| The InAs/AlSb high electron mobility transistor?HEMT?device has many advantages in high speed,high frequency,and low power consumption.InAs channel materials has high peak velocity?4×l07 cm/s?,and InAs/AlSb heterojunction has large conduction band offset?Ec?1.35 eV?,resulting in elevating the concentration of 2DEG.In this dissertation the process of InAs/AlSb HEMT is creatively developed.The high-k dielectric layer/InAlAs MOS capacitors are manufactured and characterized.InAs/AlSb HEMT devices with different epitaxial structures and low temperature characteristics are simulated and optimized.The main contributions are summarized as follows.Firstly,the InAs/AlSb HEMT simulation models are constructed with Sentaurus TCAD,and two kinds of epitaxial structures of single delta doping and double delta doping are simulated.A band diagram of the InAs/AlSb HEMT structure was obtained using a calibrated density simulation.DC and AC characteristics of devices with 0.25?m gate length are compared.The saturation drain current Ids and peak transconductance gm VDS=0.6 V for the single delta doping are 576 mA/mm and 1047 mS/mm,while Ids and gm for the double delta doping are 832 mA/mm and 1208 mS/mm,respectively.It is found that the turn-off voltage and the transconductance of double doping are larger than that of the single doped structure.At the same time,the peak transconductance of the device is about 680 ms/mm at 300 K,while the peak value of transconductance is 1250 ms/mm at150 K,nearly 2 times larger than the value at 300 K.Meanwhile,the high carrier mobility,better grid control ability and small conduction channel resistance at low temperature are obtained.Finally,the threshold voltage and saturation drain current of InAs/AlSb MIS-HEMT device is simulated and enhanced compared with InAs/AlSb HEMT.Secondly,the designed epitaxial structures of single delta surface doped and double delta doped InAs/AlSb HEMT have been manufactured by MBE.The InSb is a key interface to obtain higher carrier mobility compared with the AlAs interface,which is formed by depositing In monolayer in the interface of InAs/AlSb and combining with Sb.The electron mobility and electron concentration for HEMT epitaxial wafer of single surface and double delta doped delta surface doped were 16780 cm2/Vs and 1.98?1012/cm2,12562cm2/Vs and 7.71?1012/cm2,respectively,from van der Pauw Hall measurement.From 175K to 300 K,the electron mobilities of two structures show identical variation trend.Photon scattering is the main scattering mechanism.However,below 175 K,the dominant scattering mechanism is the ionized impurity scattering,which shows the electron mobility is only related to the N-1 doping concentration.The higher electron mobility is obtained from samples of single doping due to the low ionized impurity scattering caused by the single low?doping concentration.Thirdly,the fabrication process of InAs/AlSb HEMT devices are developed by electron beam evaporation technique on InAs.The ohmic contact resistance is a part of the extrinsic parameters of the device.Reducing the ohmic contact resistance is beneficial to the device's transfer characteristics and transconductance enhancement,and further affects the device's high frequency characteristics.Two metal materials,Au/Ge/Ni/Au and Pd/Pt/Au,have been used to study the ohmic contact.The alloy Au/Ge/Ni/Au?20/40/14/220 nm?ohmic contact was measured by the TLM at 300 K and the ohmic contact resistance Rc was 0.65?-mm.The ohmic contact resistance Rc,made from non-alloy Pd/Pt/Au?12/35/60 nm?,is 0.27?-mm.The thermal stability of the alloy is worse compared to non-alloy ohmic contact,while the non-alloy ohmic contact resistance is low and has good thermal stability.The non-alloy ohmic contact system is used in the fabrication of InAs/AlSb HEMT device.Fourthly,the Schottky gate InAs/AlSb HEMT devices are fabricated,and the fabrication process of the device is designed.The technological process of the ohmic contact,device mesa isolation,gate groove corruption and schottky gate contact formation is included.Ohmic contact is formed using non-alloy Pd/Pt/Au.Dry mesa etching is used,because the material etching damage the RF characteristic will deteriorate in the process of the dry mesa etching.The gate groove of selective wet etching device for InAlAs material and InAs material was selected by using citric acid grid etching solution.A Schottky gate InAs/AlSb HEMT device with a gate length of 2?m and a gate width of 2×30?m has been fabricated,which fills the gap of the research on In As/AlSb HEMT devices in china.Fifthly,study on reduction of the larger gate leakage current of the InAs/AlSb HEMT gate has been performed,and the solution is to insert the high-k dielectric between the InAlAs and the gate metal to form MIS-HEMT.The high-k/InAlAs MOS capacitor was fabricated and characterized.First,Al2O3 and HfO2 were deposited into InAlAs to form MOS capacitors.According to the measurement results,oxide trap charge Qot is respectively8.76?1011 cm-22 and 7.19?1010 cm-2 for the HfO2 and Al2O3 capacitors.Obviously the oxide trapped charge of Al2O3/In AlAs capacitor is smaller.The interface state density of HfO2and Al2O3 is 3×10122 cm-22 eV-11 and 2.5×1011 cm-2eV-1 respectively,which indicates that Al2O3 can effectively reduce the interface state density and improve the interface characteristics of InAlAs layer.When the gate voltage is 1V,the leakage currents of Al2O3and HfO2 samples are 1.37×10-6 A/cm2 and 3.22×10-66 A/cm2 respectively.It shows that the Al2O3 sample has less leakage than the HfO2 sample.The results show that the Al2O3intercalation layer can effectively prevent the interface oxidation and trap charge formation.XPS also showed that HfO2 deposited on InAlAs formed MOS had worse interface characteristics than Al2O3.In the later preparation of insulated gate dielectric InAs/AlSb MIS-HEMT devices,Al2O3 is preferentially selected as high-k dielectric material. |
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