Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Junction field effect transistor
Reexamination Certificate
2002-03-12
2003-07-22
Wilson, Allan R. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Junction field effect transistor
C257S257000, C257S465000
Reexamination Certificate
active
06597025
ABSTRACT:
The invention relates to a light-sensitive semiconductor component that includes a channel region with a first type of doping and zones with an opposed type of doping that are in contact therewith, a pn junction being formed at the area of contact between the channel region and the zones.
A semiconductor component of the kind set forth is known from EP 0 883 187 A1. The component is constructed in principle as a semiconductor diode in CMOS technology. On a p doped substrate therein there is provided a p- (that is, weaker) doped channel region in which a “dot-shaped”, or slightly laterally expanded, n doped zone is inserted by diffusion or preferably by implantation. This n doped zone will be referred to hereinafter as a “dot zone” because of its shape relative to the channel region. A pn junction is formed in known manner between the n doped dot zone and the p doped channel region.
The known semiconductor diode is connected so as to detect light quanta. A light quantum that is incident in the laterally extending channel region generates a pair of charge carriers therein, that is, a hole and a free electron. The charge carriers exist until they are “destroyed” again by recombination. When the doping of the channel region is weak enough, however, they will have a long life during which notably the minority charge carriers (for example, an electron in the p region) can travel a comparatively large distance under the influence of diffusion processes. The minority charge carrier can then notably reach the barrier layer between the p doped channel region and the dot zone, said minority carrier then being pulled into the dot zone because of the electrical field prevailing at that area. A substantial part of the minority charge carriers of the channel region that are produced by light quanta thus accumulates in the dot zone in which their presence can be detected by means of appropriate electronic evaluation circuitry.
As opposed to conventional light-sensitive semiconductor diodes with a planar pn junction, the dot zone in the diode disclosed in EP 0 883 187 A1 does not extend over a large area in the channel region, but is rather limited to a minimum part of the surface area of the channel region. This is due to the fact that the remaining surface area over the channel region should be reserved so as to accommodate integrated electronic evaluation circuitry for reading out the charge of the semiconductor diode. Therefore, this surface area over the p doped channel region is provided with n++ doping so as to create a basis for said electronic circuitry that is electrically insulated from the channel region. The size of a pixel, that is, the surface area of a light-sensitive semiconductor component that can be selectively read out by the electronic circuitry, is limited by the diffusion length of the charge carriers in the p doped channel region in the diode disclosed in EP 0 883 187 A1. This is because only minority charge carriers that can reach a dot zone via diffusion can be detected in this dot zone. Therefore, only comparatively small pixels can be realized. Furthermore, because of the undirected nature of the diffusion, crosstalk of the signals occurs between neighboring pixels in a component as disclosed in EP 0 883 187 A1.
For many applications it is desirable to have available light-sensitive semiconductor components with pixels of comparatively large area. An example in this respect is an optical detector for reading out X-ray scintillators. The detection of X-rays usually takes place in two stages where an X-ray quantum first causes the emission of visible light in a scintillator crystal and this visible light is subsequently detected by a light-sensitive semiconductor diode in a second step. It is a typical aspect of this process that it is necessary to detect very small quantities of light that, moreover, are distributed across a large area in space because of the undirected emission. For example, the pixel of a semiconductor detector for an X-ray scintillator typically has a surface area of 2 mm
2
and the entire detector is then usually composed of a number of such pixels and has a format of, for example 4×100 cm
2
for a typical application.
It is known that light-sensitive semiconductor components with integrated amplifier circuits exhibit noise which increases as the capacitance of the semiconductor diode increases. Therefore, the reduction of the capacitance of detector elements is a crucial condition in realizing low-noise optical front ends that include a light-sensitive element and an amplifier. In this respect it is to be noted that the component is advantageously manufactured by means of customary integrating semiconductor techniques, for example, CMOS techniques, that define numerous parameters of the component for technical reasons. This holds notably for the doping strength of the channel region so that, generally speaking, it is no longer available for adaptation of the capacitance of the semiconductor diode.
Considering the foregoing it was an object of the present invention to provide a light-sensitive semiconductor element for an X-ray detector in an X-ray examination apparatus that has a comparatively large pixel surface area in conjunction with a low capacitance and hence is suitable in particular for use in X-ray scintillators. It should preferably be possible to manufacture the semiconductor component in standard (CMOS) production steps and the component should also have a high radiation hardness in respect of ionized X-rays, a suitable channel separation and an exactly defined geometry.
This object is achieved by means of a semiconductor component as disclosed in the characterizing part of claim
1
. Advantageous further embodiments are disclosed in the dependent claims.
The semiconductor component thus includes a channel region with a first type of doping as well as dot zones that are in contact with the channel region and have a type of doping that opposes the doping of the channel region. The channel region may thus be p doped and the dot zones may be n doped or vice versa. Because of such different doping, a respective pn junction arises in the area of contact between the channel region and a dot zone, that is, a semiconductor diode. Furthermore, the semiconductor component includes at least one group of several dot zones, the dot zones of said group being connected electrically in parallel with one another.
Because of the electrical connection of the dot zones of a group, the charge accumulated in the dot zones can be uniformly read out. This means that the dot zones of the group together constitute a pixel. The size of this pixel is then determined by the surface area across which the group of dot zones is distributed; it can quasi be made arbitrarily large. Preferably, many of such groups, each time constituting a respective pixel, are arranged on the semiconductor component so as to cover the surface area thereof.
Preferably, the dot zones of a group are arranged so as to be coherent. This means that each dot zone of the group has at least one neighboring dot zone that is also a member of the same group. The surface area that is covered by the dot zones is typically bounded so as to be rectangular or compact in a different way.
Furthermore, the dot zones in the channel region are preferably distributed according to a hexagonal pattern. This means that each dot zone is situated at the center of a hexagon whose corners are occupied by dot zones. The smallest geometrical cell of this arrangement is an equilateral triangle with dot zones provided at its corners.
The formation of a pixel from hexagonally arranged and electrically coupled dot zones results in a detector element that has a substantially reduced capacitance and at the same time a suitable sensitivity across the entire pixel surface that is occupied by the group. As opposed to conventional detector diodes, the barrier layer (on whose surface area the capacitance of the diode is approximately proportionally dependent) does not extend across the entire pixel surface,
Brockherde Werner
Hausschild Ralf
Kemna Armin
Lauter Josef
Vodopia John
Wilson Allan R.
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