When engineers and hobbyists refer to the diode in MOSFET, they are talking about the intrinsic body diode. This is not an intentionally added component; it is a parasitic P-N junction formed during the semiconductor manufacturing process between the drain and source regions. For an N-channel MOSFET, the anode is at the source and the cathode is at the drain. It conducts current when the MOSFET is reverse-biased (source voltage is higher than drain voltage), typically exhibiting a forward voltage drop ($V_f$) of 0.7V to 1.2V in silicon devices. While often treated as a nuisance in high-frequency switching, this diode is a critical freewheeling path in motor drives and power supplies.
The Intrinsic Body Diode: Symbol, Pinout, and Physical Origin
To understand the body diode, you have to look at the physical silicon die. In a standard vertical N-channel power MOSFET, the drain is connected to the N- epitaxial layer at the bottom, while the source is connected to the P-body and N+ source regions at the top. Because the P-body and N- epitaxial layer are physically adjacent, they form a P-N junction. The manufacturer intentionally shorts the P-body to the N+ source region via the source metallization to prevent the parasitic NPN bipolar transistor (formed by the source, body, and drain) from turning on and destroying the device.
In a standard 3-pin TO-220 package (viewed from the front, pins pointing down), the pins are Gate (1), Drain (2), and Source (3). The metal tab is internally connected to the Drain. On the schematic symbol for an N-channel MOSFET, the body diode is drawn with the diode arrow pointing from the Source (anode) to the Drain (cathode), with the bar across the arrow touching the Drain line. For a P-channel MOSFET, the diode is reversed: anode at Drain, cathode at Source.
MOSFET Operating States and Safe Default Part Numbers
The behavior of the MOSFET channel and the body diode change depending on the biasing of the Gate-Source ($V_{GS}$) and Drain-Source ($V_{DS}$) terminals. The table below maps out these operational regions, which is critical for predicting when the diode will conduct.
| Region / State | $V_{GS}$ Condition | $V_{DS}$ Condition | Channel State | Body Diode State |
|---|---|---|---|---|
| Cutoff | < $V_{GS(th)}$ | $V_{DS}$ > 0 | OFF (High Impedance) | Reverse Biased (OFF) |
| Ohmic / Linear | > $V_{GS(th)}$ | $V_{DS}$ < ($V_{GS}$ - $V_{th}$) | ON (Resistive) | Reverse Biased (OFF) |
| Saturation | > $V_{GS(th)}$ | $V_{DS}$ > ($V_{GS}$ - $V_{th}$) | ON (Current Source) | Reverse Biased (OFF) |
| Third Quadrant (Freewheeling) | Any | $V_{DS}$ < -0.7V | OFF or ON | Forward Biased (ON) |
| Avalanche Breakdown | < $V_{GS(th)}$ | $V_{DS}$ > $V_{(BR)DSS}$ | OFF | Avalanche Conducting |
When selecting a MOSFET, you must check the datasheet for the body diode's specific ratings: continuous source-drain current ($I_{SD}$), forward voltage ($V_{SD}$ or $V_f$), and reverse recovery time ($t_{rr}$). Here are safe default part numbers across common use cases, complete with their diode-specific ratings.
| Part Number | Type / Package | $V_{DS}$ / $I_D$ | $R_{DS(on)}$ @ 10V | Diode $V_f$ (Typ) | Diode $t_{rr}$ |
|---|---|---|---|---|---|
| IRLZ44N | N-Ch Logic / TO-220 | 55V / 47A | 22 mΩ | 1.2V | 130 ns |
| IRF3205 | N-Ch Standard / TO-220 | 55V / 110A | 8 mΩ | 1.0V | 105 ns |
| BSC060N10NS3 | N-Ch OptiMOS / SuperSO8 | 100V / 100A | 6.0 mΩ | 0.84V | 32 ns |
| C3M0060120D | SiC N-Ch / TO-247-3 | 1200V / 36A | 60 mΩ | 3.2V | ~0 ns (None) |
Sources: Datasheets from Infineon Technologies and onsemi.
Biasing, Selection, and the Freewheeling Application Circuit
You do not 'bias' the body diode directly; it is a passive junction that conducts automatically when forward-biased by the circuit's inductive kickback. However, you select the MOSFET based on how the diode will be used. If your application involves hard-switching an inductive load at high frequencies (e.g., a 50kHz buck converter), the slow reverse recovery time ($t_{rr}$) of a standard silicon body diode will cause massive switching losses and potential shoot-through. In motor drives running at 10kHz to 20kHz, the silicon body diode is usually sufficient.
Below is a complete 12V DC motor PWM speed control circuit. While the IRLZ44N's body diode can handle the flyback energy, we include an external Schottky diode to bypass the body diode's slow recovery, reducing heat and EMI.
| Component | Value / Part | Connection / Purpose |
|---|---|---|
| Q1 (Switch) | IRLZ44N (N-Ch MOSFET) | Gate to PWM, Drain to Motor(-), Source to GND |
| R1 (Gate Resistor) | 100Ω (1/4W Carbon Film) | Between MCU PWM pin and Gate (limits $dI/dt$, prevents ringing) |
| R2 (Pull-down) | 10kΩ (1/4W Carbon Film) | Gate to Source (ensures Q1 turns off if MCU pin floats) |
| D1 (Flyback) | 1N5819 (Schottky, 40V 1A) | Cathode to 12V, Anode to Drain (bypasses slow body diode) |
Failure Modes and Multimeter Testing Procedures
The body diode typically fails in one of three ways: thermal runaway (dissipating $V_f \times I_{SD}$ generates heat, which lowers the forward voltage threshold, pulling more current until the silicon melts), avalanche failure (exceeding the single-pulse avalanche energy $E_{AS}$ rating during an inductive spike), or $dV/dt$ induced turn-on (a fast-rising voltage spike across the drain-source capacitance couples into the gate, accidentally turning the channel on and causing a short circuit).
Testing the body diode is the fastest way to confirm if a MOSFET is blown. A shorted diode almost always means a dead MOSFET. Follow these numbered steps using a digital multimeter (DMM):
- Isolate the Component: Remove the MOSFET from the circuit. In-circuit testing will yield false readings due to parallel paths.
- Discharge the Gate: Touch a bare wire or your finger across the Gate and Source pins. This bleeds off any trapped charge in the gate capacitance that might accidentally turn the channel on during testing.
- Set DMM to Diode Test: Turn the multimeter dial to the diode symbol.
- Forward Bias Test (N-Channel): Place the Red probe on the Source pin and the Black probe on the Drain pin. You should read a forward voltage drop between 0.40V and 0.85V for silicon, or ~3.0V for SiC. If it reads 'OL' or '1', the diode is open (dead).
- Reverse Bias Test: Swap the probes (Black on Source, Red on Drain). The meter should read 'OL' (Over Limit). If it reads near 0.00V or beeps continuously, the diode (and the MOSFET) is shorted and must be replaced.
Silicon vs. SiC: When the Intrinsic Diode Falls Short
As switching frequencies push past 50kHz and bus voltages exceed 400V (such as in solar inverters or EV chargers), the silicon body diode becomes a severe bottleneck. Its reverse recovery charge ($Q_{rr}$) causes massive current spikes when the MOSFET turns back on. This is where Silicon Carbide (SiC) MOSFETs change the paradigm.
| Criterion | Silicon (Si) MOSFET | Silicon Carbide (SiC) MOSFET |
|---|---|---|
| Reverse Recovery ($t_{rr}$) | Slow (50ns - 200ns); high $Q_{rr}$ | Virtually zero (majority carrier device) |
| Forward Voltage ($V_f$) | Low (~0.7V - 1.2V) | High (~2.8V - 3.5V) |
| Switching Losses | High at >50kHz due to $Q_{rr}$ | Extremely low; enables MHz switching |
| Cost (per Amp) | Low ($0.50 - $2.00) | High ($4.00 - $15.00+) |
| Best Application | <20kHz motor drives, low-side switches | >100kHz hard-switching, 600V+ bus systems |
When designing with SiC MOSFETs like the Wolfspeed C3M series, you must account for the higher $V_f$ of the body diode. If the diode will conduct continuously for long dead-times, the conduction losses ($I^2R$ and $V_f \times I$) will spike. In those specific continuous-conduction scenarios, designers still place an external SiC Schottky diode in parallel to share the current, but for standard hard-switching dead-times, the intrinsic SiC body diode is more than capable and vastly superior to its silicon counterpart.
