Device Physics And Related Experimental Studies Of X-band AlGaN/GaN High Electron Mobility Transistors

Wide bandgap AlGaN/GaN high-electron-mobility transistors (HEMT), which are suitable for high temperature, high frequency and high power device applications, have been regarded as the next generation technology all over the world. However, many problems of the lattice mismatch and thermal mismatch exist in AlGaN/GaN HEMT materials grown by heterogeneous epitaxial technology. Especially in wurzite GaN based transistor structures, the spontaneous polarization and piezoelectric polarization is very large, lots of mechanism of device physics effects are still ambiguous. And the related experimental studies in AlGaN/GaN HEMT devices such as characterization and strain analyse of heterostructure materials, ion implantation doping of GaN, ohmic contact and current collapse effects have not been fundamentally solved.The work presented in this dissertation focused on the key device physics problems and related experiments of X-band AlGaN/GaN HEMTs, and the major achievements and results of the dissertation are listed as followings.1. Based on the research of spontaneous polarization and piezoelectric polarization in AlGaN/GaN HEMTs, the relationship between the polarizability and lattice constants and the elastic coefficient is obtained in GaN-based heterojunction. With the principle of electroneutrality equilibrium, the physical model between the polarization induced sheet charge and the sheet 2DEG concentration for undoped Ga-face AlGaN/GaN HEMT structures is achieved.2. By the high resolution X-ray diffraction (HRXRD) technology, the accurate measurement methods of lattice parameters in GaN-based materials have been established, and the horizontal and vertical strain of the GaN heterojunction is conducted by further research. By means of Williamson-Hall method, the mosaic structure parameter of the epitaxial materials is measured in order to clarify the dislocation types and calculate the dislocation density in GaN-based materials. Then the accurate calculations of the dislocation densities of screw dislocation and edge dislocation in GaN materials have been made, and the total dislocation density is about 109cm^-2. 3. Si-ion implantation for nonalloyed contacts to an GaN heterostructure materials has been investigated, and highly-doped GaN material is obtained by means of the rapid thermal annealing(RTA) technology at relatively lower temperature (<1100℃), which offers a new technical approach for high quality ohmic contact. Results shows that the sheet electron concentration is 2×10¹⁵cm^-2 and the square resistance is 100?/â–¡after rapid thermal annealing at 1100°C for the Si ion implantation into GaN with doses of 10¹⁶cm^-2 at energy of 100keV,and the threading dislocation densities of the samples is decreased to 1.55×10⁹cm^-2. Meanwhile, the results of photoluminescence(PL) spectra show two blue luminescence (BL) bands with energy of 2.61eV and 2.67 eV respectively, both of which differ from the previous reported results. Further studies indicate that the 2.61 eV BL band emission is attributed to an electron transition between the donor level ON and the deep acceptor complex level VGa-SiGa, and the energy of 2.67 eV BL band is considered as the transition from the shallow donor level SiGa at 20meV under the conduction band to the deep complex acceptor level VGa-ON.4. High quality and multilayer metal ohmic contact to the unintended doped GaN with low specific contact resistivity has been achieved. Ohmic contact containing Ti(15nm)/Al(220nm)/Ni(40nm)/Au(50nm) four-layer metals have displayed the lowest contact resistivity on unintended doped GaN. After rapid thermal annealing at 900°C for 1 min with N2 ambient, the lowest contact resistivity of 1.26×10-7Ω·cm² is obtained. The studies of X-ray diffraction and Auger electron spectroscopy have been made to investigate the microstructure of the annealed contacts. The key to the success of the ohmic contact is the Ti layers placed between the Al layer and GaN. Upon rapid thermal annealing, there occurrs both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film, Ti reduces the native oxide on GaN and the in-diffusion of this leads to the formation of TiN when Ti reacts with GaN, a high concentration of nitrogen vacancies is created near the interface, causing the GaN to be heavily doped n-type. While the out-diffusion of this leads to the formation of low work function Ti-Al intermetallic phase. The intimate contact between the low work function intermetallic and n-GaN results in a low barrier height, allowing electron to flow in direction through the heterojunction interface by tunnel effect emission.5. Based on virtual gate model, the physical mechanism of current collapse has been analyzed. In order to eliminate the self-heating effects of AlGaN/GaN HEMT , A special device with only 10μm gate-width is fabricated to investigate current collapse effects. A new experimental method with pulsed signal to study current collapse phenomena is established. Results shows that the variation of pulse frequency and pulse width are both induced a change of current collapse of GaN HEMTs, which is related immediately with the mechanism of electron capture and release in the surface states in the device. The optimization of the structure parameters for the field-plated GaN-based HEMT is achieved to eliminate current collapse by reducing the peak electron temperature in the device channel. The degree of current collapse of the GaN HEMTs after passivation is reduced to only 4.7%.6. Optimization design of various structure parameters and device processes, which is influenced of characteristics of working frequency and power-output, is achieved. The key device fabrication processes, with or without mesa isolation, have been developed, and the AlGaN/GaN HEMTs with superior frequency and power characteristic have been fabricated.The AlGaN/GaN HEMTs grown on semi-insulating 6H-SiC substrates with 0.25μm gate-length and 100μm gate-width is presented. The DC measurement results exhibit a maximum drain current density of 1112mA/mm at a zero gate voltage and the peak extrinsic transconductance of 250mS/mm. On-wafer RF measurements show the values of unity current gain cutoff frequency (fT) of 41.5GHz and maximum frequency of oscillation (f_max) of 108GHz is evaluated by extrapolation of the unity gain and MSG data at 20dB/decade. The device under 8GHz continuous wave conditions biased at a drain-source voltage of 28V and gate-source voltage of -3.2V shows a saturated output power of 5.62W/mm with an associated gain of 7.49dB and PAE of 31%. And the total gate periphery of 3mm AlGaN/GaN HEMTs after shell package shows a maximum drain current of 2.5A at zero gate voltage and the peak extrinsic transconductance of 660mS; Biased at drain-source voltage of 40V and gate-source voltage of -2.5V, 15.85W total power is achieved at 8GHz CW conditions, with gain of 6.95dB and PAE of 36%.