| Summary: | 博士 === 國立交通大學 === 材料科學與工程系所 === 94 === In this paper, high-electron-mobility transistors (HEMTs) with doping profile modification are discussed for device linearity improvement. The modification was based on the third-order intermodulation distortion (IM3) and the third-order intercept point (IP3) analysis through simple equivalent circuit of the devices. The correlation of the extrinsic transconductance (Gm) with IM3 and IP3 indicates that flatter Gm distribution vs gate bias voltage causes lower IM3 level and that high Gm with flatter Gm distribution result in higher IP3 of the devices. Therefore, doping modification that improves the flatness of the Gm distribution will improve the device linearity.
The study is divided into four parts: First, a metamorphic high-electron-mobility transistor (MHEMT) with In0.55Ga0.45As/In0.67Ga0.33As/In0.55Ga0.45As composite channel layers was developed for low noise and high-linearity applications. The use of a composite channel results in high electron mobility and good confinement of electrons in the channel region which are the desired characteristics of a low-noise and high-linearity device. The device shows great potential for high-linearity and low-noise applications at high frequencies.
Second, the uniformly-doped and the δ doped In0.52Al0.48As/In0.6Ga0.4As MHEMT were fabricated and the DC characteristics and the third-order intercept point (IP3) of these devices were measured and compared. Due to more uniform electron distribution in the quantum well region, the uniformly-doped MHEMT exhibits flatter Gm (transconductance) vs IDS ( drain to source current ) curve and much better linearity with higher IP3 and higher IP3 to PDC ratio as compared to the δ doped MHEMT, even though the δ doped device exhibits higher peak transconductance. As a result, the uniformly doped MHEMT is more suitable for communication systems that require high linearity operation.
Third, a low noise InGaP/InGaAs pseudomorphic high-electron-mobility transistors (PHEMTs) with high IP3 was developed. The device utilizes InGaP as Schottky layer to achieve a low noise figure and uses AlGaAs as the spacer to improve the electron mobility and the device also uses dual delta doped layers for uniform electron distribution in the channel to improve the device linearity.
Finally, doping modification in the Schottky layer (Schottky layer doped) and in the channel layer (channel doped) of the conventional δ doped InGaP/InGaAs PHEMT were experimented to see the extra doping effect on the HEMT device linearity. DC and RF performances of these devices were measured and compared. It is found that extra doping either in the channel region or in the Schottky layer can improve the flatness of the Gm distribution under different gate bias conditions and thus achieve lower IM3 and higher IP3 of these devices with small scarification in the peak Gm value as compared to the conventional delta doped devices. The power performances of these devices were tested with different drain to source voltage (VDS) bias points. When the VDS bias was increased, the Gm values of the channel doped device and the Schottky layer doped device increased and decreased respectively with the increasing VDS bias. The adjacent-channel power ratio (ACPR) measurements of these devices were performed at different DC bias power levels. Overall, it was found that channel doped device demonstrated best linearity performance among these three different types of devices studied with highest IP3 level, lowest IM3 and best ACPR under CDMA modulation even though it has the lowest electron mobility among these devices. Overall, different structures and doping profiles of InGaP/InGaAs PHEMT and InAlGs/InGaAs MHEMT devices were experimented for device linearity improved. It’s found with paper design of the device structure and doping profile, the linearity of the HEMT device can be greatly improved and the experimental results match well with the theoretical analysis in this thesis.
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