Active solid-state devices (e.g. – transistors – solid-state diode – Avalanche diode
Reexamination Certificate
2002-04-08
2004-09-14
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Avalanche diode
C257S106000, C257S175000, C257S199000, C257S481000, C257S551000, C257SE29335, C257SE21355
Reexamination Certificate
active
06791161
ABSTRACT:
COPYRIGHT NOTICE AND AUTHORIZATION
A portion of the disclosure of this patent document contains material, which is subject to mask work protection. The mask work owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all mask work rights whatsoever.
FIELD OF THE INVENTION
This invention relates to semiconductor devices, methods of manufacturing semiconductor devices, and more particularly, to Zener diodes and methods of manufacturing high-yield, high-precision Zener diodes.
BACKGROUND OF THE INVENTION
Semiconductor devices are critical components in a vast array of modern products. Purified crystalline semiconductor materials have highly useful electrical characteristics when specific types of impurities, called dopants, are introduced into the semiconductor's crystalline structure. Depending on the nature of the dopant introduced into the semiconductor material, the semiconductor material will take on a particular conductivity type, either P-type or N-type. In connection with the present invention, to say that a material has one conductivity type that is consistent with another conductivity type means that both conductivity types are either P-type or N-type.
The element silicon is a an example of a semiconductor material. Pure silicon has four electrons in the valence shell of each of its atoms. Pure crystalline silicon forms a lattice structure, in which silicon's valence electrons form stable covalent bonds with other silicon atoms.
An example of P-type material is pure silicon doped with an impurity such as boron, aluminum, gallium, or indium. These materials are referred to as “acceptor” impurities because their valence shells contain only three electrons. When these materials are introduced into a semiconductor crystal, the uniform lattice structure of the silicon is affected because the three-electron valence shells of the doping material can't complete the lattice. The vacancy created by the lack of a fourth electron is called a hole. Holes are loosely held to the impurity atom so that, when affected by an electric field, electrons can drift into the hole, thus causing the hole to appear to drift. In this way, the hole acts as a positive-charge current carrier.
An example of N-type material is pure silicon doped with a very small amount of impurities containing five electrons in the valence shell. These materials can be antimony, phosphorus, or arsenic. Because of their extra electrons, they are called donor impurities. When these materials are blended with pure silicon, the uniform lattice structure of the silicon is affected because the five-electron valence shell of the doping material has too many electrons to simply complete the lattice structure. These extra electrons are loosely held to their impurity atoms so that, when affected by an electric field, the electrons can drift, thus acting as a negative-charge current carrier.
When a junction is formed between P material and N material, (“P/N junction”) the extra current carriers tend to cross the junction so that the lattice structure in the vicinity of the junction tends to have four electrons associated with each atom. The region where this phenomenon occurs is called the depletion region since both the P- and N-type materials have been depleted of their current carriers in this region.
In a P/N junction device, sometimes called a diode or rectifier, the electrode connected to the P-type material is referred to as the anode, and the electrode connected to the N-type material is called the cathode. The depletion region of a P/N junction has the useful property of causing a P/N junction device to conduct current when a positive voltage (above a forward voltage drop threshold) is applied across the P/N junction and to block the flow of current when a negative voltage is applied across the P/N junction. A positive voltage applied from anode to cathode is referred to as forward bias, and a negative voltage is referred to as reverse bias.
Accordingly, diodes conduct current from anode to cathode at forward biased voltages above the forward voltage drop, and diodes block current under reverse bias up to a point at which the diodes break down under a sufficiently high reverse biased voltage. Diodes that take advantage of breakdown characteristics are called Zener diodes.
Zener diodes have been used since the late 1950's as voltage references or for voltage regulation, originally as an alternative to the vacuum tube. Zener diodes have the useful property of blocking current under reverse bias, up to a threshold or breakdown voltage. When installed in parallel with a load, reverse biased Zener diodes clamp the voltage across the load at the Zener diode's breakdown voltage.
Zener diodes are P/N junction devices that are designed to operate nondestructively in reverse bias breakdown mode. While every P/N junction will break down under a sufficiently high reverse bias, a low-power rectifying diode will break down at a fairly high voltage and would likely be damaged by the resulting current. However, Zener diodes are designed to operate in breakdown mode, at specified currents, without sustaining damage.
A relatively lightly doped P/N junction will exhibit avalanche breakdown at the relatively high voltage of approximately 30V to 50V. Avalanche breakdown is the result of energizing thermally produced electron/hole pairs in the depletion region surrounding a P/N junction with the electric field associated with the reverse biased P/N junction. Given a sufficiently large electric field, energized electrons eventually take on enough energy to ionize atoms of the semiconductor material in the depletion region. Next, electrons that are released by ionization themselves become energized by the electric field, resulting in further ionization. The result of the chain reaction of ionization is the occurrence of sufficient numbers of charge carriers to enable the P/N junction to conduct electrical current. Observers have remarked that this chain reaction is like an avalanche on a snow covered mountain. Accordingly this type of breakdown is called avalanche breakdown.
Zener diodes are designed to break down at a specific voltage with a sharp reproducible characteristic. The diodes are designed to conduct the breakdown current nondestructively.
Zener diodes are generally used either as a voltage reference or as transient voltage suppressors. When used as voltage references, a high degree of precision is required for some electrical circuit designs. Accordingly, Zener diodes are frequently specified in terms of a ± percentage error in breakdown voltage tolerance.
Known processes of manufacturing Zener diodes consist of fabricating them on thinly sliced wafers of crystalline semiconductor substrate. Conventional substrate wafers are formed with a high purity, monocrystalline, semiconductor material by a known monocrystalline growth method. In the growth method, a pool of doped molten liquid semiconductor material is seeded with a small semiconductor crystal. The seed is slowly drawn out of the pool, and as it is drawn out, the molten semiconductor atoms or molecules align with the lattice structure of the seed crystal to form a generally cylindrical ingot of semiconductor material. The crystalline semiconductor material can also be fabricated with known float zone methods. The ingot is then sliced into generally circular substrate wafers, of a conductivity type determined by the dopant type and concentration introduced into the molten semiconductor.
Ideally, each such semiconductor wafer has precisely the same doping concentration and resistivity. However, in practice, this is not the case. Because of inherent properties of dopant materials and the way the dopant materials are introduced into the semiconductor material there are differences in dopant concentration and resistivity along the length of a semiconductor ingot. Further, there are also differences
Blackwell Sanders Peper Martin LLP
Fab-Tech Inc.
Huynh Andy
Nelms David
LandOfFree
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