Define the Vertical Doublediffused MOSFET Structure

# Define the Vertical Double-Diffused MOSFET Structure

## 2. Define the Vertical Double-Diffused MOSFET Structure

The structure that answered the bipolar transistor's limits is the vertical double-diffused MOSFET, commonly called VDMOS: a planar MOS gate sitting atop a double-diffused body region, with current flowing vertically down through the chip from a top-side source to a bottom-side drain, instead of laterally across the surface the way a conventional small-signal MOSFET does. The word double-diffused names the fabrication sequence directly: a p-type body region and an n-type source region are diffused into the same masked opening, one after the other, so the lateral difference in how far each diffusion spreads under the mask edge sets the channel length precisely — without needing a separate, hard-to-control lithography step to define that tiny dimension.

Turning the current path vertical is what makes the device a power device rather than a small-signal one. A lateral MOSFET's current-carrying capacity is limited by the chip's surface area available for its channel width; a vertical device instead spreads current down through the full thickness of the silicon and out through a drain contact covering the entire backside of the die, so thousands of small identical cells can be built in parallel across the chip's surface, each contributing its own channel, and all sharing one common drain underneath. The result is a single power MOSFET assembled from a dense, repeating array of tiny self-aligned cells.

The Vertical Double-Diffused MOSFET Cross-Section 2. Define the Vertical DMOS Structure VDMOS SINGLE-CELL CROSS-SECTION (TOP TO BOTTOM) Source metal + gate oxide/polysilicon gate at the top surface Double-diffused p-body region sets channel length by diffusion, not lithography n-type drift region carries current vertically downward n+ drain substrate forms one continuous backside contact # c 9 d 1 d 9 diffusion sets the channel; geometry sets the current path
Why Many Small Cells Beat One Large Channel 2. Define the Vertical DMOS Structure CELL ARRAY vs LATERAL SURFACE CHANNEL Lateral MOSFET: current capacity limited by surface channel width available Vertical cell: current flows downward, one drain contact shared by the whole backside Thousands of identical small cells tile the die surface, all in parallel Total current capacity scales with die area, not with a single long channel edge a power MOSFET is an array of cells, not one wide transistor

Take define the vertical doublediffused mosfet structure further

Ask the copilot about this term, or have our engineers assess it against your process.

Ask CFSGPT about this → Request a technical assessment → Open the CFS app