By Paul M. Brown Jr.
Program particular built-in circuits (ASICs), either analog and electronic, became normal approach point bulding blocks. ASIC proprietors have tried to supply instruments that they wish will allow particularly green IC designers (i.e. platforms engineers) to layout refined customized built-in circuits. This philosophy has been extra profitable in electronic expertise than in analog. considerably extra paintings is concerned with analog layout and much fewer automated instruments can be found. nearly each analog ASIC seller bargains various semiconductor applied sciences, software 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 may possibly use analog ASICs lack the technical info to take action. they aren't yes whilst customized analog ICs are competitively priced or which seller will most sensible serve their wishes. additionally, many engineers wouldn't have enough analog layout adventure, specially with built-in circuits. Consqeuently, many that may benefit from analog ASIC know-how don't use it whereas others have undesirable stories that may have simply been refrained from.
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6 mils), which makes the effective base width very wide, increasing the chance of carrier recombination in the base region. This increases the base cur rent for a given collector current, resulting in a relatively low beta. The emitter-to-collector current flow is lateral, as the name im plies. The purpose of the buried layer in a lateral pnp is to keep carriers from being collected by the reverse-biased, epi-substrate junction. Some collection, however, does occur at the epi-isolation junction.
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. Integrated Circuit Components E m i t t e r diffusion length ,1 1 Τ 1 Resistor body width (a) E p i island Resistor contact contact Oxide η emitter Pinched Resistor region contact jab -ite Aluminum ί -X. v ρ base ρ isolation <: η buried layer ρ substrate (b) Figure 3-7 Pinched resistor geometry. 5 V. Their absolute value, tolerance, and matching are very poor.
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. Base resistors have relatively good high-frequency performance but are limited somewhat by the base-epi parasitic capacitance.