User:John R. Brews/Sample

In electronics, the mode of an electrical device refers to its steady-state bias condition or operating point in the absence of signals. 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, devices toggle between the cutoff mode (off) and the ohmic mode (on), and visit the active mode only briefly during the transition between the on and off modes.

Bipolar transistor	B-E Junction Bias	B-C Junction Bias	Mode
E injects, C collects	Forward	Reverse	Active
E and C inject	Forward	Forward	Saturation
No injection	Reverse	Reverse	Cut-off
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, some confusing nomenclature has arisen that is only slowly being replaced. For both bipolar and MOSFET devices, it is common to refer to the off mode as cutoff. However, the active mode of the bipolar transistor often is called the saturation mode of the MOSFET, while the on mode of the bipolar transistor often is called its saturation mode. This confusion of terminology does nothing to clarify the discussion of circuitry.

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 reversing polarity 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 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.

Due to a fixation upon the bipolar transistor in the early history of the MOSFET, it was felt necessary to maintain an analogy between the bipolar transistor and the MOSFET, and the MOSFET was forced to become a three-terminal device by shorting the source to the body. That way, one could use V_DS in the MOSFET as an analog for V_CB in the bipolar. As a result, the early Shichman-Hodges model of the MOSFET engrained the use of the variable V_DS in the lore of the MOSFET at the expense of a more physical modeling based upon a four-terminal model, and introduced an unphysical separation of modes of operation of the device based upon some very model-specific voltage ranges, rather than upon the physical operation of the device. Thus, rather than separating modes based upon the presence or absence of the inversion layer at one or both ends of the channel region, the distinction is made based upon voltages: V_DS < ( V_GS – V_T ) or V_DS > ( V_GS – V_T ). More general four-terminal numerical models do not support this approach.