Bicmos Logic 1980s Measure Output Drive Current Against Pure CMOS

# BiCMOS Logic (Late 1980s): Step 9 — Measure Output Drive Current Against Pure CMOS

## 1. Current, Not Just Delay, Is the Direct Metric

Step 8 measured the downstream consequence — delay — of a drive-current difference; this step measures the drive current itself, directly, as the more fundamental quantity. A representative minimum-size CMOS output transistor of this era's process generation sources or sinks output current on the order of a few milliamps before its own saturation behavior limits further increase; a comparably sized bipolar output transistor, operating in saturation with adequate base drive supplied by the CMOS stage per Step 7, commonly delivers several times that current from a similar or smaller silicon footprint.

## 2. Why Fan-Out and Bus-Driving Specifically Benefit

This drive-current gap is what makes BiCMOS output stages specifically valuable for fan-out-heavy and bus-driving applications: a single BiCMOS output stage can often replace what would otherwise require either an oversized CMOS output transistor (paying the upstream input-capacitance penalty described in Step 1) or multiple parallel CMOS drivers (paying area and routing complexity instead). The bipolar device's current gain gives the extra output current largely for free in area terms, since the added area is the small bipolar structure from Step 5, not a proportionally wider MOSFET.

## 3. Companion Diagram One: Quantitative View

Output Drive Current: Comparable Footprint, Different Devices maximum sustained output current, matched die area, two output stage types CMOS output transistor bipolar output transistor the multiple this step actually measures limited by MOSFET saturation current at this size several times higher, from bipolar current gain comparable footprint, markedly different available output current the gain comes from the device, not from spending more area

## 4. Companion Diagram Two: Structural View

Two Ways to Get More Output Current the area cost of reaching a given drive-current target, two routes WIDEN THE CMOS DEVICE more width, more input capacitance upstream too PARALLEL CMOS DRIVERS more area, more routing, same stage per driver ONE BIPOLAR STAGE small added area, current gain supplies the rest the bipolar route reaches the same target without either CMOS penalty

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