Silicon carbide power devices having trench-based silicon...

Details Silicon carbide power devices having trench-based silicon... Silicon carbide power devices having trench-based silicon...

1999-05-17

2001-11-06

Chaudhuri, Olik (Department: 2814)

Active solid-state devices (e.g., transistors, solid-state diode

Specified wide band gap semiconductor material other than...

Diamond or silicon carbide

C257S168000, C257S266000, C257S267000, C257S279000

Reexamination Certificate

active

06313482

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to semiconductor switching devices, and more particularly to silicon carbide switching devices for high power applications.
BACKGROUND OF THE INVENTION
Integrated circuit semiconductor devices typically include many active devices (e.g., rectifiers, transistors) in a single semiconductor substrate. Power semiconductor devices, which may be integrated circuit or discrete devices, are typically designed to carry large currents and/or support high voltages. Recently developed silicon power semiconductor devices are described in an article by G. Deboy et al. entitled “A New Generation of High Voltage MOSFETs Breaks the Limit Line of Silicon”, International Electron Device Meeting (IEDM) Proceedings, pp. 683-685 (1998). In particular, the Deboy et al. article describes a 600 volt COOLMOS™ silicon device having an area specific on-resistance of typically 3.5 &OHgr;mm
2
. In an attempt to overcome the tradeoff relationship between breakdown voltage and on-state resistance of conventional semiconductor devices, semiconductor superjunction (SJ) devices have also been proposed. Such devices are described in an article by T. Fujihira, entitled “Theory of Semiconductor Superjunction Devices”, Japanese Journal of Applied Physics, Vol. 36, Part 1, No. 10, pp. 6254-6262 (1997).
Notwithstanding the excellent performance characteristics provided by state-of-the-art silicon power devices, silicon carbide power devices have also been considered because silicon carbide has a wide bandgap, a high melting point, a low dielectric constant, a high breakdown field strength, a high thermal conductivity and a high saturated electron drift velocity compared to silicon. These characteristics allow silicon carbide microelectronic devices to operate at higher temperatures and higher power levels than conventional silicon based devices. In addition to the above advantages, silicon carbide power devices can typically operate with lower specific on-resistance than conventional silicon power devices. Some of the advantages of using silicon carbide for forming power semiconductor devices are described in articles by K. Shenai, R.S. Scott and B. J. Baliga, entitled
Optimum Semiconductors for High
-
Power Electronics
, IEEE Transactions on Electron Devices, Vol. 36, No. 9, pp. 1811-1823 (1989); and by M. Bhatnagar and B. J. Baliga entitled
Analysis of Silicon Carbide Power Device Performance
, ISPSD '91, Abstr. 8.3, pp 176-180 (1991).
It has also been demonstrated experimentally that high voltage Schottky rectifiers can be made from silicon carbide, with low forward on-state voltage drop. Such rectifiers are described in articles by M. Bhatnagar et al., entitled “Silicon-Carbide High Voltage (400V) Schottky Barrier Diodes”, IEEE Electronic Device Letters, Vol. 13, No. 10, pp. 501-503, (1992) and “Comparison of 6H-SiC, 3C-SiC, and Si for Power Devices” IEEE Trans. on Electron Devices, Vol. 40, No. 3, pp. 645-655 (1993). Unfortunately, such devices may suffer from unusually high leakage currents when operated in a reverse blocking mode, because of the occurrence of very high electric fields on the silicon carbide side of the Schottky rectifying junction. Excessive forward on-state series resistance may also be present in the silicon carbide drift regions of such devices when designed to block reverse voltages above 2,000 volts. For example, as described in an article by Q. Wahab et al. entitled “A 3 kV Schottky Barrier Diode in 4H-SiC”, Applied Physics Letters, Vol. 72, No. 4, pp. 445-447, January (1998), a 4H-SiC Schottky rectifier designed to block voltages of 3,000 volts may experience a forward on-state voltage drop of 7.1 volts when forward on-state current densities of 100 A/cm
2
are present. These devices may also have insufficient thermal stability for many high power applications because the reverse leakage current in such devices typically increases rapidly with increasing reverse bias. To illustrate this problem, simulations have been performed on a one-dimensional 4H-SiC Schottky rectifier using an N-type drift region doping concentration of 5×10
15
cm
−3
land a drift region thickness of 25 &mgr;m. The results of one such simulation are illustrated by FIG.
1
. In particular,
FIG. 1
illustrates a typical triangular shaped electric field profile in the drift region with a maximum electric field at the Schottky rectifying contact. Because the magnitude of the electric field at the Schottky contact is high at about 2.4×10
6
V/cm at a reverse bias (RB) of 3,000 volts, the reverse leakage currents will typically become excessive due to barrier lowering and field emission effects.
In silicon Schottky rectifiers, it has been shown that a combination of a trench-based insulated anode electrode and a graded doping profile can produce Schottky rectifiers having superior forward and reverse characteristics. Such devices are described in U.S. Pat. No. 5,612,567 to B. Jayant Baliga, entitled “Schottky Barrier Rectifiers and Methods of Forming Same” and in U.S. application Ser. No. 09/167,298, filed Oct. 6, 1998, entitled “Rugged Schottky Barrier Rectifiers Having Improved Avalanche Breakdown Characteristics”, now abandoned, the disclosures of which are hereby incorporated herein by reference. However, the use of a trench-based anode electrode typically cannot be extended to a silicon carbide device because the magnitude of the electric field in the insulating region (e.g., oxide) lining the trench may become excessive and exceed the rupture strength of the insulator, which for silicon dioxide is 1×10
7
V/cm. Simulations of silicon carbide Schottky rectifiers having the structure illustrated by the aforementioned '567 patent reveal that when blocking reverse voltages, almost all of the reverse bias voltage will appear across the insulating region. If the insulating region comprises silicon dioxide, the maximum reverse blocking voltages for such devices may be limited to only 200 volts.
Thus, notwithstanding the above-described power semiconductor devices, there continues to be a need for improved power semiconductor devices that utilize the preferred material characteristics of silicon carbide more advantageously.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide improved semiconductor switching devices for high power applications.
It is another object of the present invention to provide semiconductor switching devices having low on-state resistance and high blocking voltage capability.
It is still another object of the present invention to provide power semiconductor devices having improved ruggedness.
These and other objects, advantages and features of the present invention are provided by silicon carbide switching devices that comprise a silicon carbide substrate having a silicon carbide drift region of first conductivity type (e.g., N-type) and a trench therein at a first face thereof. A uniformly doped silicon carbide charge coupling region of second conductivity type (e.g., an in-situ doped epitaxial P-type region) is also provided in the trench. This charge coupling region forms a P-N rectifying junction with the drift region that extends along a sidewall of the trench. According to a preferred aspect of the present invention, the drift region and charge coupling region are both uniformly doped at equivalent and relatively high net majority carrier doping concentrations (e.g., 1×10
17
cm
−3
) so that both the drift region and charge coupling region can be depleted substantially uniformly when blocking reverse voltages. This combination of preferred drift and charge coupling regions improves the electric field profile in the drift region to such an extent that very low forward on-state drift region resistance can be achieved simultaneously with very high reverse blocking voltage capability. Silicon carbide switching devices that can advantageously use the preferred combination of drift and charge coupling regions include Schottky barrier rectifiers (SBRs), junction fi

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