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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)

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