect, but also to ambient temperatures. Due to the electrothermal effect in high/low temperature environment, the double thermal sub-circuit models have been developed by using the finite element method thermal simulations based on the physical mechanism of the electrothermal effect and heat transfer theory. The impact of self-heating and ambient thermal effects on the channel temperature of GaN HEMT have been investigated, and the related equivalent thermal resistances and thermal capacitances are extracted. The predictions of channel temperatures by the proposed model are more accurate than the results predicted by the traditional thermal sub-circuit model over the ambient temperature range of-55 oC to 175 oC. Moreover, the nonlinear formulas of drain-source current and gate capacitances are modified according to the electrothermal property of high/low temperature. The measured and simulated results show that good agreement has been achieved on the small signal S-parameters, the fundamental output power, the second and third harmonic output power, and the efficiency characteristics under the high/low temperature conditions.4. Research on scalable large signal model of GaN HEMT. The scalability of large signal model is an important tool to model large-size devices. The large signal model of GaN HEMT with moderate gate width has been used as a reference. In view of the physical geometry properties of gate finger number and unit gate width of GaN HEMT devices, as well as the thermal coupling effect and parasitic characteristics within multi-finger device, the scale rules of the large signal model, based on the thermal sub-circuit dependent on the device size dimension and the distributed region structure have been proposed. Compared with the on-wafer measurement results of the Ga N HEMT with different finger numbers and unit gate width, the predictions of the model has good agreement on the impedances characteristics of fundamental and high order harmonics, and the output power of fundamental and high order harmonics even under mismatching conditions. Furthermore, the simulated results of the scalable model are in high accuracy for the high power/efficiency power amplifier MMIC. The proposed scalable large signal model could be useful for the design and optimization of high efficiency power amplifier MMIC with large size of gate width, and to some extent be helpful for the large-size devices and the models for further research.