(1) Field of the Invention
The present invention relates to semiconductor devices. More specifically, the present invention relates to transistors and circuits utilizing transistors.
(2) Background Information
In the development of integrated electronic circuits, increased interest has been displayed in the possibility of obtaining analog functions and digital functions on the same circuit. Although, bipolar technologies are considered to be better for purely analog circuits, MOS technologies are preferred for the implementation of a digital part of a circuit. A lateral bipolar transistor is an example of a device that integrates analog and digital circuits on the same chip by combining bipolar technologies and MOS technologies.
A lateral bipolar transistor may be obtained from a bipolar transistor that has a collector not tied to the substrate. One type of lateral bipolar transistor may include an emitter, two collectors, a gate, a base and a source. By properly biasing the base, the drain and the source (emitter and collector respectively), a bipolar operating mode is obtained. Typically, the emitter-base junction is forward biased and the collector-base junction is reversed biased. Also, the gate is brought to a potential sufficiently negative (with respect to the base) to avoid inversion of the conductivity type of the zone situated between two diffusion zones constituting the collector and the emitter of the transistor. This prevents MOS transistor operation between the two diffusions which constitute the collector and emitter of the lateral bipolar transistor. To favor the lateral bipolar transistor, the gate length is typically kept small and the perimeter/surface ratio of the emitter is maximized. A discussion about MOS transistors operated in lateral bipolar mode may be found in Eric A.
Vittoz's paper, "MOS Transistors Operated in the Lateral Bipolar Mode and their Application in CMOS Technology", IEEE. Journal of Solid-State circuits, Vol. SC-18, No. 3, June 1983.
In one application, two transistors operated in a lateral bipolar mode may be used in a differential amplifier. Such differential amplifier may include a circuit with a pair (differential pair) of lateral bipolar transistors that may have their emitters connected to a current source the collector currents are defined by signals applied to the bases of the two transistors. The differential amplifier may be used to amplify the difference between two signals coupled to the bases of the two transistors operated in a lateral bipolar mode. The base-emitter voltages of the two transistors operated in a lateral bipolar mode may be changed differentially. A problem with this configuration is the mismatch between the two transistors operated in the lateral bipolar mode.
It is desirable to provide a single lateral bipolar transistor device that may be used by itself to form a differential amplifier. It is desirable that such lateral bipolar transistor exhibits better matching than the two transistors operated in a lateral bipolar mode of the above-mentioned differential amplifier.