shoc rectifier: a new metal-semiconductor device with excellent forward and reverse characteristics
TRANSCRIPT
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Shielded Ohmic Contact (ShOC) Rectifier: A New
Metal-Semiconductor Device with Excellent Forward
and Reverse Characteristics
M. Jagadesh Kumar
Department of Electrical Engineering,
Indian Institute of Technology, Delhi,
Hauz Khas, New Delhi 110 016, INDIA.
Email: [email protected] Fax: 91-11-2658 1264
Abstract
We report a new structure, called the Shielded Ohmic Contact (ShOC) rectifier which utilizes
trenches filled with a high barrier metal to shield an Ohmic contact during the reverse bias.
When the device is forward biased, the Ohmic contact conducts with a low forward drop.
However, when reverse biased, the Ohmic contact is completely shielded by the high barrier
Schottky contact resulting in a low reverse leakage current. Two dimensional numerical
simulation is used to evaluate and explain the superior performance of the proposed ShOC
rectifier.
Index Terms: Schottky barrier, Ohmic Contact, forward voltage drop, reverse leakage
current, Rectifier, Diode, breakdown, Simulation
Final Manuscript submitted to
IEEE Trans on Electron Devices17 November 2004
M. Jagadesh Kumar, "ShOC Rectifier: A New Metal-Semiconductor Device with Excellent Forward and ReverseCharacteristics," IEEE Trans. on Electron Devices, Vol.52, pp.130-132, January 2005.
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IntroductionApplications requiring low power dissipation and fast switching speed widely use low barrier
Schottky rectifiers. One has to always make a trade-off between the forward voltage drop and
the reverse leakage current, since a low barrier metal gives small forward voltage drop but at
the cost of high leakage current and the high barrier metal gives a lower leakage current but
with a high forward voltage drop. To overcome the above problem, several novel lateral
Schottky rectifiers have been reported in the recent past [1-5]. Among them, in the lateral
merged double Schottky (LMDS) rectifier [1,2], two Schottky barriers are used - the low
barrier Schottky contact conducts during forward bias and the high barrier Schottky contact
effectively shields the low barrier Schottky contact during the reverse bias. One drawback of
the LMDS rectifier is that it requires two Schottky metals. In this paper, we report a new
device called the Shielded Ohmic Contact (ShOC) rectifier in which the low barrier Schottky
contact in the LMDS rectifier is replaced by an Ohmic contact so that during the forward
bias, the Ohmic contact conducts and is effectively shielded by the high barrier Schottky
contact during the reverse bias. Using numerical simulations we demonstrate that using the
ShOC rectifier, which uses only one high barrier Schottky contact, we can still realize low
forward drop, low reverse leakage current and high breakdown voltage similar to that of the
LMDS rectifier which uses two Schottky contacts.
Simulation results and discussion
A cross-sectional view of the ShOC rectifier is shown in Fig.1 which is implemented in
MEDICI[6], a 2D device simulator. The tunneling Ohmic contact formed by the high barrier
metal on the diffused N+
region between the two high-barrier metal trenches forms the anode
contact of the device. This N+ region can be formed before creating the trenches in the oxide
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window region. The cathode contact is placed symmetrically on both sides of the anode. To
reduce the electric field crowding at the trench edges, metal field termination is used[7]. We
have compared the ShOC rectifier with LMDS and the conventional low barrier Schottky
(LBS) and high barrier Schottky (HBS) structures. The parameters used for the simulation of
these rectifiers are given in Table 1.
The simulated current voltage characteristics of the ShOC rectifiers are compared
with that of the LMDS, conventional low barrier and high barrier Schottky structures in Fig.
2. We can see that the forward characteristics (as shown in Fig. 2(a)) and the reverse
characteristics (as shown in Fig. 2(b)) are approximately identical for the ShOC and LMDS
rectifiers. Just as in the case of the LMDS rectifier, the ShOC rectifier also exhibits a higher
breakdown voltage when compared to the LBS and HBS rectifiers. The improved
performance of the ShOC rectifier can be understood from the current flow lines shown in
Fig.3. When the ShOC rectifier is forward biased, the Ohmic contact conducts all the forward
current giving rise to a low forward voltage drop. However, under reverse bias conditions, we
notice that the Ohmic region is effectively shielded by the high barrier trench and the reverse
leakage current corresponds to that of the high-barrier trench. In the above simulations, we
have taken an optimum value for the Ohmic contact width (m=0.2 m) at the given trench
depth (d=1.5 m). Although we have chosen a trench aspect ratio(d/w) equal to 7.5, our
simulations show that the trench width w can be increased to reduce the trench aspect ratio
without affecting the device characteristics.
The performance of the ShOC rectifier, i.e. the how efficiently the Ohmic region can
be shielded by the high-barrier trench is, however, dependent on the d/m ratio. The forward
voltage drop (at a forward current density of 100 A/cm2) and the reverse current density (at a
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reverse voltage of 25V) versus the d/m ratio are shown in Fig. 4 ( with d fixed at 1.5 m and
m as a variable) and in Fig. 5 ( with m fixed at 0.2 m and das a variable) We can easily
observe from Fig. 4 that as m increases for a fixed d, the shielding provided by the high
barrier trench becomes inefficient and the Ohmic contact region plays a greater role in current
conduction. This will result in a reduced forward voltage drop but will also increase the
reverse leakage current. Therefore, the d/m ratio should be chosen such that the reverse
leakage current is at its minimum. Both the forward voltage drop and reverse leakage current
will approximately match with that of the LMDS for a d/m ratio of 7 or above. On the other
hand, we observe from Fig. 5 that for a fixed m, ifdis decreased, the Ohmic contact region is
again not efficiently shielded for a d/m ratio less than 7. The important observation that needs
to be made here is that the Ohmic contact region is not efficiently shielded if m is long in
relation to d. Therefore, for the given trench depth in our simulations, we have chosen the
Ohmic contact width m to be 0.2 m so that d/m ratio is greater than 7. Thus for a given
trench depth, by choosing an appropriate width for the Ohmic region, we can easily realize the
same current voltage performance for the ShOC rectifier as compared to the LMDS rectifier.
Conclusion
A novel shielded ohmic contact rectifier (ShOC) having a high barrier metal filled in a trench
surrounding an Ohmic contact is presented. The results show that this structure is as good as
the previously reported LMDS structure with the advantage of using only one high barrier
Schottky metal contact. The ShOC rectifier also exhibits significantly enhanced breakdown
voltage compared to the conventional high barrier or low barrier Schottky rectifiers. The
combined low forward voltage drop and excellent reverse blocking capability make the
proposed ShOC rectifier attractive for use in low-loss high speed smart power IC applications.
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References
[1] Y. Singh and M.J. Kumar, A new 4H-SiC lateral merged double Schottky (LMDS) rectifier
with excellent forward and reverse characteristics, IEEE Trans. on Electron Devices, Vol.48,pp. 2695-2700, Dec 2001.
[2] Y. Singh and M.J. Kumar, Novel lateral merged double Schottky (LMDS) rectifier: proposaland design,IEE Proc. Circuits, Devices and Systems, Vol. 148 pp. 165-170, June, 2001.
[3] Y. Singh and M.J. Kumar, Lateral thin-film Schottky (LTFS) rectifier on SOI: a device with
higher than plane parallel breakdown voltage, IEEE Trans. on Electron Devices, Vol.49, pp.181-184, January 2002.
[4] M.J. Kumar and Y. Singh, A new low-loss lateral trench sidewall Schottky (LTSS) rectifieron SOI with high and sharp breakdown voltage, IEEE Trans. on Elec. Devices, Vol. 49 pp.
1316-1319, July 2002.
[5] M. J. Kumar and C.L. Reddy, "A New High Voltage 4H-SiC Lateral Dual SidewallSchottky (LDSS) Rectifier: Theoretical Investigation and Analysis", IEEE Trans. on
Electron Devices, Vol.50, pp.1690-1693, July 2003.
[6] MEDICI 4.0, a 2D device simulator. TMA, Palo Alto, CA.
[7] V. Saxena, J. N. Su, and A. J. Steckl, High-voltage Ni- and Pt-SiC Schottky diodes utilizing
metal field plate termination, IEEE Trans. on Electron Devices, Vol. 46, pp. 456-464, Mar.
1999.
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Table 1. MEDICI parameters for the LBS, HBS, ShOC and LMDS structures
Parameter Value
N+
doping for Ohmic contact 1019
cm-3
Drift region doping, ND 1016cm-3
Drift region length, L 12 m
Drift region thickness, tsi 2 m
Buried oxide thickness, tbox 3 m
Field oxide thickness, tox 0.5 m
Trench depth, d 1.5 m
Trench width, w 0.2 m
Mesa width, m 0.2 m
Low Schottky barrier height(Ni), L 0.57eV
High Schottky barrier height(W), H 0.67eV
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Figure captions
Fig. 1 Cross-section of the Shielded Ohmic Contact (ShOC) rectifier.
Fig. 2 (a) Forward characteristics and (b) Reverse characteristics of the ShOC
rectifier compared with that of the LMDS, conventional low-barrierSchottky (LBS) and high-barrier Schottky (HBS) rectifiers
Fig. 3 (a) Forward current flow lines of ShOC rectifier at a forward voltage of 0.3
V and (b) Reverse current flow lines of ShOC rectifier at a reverse voltage of120 V.
Fig. 4 (a) Forward voltage drop versus d/m ratio at a forward current density of
100A/cm2
and (b) Reverse leakage current density versus d/m ratio at areverse bias of 25 V for the ShOC and the LMDS rectifiers. Here m is varied
for a fixed trench depth w.
Fig. 5 (a) Forward voltage drop versus d/m ratio at a forward current density of 100A/cm2 and (b) Reverse leakage current density versus d/m ratio at a reverse
bias of 25 V for the ShOC and the LMDS rectifiers. Here the trench width wis varied for a fixed m.
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Cathode Cathode
m
W
Buried Oxide
N+ Substrate
N+ N+
tSid
N Drift Region
L
High Barrier Schottky and Anode
N+ Ohmic contact
Field Oxide
Fig.1
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0.0 0.1 0.2 0.3 0.4 0.5 0.6
10-4
10-5
10-6
10-7
10-8
10-9
10-10
HBSLBSLMDSShOC
Forward
currentIF,A/m
Forward voltage drop VF
, [V]
Fig. 2(a)
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0 20 40 60 80 100 120 140
10-8
10-9
10-10
10-11
10-12
10-13
HBSLBSLMDS
ShOC
Revers
ecurrentIR,A/m
Reverse voltage VR, [V]
Fig. 2(b)
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Fig. 2(a)
Fig. 3(a)
Fig. 3(b)
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2 4 6 8 10 12 14 160.15
0.20
0.25
0.30
0.35
0.40
0.45
d=1.5m
(m variation)
LMDS
ShOC
Forwa
rdvoltagedropVF,[
V]
d/m
Fig.4(a)
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2 4 6 8 10 12 14 16
d=1.5m
LMDSShOC
(m variation)
-2
2
0
4
6
8
10
12
14
16
18
Reversec
urrentIR,A/m(
1X10-6)
d/m
Fig.4 (b)
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0 2 4 6 8 10
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
m=0.2m
ShOC
(d variation)
LMDS
ForwardvoltagedropVF,
[V]
d/m
Fig.5 (a)
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0 2 4 6 8 10
m=0.2m
LMDSShOC
(d variation)
0
1
2
4
3
5
Reverse
currentIR,A/m(
1X1
0-4)
d/m
Fig. 5 (b)