SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Power MOSFET (3) 2013. 1. 10. SD Lab....

SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.

Power MOSFET (3)

2013. 1. 10.SD Lab. SOGANG Univ.

BYUNGSOO KIM


Contents1. Power U-MOSFET On-Resistance

2. Power U-MOSFET Cell Optimization

3. Device Capacitances


1. Power U-MOSFET On-Resis-tance

• The exception of the JFET region resis-tance

- a significant reduction of the overall specific on-resistance - much smaller cell pitch =>reduction of the specific resistance

• The total on resistance:



1.1 Source Contact Resis-tance

• The contact resistance:

• The specific contact resistance:

• The cell width:

Ex) a trench width(WT): 1μma mesa width (WM): 4μma source contact window width (WC): 3μma source boundary width (WS): 2μmthe width of the contact: 0.5μmthe cell width (WCell): 5 μm

the specific contact resistance: Ω



1.2 Source Region Resis-tance

• The N+ source region contact:

• The length of the N+ source region:

• The specific resistance of the N+ source region:

- usually negligible in the power U-MOS-FET

- Ex) cell pitch: 5 μm the length of source: 1 μm a typical sheet resistance: 20 Ω the specific resistance: 0.0005 mΩ



1.3 Channel Resistance• The channel resistance:

• The channel length:

• The specific resistance contributed by the channel:

Ex) a cell pitch: 5 μm The specific on-resistance(Vg=10V): 0.229 mΩ (the channel length: 1 μm, the gate oxide thickness: 500 Å)

• The reduction in the specific on-resistance contributed by the channel in the UMOS-FET

structure is due to the increase in the channel density.

channels formed on both of the verti-cal sidewalls of the trench-gate structure



1.4 Accumulation Resis-tance

• The accumulation length:

• The resistance of the accumulation layer:

• The specific on-resistance contributed by the accumulation layer:

: 0.6

• smaller than that for the power VD-MOSFET structure

• the accumulation layer path (LA) is only 1 μm in size and the cell pitch is much smaller for the UMOSFET structure.

• In some power U-MOSFET structures, a thicker oxide is used

to reduce the gate–drain capacitance =>the accumulation layer resistance will be increased by the smaller charge within layer


• The width of the cross-sectional area for the first por-tion of the current path:

• The resistance of an elemental segment of thickness dy:

• The resistance contributed by the first portion of the drift region:

• The distance over which the cross-sectional area:

• The resistance of this portion of the drift region:

• The specific on-resistance:

• Ex) drift region doping: a blocking voltage capacity: 50V a cell width: 5 μm the specific resistance: 0.209 mΩ


1.5 Drift Region Resistance


• The specific resistance:

• It is necessary to use a substrate with a thickness of 500μm

• the thin (2–50 μm) epitaxial drift region is grown a substrate with on a thickness of 500 μm to avoid breakage during han-dling of the wafers in the processing equipment.

• The lowest available resistivity: 0.003 Ωcm

The specific resistance: 0.15 mΩ (<50V)

• the wafer thickness reduced to 200 μm after device fabrication =>the specific resistance 0.06 mΩ

• reduce the thickness of the wafer just be-fore application of drain electrode metal


1.6 N+ Substrate Resistance



1.7 Drain Contact Resis-tance

• the current flows through the contact resistance between the drain metal and the N+ substrate.

• Due to the uniform current flow at the drain contact, its resistance is not am-plified.

• a contact resistivity(Ti): Ω



1.8 Total On-Resistance

• The ideal specific on-resistance: 0.105 mΩ (a device capable of supporting 50 V)



1.8 Simulation Example


2. Power U-MOSFET Cell Opti-mization

2.1 Orthogonal P-Base Contact Structure

• an increase in the channel density by a factor of 2.5 times resulting in a smaller specific on-resistance

• A design with a ratio (ZN+/ZP) of 10 provides an adequate design compromise



2.1 Orthogonal P-Base Contact Structure



2.2 Impact of Breakdown Voltage

• The breakdown voltage capability - a higher resistivity drift region - larger thickness

• improvements in the performance of the power U-MOSFET structure

- by improving the edge termination design



2.3 Ruggedness Improvement

• the enhanced electric field• rounding the bottom of the

trenches

• the enhanced electric field• rounding the bottom of the

trenches• the inclusion of a deep P+ region• larger cell pitch• increase of the on-resistance


• In practice the switching speed is limited by the device capacitances.

• the small thickness of the gate oxide - a significant capacitance

• The rate of switching between the on-state and off-state => determined by The rate of being charged or discharged

• A role of the capacitance between the drain and the gate electrodes - the drain current and voltage transitions during the switching event




6.1 Basic MOS Capacitance



6.2 Power VD-MOSFET Structure Capaci-tances

• The input (or gate) capacitance of the struc-ture

- the overlap of the gate electrode with the N+ source and P-base regions - determined by the oxide capacitance

• It is common design practice in power VD-MOSFET structures to overlap the source elec-trode over the gate electrode.

- a very thick metal layer (4–10 μm) to reduce its resistance =>an additional input capacitance - a thick (typically 6,000 Å) interelectrode di-electric film (tIEOX)


• The specific input capacitance:

• The gate–drain (or reverse transfer) capac-itance:


21.35nF



• CSM is decreases with increasing drain bias voltage.

• The applied drain bias:



3.0 nF



• The output capacitance (CO) is associated with the capacitance of the junction between the P-base region and the N-drift region.

• The specific junction capacitance

• the depletion region thickness at the junction4.3 nF



6.3 Power U-MOSFET Structure Capaci-tances

• The input (or gate) capacitance - the overlap of the gate electrode with the N+ source and P-base regions along the sidewalls of the trench - determined by the oxide capacitance

• The formation of a large source metal area - a very thick metal layer (4–10 μm) to reduce its resistance=>an additional input capacitance - a relatively thick (typically 6,000 Å) interelec-trode dielectric film (tIEOX)


• The specific input (or gate) capacitance:

• The input capacitance for the power U-MOS-FET is much larger than that for the power VD-MOSFET.

• the reduction of the specific on-resistance with the power UMOSFET structure is at-tended with an increase in the input gate ca-pacitance.

• The reduction of the specific input capaci-

tance of the power U-MOSFET - increasing the mesa width - accompanied by an increase in the specific on- resistance


105 nF



8.2 nF

much larger than that for the power VD-MOSFET

8.2 nF

4.0 nF

greatly reduced when the mesa width is enlarged an increase in the specific on-resistance



5.5 nF

6.0 nF

5.5 nF

the model A - without screening (KS = 0) - a much larger specific output capacitance

the model B - a screening factor KS = 0.75 - the extracted values

Model A:

Model B:

SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Power MOSFET (3) 2013. 1. 10. SD Lab....

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