Active solid-state devices (e.g. – transistors – solid-state diode – Schottky barrier
Reexamination Certificate
1999-01-19
2001-06-26
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Schottky barrier
C257S475000, C257S481000
Reexamination Certificate
active
06252288
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of high power rectifiers.
2. Description of the Related Art
Semiconductor devices are increasingly required to accommodate high currents and/or high voltages without failing. For example, a variable speed pulse-width modulated (PWM) motor control circuit typically employs a number of transistors as switches, each of which has a flyback rectifier connected across it; the switches are closed in sequence to provide variable frequency AC power to a motor. The rectifier in this type of application is required to conduct a large current when forward-biased, and to block a high voltage when reverse-biased. To maximize the efficiency of the control circuit, the flyback rectifier ideally has a low forward voltage drop V
FD
. The rectifier should also have a small stored charge Q
n
, to increase switching speed, and a “soft” recovery with a small peak reverse current I
RP
, to reduce the stress on the associated switching devices.
A number of power rectifier devices have been used to provide the high current and reverse blocking characteristics needed for such a high power application. One such device, the P-i-N rectifier, is shown in FIG.
1
. An N-type drift layer
10
is between an N
+
layer
12
and a P
+
layer
14
(X
+
denotes a carrier concentration of at least 1×10
18
/cm
3
, X
−
denotes a carrier concentration of less than 5×10
16
/cm
3
). Metal on the P
+
and N
+
regions provide the rectifier's anode
16
and cathode
18
, respectively.
When forward-biased, P
+
region
14
injects large numbers of minority carriers into drift region
10
, greatly lowering the resistance of the drift region and allowing the rectifier to carry a high current density. The P-i-N rectifier's drift region
10
is usually thick, resulting in a high “blocking voltage”; i.e., the reverse voltage which the rectifier can accommodate without breaking down. These characteristics make the P-i-N rectifier useful for high power applications.
The P-i-N rectifier has several drawbacks, however. As described in J. Baliga,
Power Semiconductor Devices,
PWS Publishing Co. (1996) at p. 153, the P-i-N rectifier suffers from a “forward voltage overshoot” phenomenon, in which its V
FD
at turn-on is higher than it is under steady-state conditions. This can be a serious problem in power circuit because the higher V
FD
may appear across the emitter-base junction of a bipolar transitor used as an active element and exceed its breakdown voltage.
Another drawback of the P-i-N rectivier is its poor reverse recovery characteristic—as described in Baliga (ibid.) at p. 154. Reverse recovery occurs when the rectifier is switched from its on-state to its reverse blocking state. To undergo this transition, the minority carrier charge stored in the drift region during forward conduction must be removed, which requires the injected minority carriers to recombine with majority carriers. During recombination, some reverse current flows through the device before eventually decaying to zero. Because so many holes are injected into the drift region during forward conduction, recombination proceeds slowly in a P-i-N rectifier and thereby produces a poor reverse recovery characteristic.
Another rectifier device used in high power applications is the “merged P-i-N/Schottky” (MPS) rectifier, shown in FIG.
2
and described in Baliga (ibid.) at pp. 187-192. As with the P-i-N rectifier, this device has an N-type drift region
20
on an N
+
region
22
. However, instead of a continuous P
+
layer over the drift region, two physically separate P
+
regions
24
,
26
are diffused into drift region
20
. Metal layers
28
and
30
provide the rectifier's anode and cathode, respectively. The interface between the anode
28
and the lightly doped drift region
20
form a Schottky contact
32
; the area under the Schottky contact is referred to herein as the Schottky region
34
.
When forward-biased, P
+
regions
24
and
26
inject holes into drift region
20
. This results in conductivity modulation of the drift region in a manner similar to the P-i-N rectifier, which reduces its resistance to current flow. The barrier height of the Schottky contact
32
is less than that of the structure's P
+
-N junctions; if the Schottky region is sufficiently wide, a significant amount of current will flow through it, with the lower barrier height providing a lower V
FD
for the device. The MPS rectifier also provides an improved reverse recovery characteristic: the injection level required to reduce the resistance in series with the Schottky region is not as large as that observed in the P-i-N rectifier; consequently, the stored charge in the MPS recitifer is smaller than that found in the P-i-N rectifier.
The MPS rectifier has several shortcomings, however. When reverse-biased, the device's Schottky contact
32
is subjected to a strong electric field. If unprotected, the Schottky contact exhibits a “barrier height lowering effect”, which allows a reverse current to leak through the device via the lowered barrier. This can be a countered by reducing the spacing between the P
+
regions, but the device then begins to resemble the P-i-N structure—increasing the forward voltage drop and losing some of the advantages provided by the Schottky region. Unfortunately, this spacing is difficult to control, due to the lateral diffusion that occurs when fabricating the device.
SUMMARY OF THE INVENTION
A rectifier device is presented that overcomes the problems noted above. The rectifier is particularly well-suited to high power switching applications, providing a high current density and reverse blocking voltage, along with a low forward voltage drop and superior reverse recovery characteristics.
The novel structure has an N
−
drift layer on an N
+
region. A pair of trenches are recessed into the drift layer opposite the N
+
region; the trenches are separated by a mesa region. Oxide side-walls line the outside of each trench. A shallow P
+
region extends from the bottom of each trench into the drift layer. The trenches each contain a conductive material that provides a conductive path between the top of the trench and its shallow P
+
region. The rectifier's anode is provided by a metal layer that contacts the conductive material in the two trenches and the mesa region; metal on the structure's N
+
region serves as the device's cathode.
The interface between the anode and the drift layer provides a Schottky contact. When forward-biased, the shallow P
+
regions inject holes into the drift region, lowering its resistance and allowing forward conduction through both the Schottky and P
+
regions. The Schottky contact's low barrier height gives the device a low forward voltage drop. When reversed-biased, depletion regions form around the shallow P
+
regions and the oxide side-walls and provide a potential barrier across the Schottky region which shields the Schottky contact, thereby providing a high reverse blocking voltage and greatly reducing reverse leakage current. The presence of the Schottky region also reduces the amount of stored charge that must be removed, further improving the device's reverse recovery performance.
The trench structures enable both the trench widths and the width of the mesa region to be precisely controlled, permitting the device's forward voltage drop and reverse recovery characteristics to be tailored to the demands of a particular application.
A number of such structures are fabricated in parallel to provide a desired current carrying capacity. Devices fabricated in this way have carried currents of 50 A with a reverse blocking voltage in excess of 1200 volts, on a die size of only 0.5 cm
2
. These characteristics make the novel rectifier device well-suited for high power applications; for example, as the flyback diode across a transistor switch in a PWM mot
Koppel & Jacobs
Loke Steven
Rockwell Science Center LLC
LandOfFree
High power trench-based rectifier with improved reverse... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High power trench-based rectifier with improved reverse..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High power trench-based rectifier with improved reverse... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2517043