Investigations of Heterostructure Field-Effect Transistors with Graded InxGa1-xAs Channel

• Main Authors: Yih-Juan Li, 李亦顓
• Other Authors: Wei-Chou Hsu
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/95278379523178718150

博士 === 國立成功大學 === 電機工程學系碩博士班 === 92 === 　　In this dissertation, a variety of compositionally graded InxGa1-xAs channels have been applied and investigated in the doped-channel field-effect transistors (DCFET’s) and the metamorphic high electron mobility transistors (MM-HEMT’s). Device performances are significantly improved due to the graded channel. 　　First, a symmetric double-side �-doped AlGaAs/InxGa1-xAs heterostructue field-effect transistor with a homogeneously Si-doped graded channel has been studied. The In composition in the graded InxGa1-xAs channel was varied linearly from 0.22 (at the bottom of channel) to 0.12 (at the upper of channel). Improved two-dimension electron gas (2DEG) density and mobility at 300 (77) K as high as 3.6 (3.2)×1012 cm-2 and 4850 (18000) cm2/V•s are achieved, respectively. Meanwhile, we can obtain a large gate voltage swing (GVS) over 2 V. Besides, the use of a superlattice AlGaAs/GaAs buffer layer can simultaneously reduce the leakage current, output conductance, the shift of threshold voltage with increasing temperature. 　　For obtaining higher carrier concentrations, double �-doped layers were adopted in the metamorphic InAlAs/InGaAs HEMT’s. In order to improve the mobility, a higher indium composition (x = 0.5) was set at the center of channel and then decreased linearly toward both sides of the channel (x = 0.3). As compared with the metamorphic In0.42Al0.58As/In0.53Ga0.47As HEMT, the metamorphic InAlAs/InGaAs HEMT with symmetric graded InxGa1-xAs channel can obtain higher 2DEG density of 4.8×1012 cm-2 while keeping the electron mobility as high as 7520 cm2/V•s. Due to the improved electron mobility and good carrier confinement in MM-HEMT’s with the symmetric graded InxGa1-xAs channel, the maximum transconductance of 276 mS/mm is obtained for the gate dimension of 1.5×125 �m2. Moreover, the measured current gain cutoff frequency and maximum oscillation frequency for a 1.5 �m gate length device are 22.4 GHz and 45.5 GHz, respectively. The device performances are among the best for 1.5 �m gate length. 　　Finally, we have fabricated a novel MM-HEMT with a symmetric graded InxGa1-xAs (x＝0.5�_0.65�_0.5) channel and an inverse step graded buffer layer. Due to the lower interface roughness scattering, an improved electron mobility as high as 9500 (30600) cm2/V•s at 300 (77) K was achieved. By using the self-consistent calculation method, three subbands in the graded channel were determined, which are identical to the Shubnikov-de Haas (SdH) characterization. By using the symmetric graded channel, In0.425Al0.575As Schottky layer and the undoped InP setback layer, a high gate-drain breakdown voltage of 24 V is obtained. Meanwhile, the measured current gain cutoff frequency and maximum oscillation frequency for a 1.5 �m gate length device were found to be 18.9 GHz and 48.4 GHz, respectively. The improved characteristics are suitable for high-power and high-frequency circuit applications.