Every MOSFET (except special structures like some RF/LDMOS) is inherently a four-terminal device: gate, source, drain, and body (substrate). In most discrete MOSFETs, the body is tied to the source. This internal connection forms a p–n junction between the body (p-type for n-channel, n-type for p-channel) and the drain.
This p–n junction behaves like a diode, often called the body diode (or parasitic diode).
So it’s not “added” externally—it’s built-in due to the MOSFET structure.
Here’s a detailed breakdown of why it exists and what its functions are.
1. Why the Body Diode Exists (The Origin)
The body diode is not a discrete component soldered in parallel. It is a parasitic element that is an unavoidable consequence of the physical structure of a Silicon MOSFET.
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Structure: A MOSFET is built on a silicon substrate (the "Body" or "Bulk"). A region of the opposite doping type (the "Drift" region) is created, and the Source terminal is connected to both the N+ and P+ regions.
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The PN Junction: This creates a PN junction between the P-type Body and the N-type Drift region (which is connected to the Drain). A PN junction is, by definition, a diode.
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Inherent Presence: Therefore, every standard planar MOSFET has a diode formed between its Drain and Body. Since the Body is shorted to the Source internally to avoid unwanted bias effects, this diode effectively appears between the Drain and the Source.
This inherent structure is shown in the schematic symbol for a MOSFET, where the diode is explicitly drawn.
2. The Functions and Implications of the Body Diode
The existence of this diode has critical consequences, both beneficial and problematic, for circuit operation.
Primary Function: Freewheeling / Clamping in Inductive Loads
This is the most important useful function of the body diode.
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The Scenario: MOSFETs are very commonly used to drive inductive loads like motors, solenoids, or transformers. A key property of an inductor is that it resists changes in current.
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The Problem: When you turn off the MOSFET, the current through the inductor cannot stop instantly. This would cause a massive voltage spike across the inductor (given by
V = L * di/dt) that could easily destroy the MOSFET or other components. -
The Solution (The Body Diode): The body diode provides a path for this inductive current to continue circulating ("freewheeling") after the switch is turned off. The collapsing magnetic field in the inductor forces current through the diode, safely clamping the voltage spike and dissipating the stored energy.
This is absolutely crucial in H-Bridge and half-bridge circuits used for motor control and switching power supplies. In these circuits, one MOSFET's body diode provides the freewheeling path when the complementary switch is turned off.
Without this inherent diode, these circuits would not work and would self-destruct. In some high-performance applications, an external Schottky diode is added in parallel because it has a lower forward voltage than the body diode, improving efficiency.
Other Key Implications:
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Blocking Direction: A MOSFET can only block voltage in one direction. Because of the diode, it cannot block reverse voltage (from Source to Drain). If a reverse voltage is applied that exceeds the diode's forward voltage (typically ~0.7V - 1.5V), the diode will conduct, potentially causing a short circuit.
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Solution: For applications requiring bidirectional blocking (like AC switches), two MOSFETs are connected in series, source-to-source, so their body diodes are back-to-back, blocking current in both directions.
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Switching Losses (A Drawback): The body diode is a major source of power loss and complexity in switching circuits.
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Reverse Recovery: When the MOSFET is turned on to carry current in its forward direction (Drain-to-Source), the body diode that was just conducting (in the complementary switch) must quickly turn off. Silicon diodes don't turn off instantly; they have a reverse recovery time (
t_rr), where they briefly conduct current in reverse. This creates a significant current spike and switching loss in the MOSFET that is turning on. -
Solution: This is a key reason for the development of wide-bandgap semiconductors like Silicon Carbide (SiC) MOSFETs. Their body diodes have much better (faster) reverse recovery characteristics, leading to far higher efficiency.
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Summary Table
| Aspect | Explanation |
|---|---|
| Origin | A parasitic PN junction inherent to the silicon structure of a MOSFET. It is not an external component. |
| Primary Function | Freewheeling Diode: Provides a path for inductive kickback current, protecting the circuit from voltage spikes. |
| Key Implication #1 | Unidirectional Blocking: A MOSFET can only block voltage from Drain-to-Source. It cannot block reverse voltage. |
| Key Implication #2 | Switching Losses: The diode's reverse recovery characteristic is a major source of power loss and heat in high-frequency switching circuits. |
Conclusion
Think of the body diode not as an intended feature but as a structural byproduct that circuit designers must account for and exploit. Its ability to handle inductive energy makes it invaluable, but its sluggish reverse recovery performance is a fundamental limitation of traditional silicon power MOSFETs that continues to drive innovation in semiconductor technology.
