Modeling And Amplifier Design Based On AlGaN/GaN HEMTs

After Silicon (Si) and Gallium Arsenide (GaAs), Gallium Nitride (GaN) is becoming acknowledged and honored as the third generation of semiconductor because of its wider band-gap, higher break-down voltage and saturated electron speed, better stability and thermal conductivity. And these good characters give the AlGaN/GaN high electron mobility transistors (HEMTs) great advantage over other devices in high power, high temperature and high frequency applications.As the market of wireless communication keeps on developing, people are giving more expectation on reliability and circuit performance of AlGaN/GaN HEMTs. In decades, a lot of work has been done on the GaN material and devices. However, the theories of reliability, small-signal and large-signal models are far from maturity. Most charactering methods of AlGaN/GaN HEMTs is still the same as ones applied for MESFET and GaAs, which can not precisely describe the high frequency behavior of GaN-based HEMTs.In this paper, a new 18-element, 19-parameter small-signal equivalent circuit model is proposed. Comparing to classic FET model, the new small-signal model has taken distributing capacitance in high frequency and the impact caused by gate-close-to-source structure into our consideration. Based on the topology of our new equivalent circuit model, we have developed a corresponding direct parameter extraction method written in MATLAB. The parasitic elements are extracted from both Cold-FET and pinch-off bias to obtain more precise results. And the intrinsic part is directly extracted. All parameters needed in this process are determined by the device structure rather than optimization methods. This guarantees the consistence of parameter value with physical meaning. Our new small-signal model can accurately predict small-signal behavior of HEMTs up to 20GHz. And based on this small signal model, we also build up a Large-signal model for AlGaN/GaN HEMTs.Load-Pull measuring system plays an important role in characterizing Large-signal behavior of radio-frequency devices. But we can hardly find any systematic reference or books on this subject. In this paper, based on our using experiences, we summarized the key points about how to build up, calibrate and make use of the Load-Pull system. Also, several advices are proposed for purchasing and utilizing the system.Based on self-developed AlGaN/GaN HEMT with 2.5-mm gate width technology on SiC substrate, the X-band two-way combined GaN solid-state power amplifier module is fabricated. The module consists of AlGaN/GaN HEMT, Wilkinson power hybrids, DC-bias circuit and micro-strip matching circuits. For the stability of amplifier module, the special RC networks at the input and output, the resistor between the DC power supply and gate of transistor at the input and the 3/4λWilkinson power hybrids are used for cancellation of low frequency self-oscillation and crosstalk of each amplifier. Under Vds=27V,Vgs=-4.0V, CW operating condition at 8GHz, the amplifier module exhibits 41.46dBm maximum output power with power added efficiency of 23.4%, and the synthesis efficiency is 82.3%.