The Characteristic Research Of AlGaN/GaN Heterostructure Field Effect Transistors

In the 21st century, wireless communication is one of the most important information technology. Next generation wide band wireless mobile communication requires high frequency/high power solid state electron devices. GaN is one of the most important 3nd generation semiconductors and one of the best candidates for developing high tempreture, high frequency, and high power microwave electron devices due to its intrinsic excellent physical properties and chemical stabilities, such as wide band gap, high breakdown electric field, high electron saturated velocity and high thermal conductivity.AlGaN/GaN heterostructures have been a subject of intense investigation because they are currently the most important and most essential structures for developing GaN based high tempreture, high frequency, and high power microwave electron devices. AlGaN/GaN heterostructures have large band offset and strong polarization, which make the sheet carrier concentration of the two dimensional electron gas up to～10¹³/cm² without intentional doping. AlGaN/GaN heterostructure field effect transistors (AlGaN/GaN HFETs) based on AlGaN/GaN heterostructures have many excellent performances, such as high transconductance, high saturated current, high cutoff frequency and high breakdown voltage. In the last decade, though the performance of AlGaN/GaN HFET have been improved a lot, the reliable AlGaN/GaN HFETs have not been utilized commercially now. Because there are still many problems in the investigation of AlGaN/GaN HFET, such as the influence of the device process on the characteristics of AlGaN/GaN HFET, the reliability of the AlGaN/GaN HFET working in high temperature, et al. Therefore, it is very important to investigate the basic characteristics of AlGaN/GaN HFET. The work in this dissertation is based on the focus in the investigation of AlGaN/GaN HFET, including the influence of Schottky contact metals on the strain of AlGaN barrier layer, the influence of device processing (such as the formation of Ohmic contacts and Schottky contacts) on the 2DEG electron mobility and the thermal stability of Ni Schottky contacts on AlGaN/GaN heterostructures. The main conclusions of the dissertation are listed below.1. The accurate calculation of the relative permittivity of AlGaN barrier layer. Using the measured C-V characteristics and the photocurrent spectrum of the Ni Schottky contacts on AlGaN/GaN heterostructures at room temperature, considering the 2DEG capacitance, the value of the relative permittivity of the AlGaN barrier layer was analyzed and calculated by self-consistently solving Schrodinger's and Poisson's equations accurately. It is shown that the calculated value of the relative permittivity decreases as the reverse bias of the Ni Schottky contacts increases in negative direction and the reason is carefully analyzed.2. The influence of Schottky contact metals on the strain of AlGaN barrier layer.(a) The infuence of different Shottky metals on the strain of AlGaN barrier layer. Ir and Ni Schottky metals were deposited on AlGaN/GaN heterostructures. Using their C-V characteristics and I-V characteristics, the polarization charge density under the Schottky metals was calculated by self-consistently solving Schrodinger's and Poisson's equations. It is found that the polarization charge density under the two Schottky metals is different unless they have much different Schottky barrier heights. However, from the I-V characteristics of the two samples, it can be found that their Schottky barrier heights are merely the same. Conclution can be made that the influence of different Schottky metals on the strain of AlGaN barrier layer is different.(b) The influence of different Ni Schottky contact areas on the strain of AlGaN barrier layer. Ring Ni Schottky metals with different areas were deposited on AlGaN/GaN heterostructures. With the same method as in (a), the polarization charge density under the Schottky metals was calculated. It is shown that as the Ni Schottky contact area increases, the polarization charge density under the Schottky metals decreases, and therefore the strain of the AlGaN barrier layer decreases.(c) The influence of different Ni Schottky contact thicknesses on the strain of AlGaN barrier layer. Ni/Au Schottky metals with different thicknesses were deposited on AlGaN/GaN heterostructures. Using C-V and I-V characteristics, the 2DEG density in the AlGaN/GaN heterostructures was calculated under Ni Schottky contact metals with different thicknesses. It is found that the 2DEG density for the 600A/2000A thick Ni/Au Schottky contact is merely twice of that for the 50A/50A thick one. This is because the strain under the thick Ni Schottky contact is much higher than that under the thin one, and much more donor-like surface defects are introduced during the depositing of the thick Ni Schottky contact.3.The 2DEG electron mobiliy in circular AlGaN/GaN HFET.(a) The influence of different Ni Schottky contact areas on the 2DEG electron mobility. Based on the Ohm Law, we deduced the I-V relationship in the linear region of the circular AlGaN/GaN HFET. The 2DEG electron mobility at the source-drain bias of 100mV under different gate biases were calculated using the measured C-V and I-V characteristics of the circular AlGaN/GaN HFETs with different gate areas. It is shown that when the gate bias increases from negative to positive, the 2DEG electron mobility increases with the 2DEG density for the small gate area and at a given gate bias, as the gate areas increase, the 2DEG electron mobility increases while the 2DEG density decreases. In addition, we also calculated the averaged 2DEG electron mobility in the linear region of the AlGaN/GaN HFETs.(b) "Strain polarization gradient Coulomb field" scattering mechanism was proposed for the first time. When Ni Schottky contact metals are deposited on the surface of the AlGaN barrier layer, the strain in the AlGaN barrier layer will be changed, and therefore the polarization charge density under the Ni Schottky contact is different from the ungated region which results in the polarization charge density gradient along the 2DEG channel. Usually, in the calculation of the electron states in the quantum well, the strain in the AlGaN barrier layer is assumed to be uniform along the 2DEG channel, therefore the presence of the polarization charge density gradient will scatter the electrons in the 2DEG as an addional potential. This scattering mechanism are named "Strain polarization gradient Coulomb field". When the Ni Schottky contact area is smaller, the 2DEG density is larger, and the polarization charge density gradient is larger which make the "Strain polarization gradient Coulomb field" scattering more significant.(c) The influence of annealed Ohmic contacts on the 2DEG electron mobility. The 2DEG electron mobility of AlGaN/GaN heterostructures device without gate was calculated and analyzed. It is shown that after depositing and annealing the Ohmic contacts, the value of the 2DEG density of the ungated AlGaN/GaN heterostructure just has a little change compared with that of the Hall measurement, but the calculated 2DEG electron mobility is decreased by more than 50% . During the depositing and annealing process of source and drain Ohmic contacts, some defects are introduced into the AlGaN barrier layer, meanwhile, the strain of the AlGaN barrier layer near the Ohmic contacts is changed, resulting in a "Strain polarization gradient Coulomb field" near the soure and drain, respectively. With scattering of the defects and the "Strain polarization gradient Coulomb field", the 2DEG electron mobiliy is greatly decreased.4. The thermal stability of Ni Schottky contacts on strained AlGaN/GaN heterostructures.(a) The influence of different annealing temperatures on the characteristics of Ni Schottky contacts on strained AlGaN/GaN heterostructures. Ni Schottky contacts on strained AlGaN/GaN heterostructures were annealed for 30 minutes at different temperatures in a N₂ ambient. The reverse leakage current decreases after thermal annealing at up to 800℃. Ni Schottky contacts on the heterostructures with 30min thermal stressing at 700℃exhibit lower reverse leakage current by more than three orders of magnitude lower than the unannealed sample. Using C-V characteristics and photocurrent spectrum, it is found that the Schottky barrier heights of Ni Schottky contacts on AlGaN/GaN heterostructures increase and the 2DEG density decreases after thermal stressing. By self-consistently solving Schrodinger's and Poisson's equations, it is shown that as the annealing temperature increases, the Fermi level decreases, the relative permittivity and polarization of the AlGaN barrier layer decreases, the triangular quantum well becomes wider and therefore the distribution of the 2DEG spreads deep into GaN. This can be attributed to the interaction between Ni and AlGaN, which reduces the strain energy and the porlarization electron field in the AlGaN barrier layer.(b) The influence of different annealing time on the characteristics of Ni Schottky contacts on strained AlGaN/GaN heterostructures. Ni Schottky contacts on strained AlGaN/GaN heterostructures were annealed at 600℃for different time in a N₂ ambient and decreases in the reverse leakage current after thermal annealing was observed. When the annealing time is less than～13 hours, the reverse leakage current decreases with increasing annealing time. This can be explained by the initial interaction between the Ni atoms and the surface states of the AlGaN barrier layer which decreases the 2DEG density. However, when after more than 13 hours annealing, the reverse leakage current of the Ni Schottky contacts will increase with increasing annealing time. This can be attributed to the reduction of the polarization field and the Schottky barrier height due to the deep diffusion of Ni atoms into GaN layer after long time annealing which results in the partial strain relaxation or reduced effective strained thickness of AlGaN layer.