Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-05-05
2004-06-22
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S088000, C257S040000, C257S471000, C257S474000
Reexamination Certificate
active
06753580
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a diode structure and method for its manufacture, and more specifically relates to a novel diode which has a reduced reverse recovery voltage characteristic and a soft turn-off characteristic.
BACKGROUND OF THE INVENTION
Semiconductor diodes are well known, and are commonly made by diffusing a deep P/N junction into the surface of a monocrystalline silicon substrate. For example, a wafer of silicon having an epitaxially formed N type top layer may have a deep P diffusion formed in its top surface. Such diffusions are typically 4-20 &mgr;m microns deep. Anode and cathode electrodes are then directly or indirectly connected to the P and N regions respectively. A suitable termination structure is also provided. Heavy metals such as gold and platinum may be diffused into the body of the silicon to “kill” or reduce the lifetime of minority carriers, thus increasing the turn-off speed of the device. Alternatively, E-beam radiation can be used for lifetime killing alone or with the heavy metals.
Such diodes have turn-off characteristics which can be detrimental in certain circuit applications. For example, when the diode becomes reverse biased for turnoff, the current will reverse through zero and then recover during a time to. As the current rises toward zero, it will have a recovery rate di/dt that will induce high voltage spikes in inductive circuits. U.S. Pat. No. 5,747,872 describes a soft turn-off diode, employing highly doped N and P layers separated by a lightly doped N layer and using heavy metal diffusion and irradiation.
It would be very desirable to have a diode with a “soft” turn off characteristic and reduced recovery voltage characteristics and of simple structure for use in inductive circuits such as power factor correction circuits. Such a diode would reduce the production of high voltage spikes during diode turn-off in such circuits.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the invention, a diode is made with a “weak” anode, that is, an intentionally weak injecting anode. Such an anode will produce a diode with soft turn-off characteristics.
Weak anodes, sometimes termed “transparent” anodes, are known for use with non-punch through insulated gate bipolar transistors (“NPT IGBTs”) where they are used as a collector region of reduced injection efficiency. Such anodes are used to replace the need for buffer layers and lifetime killing in NPT IGBTs. These device will have a DMOS type top structure in a wafer, and the weak anode in the bottom surface of an ultra thin float zone silicon wafer.
In accordance with the invention, the conventional deep and heavily doped P diffusion into N type silicon of a conventional diode is replaced by a much shallower weak anode. The weak anode can be formed by a very shallow (0.5 micron) boron implant having a dose of about 1E14 cm
−2
at 80 KeV. It should be noted that only about 5% to 10% of the implanted dopant is electrically active because a conventional activation procedure of annealing at above about 700° C. is not used. Other doses can be used, in the range of 1E11 to 1E16, depending on the forward voltage drop and reverse recovery required.
An aluminum layer is then sputtered or evaporated atop the boron implant and is sintered at about
420
° C. for 30-60 minutes. This may be followed with E-beam radiation of the wafer to further reduce the minority carrier lifetime.
Alternatively, the weak anode can be formed by the deposition of P type amorphous silicon on the wafer surface, as disclosed in copending U.S. application Ser. No. 09/566219, filed May 5, 2000 (IR-1462). The silicon surface receiving the weak anode may also be heated in vacuum as disclosed in copending application Ser. No. 09/565928 (IR-1706) to eliminate the need for the anneal step to activate the weak anode junction. The spaced activated weak anode described in copending U.S. application Ser. No. 09/565922 (IR-1708) may also be used.
The resulting diode formed by any of these processes will have a soft turn-off characteristic and reduced recovery charge. These characteristics make it ideal for use in motor control circuits, and as a power factor correction diode.
REFERENCES:
patent: 3667009 (1972-05-01), Rugg
patent: 4997788 (1991-03-01), Martens
patent: 5214276 (1993-05-01), Himoto et al.
patent: 5362975 (1994-11-01), von Windheim et al.
patent: 5859465 (1999-01-01), Spring
patent: 6175143 (2001-01-01), Fujihira
patent: 6229180 (2001-05-01), Yoshida
patent: 6373076 (2002-04-01), Alok et al.
patent: 402187071 (1990-07-01), None
Francis Richard
Ng Chiu
Redda Fabrizio Ruo
Fahmy Wael
Ha Nathan W.
International Rectifier Corporation
Ostrolenk Faber Gerb & Soffen, LLP
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