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A Guide to Analog Asics by Paul M. Brown Jr.

By Paul M. Brown Jr.

Program particular built-in circuits (ASICs), either analog and electronic, became ordinary approach point bulding blocks. ASIC proprietors have tried to supply instruments that they wish will let particularly green IC designers (i.e. structures engineers) to layout subtle customized built-in circuits. This philosophy has been extra winning in electronic know-how than in analog. considerably extra paintings is concerned with analog layout and much fewer automated instruments can be found. virtually each analog ASIC seller bargains diverse semiconductor applied sciences, instrument units, documentation (usually missing intimately and never offering the correct historical past and guidelines), and ranging degrees of engineering help. the result's that many engineers who might use analog ASICs lack the technical details to take action. they don't seem to be yes while customized analog ICs are low-cost or which seller will top serve their wishes. furthermore, many engineers should not have sufficient analog layout adventure, specifically with built-in circuits. Consqeuently, many that may benefit from analog ASIC expertise don't use it whereas others have undesirable reviews that can have simply been kept away from.

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25%/V that describe ion-implanted resistor values are the same as those for diffused-base resistors. Table 3-2 summarizes typical ion-implanted re­ sistor characteristics. Pinched Resistors Pinched resistors are base-diffused resistors with an emitter diffusion overlaid and connected to the epi layer. The resulting geometry is il­ lustrated in Figure 3-7. The net effect is to severely reduce the crosssectional area of the p-type diffusion and thereby achieve a very high value of sheet resistivity.

The forward voltage drop φ is directly proportional to absolute temperature. s These characteristics can be used to perform an amazing array of precision analog functions. Examples will be presented in Chapter 5. Junction Breakdown A reverse-biased pn junction will exhibit a small current flow due to minority carrier holes and electrons that are accelerated across the de­ pletion region by the electric field (there could also be leakage contrib­ uted by ionic contamination on the surface of the wafer).

Where R L W p s p = = = = resistance in ohms emitter diffusion length resistor body width pinched base sheet resistivity in Ω/sq These devices have several unique characteristics. The value of the resistor is almost entirely determined by the pinched region. The contact resistance and the nonpinched portion of the p-type material make an insignificant contribution to the total resistor value. Pinched resistors are basically JFETs with a very high pinch-off voltage and therefore exhibit nonlinear voltage characteristics.

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