Mode (electronics)

In electronics, the mode of an electrical device refers to its bias condition or, when no signals are present, to its operating point or Q-point (quiescent point). In analog circuits the so-called active mode of the device is chosen by the circuit designer to allow adequate signal amplitude and adequate voltage or current gain, along with acceptable signal distortion. In digital circuits, a device toggles between the off-mode (or cutoff mode) and the on-mode (or saturation mode in bipolar transistors, or ohmic mode for MOSFET's), and visits the active mode only briefly while switching between the on and off modes.

Bipolar transistor	B-E Junction Bias	B-C Junction Bias	Mode
E injects, C collects	Forward	Reverse	Active (Forward-active)
E and C inject	Forward	Forward	Saturation
No injection	Reverse	Reverse	Cutoff
C injects, E collects	Reverse	Forward	Reverse-active

MOS transistor	G-S Bias	G-D Bias	S-B Bias	D-B Bias	Mode
Channel at source end only	≥ VT(S)	≤ VT(D)	Zero or Reverse	More reverse than S-B	Active (Saturation)
Channel at both ends	≥ VT(S)	> VT(D)	Zero or Reverse	More reverse than S-B	Ohmic (Triode)
No channel	< VT(S)	< VT(D)	Zero or Reverse	Zero or reverse	Cutoff (Subthreshold)

For historical reasons, the saturation mode of the MOSFET refers to its active mode, while the saturation mode of the bipolar transistor invariably refers to its on mode. This confusion of terminology does nothing to clarify discussion.

In the bipolar device, the emitter is designed for efficient injection, while the collector is designed to collect with low capacitance between collector and base. Thus, the bipolar device is inherently asymmetrical, and a distinction between forward and reverse modes of operation makes sense. In the MOSFET, the source and drain are interchangeable, so reverse operation simply exchanges the source for the drain. An exception is the power MOSFET, which like the bipolar transistor, has source and drain separately optimized for their particular function.

In the MOSFET, the threshold at source (or drain) is altered by the source-to-body (or drain-to-body) voltage, requiring a larger gate-to-source (gate-to-drain) voltage for channel formation the larger the reverse bias. Hence, the two threshold voltages V_T(S) and V_T(D) are different if the two reverse biases differ.

The term "cutoff" seems to imply no current flow at all, so for the MOSFET the term "subthreshold" sometimes is preferred. In subthreshold operation the current in the MOSFET decreases to very low values exponentially with gate voltage, and has very weak dependence upon drain-to-source voltage.

The table separates modes based upon the presence or absence of an inversion layer, or channel, at one or both ends of the channel region, concepts that retain meaning even for modern MOSFETs with very small dimensions and/or four-terminal operation. On the other hand, the obsolete Shichman-Hodges model that short-circuits the source to the body to obtain a three-terminal representation, bases the distinction between modes upon voltages: V_DS < ( V_GS – V_T ) or V_DS > ( V_GS – V_T ), a limited approach.