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AN605

Power MOSFET Basics: Understanding MOSFET Characteristics Associated With The Figure of Merit

Vishay Semiconductors 

a) TrenchFET Power MOSFET Cross Section

Source

Gate

P-Body

AN605

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b) Trench Vertical DMOS Components of Resistance

Source

Interconnect

Gate

Interconnect

N+

+

N+

Channel

P-Body

Spreading

N-Epi

N-Sub

Drain

Epi

Substrate

Drain

Interconnect

FIGURE 2. Trench DMOS 3D cross-section, with associated resistive elements.

a)

b)

Source

Cgs

Gate

Cgs Body

Gate

rDS

rDS

rDS

rDS

Cgd

Cgd

Drain

FIGURE 3. Ultra-high-density die cross-section with equivalent parasitic parameters

The on-resistance of the DMOS Trench MOSFET is the sum

of all the individual regions through which the mobile carriers

must flow (as shown in Figure 2).

rDS(on) = RSOURCE + RCH + RA + RD + Rsub + Rwcl

(1)

It should be noted that for a Planar MOSFET, the rDS(on) figure

also includes the JFET component resistance.

Drain

Gate

Source

Drain

PARASITIC CAPACITANCE IN A MOSFET

The simplest view of an n-channel MOSFET is shown in

Figure 4, where the three capacitors, Cgd, Cds, and Cgs

represent the parasitic capacitances. These values can be

manipulated to form the input capacitance, output

capacitance, and transfer capacitance, as described in

Table 1.

Document Number: 71933

08-Sep-03

Gate

CGD

RG

CGS

npn

CDS

RB

Source

FIGURE 4. Simple equivalent circuit for a n-channel MOSFET,

showing the parasitic capacitance, npn transistor

and Rb resistor.

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