Trench Power MOSFET is the vertical transistor with gate electrode in trench enabling compact high-voltage, low-resistance switching — dominating power electronics through super-junction structures balancing on-resistance and breakdown voltage tradeoffs.
Vertical Trench Gate Structure:
- Trench architecture: narrow vertical trench etched into Si; gate oxide and polysilicon gate fill trench
- Gate position: gate electrode vertical in trench; enables planar cell arrangement; higher cell density than lateral gate
- Channel formation: inversion layer forms on trench sidewalls; multiple channel widths sum to total transconductance
- Current path: current flows vertically from drain through channel to source; vertical orientation enables thick drift region
- Depth scaling: shallower trenches increase channel width; trench aspect ratio (depth/width) affects manufacturability
Super-Junction Concept:
- Compensation doping: alternating p-type and n-type pillars in drift region; compensation reduces average dopant concentration
- Voltage advantage: lower average doping allows thinner/lower-resistance drift region for same breakdown voltage
- Breakdown physics: fully-depleted drift region sustains high voltage; compensation enables thick depletion region
- Efficiency gain: Rdson reduced 2-4x vs conventional MOSFET for same BV; super-junction fundamental advantage
- Cell pitch: spacing between compensation pillars set by voltage rating; smaller pitch → lower Rdson but higher complexity
Body Diode Characteristics:
- Intrinsic diode: parasitic p-n junction between source (p-channel) and drain (n-drift); freewheeling diode
- Reverse recovery: minority carrier storage in drift region; slow recovery causes switching losses and EMI
- Forward voltage: ~0.7-1 V typical; body diode conducts when switch turned off and current direction reverses
- Dynamic behavior: reverse recovery charge (Q_rr) specified; affects switching losses in synchronous converter applications
- Soft recovery: design reduces dI/dt during recovery; minimizes voltage overshoot and EMI
Gate Charge (Qg) Characteristics:
- Total gate charge: Qg sum of plateau charges; divided into Miller charge (Q_gd) and accumulation charges
- Qgs: charge to reach threshold voltage; charge to establish channel
- Qgd (Miller): charge while V_ds changes at constant current; capacitive charge in Miller plateau
- Gate current source: driving gate requires specified charge delivery; affects driver design and switching speed
- Qg-V_ds curve: multiple operating points (at various V_ds) specified in datasheet
Rdson × Breakdown Voltage Tradeoff:
- Fundamental limit: silicon physics limits Rdson·BV product; lower is better but fundamental limit exists
- Trade-off relationship: Rdson ∝ 1/BV²; higher voltage rating requires higher resistance
- Drift region: thicker drift region for higher voltage; reduces conductivity
- Super-junction advantage: compensation allows breaking fundamental limit; reduces Rdson·BV product
- Temperature coefficient: Rdson increases with temperature (~+0.5%/°C typical); affects power loss calculations
Epi-Layer Optimization:
- Epitaxial layer: grown on substrate; determines drift region thickness and doping profile
- Doping profile: uniform vs graded profiles; grading reduces resistance but complicates fabrication
- Quality: crystal defects increase leakage current; low defect density critical for high-voltage devices
- Growth control: precise thickness/doping control essential; slight variations affect device characteristics
- Substrate choice: higher-dopant substrate reduced resistivity; improves backside contact and thermal spreading
Cell Pitch and Cell Design:
- Hexagonal cell: typical cell shape; optimizes current spreading and transconductance
- Cell density: higher density → lower on-resistance for same area; tradeoff with reliability and yield
- Current concentration: non-uniform current distribution; edge cells carry more current; failure mechanism
- Stress concentration: corners of cells subject to high electric field; design reduces field crowding
Applications in Power Conversion:
- DC-DC converters: synchronous buck converters; Rdson determines efficiency and heat dissipation
- Motor drives: MOSFET inverters driving 3-phase motors; switching losses and efficiency critical
- Electric vehicle (EV): inverters and motor drives; high power handling; efficiency and thermal management essential
- Lighting/LED drivers: switching regulators for LED driver circuits; power and efficiency requirements
Breakdown Voltage Specifications:
- V_DSMAX: maximum V_ds safe operating voltage; specified at I_d = 250 μA; voltage rating
- V_DSEO: drain-source voltage with emitter open (turned off); = V_DSMAX
- V_GS max: maximum gate-source voltage; typically ±20 V; gate oxide stress limits
- V_BD: breakdown voltage; gate current specified at 250 μA; failure point
Trench power MOSFETs dominate high-voltage switching applications through super-junction compensation — enabling compact devices with favorable on-resistance/breakdown voltage tradeoffs suitable for power conversion and motor drives.