High power rectifier

Details High power rectifier High power rectifier

1999-08-10

2001-06-26

Tran, Minh Loan (Department: 2826)

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

Regenerative type switching device

Combined with field effect transistor

C257S658000, C257S119000, C257S128000, C257S427000

Reexamination Certificate

active

06252258

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
Π
, to reduce switching loss and 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 − 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+ layers 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 transistor used as an active element and exceed its breakdown voltage.
Another drawback of the P-i-N rectifier 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 with a large I
RP
and large voltage overshoot. This poor reverse recovery characteristic adds a considerable amount of stress to the power switches the rectifier is typically connected across, and requires the rectifier to dissipate a significant amount of power when transitioning to a reverse blocking state.
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, while exhibiting very low switching losses.
The novel rectifier device has an − drift layer on an N+ layer. A pair of trenches are recessed into the drift layer opposite the N+ layer; the trenches are separated by a mesa region. Oxide layers line the walls and bottom of each trench. A shallow P region extends from the bottom of each trench into the drift layer and around the corners formed at the intersections of its respective trench's oxide side-walls and its bottom. The rectifier's anode is provided by a metal layer that contacts conductive material in the two trenches and the mesa region; metal on the structure's N+ layer serves as the device's cathode.
The interface between the anode and the mesa region provides a Schottky contact. Forward conduction begins when a voltage is applied across the device sufficient to overcome the Schottky barrier height, which allows current to flow from anode to cathode via the Schottky contact. When reversed-biased, depletion regions form around the shallow P regions and the oxide side-walls which provide a potential barrier across the mesa region that shields the Schottky contact from a high electric field, thereby providing a high reverse blocking voltage and greatly reducing reverse leakage current. The device is unipolar; as such, its reverse recovery time is negligible, which enables it to switch from forward conduction to reverse blocking mode very quickly. This also enables the rectifier to exhibit very low switching losses—i.e., very little power is dissipated when transitioning from an on-state to a blocking state.
The rectifier's N− drift and N+ layers may be conventional silicon (Si) but are preferably made from semiconductor materials having a bandgap voltage higher than that of silicon, such as silicon carbide (SiC), gallium nitride (GaN), gallium arsenide (GaAs), or diamond. The use of a material with a higher bandgap voltage enables the use of a drift layer that, for the same blocking voltage, is much thinner than would be necessary with an Si implementation. These materials also permit the drift region's doping density to be much higher than an Si version capable of providing the same blocking voltage, which reduces the device's on-resistance.
The trench structures enable both the trench width and the width of the mesa region to be precisely controlled, permitting the forward voltage drop and reverse blocking 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.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.


REFERENCES:
patent: 4982260 (1991-01-01), Chang et al.
patent: 5488236 (1996-01-01), Baliga et al.
Baliga, B. Jayant.Power Semiconductor Devices,pp 153-154 and 575-577. Boston, MA:PWS Publishing Company, 1996.

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