Radiation detector

A layer of transparent conductive material insulatingly overlies a major surface of a substrate of semiconductor material to provide a CIS (conductor-insulator-semiconductor) capacitor. A region of opposite conductivity type is provided in the substrate adjacent the major surface of the substrate. The capacitor is biased in accumulation and the region of opposite conductivity type is reversely biased with respect to the substrate. Minority charge carriers generated in the semiconductor substrate underlying the conductive layer in response to applied radiation diffuse to the region of opposite conductivity type and are sensed.

The present invention relates in general to radiation detecting devices and 
in particular to such devices for converting radiation in the form of 
X-rays and light into electrical signals. 
The present invention is an improvement over the invention described and 
claimed in copending patent application, Ser. No. 807,080, filed June 16, 
1977, now U.S. Pat. No. 4,101,924 and assigned to the assignee of the 
present invention. 
An object of the present invention is to provide a radiation detector of 
high collection efficiency. 
Another object of the present invention is to provide a radiation detector 
having high sensitivity to radiation yet providing low output capacitance 
for electrical signals. 
A further object of the present invention is to provide a radiation 
detector of enhanced signal to noise ratio. 
In carrying out the invention in an illustrative embodiment thereof, there 
is provided a substrate of semiconductor material of one conductivity 
type. A first layer of a transparent insulating material is provided 
overlying a major surface of the semiconductor substrate. A second layer 
of a transparent conductive material overlies the first layer and forms a 
capacitor with the substrate and the first layer. A region of opposite 
conductivity type is provided in the substrate contiguous to the surface 
adjacent region of the substrate underlying said layer of transparent 
conductive material. The area of the second layer of transparent 
conductive material is substantially larger than the area of the region of 
opposite conductivity type in the major surface of the substrate. Means 
are provided for applying a biasing voltage between the second layer and 
the substrate to establish a region of accumulation in the surface of the 
substrate underlying the second layer. Output means are connected in 
circuit with the PN junction for sensing the flow of minority carrier 
charge generated in the substrate.

Referring now to FIGS. 1, 2 and 3 there is shown a radiation sensing device 
10 including a substrate 11 of monocrystalline silicon semiconductor 
material of N-type conductivity and relatively high resistivity, for 
example 10 ohm centimeters. A thick layer 13 of insulation, such as 
silicon dioxide, is formed on a major surface 12 of the substrate. The 
substrate 11 may conveniently be 10 mils thick and the layer of thick 
insulation may be 1 micron thick. An elongated rectangular recess 14 is 
formed in the thick insulating layer extending to within a short distance 
of the major surface of the semiconductor substrate 11 to provide a thin 
layer of insulation, for example, 0.1 microns thick, over the major 
surface. Overlying substantially one-half of the recess 14 and also 
extending over an adjacent portion of the thick layer of insulation is 
transparent conductive member 17. The longitudinal axis 18 of the 
conductive member 17 is parallel to the long dimension of the recess 14. 
Overlying substantially the other half of the recess 14 and also extending 
over an adjacent portion of the thick insulation is another conductive 
member 19 having a longitudinal axis 20 parallel to the long dimension of 
the recess 14. Adjacent edges of the elongated conductive members 17 and 
19 are spaced apart to form a narrow gap 21 having a longitudinal axis 
parallel to the long dimension of the recess 14. The conductive members 17 
and 19 are constituted of a transparent conductive material such as 
antimony tin oxide, indium tin oxide, or tin oxide. The conductive members 
17 and 19 may also be constituted of thin layers of metal, for example 
molybdenum approximately 10 Angstroms thick. A thin layer 23 of silicon 
dioxide is formed over the transparent electrodes 17 and 19 to provide 
protection for the electrodes. Terminals 24 and 25 provide electrical 
connection to the conductive members 17 and 19, respectively. Also 
provided in the semiconductor substrate is an elongated region 26 of 
P-type conductivity having a longitudinal axis parallel to the long 
dimension of the recess 14. The P-type region 26 underlies the gap 21 with 
one edge 27 underlying electrode 17 and the other longitudinal edge 28 
underlying conductive member 19. Preferably, the edge 27 underlies the 
adjacent edge of transparent conductive member 17, and edge 28 underlies 
the adjacent edge of transparent conductive member 19 to keep capacitive 
coupling between the conductive members 17, 19 and P-type region 26 to a 
minimum. Terminal 30 provides conductive connection to the P-type region 
26. Terminal 31 secured to the bottom surface of the substrate 11 provides 
conductive connection thereto. The elements 17, 15 and 11 constitute a 
first CIS (conductor-insulator-semiconductor) capacitance 32 and the 
elements 19, 15 and 11 constitute a second CIS capacitance 33. 
FIG. 1 also shows a circuit for operating the device 10. The circuit 
includes a bias source 34 having the positive terminal thereof connected 
to ground and to the substrate terminal 31 and having the negative 
terminal thereof connected to terminals 24 and 25. The source 34 biases 
the CIS capacitances 32 and 33 in accumulation (i.e. the voltage at which 
minority carriers generated in the substrate are repelled from the 
surface. Such a bias causes minority carriers generated in the substrate 
to be repelled away from the surface of the substrate and thus maintains 
collection efficiency in the substrate underlying electrodes 17 and 19. 
Bias potential for the P-type region 26 is provided by the bias source 37 
and high gain differential amplifier 38. The differential amplifier 38 
includes an inverting terminal 39, a noninverting terminal 40, and an 
output terminal 41. The positive terminal of the source 37 is connected to 
ground and the negative terminal thereof is connected to the noninverting 
terminal 40. The inverting terminal 39 is connected to the terminal 30 for 
the P-type region 26. A feedback resistance 42 is connected between the 
output terminal 41 and the noninverting terminal 39. The high gain 
differential amplifier 39 with resistive feedback functions to maintain 
the differential in voltage between the inverting terminal 39 and the 
noninverting terminal 40 close to zero. The negative potential of source 
37 appears on the P-type region 36 and establishes a depletion region 43 
in the substrate, as shown in FIGS. 2 and 3. The absolute value of the 
potential of source 37 is greater than the absolute value of the voltage 
of source 34. Thus, the absolute value of the potential of the diffused 
region 26 is greater than the absolute value of the surface potential of 
the substrate underlying the conductive members 17 and 18. Any minority 
charge generated in the surface adjacent regions of the substrate 
underlying electrodes 17 and 19 as a result of incident radiation or from 
thermal excitation flows into the P-type region 26 and to the terminal 39. 
As terminals 39 and 40 are maintained at the same potential as a 
consequence of the action of the amplifier 38, the charge generated in the 
surface adjacent regions of the substrate underlying electrodes 17 and 19 
flows through the resistor 42 to the output terminal 41. Thus, the 
potential on the terminal 41 is proportional to the radiation induced 
current flow through the resistance 42. 
The device of both the present invention and the device of aforementioned 
patent application Ser. No. 807,080 have the advantage that the charge 
generation function and the charge sensing function are separated. The 
areas of the transparent conductive members 17 and 19 in both devices may 
be made very large in relation to the surface area of the P-type region 
26, for example 100 times, to provide high sensitivity while the area of 
the P-type region 26 may be kept quite small to provide low output 
capacitance and hence high voltage output in relation to photon input. By 
utilizing high values of reverse bias on the P-type region, the 
capacitance of this region may be further reduced and hence the 
sensitivity of the detector further improved. 
The device of the present application has the additional advantage that by 
operating the CIS capacitors 32 and 33 below threshold voltage and in 
accumulation rather than above threshold voltage the signal to noise ratio 
of the device is substantially increased, as will be apparent from a 
consideration of FIG. 4 to which reference is now made. FIG. 4 shows a 
pair of graphs 44 and 45 of current flowing in the line between terminal 
39 and terminal 39 as a function of various voltages applied across the 
CIS capacitors 32 and 33 for a device constituted of silicon 
semi-conductor material of a resistivity of 10 ohm centimeters, having an 
oxide thickness of 0.1 microns and having an aggregate transparent 
electrode area of 4 square millimeters. The minority carrier lifetime in 
the semiconductor material was approximately 20 micro-seconds. Graph 44 
shows the manner in which the current varies as the voltage on the 
transparent electrodes is increased in the negative direction in the 
absence of applied radiation, i.e. dark current. As the voltage on the 
transparent electrode 17 and 19 is increased from zero volts, dark current 
decreases slightly from a value of 3 .times. 10.sup.-11 amperes to a value 
of about 2.5 .times. 10.sup.-11 at just below threshold voltage (about -14 
volts). As the voltage is increased beyond threshold voltage, the dark 
current increases very rapidly until it reaches a saturation value of 
about 4 .times. 10.sup.-10 amperes. Thus, operating the device below 
threshold and in accumulation reduces the dark current by a factor of 16. 
As noise is proportional to the square root of the dark current, noise is 
reduced by a factor of 4. Graph 45 shows output current including signal 
and dark current components of the same device as a function of bias 
voltage applied across the CIS capacitors 32 and 33 while light of 
constant power of a particular value and a particular wavelength (5000 
Angstroms) is directed through the transparent electrodes 32 and 33 onto 
the surface of the semiconductor substrate. At zero bias voltage, the 
output current is about 3 .times. 10.sup.-9 amperes. As the bias voltage 
on the transparent electrodes 17 and 19 is increased in the negative 
direction, the output current remains substantially constant until a 
voltage close to threshold voltage, nominally 14 volts, is reached at 
which time the output current decreases to a value 1.25 .times. 10.sup.-9 
amperes at threshold and thereafter increases to a saturation value above 
threshold of 4.8 30 10.sup.-9 amperes. Thus, below threshold the signal 
component of current is 2.75 .times. 10.sup.-9 (3 .times. 10.sup.-9 minus 
0.25 .times. 10.sup.-9) and at a voltage above threshold the signal 
component of output current is 4.5 .times. 10.sup.-9 (4.9 .times. 
10.sup.-9 minus 4 .times. 10.sup.-10). Thus, the decrease in the signal 
component of current in operating the device below threshold over that in 
operating it above threshold is 1.7. Accordingly, the signal to noise 
ratio improvement in operating the device in the accumulation mode, i.e. 
below threshold over that of operating it above threshold is improved by 
the ratio of 4 divided by 1.7 or 2.3. For wavelengths of visible radiation 
in the range of 4,000 to 9,000 Angstroms and of the same constant power, 
graphs similar to graph 45 were obtained from which the same 2.3 to 1 
improvement in signal to noise ratio was obtained. 
For the utilization of the device to detect radiation to which the 
semiconductor is transparent such as, for example, X-rays, suitable 
conversion means for converting X-rays to light to which the 
semiconductive material is more responsive is provided. To this end, in 
FIGS. 1, 2 and 3 is shown a scintillator 45 suitable for the conversion of 
X-rays into visible light to which silicon is responsive. The scintillator 
45 may be constituted of a material such as cesium iodide. 
In the detector of the present invention, charge carriers generated in the 
surface adjacent regions of the semiconductor substrate underlying 
electrodes 17 and 19 move by diffusion to the output region 26 where they 
may be stored or read out. The time to travel from the point of generation 
to the output region is a function of the square of the distance between 
the point of generation and the output region. Thus, the speed of response 
of the detector is a function of the size of the charge generation region 
underlying the transparent electrodes along the surface in relation to the 
output region of opposite conductivity. The speed of response is also a 
function of the location of the output region in relation to the charge 
generation region. In the detector described in connection with FIGS. 1-3, 
the provision of elongated conductive members 17 and 19 and an elongated 
region 26 of opposite conductivity type underlying both electrodes 17 and 
19 keeps the distance that carriers generated in the depletion regions 
must travel to reach the output region 26 to a minimum, and thus provides 
a high speed of response. This high speed of response is at the expense of 
some decrease in sensitivity due to increased area of the output PN 
junction. A number of electrically connected elongated regions of opposite 
conductivity type may be provided on a common substrate, each with its 
pair of associated elongated conductive members to enhance the speed of 
response. The number and spacing of such elongated regions is chosen so as 
to obtain the desired speed of response. 
A particular advantageous organization of materials for the detector of the 
present invention comprises a substrate of silicon, an insulating layer of 
silicon dioxide overlying the substrate and a thin layer of a metal, for 
example, a layer of molybdenum about 10 Angstroms thick. The thin layer of 
metal passes radiation over a broad band from deep in the ultraviolet 
portion of spectrum to well into the infra-red portion of the spectrum. 
The silicon dioxide has a band gap of about 10.5 electron volts and hence 
would also transmit radiation from deep in the ultraviolet portion of the 
spectrum well into the infra-red portion of the spectrum. Thus, the 
detector response would be determined primarily by the response of the 
silicon substrate. As pointed out below this response in terms of quantum 
efficiency as a function of wavelength would be excellent from deep in the 
ultraviolet portion of the spectrum into the short wavelength part of the 
infra-red portion of the spectrum. 
While the invention has been described in connection with devices made of 
silicon semiconductor material, it is understood that the invention is 
equally applicable to devices made of other semiconductor materials, such 
as germanium, gallium phosphide and gallium arsenide. 
While the invention has been described in connection with devices 
constituted of a semiconductor substrate of N-type conductivity with a 
P-type output region, it will be understood that P-type substrates with an 
N-type output region could as well be used. In such a case, the applied 
potentials would be reversed in polarity. 
While in the circuit of FIG. 1, the P-region 26 was operated with reverse 
bias, it will be understood that the P-region 26 may be operated with zero 
bias with respect to the substrate. Elimination of the source 37 would 
provide such a circuit. Other output circuits may be utilized with the 
device of FIG. 1, for example, the output circuit described and claimed in 
patent application Ser. No. 846,543, filed Oct. 28, 1977, and assigned to 
the assignee as the present invention. 
While a pair of separate electrodes 17 and 19 were utilized in the device 
of FIG. 1, it will be understood that a single electrode could have been 
provided in the recess 14 and overlying the P-type region 26 as well as 
the N-type substrate with some advantage in simplicity of fabrication. 
Such a device, however, would have increased output capacitance. 
The detector of the present invention is particularly advantageous over PN 
junction detectors. In the detector of the present invention the region in 
which charge generation occurs can be made very large in relation to the 
size of the region of opposite conductivity type with greatly increased 
sensitivity of the detector, as pointed out above. Also, in the detector 
of the present invention the region in which charge generation occurs 
extends to the surface of the semiconductor substrate. Thus, charge 
carriers which are generated near the surface of the substrate by 
radiation in the blue and ultraviolet portions of the spectrum are 
substantially all collected with resultant high efficiency of conversion 
of radiation into electrical signal. In PN junction detectors, the PN 
junction is located below the surface of the semiconductor substrate. 
Carriers generated at the surface of the substrate in response to 
radiation must travel through heavily doped surface adjacent regions of 
the substrate to the PN junction to be detected. A large proportion of the 
carriers generated at the surface recombine both at the surface and in the 
bulk of the substrate before they reach the PN junction with resultant 
lower efficiency of conversion of radiation into electrical signal. 
While the invention has been described in specific embodiments, it would be 
understood that modifications may be made by those skilled in the art and 
it is intended by the appended claims to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.