The Interface Defect States And Schottky Characteristics Of InAlN/AlN/GaN HFETs

In recent years, as typical representative of the third generation of wide band gap semiconductor materials, the nitrogen families of GaN-based semiconductor devices have broad application prospects in high-temperature, high-frequency and high-power devices field and has been very extensively and in-depth researched at home and abroad. AlGaN/GaN heterostructure field-effect transistors (AlGaN/GaN HFETs) are highly representative of one of the device structures, which have the superior performances of GaN material, namely high breakdown field strength, high saturated electron drift velocity, high thermal conductivity and stability chemical properties. So they have fine features such as high transconductance, high saturation current,high cut-off frequency and high breakdown voltage. Besides AlGaN/GaN has a big energy gradient and strong polarization effect. Only by spontaneous polarization and piezo polarization, AlGaN/GaN is sufficient to produce up to to-1013/cm2two-dimensional electron gas density(the2DEG in the quantum well of AlGaN/GaN hetero interface) even without any doping, or bias. It has been one of the important areas of international research on GaN devices. A lot of progress has been made, including the performance of the devices and process preparation techniques.However, due to the lattice mismatch between the AlGaN and GaN substrate material, the AlGaN barrier layer has strong tensile strain and piezoelectric polarization. The inverse piezoelectric effect has a serious impact on the reliability of the device coupled with the defects caused by AlGaN and GaN lattice mismatch. As a new type of barrier layer material, InAIN is lattice matched with GaN when its in constituent is around0.17to0.18, so there is no stress and the generation of the piezoelectric polarization, which can improve the reliability of the device. Under the ultra-thin barrier layer, there still exists high dimensional electron gas density due to the spontaneous polarization caused by high al component (al component is about 0.82),which is very suitable for millimeter-wave power devices. However because there are big differences in growth temperature(AlN needs high-temperature, inn needs low temperature), lattice constants, bond lengths, thermal stability between AIN and inn and InAlN thin film is prone to phase separation and component uneven during growth process, the epitaxial growth of high quality inain material is very difficult. InAlN/GaN heterostructure field-effect transistors(InAlN/GaN HFETs) began to be researched from2000. Now the frequency characteristics of InAlN/GaN HFETs have achieved good results with the characteristic frequency can breakthrough370Ghz (1g=30nm), the sheet resistance for260/□, the carrier concentration of up to1.72×1013cm-2, the mobility of1240cm2/(vs).InA1N and GaN has good lattice matching (corresponding to the in composition0.17to0.18), but research on interface defects of InAlN/AIN/GaN HFETs is relatively little. This paper carried out research on the interface defects states of InAlN/AIN/GaN HFETs and found that there are still high densities of interface defect states InAlN/Aln/GaN HFETs. These high densities of interface defects were analyzed and researched in this paper. Series resistor in InAlN/AIN/GaN HFETs is also a very important parameter of the field effect device. A small series resistor can reduce the power consumption and increase the output current of the device. Also the smaller the resistance means smaller time delay, thereby improving the frequency characteristic of the device. Many studies have shown that when the traditional drain ohmic contact are changer to schottky drain, the device characteristics of AlGaN/GaNHFETs have been greatly improved, such as the increase of breakdown voltage, the large reduced gate-drain series resistance.to this end, the impact of the the schottky leakage process and ohmic contact process on the gate-drain series resistance was also studied in this paper. Specific studies were included in the following:1、Interface defect states of InAlN/AIN/GaN HFETs. The capacitance-voltage (CV), conductance-voltage (GV) and the current-voltage IV test of devices were conducted with changing frequencies. Using parallel conductance method and test data, the information of interface defect states including interface defect densities, the time constants and the interface defect energy levels was got in different sizes, different components of the devices. The results have shown that there still exists high densities of interface defect states in in0.18Al0.82N/AlN/GaN HFETs and in0.17Al0.83N/AlN/GaN HFETs; the device sizes whether they are round or square have little impact on interface defect states; InAlN with18%in constituent has better lattice matching with GaN undergrowing conditions in this paper than that17%in constituent.2、Series resistance in InAlN/AlN/GaN HFETs with schottky drain. Different sizes of device structures were designed and analyzed. It is found that different from the AlGaN/GaN HFETs, InAlN/AlN/GaN HFETs have its own complexity and the effects of device size are significant. When the gate length share of the source and drain spacing ratio is very small (about0.1to0.2) or gate length decreases to a certain extent (10μm), gate-drain series resistances of the devices with schottky drain process decreased compared to the traditional InAIN/AlN/GaN HFETs. However when the ratio is greater than the proportion, gate-drain scries resistances of the devices with schottky drain process increase a lot. The paper analyzed reason of the series resistance variation by calculating the two-dimensional electron gas density and mobility.