In electronic design, MOSFETs and bipolar transistors are two of the most commonly used switching devices. Although both can achieve current control and power conversion, their operating characteristics in the ON state differ significantly. Typically, a transistor’s ON state is described using the saturation voltage Vce(sat), while a MOSFET’s ON state is characterized by its on-resistance Rds(on). So, is there a case where we can reverse this and use saturation Rce or saturation Vds instead? Many distributors offer a wide range of electronic components to cater to diverse application needs, like BSS84
What does a transistor describe its conduction characteristics?
When a transistor operates in the ON state, it is in the saturation region. At this time, the current Ice is jointly affected by Ib (base current) and Vce:
Ib cannot remain constant (affected by drive capability, temperature, and β variation)
Therefore, ice cannot be determined solely by Vce
So a fixed “saturation Rce” cannot be obtained
The Vce(sat) in the saturation region is typically only 0.1–0.3V, very small and with a limited range of variation, so using the saturation voltage Vce(sat) to describe the conduction characteristics of a transistor is the most reasonable.
In other words, because a transistor is a current-driven device, the determination of Ice is not sufficiently predictable, and thus it cannot obtain a stable equivalent resistance like a MOSFET.
Why can a MOSFET use Rds(on) to describe its conduction characteristics?
When a MOSFET is in the ON state, it operates in the linear region, and its behavior is completely different from that of a traditional bipolar transistor. In the linear region, the main driving quantity is Vgs (gate-source voltage). As long as Vgs is fixed at a certain value (such as 4.5V or 10V), the channel will maintain stable conductivity. Under this condition, the MOSFET’s Ids behaves more like flowing through a resistor, so we usually use Rds(on) to describe its conduction performance.
Furthermore, once the channel is formed, the MOSFET’s current is primarily affected by Vds, and when Vgs is fixed, Rds(on) is relatively stable, making it very suitable for calculating conduction loss. This characteristic makes MOSFETs especially suitable for switching power supplies, synchronous rectification, motor drives, etc., where engineers can evaluate efficiency simply by looking at Rds(on).
More importantly, due to the presence of a body diode + bidirectional channel structure, current can flow in both directions between D and S. This is the key foundation for synchronous rectification, Buck-Boost, and bidirectional power conversion, and further supports the rationality of describing conduction characteristics using “resistance.”
If D and S are interchangeable, why define D and S at all?
During the manufacturing process of MOSFETs inside ICs, the source and drain structures are symmetric, and their physical appearance looks almost identical. To facilitate model calculations, circuit descriptions, and direction definitions, engineers artificially distinguish Drain and Source in documentation, but this is more of a “naming need” rather than an absolute physical difference.
However, in power MOSFETs, the situation is not entirely the same. To improve breakdown voltage, the drain of power NMOS devices is usually designed with a lightly doped drift region (LDD). This structure increases the drain’s voltage capability, while the source is paired with a large metal area to reduce parasitic resistance. In this case, although D and S can still be interchanged electrically, they are no longer physically symmetric, and improper wiring may affect breakdown voltage, thermal performance, and parasitic effects.
Therefore, the concepts of D and S remain necessary in circuit applications, but their actual functional roles depend on operating conditions rather than the printed pin names.
What happens to the MOSFET’s electrical characteristics when D and S are swapped?
When Vgs = 0, if D/S are forcibly swapped, the circuit may still conduct as long as Vds exceeds the body diode’s forward voltage drop (typically around 0.7V). This is because the internal body diode has a fixed direction, so even without a formed channel, reverse conduction occurs through the diode.
When Vgs > Vth and the channel is built, the current flows primarily through the channel, and in this state, swapping D and S has very little effect on most electrical parameters. Threshold voltage Vth, parasitic capacitance, and the Miller effect remain nearly unchanged, and Rds(on) also changes only slightly. Therefore, from an engineering perspective, once a MOSFET is turned on, D and S can basically be regarded as interchangeable.
However, due to the fixed direction of the body diode, reverse connection may lead to lower reverse breakdown voltage. This is the most significant change when D and S are swapped. Designers must take note of this to avoid overvoltage damage.
The final determination of D and S
The fundamental operating principle of a MOSFET determines that: at any moment, the true Source is the terminal with the lower potential that forms a valid Vgs with the Gate — not necessarily the pin labeled “S” on the package. This means that in different topologies or during transient changes, the “working Source” of a MOSFET may change.
This characteristic explains why MOSFETs can flexibly switch current direction in synchronous rectification, charge pumps, Buck-Boost circuits, and other bidirectional topologies. Regardless of current direction, as long as the gate drive maintains an effective Vgs, the device automatically determines which terminal becomes the Source.
Therefore, the D/S functionality of a MOSFET is not fixed but dynamically determined by operating conditions. This is a key difference between MOSFETs and bipolar transistors.
Summary
Through a systematic analysis of MOSFET behavior, we can draw several key conclusions:
First, using Rds(on) to describe MOSFET conduction characteristics is based on its voltage-driven nature and stable channel resistance, whereas the transistor, being current-driven, is more suited to using Vce(sat) to describe saturation behavior.
Second, although MOSFETs have defined D and S in structure and labeling, the true Source is the terminal that forms an effective Vgs with the Gate, giving MOSFETs natural D/S interchangeability under conduction conditions.
Additionally, due to the presence of a body diode, a MOSFET can conduct even when reverse-connected and supports bidirectional current flow, which is the fundamental reason it is widely used in synchronous rectification and bidirectional circuits.