Provide guard structures to divert carriers from where they might harm in Software Printing Code-39 in Software Provide guard structures to divert carriers from where they might harm

Provide guard structures to divert carriers from where they might harm using software todevelop code39 on web,windows application QR Code ISO speicification A highly e ective way o Software 3 of 9 barcode f preventing substrate or well currents from forward-biasing a parasitic BJT is to provide a low-resistance bypass to the emitter or to some other node of suitable potential. In the case of an inverter, this implies shortening the MOSFET bodies to their respective source terminals as illustrated in g.11.

32b. Di usion islands that share the polarity of the embedding substrate or well are used to galvanically connect to vss or vdd respectively.30 Such features are referred to as body ties with substrate.

well body plugs contacts taps and di usion picku USS Code 39 for None ps being used as synonyms. Body ties are generously distributed over the chip, not a single well must be left oating!31 Body ties are placed close to the parasitic bipolars. The smaller the bypass resistances Rs and Rw are made, the more current can safely be absorbed without having the base emitter drop exceed the critical threshold of 0.

6 V or so at which BJTs begin to conduct.. As rem ains to b e seen in section 14.1.3, the highly dop ed p + and n + islands are necessary to avoid the Schottky junction that would form if m etal were allowed to connect to lightly dop ed p and n regions directly.

Floating wells and/or a oating substrate not only render a chip vulnerable to latch-up, but also op en the do or to undesirable current leaks, capacitive coupling phenom ena, and M OSFET back gate e ects.. 11.6 PREVENTING ELECTRICAL OVERSTRESS A popular layout arrang ement termed butted contacts is shown in g.11.32a.

A p+ (n+ ) island is made to abut with the n+ (p+ ) source region of an n-channel (p-channel) MOSFET. The two are then connected with a short metal strap and tied to vss (vdd). Larger body ties shaped into elongated stripes help to lower base-to-emitter resistances further by sidestepping the poor conductivity of the lightly doped well and substrate materials.

This type of protective structure is referred to as a guard bar; an example has been given in g.11.21.

Even more e ective are guard rings, where a p+ di usion stripe fully encloses n-type MOSFETs, and vice versa. Low resistance requires guard structures to be contacted from metal at regular intervals; connecting via di usion or poly lines must be avoided because of their mediocre conductance. Body ties aim at protecting parasitic BJTs from majority carriers.

That is, the p+ island next to the n-channel MOSFET collects holes that reach the p base region of the parasitic npn-bipolar and provides a safe current path to ground. The opposite is true in the n well..

Provide guard rings to absorb carriers where they might go astray A reciprocal approach c onsists in collecting unwanted carriers close to their place of origin before they could possibly nd their way to some vulnerable BJT. The goal is to absorb stray currents so as to keep Is and Iw small. To attract unwanted electrons, an n+ implant is placed around those devices from which dangerous carriers might emanate and connected to vdd, see g.

11.33. The converse applies to holes.

Another way of looking at such structures is to consider them as extra collectors added to the parasitic bipolars in order to divert most of their undesirable collector currents to vdd and vss respectively. The necessary layout structures look very much like guard rings and are in fact subsumed under this more general term, yet they do attract minority carriers. In order to collect carriers that have managed to penetrate deeper into the substrate, minority guards are often prolonged vertically by implanting a well of identical polarity underneath.

This is shown as an option on the left-hand side of g.11.33b.

An extra di usion ring running along the circumference of a chip and contacted from metal at regular intervals further helps to lower overall substrate resistance and to provide maximum dispersal of substrate currents.32 Such a ring is particularly important when no backside die contact is being used..

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