Low-Voltage High-Power GaAs FET’s for Digital Wireless Communication System Applications

• Main Authors: CHEN SZU HUNG, 陳仕鴻
• Other Authors: Edward Yi Chang
• Format: Others
• Language: en_US
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/54885830319262866505

博士 === 國立交通大學 === 材料科學與工程系 === 91 === In this dissertation, high power GaAs-based FETs were developed for low voltage wireless communication applications. The developed GaAs-based FETs include MESFETs (Metal Semiconductor Field Effect Transistors), D-PHEMTs (depletion-mode pseudomorphic high electron mobility transistors), and enhancement-mode PHEMTs (E-PHEMTs). The process of the ion-implanted planar gate MESFET contains double Be implantation to trim the drain saturation current of the device to the desired level and reduce the surface and substrate defect trapping effects. The MESFETs shows good device uniformity and excellent power performance. The average pinch-off voltage of the 20 mm devices is -2.81 V with a standard deviation of 120 mV across a 3-inch wafer. The 1 µm × 20 mm MESFET exhibits a high output power of 33.85 dBm (121.3 mW/mm) with associated power-added efficiency (PAE) of 54.6 % at Vds =3.4 V with quiescent Ids of 1000 mA at 1.9 GHz. Under IS-95 CDMA modulation signal at 1.9 GHz and biased at Vds =3.6 V with quiescent Ids of 80 mA, the device has adjacent channel leakage power (Padj) of —29.5 dBc at 1.25 MHz offset and —44.9 dBc at 2.25 MHz offset from the center frequency at output power level of 28 dBm. The test data show that the developed double Be-implanted power MESFETs deliver high power, high efficiency and demonstrate good linearity under digital modulation signal. The superior performance of the MESFETs shows the developed devices are suitable for low-voltage digital wireless communication system applications, such as DCS (Digital Cellular System) and CDMA (Code Division Multiple Access). High power dual planar-doped AlGaAs/InGaAs D-PHEMTs and E-PHEMTs are also developed. When testing condition is at 1.9 GHz and Vds= 1.2 V under class AB condition, the 6.72-mm D-PHEMT has a maximum PAE of 43.62 % with an associated output power of 22.18 dBm. The performance of the D-PHEMTs under digital modulation signal is also evaluated. At Vds= 1.2 V under 1.9 GHz π/4-shifted QPSK PHS signal, the device shows an output power of 22 dBm with a Padj of -56.86 dBc at 600 kHz apart from 1.9 GHz center frequency and the linear PAE is 41.31 %. The 6.72-mm PHEMT meets the PHS specifications and the measured results are the first report on the power PHEMT for 1.2-V PHS application. When 20.16-mm PHEMT was qualified under 1.9 GHz IS-95 CDMA modulation signal, the results show the device meets the CDMA specifications at both Vds= 3.0 V and Vds= 2.4 V. When the linear output power is 28 dBm, the device has a PAE of 37.8 % at Vds= 3.0 V and PAE of 30.2 % at Vds= 2.4 V. At Vds= 2.4 V, the device has Padj of -31.46 dBc at 1.25 MHz and —48.83 dBc at 2.25 MHz offset from the center frequency when output power is 28 dBm. This is also the first report on the power PHEMT for 2.4-V CDMA application. In addition, high power E-PHEMTs were also fabricated. The device has VT of 90 mV, maximum gm of 490 mS/mm, and drain current density of 350mA/mm (at Vgs = 1.0 V). When tested at 1.9 GHz, the 3.36-mm E-PHEMT exhibits a high power density of 247 mW/mm at 3.6 V, 196 mW/mm at 3 V, 152 mW/mm at 2.4 V and 63 mW/mm at 1.2 V. The 20.16-mm E-PHEMT shows 34.1 dBm (128 mW/mm) output power with PAE of 64.5 % at Vds = 3.0V. At Vds = 2.4 V, a maximum saturated output power of 32.25 dBm and maximum PAE of 78.5 % is achieved. At 1.2 V, the E-PHEMT also delivered a high output power of 29.75 dBm (47 mW/mm) with maximum PAE of 51.6 %. The 3.36-mm E-PHEMT was also qualified by 1.9 GHz π/4-shifted QPSK modulated PHS signal. The test condition is at Vds = 2.4 V and quiescent Ids = 30 mA. The device shows a linear PAE of 35.1 % with associated power gain of 15.5 dB at output power of 22.4 dBm. The Padj of the device is —56.6 dBc at 600 kHz apart from the center frequency. The 3.36-mm E-PHEMT meets PHS specifications at Vds = 2.4 V. The results demonstrate both MESFETs and PHEMTs developed show high power, high efficiency and high linearity at low voltage operation. The developed GaAs FETs have great potential for advanced low voltage wireless communication system applications. Finally, a novel T-shaped gate process with PSM and silicon nitride etch-back technologies is also described in this dissertation. The phase shift mask and i-line stepper with a small σ were used to improve the resolution and DOF of the lithography process. The T-shaped gate with a length of 0.167 μm was achieved. This novel process is a high-throughput process of fabrication of T-shaped gate by using i-line exposure with PSM compared to the conventional E-beam lithography technology and can be used for mass production of integrated circuits.