Download PDF by Paul M. Brown Jr.: A Guide to Analog Asics

By Paul M. Brown Jr.

ISBN-10: 0121369706

ISBN-13: 9780121369705

ISBN-10: 032315476X

ISBN-13: 9780323154765

Program particular built-in circuits (ASICs), either analog and electronic, became average approach point bulding blocks. ASIC owners have tried to supply instruments that they wish will allow quite green IC designers (i.e. platforms engineers) to layout refined customized built-in circuits. This philosophy has been extra winning in electronic expertise than in analog. considerably extra artwork is concerned with analog layout and much fewer automated instruments can be found. virtually each analog ASIC seller deals diversified semiconductor applied sciences, software units, documentation (usually missing intimately and never offering the right kind history and guidelines), and ranging degrees of engineering help. the result's that many engineers who may possibly use analog ASICs lack the technical details to take action. they aren't convinced whilst customized analog ICs are inexpensive or which seller will most sensible serve their wishes. additionally, many engineers shouldn't have sufficient analog layout event, specially with built-in circuits. Consqeuently, many that may benefit from analog ASIC expertise don't use it whereas others have undesirable reports which may have simply been shunned.

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In custom applications, or standard products, base 3. Integrated Circuit Components 40 resistors have been routinely made in the range of 50 kil. There is, how­ ever, a significant die area penalty for these large values. The absolute values of base resistors can vary by as much as ± 3 0 % from wafer to wafer due to variations in the base diffusion sheet resistivity and mask­ ing and etching tolerances. The resistors match and ratio to within 1 to 2 % on a given die. The breakdown voltage for these resistors is the same as the npn collector-base breakdown for the process they are fabricated on.

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. The resistor value depends on the basewidth and therefore the beta of npn transistors. Since the top junction of the resistor is an npn emitter-base junction, the breakdown voltage of a pinched resistor is B V , which is typically ebo 46 3.

Even with this control the sheet resistivity can vary by as much as ± 2 0 % from one manufacturing run to another. This is why designing with ratios rather than absolute values is so important. If the process contains implanted resistors or p-channel JFETs, the implan­ tation usually follows the base diffusion. The next diffusion (Figure 2-14) is used to make npn emitters, channel stops to prevent parasitic MOS inversion (which forms con­ ducting channels in high-voltage processes), and epi contacts.

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