| Summary: | 博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 95 === In this dissertation, we have investigated InAlAs/InGaAs heterostruture field-effect transistors (HFETs) with different InxGa1-xAs channel structures, including graded, compressively-strained, tensile-strained, and coupled δ-doped channels. Through the channel engineering, we have studied the influences on comprehensive device performances, such as drain current (IDSS0), breakdown (VBK), extrinsic transconductances (gm), output conductance (gd), cut-off frequency (fT), maximum oscillation frequency (fmax), and output power (Pout).
First, we investigate the InP-based In0.52Al0.48As/In0.53Ga0.47As high-electron-mobility transistors (HEMTs) with inversely-graded and symmetrically-graded channels (IGC-HEMT and SGC-HEMT). As compared to IGC-HEMT, SGC-HEMT has higher 2DEG concentration and better gate modulation efficiency. Therefore, SGC-HEMT demonstrates better current driving capability and microwave characteristics. On the other hand, IGC-HEMT exhibits the relieved impact ionization and higher breakdown voltage, making IGC-HEMT more suitable for power applications. Furthermore, less Coulomb scattering in the inversely graded channel makes IGC-HEMT exhibit better thermal stability. For a 0.65 μm gate-length device, the measured gm, fT, fmax, and Pout at 5.8 GHz (biased at VDS = 2 V) are 330 mS/mm (314 mS/mm), 50.0 GHz (46.0 GHz) and 46.0 (55.0 GHz) GHz, 84.9 mW/mm (95.3 mW/mm) for SGC-HMET (IGC-HEMT), respectively.
To meet the requirements of MMIC fabrications and to further improve device performances, we fabricate the InxAl1-xAs/In0.53Ga0.47As/InxAl1-xAs metamorphic high-electron-mobility transistor (MHEMT) with the different indium composition of the Schottky and buffer layers. The energy gap (Eg) of the Schottky and buffer layer can be increased by reducing the indium composition of from x = 0.52 in lattice-matched device to x = 0.42. Therefore, the device with compressively-strained channel (CS-MHEMT) was fabricated and compared with lattice-matched device (LM-MHEMT). Experimental results indicate that wider Eg of Schottky layer increases the breakdown voltages and 2DEG carrier concentration at the same time, leading to higher IDSS0 and Pout, but the compressive strain degrades the channel mobility. Thus LM-MHEMT exhibits superior gm, fT, and fmax to CS-MHEMT. For a 0.65 μm gate-length device, the gm, IDSS0, fT, and Pout at 5.8 GHz (biased at VDS = 3 V) for CS-MHEMT (LM-MHEMT) are 318 mS/mm (341 mS/mm), 452 mA/mm (366 mA/mm), 48.0 GHz (53.5 GHz), 185.8 mW/mm (131.5 mW/mm), respectively.
In order to improve the microwave and power characteristics simultaneously, the MHEMT device with tensile-strained channel was fabricated (TS-MHEMT). Because both the tensile strain and symmetrically-graded structures can increase the channel mobility, higher mobility and less impact ionization than those of conventional devices can be obtained in low In-content channel. The DC, RF, and power characteristics have been improved in TS-MHEMT as compared to the conventional MHEMT. For a 0.65 μm gate-length device, gm, IDSS0, fT, and Pout at 5.8 GHz (biased at VDS = 3 V) are 404 mS/mm, 514 mA/mm, 58.5 GHz, and 260.0 mW/mm, respectively.
Finally, we proposed metamorphic δ-doped channel heterostructure field-effect transistor (MDDFET). Because the partial wave function traveling in the high-speed undoped channel in the center of channel, high channel mobility can be achieved without sacrificing the high carrier concentration. For a 0.65 μm gate-length device, the measured gm, IDSS0, fT, and Pout at 5.8 GHz are 320 mS/mm, 566 mA/mm, 45 GHz, and 194.5 mW/mm, respectively. Furthermore, the variations of gm and ISDD0 are as low as 0.3% and -3.4% from 300 K to 420 K respectively, which can be attributed to the enhanced carrier confinement and the special electron transferring mechanism.
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