Passive Components:Resistors and Integrated Semiconductor Resistors.

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Resistors

Resistors have been available for use in ICs for many years. Some of these are made in silicon, so they are directly integrated with the rest of the IC processes. Others, similar to the magnetic device case, are thin- film resistors fabricated in an off-line process that is not necessarily compatible with silicon IC processing. Integrated silicon resistors offer simplicity in fabrication but have less than ideal characteristics with loose tolerances. For this reason, many circuits rely on the ratio of resistor values rather than their absolute values. Thin-film resistors, in contrast, are more superior offering tight tolerances and the ability to trim their absolute value down to very precise values. They also display more stable temperature and frequency dependence.

Usually resistors in ICs are characterized in terms of their sheet resistance rather than their absolute resistance value. Sheet resistance, Rsheet, is defined as the resistance of a resistive strip with equal length and width so that

Passive Components-0080

where r is the material resistivity (W m) and t its thickness (m). Once Rsheet is given, the resulting resistor value is obtained by multiplying by its length-to-width aspect ratio. To avoid very high aspect ratios, an appropriate sheet resistivity should be used. For example, with Rsheet = 10 W/ a 10:1 length-to-width ratio would give a 100 W resistor. However, to obtain a 1 kW resistor it would be better to use a different material, say, Rsheet = 100 W/ with the same 10:1 ratio instead of using a 100:1 ratio with the low- resistivity material.

Integrated Semiconductor Resistors

In this category the existing semiconductor is used as the resistive material. The resistor may be fabricated at a number of stages during the IC process giving rise to different resistors with different characteristics. Some of the most common are discussed below.

Diffused Resistors

These can be formed during either the base or emitter diffusion of a bipolar process. For an npn process the base diffusion resistor is a p-type of moderate sheet resistivity typically in the range of 100–200 W/ . This can provide resistors in 50–10 kW range. The heavily doped n+ emitter diffusion will produce an n+-type resistor with low sheet resistivity of 2–10 W/ . This can provide resistors with low values in 1–100 W range. Owing to tolerances on the photolithographic and etching processes, the tolerance on the bsolute resistance can be as high as ±30%. However, resistor pairs can be matched closely in temperature coefficients and doping profiles especially when placed side by side on the chip so that the resultant tolerance on the resistor ratio can be made to be less than ±1%. Since a diffusion resistor is based on a p-type base over an n-type epitaxy or an n+-type emitter over a p-type base, it is essential that the formed p–n junctions are always reverse-biased to ensure that current flows in the intended portion of the resistor. The presence of such a reverse-biased p–n junction also introduces a distributed capacitance from the resistor body to the substrate. This will cause high-frequency degradation whereby the resistor value drops from its nominal design value to a lower impedance value due to the shunting capacitance.

Pinched Resistors

A variation to the diffused resistor that is used to increase the sheet resistivity of base region is to use the n+-type emitter as a means to reduce the cross-sectional area of the base region, thereby increasing the sheet resistivity. This can increase the sheet resistance to ~1 kW/ . In this case, one end of the n+-type emitter must be tied to one end of the resistor to contain all current flow to the pinched base region.

Epitaxial Resistors

High resistor values can be formed using the epitaxial layer since it has higher resistivity than other regions. Epitaxial resistors can have sheet resistances around 5 kW/ . However, epitaxial resistors have even looser tolerances due to the wide tolerances on both epitaxial resistivity and epitaxial layer thickness.

MOS Resistors

A MOSFET can be biased to provide a nonlinear resistor. Such a resistor provides much greater values than diffused ones while occupying a much smaller area. With the gate shortened to the drain in a MOSFET, a quadratic relation between current and voltage exists and the device conducts current only when the voltage exceeds the threshold voltage. Under these circumstances, the current flowing in this resistor (i.e., the MOSFET drain current) depends on the width to length ratio of the channel. Hence to increase the resistor value, the aspect ratio of the MOFET should be reduced to give a longer channel length and narrower channel width.

Thin-Film Resistors

As mentioned before in the magnetic core case, a resistive thin-film layer can be deposited (e.g., by sputtering) on the substrate to provide a resistor with very tight absolute value tolerance. In addition, given a large variety of resistor materials a wide range of resistor values can be obtained in small footprints, thereby having very small parasitic capacitances and small temperature coefficients. Some common thin- film resistor materials include tantalum, tantalum nitride, and nickel–chromium. Unlike semiconductor resistors, thin-film resistors can be laser trimmed to adjust their values to very high accuracies of up to 0.01%. Laser trimming can increase the resistor value since the fine beam evaporates a portion of the thin-film material. By its nature, laser trimming is a slow and costly operation that is justified when very high accuracy on absolute values is necessary.

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