6 - Active components for RF uses

Abstract

This chapter focuses on various active components used in radio-frequency (RF) equipments. The simplest semiconductor active device for RF applications is the diode, which like its thermionic forebear conducts current in one direction only. Although, semiconductor diodes are not active devices, simply non-linear passive ones, but their mode of operation is so closely linked with that of the transistor that they are usually considered together. The earliest semiconductor diode is of the point contact variety, the user-adjusted crystal and cat's whisker, used in the early days of wireless. The varactor diode or varicap is designed solely for reversed biased use. A special doping profile giving an abrupt or “hyperabrupt” junction is used, which results in a diode whose reverse capacitance varies widely according to the magnitude of the reverse bias. The diode behaves like a normal mechanical variable capacitor, except that the capacitance is controlled by the reverse bias voltage rather than by a rotary shaft. The P type/Intrinsic/N type or PIN diode is a PN junction diode, but fabricated with a third region of intrinsic (undoped) silicon between the P and N regions. When forward biased by a direct current, it can pass RF signals without distortion, down to some minimum frequency set by the lifetime of the carriers, holes and electrons, in the intrinsic region.