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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 1, JANUARY 2005
ShOC Rectifier: A New Metal–Semiconductor Device With
Excellent Forward and Reverse Characteristics
M. Jagadesh Kumar
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—Breakdown, diode, forward voltage drop, Ohmic contact,
rectifier, reverse leakage current, Schottky barrier, simulation.
Fig. 1.
Cross section of the ShOC rectifier.
I. INTRODUCTION
Applications 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
brief, 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.
TABLE I
MEDICI PARAMETERS FOR THE LBS, HBS, SHOC, AND LMDS STRUCTURES
II. SIMULATION RESULTS AND DISCUSSION
A cross-sectional view of the ShOC rectifier is shown in Fig. 1 which
is implemented in MEDICI [6], a two-dimensional 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 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 I.
Manuscript received June 4, 2004; revised November 17, 2004. The review
of this brief was arranged by Editor V. R. Rao.
The author is with the Department of Electrical Engineering, Indian Institute
of Technology, New Delhi 110 016, India (e-mail: [email protected]).
Digital Object Identifier 10.1109/TED.2004.841336
The simulated current–voltage characteristics of the ShOC rectifier
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
0018-9383/$20.00 © 2005 IEEE
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 1, JANUARY 2005
Fig. 2. (a) Forward characteristics and (b) reverse characteristics of the ShOC
rectifier compared with that of the LMDS, conventional low-barrier Schottky
(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
of 120 V.
131
Fig. 4. (a) Forward voltage drop versus d=m ratio at a forward current density
of 100 A/cm 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 m is varied
for a fixed trench depth w .
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 reverse voltage of 25 V) 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 d as 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, if d
is 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.
III. CONCLUSION
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.
A novel ShOC rectifier 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
132
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 1, JANUARY 2005
An Analysis of Base Bias Current Effect on SiGe HBTs
Yo-Sheng Lin and Shey-Shi Lu
Abstract—The anomalous dip in scattering parameter
of SiGe heterojunction bipolar transistors (HBTs) is explained quantitatively for the
first time. Our results show that for SiGe HBTs, the input impedance can
be represented by a “shifted” series RC circuit at low frequencies and a
“shifted” parallel RC circuit at high frequencies very accurately. The apin a Smith chart is caused by this
pearance of the anomalous dip of
inherent ambivalent characteristic of the input impedance. In addition, it
), an increase
is found that under constant collector–emitter voltage (
of base current (which corresponds to a decrease of base–emitter resistance
( ) and an increase of transconductance ( )) enhances the anomalous
dip, which can be explained by our proposed theory.
Index Terms—Anomalous dip, base current, heterojunction bipolar transistor (HBT), -parameters, SiGe.
I. INTRODUCTION
Fig. 5. (a) Forward voltage drop versus d=m ratio at a forward current density
of 100 A/cm 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 w is varied for a fixed m.
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.
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. Electron Devices, vol. 48, no. 12, pp. 2695–2700,
Dec. 2001.
, “Novel lateral merged double Schottky (LMDS) rectifier: Pro[2]
posal and design,” IEE Proc. Circuits, Devices Systems, vol. 148, pp.
165–170, Jun. 2001.
, “Lateral thin-film Schottky (LTFS) rectifier on SOI: A device with
[3]
higher than plane parallel breakdown,” IEEE Trans. Electron Devices,
vol. 49, no. 1, pp. 181–184, Jan. 2002.
[4] M. J. Kumar and Y. Singh, “A new low-loss lateral trench sidewall
Schottky (LTSS) rectifier on SOI with high and sharp breakdown
voltage,” IEEE Trans. Electron Devices, vol. 49, no. 7, pp. 1316–1319,
Jul. 2002.
[5] M. J. Kumar and C. L. Reddy, “A new high voltage 4H-SiC lateral dual
sidewall Schottky (LDSS) rectifier: Theoretical investigation and analysis,” IEEE Trans. Electron Devices, vol. 50, no. 7, pp. 1690–1693, Jul.
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.
Electron Devices, vol. 46, no. 3, pp. 456–464, Mar. 1999.
The anomalous dips in scattering parameters S11 and S22 of
MOSFETs/MESFETs and scattering parameter S22 of bipolar junction
transistors/heterojunction bipolar transistors (BJTs)/HBTs, which have
been explained quantitatively [1]–[3], can be seen frequently in the
literature [4], [5]. However, the anomalous dip in scattering parameter
S11 of BJTs/HBTs has never been reported. In this paper, the anomalous
dip in S11 of SiGe HBTs is reported and explained quantitatively for
the first time. It is found that under constant collector–emitter voltage
(VCE ), an increase of base current (IB ) enhances the anomalous dip.
That is to say, for devices with lower base–emitter resistance (r ) and
higher transconductance (gm ), the anomalous dip is more prominent.
The concept of dual-feedback circuit methodology is used to simplify the circuit analysis of the hybrid -model of a SiGe HBT and then
the input impedance of the HBT is derived. The formula shows that the
input impedance of the HBT follows a “shifted” constant resistance (r)
circle at low frequencies and then a “shifted” constant conductance (g )
circle at high frequencies, which is in good agreement with the experimental results.
II. DEVICE STRUCTURE AND S11 VERSUS BASE CURRENT
The HBTs studied in this paper were fabricated with a 0.35-m
BiCMOS technology. The frequency-dependent S -parameters measurements were performed from 0.1 to 50 GHz in a common-source
configuration by an HP-8510C network analyzer. Before the measurements, a full two-port short load open through calibration was done
on a separate alumina impedance standard substrate to calibrate the
measurements to the probe tips. This setup together with Cascade air
coplanar ground–signal–ground probes was able to provide reliable
RF measurements.
Small-signal hybrid -models of the SiGe HBTs were created for
studying the anomalous dip in scattering parameter S11 [see Fig. 2(a)].
The pad parasitic effect could be modeled by an equivalent C component parallel to a series LR circuit. The followings are a brief description of the steps. First, the S -parameters of the four on-wafer dummy
Manuscript received August 9, 2004; revised November 1, 2004. This work
was supported in part by the National Science Council, Taiwan, R.O.C. under
Contract NSC92-2212-E-260-001 and in part by NSC92-2212-E-002-091 and
the Ministry of Education under Contract 89-E-FA06-2-4. The review of this
brief was arranged by Editor J.N. Burghartz.
Y.-S. Lin is with the Department of Electrical Engineering, National Chi-Nan
University, Puli, Taiwan, R.O.C. (e-mail: [email protected]).
S.-S. Lu is with the Department of Electrical Engineering, National Taiwan
University, Taipei, Taiwan, R.O.C. (e-mail: [email protected]).
Digital Object Identifier 10.1109/TED.2004.841347
0018-9383/$20.00 © 2005 IEEE