VARACTOR DIODES The Nuts and Bolts of Tuning Varactors

M any electronic systems must be tuned through the variation of reactance of one or more circuit elements. In these applications it is often desirable to perform this tuning under electronic rather than manual control. Tuning varactor diodes, which are pn junction devices that act as voltage controlled variable capacitors, can perform this function, and are used in a wide range of product applications. This paper describes the construction of tuning varactor diodes, the effects of construction on varactor diode performance, the types of silicon (Si) varactor diodes available, and modeling of varactor diodes. The PN diode has been described extensively in the literature [1, 2]. As shown in Figure 1, a practical tuning varactor PN junction diode consists of two layers, the P and the N layers, which are formed on top of a low-resistance substrate layer, typically known as the N + layer. The N layer is doped with donor atoms, thereby producing excess electrons which result in a net negative charge. The N layer is in direct contact with the P layer, which is doped with acceptor atoms, thereby producing excess holes and a net positive charge. The N + layer is very heavily doped N type material and can be considered as a mechanically-rugged platform upon which the other layers that form the diode junction are deposited or formed. The N + layer does not figure prominently in the electrical operation of the varac-tor diode. In the immediate vicinity of the junction of the P and N layers, excess electrons from the N layer nearest the junction will recombine with holes from the P layer, also nearest the junction, forming a volume about the junction of these layers in which no free carriers exist. This region is called the " depletion region. " An electric field exists within this region, since it separates a volume with net negative charge from another with net positive charge. Equilibrium is established when this electric field is sufficiently strong that it restrains dif-Here is a review of the stucture and operation of varactor diodes, intended to develop a better understanding of their parameters , including how they are modeled for EDA tools Figure 1 · Varactor diode cross section. Figure 2 · The layers of a varactor diode.