Whitepaper

Comparing Trench and Planar Schottky Rectifiers
Trench technology delivers lower Qrr, reduced switching losses and wider SOA

By Dr.-Ing. Reza Behtash, Applications Marketing Manager, Nexperia

The Schottky diode - named after its inventor, the German physicist Walter Hans
Schottky - consists essentially of a metal-semiconductor interface. Because of its low
forward voltage drop and high switching speed, the Schottky diode is widely used in avariety of applications, such as a boost diode in power conversion circuits. The electricalperformance of a Schottky diode is, of course, subject to physical trade-offs, primarily
between the forward voltage drop, the leakage current and the reverse blocking
voltage. The Trench Schottky diode is a development on the original Schottky diode,providing greater capabilities than its planar counterpart. In this paper the structure
and the benefits of Trench Schottky rectifiers are discussed.
Schottky operation
The ideal rectifier would have a low forward voltage drop,a high reverse blocking voltage, zero leakage current anda low parasitic capacitance, facilitating a high switching
speed. When considering the forward voltage drop,
there are two main elements: the voltage drop across thejunction - PN junction in case of PN rectifiers and metalsemiconductor junction in case of Schottky rectifiers; andthe voltage drop across the drift region. While the forward
voltage drop across a PN junction is intrinsically determinedby the built-in voltage and hence, mainly by the specified
semiconductor material, the forward voltage drop acrossthe metal-semiconductor interface in a Schottky barrier
rectifier can be modified by the choice of the Schottky
metal, with the Schottky barrier being the result of thedifference between the metal work (MW) function andthe electron affinity of the semiconductor.
By using Schottky metals with a low MW function, thevoltage drop across the metal semiconductor interface can be
minimized. However, there is a trade-off between the forward
voltage drop across the junction and the leakage current of
the Schottky rectifier, as the level of the leakage current isalso determined by the Schottky barrier and the electricalfield across the metal-semiconductor interface. In addition tothis trade-off, the advantage of the low voltage drop acrossthe junction can disappear when the thickness of the drift
region is increased to achieve a high reverse blocking voltage.
This is why the reverse blocking voltage of Schottky rectifiers
is traditionally limited to well below 200 V.

Trench technology
Given this explanation, one might ask how the majoradvantage of Schottky rectifiers - namely, a low voltage
drop across the metal semiconductor interface - is preserved,
despite the demand for a low leakage current and a highreverse blocking voltage? Here, Trench rectifiers prove very
useful. The concept behind the Trench Schottky rectifier
is termed ‘RESURF’ (reduced surface field). The RESURF
effect is illustrated in Figure 1. In a planar Schottky rectifier,the equipotential lines are concentrated close to the top
electrode, resulting in a high electrical field near the surface.
This results in a strong increase of the leakage current withincreasing reverse voltage, and an early breakdown when
the critical electrical field is exceeded near the surface.

metal
potential
lines

n-type drift
regionn+ substrate

aaa-029509

Figure 1: Equipotential lines in a planar Schottky rectifier (left)
and in a Trench Schottky rectifier (right) in reverse direction.

Competitor part
100801lower IR → performance gain

IR @60 V, 125 °C (mA)

10sch type

V - Tren

200.44

0.46

0.48

0.50

02550

75
100125Junction temperature (oC)150

Figure 3: Comparison of safe operating area of Nexperia’s
PMEG100T080ELPE Trench Schottky versus a Trench Schottky
part from a competitor.

Therefore, in applications where higher ambient temperatures
have to be tolerated, for example, automotive, the Trench
Schottky rectifiers are a suitable choice since they are morerobust against thermal runaway - the instability which
occurs when the increase of the dissipated power caused by
the leakage current of the rectifier is faster than the heatdissipation through the thermal resistance of the system.

Anode

0.52

0.54

0.56

0.58

0.60

VF @IFMAX, 125 °C (V)aaa-029510

RTrenchCDiodeFigure 2: Vf / IR trade-off at maximum reverse voltage and maximum
forward current at 125°C

Wide safe operating area (SOA)
The lower leakage current of Trench Schottky rectifiers
compared to their equivalent planar counterparts with acomparable forward voltage drop shows the Trench devices
have a wider safe operating area (SOA) of these devices.
SOA plots the maximum reverse voltage that can be applied
against the junction temperature. As well as the intrinsic SOA
advantages offered by Trench rectifiers compared to planar
Schottkys, Nexperia Trench products have additionally been
designed with special attention to a wide SOA. Figure 3 shows
the SOA at a thermal system resistance (Rth( j-a)) of 90 K/W ofa Trench Schottky (PMEG100T080ELPE - orange line) versus asimilarly-rated Trench Schottky device from another supplier(blue). At 125°C junction temperature the maximum allowable
reverse voltage of the Nexperia Trench device is almost 40 Vhigher than the competitor’s Trench product.

2175

blue: Nexperia
red: Manufacturer A
black: Manufacturer B

lower VF → performance gain
10-1
0.42

Rth(j-a)= 90 K/W40

The equivalent circuit diagram of a Trench Schottky rectifier
is shown in Figure 4. Besides the usual parasitic capacitanceof the Schottky diode, CDIODE, there is a second parasitic
capacitance caused by the electrode and the thin dielectricin the Trench structures, C TRENCH. This means that per unit
area, the total parasitic capacitance of a Trench Schottky
rectifier is higher than its planar counterpart.

planarNEW 60

40 V

600At Nexperia, a portfolio of Trench Schottky rectifiers has
been developed and launched with a voltage range of 45-100
V (PMEG*T family). These devices achieve a well-balanced
trade-off between the forward voltage drop (Vf ) and theleakage current (IR). As an example, the Vf / IR trade-off for
60 V products is shown in Figure 2. In this figure, the leakage
current at maximum reverse voltage is plotted against theforward voltage drop at maximum forward current at 125°C.
For comparison reasons, Trench and planar Schottky rectifiers
from two other manufacturers are shown in this graph. For agiven forward voltage drop, the Nexperia device exhibits thelowest leakage current.

PMEG100T080ELPE

120Max. reserve voltage (V)By etching trenches into the silicon and filling them with
poly-silicon - electrically separated from the drift region bya thin dielectric - the trenches act like a field plate in thesemiconductor, depleting the drift region in reverse direction
and resulting in a flattened electrical field profile along thedrift region. Therefore, the trench concept achieves a lower
leakage current by reducing the electrical field near thesurface and producing a higher breakdown voltage compared
to a planar device with the same epitaxial structure.

Whitepaper, Trench Schottky Rectifier

CTrenchRDriftCathodeFigure 4: Cross-section and equivalent circuit diagram of a Trench
Schottky rectifier, highlighting the circuit elements.

However, C TRENCH does not affect the diode switching
characteristics or EMI; in fact, Trench rectifiers haveless stored charge Qrr and show an excellent switching
performance when compared to planar Schottkys despite
this overall larger parasitic capacitance, as specified in Trench
rectifier datasheets. The switching performance of theTrench Schottky and its impact on converter efficiency will
be discussed in the next section.

96

94

Reverse Recovery Behavior
TrenchPlanar2

I [A]

10-1-2

Trench Qrr
Trench IRM
Planar Qrr-4-6-10.00

Planar IRM
-5.00

94.0
93.393.5
92.2

92.3

9291
90

89.5

130250Frequency in kHz500

Figure 6: Trench Schottky rectifiers deliver significant converter
efficiency benefits, especially at higher frequencies where theirswitching losses are less than planar devices.

Any ringing that occurs in the Trench rectifier duringswitching does not impact electromagnetic emission
levels, as confirmed by conducted and radiated emission
measurements shown in Figure 7.
A-3-5

Planar93893Trench95η in %

Reverse Recovery Behaviour and Qrr
The switching behaviour of the device is characterized byreverse recovery measurements. Such measurements arecarried out by biasing the rectifier in a forward direction,then switching the device into a reverse condition. Due tothe stored charge in the device (represented by the parasitic
capacitance CDiode in the equivalent circuit, Figure 4) whichmust be first removed before the diode blocks, a so-called
reverse recovery current of the rectifier occurs. The ramp
reverse recovery measurement for a Trench Schottky rectifier
and its planar counterpart with comparable die size andpackage is shown in Figure 5.

0.00
t [ns]

5.00

10.00

15.00

IF = 3A
VR = 48V
dI/dt = 1A/ns
tamb =85oC

20.00

25.00

Qrr (T=25°C)

Qrr (T=85°C)

IRM (T=25°C)

IRM (T=25°C)

Trench Schottky

8.6 nC

8.5 nC

2.8 A

2.7 A

Planar Schottky

26.3 nC

33.5 nC

5.2 A

5.2 A

Figure 5: Reverse recovery behaviour of Trench and planar Schottkys.

In this measurement, the current has been ramped down witha di/dit rate of 1 A/ns. The area under the zero line represents
the Qrr of the rectifier. The blue line shows the lower Qrr ofthe Trench device. The graph also shows the lower reverse
recovery current and the shorter reverse recovery time of
the Trench rectifier compared to its planar counterpart(despite its higher parasitic capacitance, as discussed earlier).
The temperature stability of Qrr for Trench rectifiers isparticularly notable, since applications very rarely operate
at 25°C. As shown in Figure 5, the Qrr of the Trench rectifier
barely changes at an elevated 85°C ambient temperature
while the Qrr of planar Schottky increases significantly.
The impact of the lower Qrr of Trench Schottky on theefficiency of a 48 V/12 V switch-mode converter is shown in
Figure 6. The low Qrr of the Trench Schottky rectifiers resultsin significant converter efficiency gains, especially at highfrequency where switching losses are larger.

3Whitepaper, Trench Schottky Rectifier

B

Figure 7: Impact on electromagnetic emission: radiated emission (A);conducted emission (B). A 48 V -12 V buck converter is used as thetest vehicle; 3 A CFP3 rectifiers

CFP2-HP (SOD323HP)
2.2 x 1.3 x 0.7 mm*Rth(j-sp) = 6 K/W

CFP3 (SOD123W)
2.6 x 1.7 x 1.0 mm*
Rth(j-sp) = 18 K/W

CFP5 (SOD128)
3.8 x 2.5 x 1.0 mm*
Rth(j-sp) = 12 K/W

CFP15B (SOT1289B)
5.8 x 4.3 x 0.95 mm*Rth(j-sp) = 7 K/W

Figure 8: Nexperia Trench Schottky rectifiers in clip-bonded FlatPower packages*Body size (lxwxh)

Conclusion
In conclusion, the following can be stated: Trench rectifiers
are the correct choice if a better trade-off between forward
voltage drop and the leakage current is required. Trench
rectifiers should also be selected in high power density
applications where the ambient temperature is elevatedsince they are more robust against thermal runaway effects.
For applications with switching speeds above 100 kHz,the reduced switching losses of the Trench devices areparticularly significant.
To find out more, visit
www.nexperia.com/trench-schottky-rectifiers

About Nexperia
Nexperia is a leading expert in the high-volume
production of essential semiconductors that arerequired by every electronic design in the world.
The company’s portfolio includes diodes, bipolartransistors, ESD protection devices, MOSFETs, GaN FETs
and analog & logic ICs. Headquartered in Nijmegen,the Netherlands, Nexperia annually ships more than
100 billion products, meeting automotive standards.
These products are recognized as benchmarks inefficiency – in process, size, power and performance —with industry-leading small packages that save valuable
energy and space. Nexperia has over 12,000 employees
across Asia, Europe and the US.
Find your regional sales office here

© 2021 Nexperia B.V.
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Date of release:
August 2021