LPT Type Trench IGBT: A Comparison with NPT Planar IGBT

Amanjyot Singh Johar

IGBT

w Becoming very popular device of choice in 500V -1700 V applicationsw Positive temperature coefficient at high current

makes them easy to parallel and construct modulesw Forward voltage drop: diode in series with on-

resistance 2.4V typicalw Ease to drive similar to MOSFETs

Competing IGBT Structures

w Punch Through IGBTs have traditionally had an advantage at voltage ratings lower than 1.2kV [*] whereas NPT devices were suitable for higher voltages

w Reassessment of above conclusions n Ultra-thin wafer processing technologyn Local lifetime control technology n Trench technology

all aim at reducing forward voltage drop

* S. Azzopardi, C. Jamet, J.M. Vinassa and C. Zardini, 'Switching performances comparison of 1200 V punchthrough and nonpunch-through IGBTs under hardswitching at high temperature', Proceedings of PESC'98, 1998, pp. 1201-1207

* V. Benda, J. Gowar and D.A. Grant, 'Power Semiconductor Devices - Theory and applications', 1999, John Wiley & Sons Ltd., Chichester

Planar IGBT

w Improving the performance of existing IGBT technology has become increasingly difficult due to the constraints of the planar IGBT structure.n The "JFET" resistance (RJFET) in

a planar IGBT exists due to the constriction of current flow in the region between adjacent cells.

n Channel forms parallel to the chip surface

w Limitations of the planar IGBTn the resistance of the JFET region

between adjacent cells in the MOSFET portion of the device

n forward voltage VF of the diode structure in the bipolar portion of the device.

Trench IGBT

w Gate oxide and conductive poly-silicon gate electrode are formed in a deep narrow trench below the chip surface

w The channel n forms along the vertical wall of the trench n perpendicular to the surface of the chipn less chip area leads to substantial increase in cell density n The consequent increase in channel width per unit area

results in a reduction in the R-channel portion of the IGBTs on-state voltage drop

w Eliminates JFET region. n Uniform current flow which, combined with greater cell density, increases the rated current density

compared to planar

w Punch through (PT) device, using local lifetime control processn Carrier lifetime is reduced in the n+ buffer layer only. n Turn-off losses can be reduced whilst maintaining a higher carrier lifetime in the n- drift region n This results in a greater carrier concentration in the drift region during conduction which reduces

the R N- component of VCE(sat)

CSTBT

w Light Punch Through Vertical Structurew Single Crystal wafer eliminates need for

epitaxial base wafer materialw Addition of an n-type layer with a relatively

high impurity density between the p-type base layer and the n- layer in the trench IGBT

w Holes movement restricted to the P-base layer

w Low on state voltagew Improved short circuit ruggedness w Reduces drive powerw Wide cell pitch for short circuit ruggedness

Cross-section of a CSTBT

Simulated CSTBT Device Structure

Vertical Device Length: 85u

Cell Pitch: 5u

Trench Depth: 1.1uBuried N layer between P body region and N drift region

Static CharacteristicsSimulated Breakdown Curve

Negligible Leakage Current

P+ substrate and N- Drift region junction supports the reverse voltage

P-base and N- drift region junction supports the forward voltage

Collector Voltage (V)

Col

lect

or C

urre

nt (A

)

Static CharacteristicsSimulated Forward Voltage Drop Characteristic

VG = 15 V

Forward Voltage Drop due to channel resistance

Wider the cell pitch, lesser the voltage drop

Minimum Forward Voltage drop is 0.5V

Voltage drop at 40A = 1.76 V

Collector-Emitter Voltage (V)

Col

lect

or C

urre

nt (A

)

Current Flow in Trench IGBT

Contours showing the flow of current in the CSTBT

Switching CharacteristicSimulated Hard Switching

Switching Conditions:

Vbus = 400 V

T = 25 C

IC = 50 A

Lgate = 1nH

R gate= 5Ω

Pulse width = 3µs

Excessive tail current seen in planar IGBT structure enhancing turn on and turn off losses

Time (us)

Col

lect

or C

urre

nt (A

)

Planar IGBTCSTBT

Turn Off Energy Loss in CSTBT

Time (us)

Col

lect

or C

urre

nt (A

)

( )

+++= tail

tailifCtfvrCCoff Q

ItVttVIE

221

Eoff = 67 µJ at 400V, 50A(simulated value)

Eoff = 1.68 mJ at 600V, 150A for planar IGBT (datasheet value)

Col

lect

or V

olta

ge (V

)

Turn On Energy Loss in CSTBT

Time (us)

Col

lect

or C

urre

nt (A

)

Col

lect

or V

olta

ge (V

) Eon = 140 µ J at 400V, 50A(simulated value)

Eon = 6.69 mJ at 600V, 150A for planar IGBT (datasheet value)

Turn Off Energy Loss in Planar IGBT

Eon = 1.1 mJ at 400V, 50A(simulated value)

Eon = 1.68 mJ at 600V, 150A for planar IGBT (datasheet value)

Col

lect

or V

olta

ge (V

)

Time (us)

Col

lect

or C

urre

nt (A

)

Turn On Energy Loss in Planar IGBT

Time (us)

Col

lect

or C

urre

nt (A

)

Eon = 3 mJ at 400V, 50A(simulated value)

Eon = 6.69 mJ at 600V, 150A for planar IGBT (datasheet value)

Col

lect

or V

olta

ge (V

)

Device CharacteristicsSimulated Capacitance

Collector Voltage(V)

Cap

acita

nce

(F)

GCGEies CCC +=

Cies = 13200 pF(simulated value)

Cies(planar) = 15458 pF(datasheet value)

MHzf

VVOVV

CC

GE

1

30

=

==

Input Capacitance Cies

Device CharacteristicsSimulated Capacitance

Collector Voltage(V)

Cap

acita

nce

(F) Reverse Transfer

Capacitance Cres

MHzf

VVOVV

CC

GE

1

30

=

==

Cres = 75 pF(simulated value)

Cres(planar) = 1056 pF(datasheet value)

Device CharacteristicsSimulated Capacitance

Collector Voltage(V)

Cap

acita

nce

(F)

Output Capacitance Cies

GCCEoes CCC +=

MHzf

VVOVV

CC

GE

1

30

=

==

Coes = 550 pF(simulated value)

Coes(planar) = 487pF(datasheet value)

Comparing CSTBT with Planar IGBT

w CSTBT 1200V/150A (based on Simulated results)n LPT type with a single crystal wafer n Forward Voltage Drop of 1.76 V at

40A

n Turn on Energy 140 µ J n Turn off Energy 67 µJ n Input Capacitance 13200 pFn Output Capacitance 550 pF

w IGBT as used in converters 1200V/150A (based on datasheet values)n NPT type with an epitaxial

layern Forward Voltage Drop of 2.1 –

2.6 V at 40An Turn on Energy 3 mJ n Turn off Energy 1.1 mJ n Input Capacitance 15458 pFn Output Capacitance 487pF

•CSTBT can be a good replacement for planar IGBT as accounted by its simulated hard switching performance

•A complete comparison can be made after analyzing the performance of the CSTBT in the converter