The increase in internal resistance (typically referring to the on-resistance RDS(on)) of MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) involves multiple factors including materials, manufacturing processes, operating conditions, and external influences. The following provides a detailed analysis:

 

1. Temperature Increase

Semiconductor Characteristics: The on-resistance RDS(on) of MOSFETs exhibits a positive temperature coefficient. As temperature rises, carrier mobility decreases, leading to increased channel resistance.

Impact: For every 10°C increase in temperature, RDS(on) may increase by 5% to 10%. In high-temperature environments (e.g., poor heat dissipation or high-power applications), internal resistance rises significantly.

 

Example: In SMPS designs with inadequate thermal management, MOSFET temperatures may exceed 100°C, doubling internal resistance.

2. Insufficient Gate Drive Voltage

Mechanism: During conduction, the gate voltage VGS must exceed the threshold voltage Vth to form a conductive channel. Insufficient VGS increases channel resistance.

Impact: Each 1V reduction in VGS may increase RDS(on) by 20%–50% (depending on device type).

 Example: Insufficient drive circuit output voltage (e.g., driving a logic-level MOSFET with 3.3V) or excessive drive resistance causing VGS attenuation.

 

3. Device Aging and Degradation

Gate oxide damage: Prolonged exposure to high temperatures or strong electric fields may induce defects in the gate oxide (SiO₂), increasing gate leakage current and raising channel resistance.

Thermal carrier effects: Under high voltage and current conditions, carriers gain sufficient energy to impact the crystal lattice, creating defects that disrupt the channel structure.

Metal migration: Long-term high temperatures cause increased resistance in metal interconnects (e.g., source/drain contacts).

 

Example: In high-frequency switching applications, MOSFETs may exhibit significant internal resistance increase within months due to the thermal carrier effect.

 

4. Manufacturing Process Defects

Channel Doping Inhomogeneity: Deviations in ion implantation or diffusion processes cause uneven channel resistance distribution, increasing local resistance.

Gate Oxide Thickness Variation: Thickness variations in the gate oxide layer affect threshold voltage and channel resistance.

Metal Layer Defects: Poor contact or insufficient thickness in source/drain metal layers increases contact resistance.

 

Example: Low-cost MOSFETs may exhibit poor internal resistance consistency due to lax process control.

 

5. External Stress Damage

 Overvoltage/Overcurrent: Transient surges exceeding rated voltage or current (e.g., ESD, power supply spikes) may damage device structure, causing permanent internal resistance increase.

 

Mechanical stress: Bending of the package or PCB can cause internal wire breaks or poor contact.

 Example: MOSFETs may be damaged by lightning strikes or power fluctuations when no protection circuitry is present at the power input.

 

6. Packaging and Thermal Issues

 High package thermal resistance: Small packages (e.g., SOT-23) have poor heat dissipation, leading to elevated internal temperatures and indirectly increasing internal resistance.

Inadequate thermal design: Insufficient heatsink area, missing thermal grease, or poor airflow exacerbate thermal effects.

 

Example: Using a TO-220 packaged MOSFET in an enclosed environment without a heatsink may cause significant internal resistance increase due to high temperatures.

 

7. Frequency and Switching Losses

 High-frequency switching losses: In high-frequency applications, switching losses (e.g., gate charge charging/discharging, output capacitance losses) cause device temperature rise, indirectly increasing on-resistance.

 Example: In a DC-DC converter, increasing the switching frequency from 100kHz to 1MHz may cause MOSFET internal resistance to rise by 10%–20% due to temperature increase.

 

Solutions and Preventive Measures

 Optimize thermal design: Add heat sinks, improve airflow, use low thermal resistance packages.

Ensure gate drive voltage: Select MOSFETs with matched drive voltage and optimize drive circuits.

Prevent overvoltage/overcurrent: Add protection circuits (e.g., TVS diodes, fuses).

Select high-quality devices: Prioritize brand-name products with stable manufacturing processes and high reliability.

Reduce operating frequency: Lower switching frequency within permissible limits to minimize losses.

Regular inspection and replacement: Conduct periodic internal resistance testing on MOSFETs in critical applications and promptly replace aged components.

 

By comprehensively analyzing these factors, the root causes of increased MOSFET internal resistance can be effectively identified, enabling targeted measures for design optimization or maintenance.