Understanding Drain and Source in NFETs
Should the Source Be at a Lower or Higher Potential Than the Drain?
In an N-channel FET (NFET), the source should be connected to a lower potential than the
drain for normal operation.
### Explanation:
1. **Conduction Condition:**
- An NFET conducts when V_GS (Gate-Source Voltage) > V_th (Threshold Voltage).
- Current flows from drain to source when V_DS (Drain-Source Voltage) > 0.
2. **Source as Reference:**
- The source is typically at the lowest potential (often ground in many circuits).
- The drain is at a higher potential to allow current flow when the FET is ON.
3. **Reverse Operation (Uncommon):**
- If the source is at a higher potential than the drain, the transistor may not function
properly or may conduct in reverse (body diode conduction in MOSFETs with bulk-source
connection).
### Conclusion:
For proper NFET operation, source should be at a lower potential than drain (e.g., source at
0V, drain at +V).
Are Drain and Source Fixed in Commercial MOSFETs?
Yes, in normal commercial MOSFETs, the drain and source are not always fixed but are
typically used in a specific way due to their internal body diode and structure.
### 1. Symmetry of MOSFETs
- In an ideal MOSFET, the drain and source are theoretically interchangeable because the
channel is formed based on voltage differences.
- However, in real commercial MOSFETs, the bulk (body) is internally connected to the
source, which makes the source act as a fixed lower potential in NFETs (or higher potential
in PFETs).
### 2. Why Drain and Source Are Usually Fixed
- **Body Diode Effect:**
- Most MOSFETs have an intrinsic body diode (PN junction between the drain and source).
- If you reverse drain and source, the body diode might conduct unintentionally, leading to
unwanted current flow.
- **Threshold Voltage Variation:**
- Since the bulk is tied to the source, the threshold voltage (V_th) is stable when using the
correct source-drain configuration.
- If you swap them, the threshold voltage may vary due to the body effect, making
performance unpredictable.
### 3. Special Cases
- **Bidirectional MOSFETs:** Some MOSFETs (especially in analog switches) are designed
symmetrically and can work with either terminal as the drain or source.
- **Power MOSFETs (with External Body Diode):** In high-power applications, reversing
drain and source is avoided because the internal body diode would conduct and cause
issues.
### Conclusion
- In most commercial MOSFETs, the source is fixed, and the drain is at a higher potential for
proper operation.
- While some MOSFETs can technically be reversed, doing so may lead to unintended
conduction due to the body diode.
What Happens If You Swap Drain and Source in a MOSFET with a Body Diode?
If you swap the drain and source in a commercial NFET, where the body (bulk) is internally
connected to the source, the MOSFET will not function correctly due to the body diode's
behavior.
### What Happens When You Reverse Drain and Source?
1. **Body Diode Becomes Forward Biased:**
- In a normal NFET, the body diode is between the source (anode) and drain (cathode).
- If you swap drain and source, you effectively reverse the diode's direction in the circuit.
- Now, when the new 'source' (which was originally the drain) is at a higher voltage than
the new 'drain,' the body diode turns ON and conducts current even when the MOSFET is
OFF.
2. **Uncontrolled Current Flow (Leakage or Continuous Conduction):**
- The body diode typically has a forward voltage drop of ~0.7V to 1V (for silicon
MOSFETs).
- If the voltage difference between the swapped drain and source exceeds this, the diode
will conduct, allowing unwanted current flow through the MOSFET even if the gate is not
turned ON.
3. **Failure to Control Current Properly:**
- Even when you try to turn the MOSFET ON (by applying V_GS > V_th), the characteristics
may not match normal operation.
- The threshold voltage can shift due to the body effect, making switching unpredictable.
4. **In Power Applications – High Heat & Losses:**
- In power electronics, the body diode conducting leads to higher power dissipation, since
diodes have a higher forward voltage drop than the MOSFET’s normal R_DS(on) resistance.
- This can cause inefficiency and overheating.
### Example Case
**Normal NFET Connection (Correct):**
- Source = 0V (ground), Drain = +V (e.g., 10V)
- Gate controls conduction:
- V_GS > V_th → MOSFET turns ON, allowing current flow from drain to source.
- V_GS < V_th → MOSFET turns OFF, and no current flows (except for small leakage).
**Reversed NFET Connection (Wrong):**
- Source = +V (e.g., 10V), Drain = 0V (ground)
- Now, the body diode (source-to-drain) becomes forward biased:
- Diode conducts whenever V_source > V_drain by ~0.7V, leading to unwanted current
flow.
- MOSFET loses its ability to fully block current when OFF.
### Conclusion
- In normal commercial MOSFETs, the source and drain are not truly interchangeable
because of the internal body diode.
- Reversing them leads to unwanted conduction through the body diode, affecting switching
behavior and efficiency.
- If a truly bidirectional MOSFET is needed, you must use a MOSFET without a body diode or
use a configuration like back-to-back MOSFETs.
