The MOSFET body diode is not an optional feature or a schematic convention; it is an unavoidable consequence of how the device is constructed. Current can flow from source to drain through that diode regardless of the gate, which means a MOSFET does not block voltage in both directions. Its conduction and recovery characteristics are often much worse than a diode you would have chosen deliberately.
Where the diode comes from
A power MOSFET is built with a body region of one doping type sitting between the source and the drift region, and the source metallization is deliberately shorted to that body to prevent a parasitic bipolar transistor from turning on. That short creates a junction between the body and the drift region that behaves as a diode connected from source to drain, oriented so that it blocks in the normal operating direction and conducts in the reverse one. There is no way to remove it without removing the body short, which would introduce a worse problem. Manufacturers therefore characterize it: forward voltage, reverse recovery charge and reverse recovery time appear in the datasheet alongside the switching parameters.
What it means for blocking and protection
The most common practical consequence is that a single MOSFET cannot serve as a bidirectional switch or as reverse polarity protection in the naive orientation, because when the source is more positive than the drain, current flows through the diode regardless of whether the device is on. Reverse battery protection circuits therefore orient the MOSFET so that the body diode blocks the fault condition and conducts harmlessly in normal operation, with the gate turning the channel on to bypass the diode and reduce the drop. Where true bidirectional blocking is required, two devices are placed back to back so that one body diode blocks each direction. Recognizing this early prevents a class of designs that appear to work and then pass current when they should not.
Reverse recovery is the expensive part
When a diode that has been conducting is suddenly reverse biased, stored charge must be removed before it blocks, and during that interval current flows backward through it. In a half bridge, that recovery current flows through the opposite switch as it turns on, adding a substantial current spike on top of the load current, producing extra loss in both devices and generating high-frequency ringing that radiates. Many silicon power MOSFET body diodes have relatively poor recovery characteristics compared with a purpose-built fast diode, which is why hard-switched bridge topologies using them can be lossy and noisy. Designers address this with devices specifically characterized for fast body diode recovery, with an external parallel Schottky to divert current, or with topologies that avoid hard commutation.
Dead time turns the diode into a design parameter
In a half bridge the two devices must never conduct simultaneously, so the control inserts a dead time during which both gates are off. Inductive load current has to keep flowing, and it does so through the body diode of the device that is about to turn on. Dead time is therefore an interval of diode conduction, with its own loss at the diode’s forward voltage, and making dead time longer to be safe increases that loss and can distort the output waveform. Making it too short risks shoot-through, where both devices conduct and the supply is briefly shorted. Synchronous rectification depends on managing exactly this: the diode carries current during the dead time and the channel carries it thereafter, at a much lower voltage drop.
Choosing devices and verifying behavior
When a topology commutates the body diode, read the reverse recovery charge and time in the datasheet, not just the on-resistance and the gate charge, because those parameters can dominate the loss budget. Compare candidate devices on the parameters your topology stresses rather than on the headline figures. Wide-bandgap devices behave differently here, and their datasheets should be consulted directly rather than assuming silicon behavior carries over. On the bench, look at the switch node and the device current during turn-on with a short-ground probe: a sharp current spike coincident with the opposite device turning on is the recovery current, and its size tells you how much the diode is costing you.
What to check before you act
- Orient protection MOSFETs so the body diode blocks the fault condition.
- Use back-to-back devices where true bidirectional blocking is required.
- Read reverse recovery charge and time for any hard-switched bridge.
- Scope the turn-on current spike to see what recovery actually costs.
Common questions
Can I disable the body diode?
No. It is inherent to the structure and cannot be removed. You can only bypass it with a parallel device or choose a part whose diode behaves better.
Does turning the MOSFET on remove the diode drop?
Turning the channel on provides a lower-resistance parallel path, so most current flows through the channel and the effective drop falls. The diode is still present and still conducts during dead time.
The practical takeaway
Assume every MOSFET conducts in reverse and design the orientation around that fact. Where the topology commutates the diode, select devices on their recovery characteristics, set dead time deliberately rather than generously, and verify the turn-on current spike on hardware before trusting the loss budget.
References and further reading
- Department of Energy: how electronics losses turn into wasted energy and heat
- NFPA: electrical safety research and guidance on overheating equipment and fire risk
- FCC: electromagnetic compatibility and radio frequency safety questions answered
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
