ABSTRACT
Passive components used in electronic circuits are resistors, inductors, and capacitors. In an integrated circuit these and other components like couplers or strip lines need to be made as part of the wafer fabrication process. The various process steps are discussed in this chapter. 17.1 ResistorsResistors are used in integrated circuits (ICs) for numerous functions that require a range of values over many decades and precision from very crude to less than 1%. Resistors are made in the semiconductor itself or deposited as a thin film. Semiconductor resistors can be made by utilizing any of the techniques used for making n-and p-type layers. Diffused resistors are possible in silicon but for III-V compounds, epilayers or ion implantation must be used. 17.1.1 Semiconductor/GaAs Resistors [1]These resistors involve the least processing because the resistor layer is already present in the device structure and the contacting and isolation processes are the same that are needed for active
devices anyway. The sheet resistances of the semiconductors that are available depend upon the technology or device type. In the case of field-effect transistor (FET)-type ICs, sheet resistance of the n+ source/drain and n-type channel layers can be utilized. These can be from 100 ohms/sq. to 1000 ohms/sq. In the case of heterojunction bipolar transistors (HBTs), lower values, those of emitter and collector layers, are available. The base layer may be used and could be in the range of one hundred to a few hundred ohms/sq. The resistance of the semiconductor resistor is the sum of the semiconductor sheet and the two contacts: R = R L
W R W
s c
. +2 (17.1)where Rc is the contact resistance per unit length, generally measured in ohm-mm. Resistors should be designed with a large L/W aspect ratio so that contact resistance is negligible. For n resistors, with sheet resistivity of 400 to 700 ohms/sq., the specific contact resistance can be high, even with n+ contact regions. Therefore these resistors may require large contact regions to ensure that the resistors scale with dimensions. Figure 17.1 shows a top view and a cross section of a semiconductor resistor in an implanted or epigrown FETtype substrate. The resistor region is defined by an ion-implanted isolation region. The minimum width of the resistor depends upon the minimum dimensions allowed for the semiconductor layer due to the constraints of the isolation process.
