Varactor Tuners Eliminated an RF Amplifier
There is an urgent demand for low noise varactor tuners for use in TV receivers to meet the requirements of FCC rules. The RF amplifier has been frequently eliminated in mechanical tuners now in use, resulting in good cross-modulation characteristics and low cost. However, the elimination of the RF amplifier has not been attained in varactor tuners, because of high series resistance of varactor diodes. The series resistance of Si diodes at 470 MHz is around 0.7Ω, and is much higher than that of mechanical capacitor, 0.15Ω. To reduce the noise figure of varactor tuners, tuning diodes with low resistance, i. e. below 0.2Ω must be developed. However, few reports have been published on low series resistance diodes for UHF use.
GaAs Varactor Tuners for UHF TV
Recently, varactor tuners have become very popular in TV tuner, but the noise figure and selectivity of present day's varactor tuners are inferior to that of mechanical tuners. Series resistance, Rs, of Si tuning diodes manufactured is around 0.8¿in mean value at 470 MHz and at diode capacitance of 9 pF. The value of 0.8¿is so high that low resistance diodes, Rs < 0.8¿, for instance, have been selected specifically for UHF tuners. Recently, low resistance diodes with Rs = 0.68¿have been fabricated by ion implantation technique. 1) Even if these diodes are employed, the Q value of tuning circuit is still much lower than that of mechanical tuner in UHF region. Such high resistance diodes lead to a higher noise figure and lower gain in tuner performance.
Liquid-phase epitaxial growth of thin GaAs layers from supercooled solutions
A technique for the liquid‐phase epitaxial growth of thin GaAs layers has been developed. This technique uses supercooled solutions free from GaAs precipitates. With the amount of the initial supercooling in the range of 1–10 °C, very smooth surfaces with a roughness smaller than 0.01 μm are obtained for layers thinner than 1μm. The layer thickness data agree well with the diffusion theory equation by assuming that the diffusion coefficient of As in Ga is 4×10−5 cm2/sec at 800 °C. The thickness uniformity within a wafer is better than ±5%, and the thickness reproducibility among different runs is within ±7%. The carrier concentration is also well controlled.