What is claimed is:
1. A method of operating a bidirectional bipolar transistor which has first and second first-conductivity-type emitter/collector regions in distinct locations separated by a bulk second-conductivity-type base region, and also has two distinct second-conductivity-type base contact regions which connect to the bulk base region in mutually separate locations, in respective proximity to first and second emitter/collector regions but not to each other, comprising the actions of:
a) when low on-state resistance is desired, applying a base drive voltage to whichever of the base contact regions is then on the collector side of the device, relative to a collector voltage; and
b) varying the base drive voltage while monitoring base current, to thereby find a target base drive voltage where differential conductance becomes detectable, above a present sensitivity threshold, with increasing base drive voltage; and operating the transistor at approximately the target base drive voltage.
2. The method of claim 1 , wherein the first and second first-conductivity-type emitter/collector regions are located on opposite surfaces of a second-conductivity-type semiconductor die.
3. The method of claim 1 , wherein the base current is initially measured with a base drive voltage of approximately zero.
4. The method of claim 1 , wherein the varying step is repeated multiple times which the device is on.
5. A system for power switching, comprising:
a bidirectional bipolar transistor which has two first-conductivity-type emitter/collector regions separated by a bulk second-conductivity-type base region, and two distinct second-conductivity-type base contact regions which connect to the bulk base region in mutually separate locations; and
a pair of transistor-mode drive circuits, separately connected to the first and second base contact regions respectively; wherein, during the transistor-ON state when low voltage drop is desired between the two emitter/collector regions, one of the drive circuits, depending on external voltage polarity, is configured to
apply an adjustable voltage to a selected one of base contact regions, relative to a respective emitter/collector voltage and
dither the adjustable voltage, to thereby find an operating point voltage where the differential conductance at the selected base contact becomes detectable, above a preset sensitivity threshold, with increasing applied voltage, and then keep the adjustable voltage at approximately the operating point voltage.