1. Field of the Invention
The present invention relates to an electronic circuit comprising a diode-connected MOSFET or MOS transistor having one of its power terminals receiving an oscillating signal, for example, an A.C. signal.
2. Discussion of the Related Art
Many electronic circuits perform a power conversion. Most often, it is a conversion of an A.C. input signal formed of a succession of positive and negative halfwaves, into a D.C. signal. One of the simplest electronic circuits to perform this function is a single-wave rectifier circuit. It is formed of a rectifying stage, followed by a filtering stage. The rectifying stage may comprise a diode which enables to keep, for example, only the positive halfwaves of the input signal. The filtering stage may comprise a capacitor and provides a substantially D.C. signal from the positive halfwaves.
To form an integrated rectifier circuit according to a CMOS technology, the diode is generally replaced with a diode-connected MOS transistor, that is, a transistor having its gate connected to the drain. Most of the power losses of the integrated rectifier circuit originate from the transistor in the on-state thereof and depend on the product of the current running through the transistor and of the transistor drain-source voltage. When the transistor is on, the channel current of the transistor varies according to the gate-source voltage, that is, to the drain-source voltage. To decreases losses in the rectifier circuit, the drain-source voltage should remain as small as possible when the transistor is on. It is thus desirable for the threshold voltage of the transistor to be as small as possible.
Conventionally, the MOS transistor is formed in and on a layer of a semiconductor material biased by a source of a reference voltage, for example, the ground. However, with such a MOS transistor structure, it appears to be difficult to significantly decrease the threshold voltage of the transistor. Further, especially during negative halfwaves of the input signal, additional losses can be observed because of the leakage current of the MOS transistor in the off state and to the conduction of parasitic diodes of the transistor.