Integrating imaging system with phototransistor having wide dynamic range

Abstract

A bipolar phototransistor comprises both an Integrating photosensor and a switching element. The base terminal of the bipolar phototransistor is utilized as the switch-control node for the pixel and its emitter is the output node of the integrating photosensor. A plurality of integrating photosensors may be placed in an array of rows and columns, wherein the bases of all bipolar phototransistors in a row are capacitively coupled together to a common row-select line, and the emitters of all bipolar phototransistors in a column are connected together to a column sense line. The input of a sense amplifier is connected to the sense line of each column of integrating photosensors. An integrating sense amplifier according to the present invention includes an amplifying element having an inverting input connected to the sense line. A capacitor, preferably a varactor, is also connected between the inverting input and output of the amplifying element. An exponential feedback element may be provided in the sense amplifiers for signal compression at high light levels. The outputs of the sense amplifiers are connected to sample/hold circuits. The rows of the array are selected one at a time and the outputs of the sample/hold circuits for each row are scanned out of the array while the pixel data for the next row are sampled.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to imaging arrays and systems. More particularly, the present invention relates to a single-chip high-sensitivity imaging system for producing serial-scanned outputs.

2. The Prior Art

Imaging circuits typically include a two-dimensional array of photosensors, each of which converts a pixel of light energy from an image into an electrical signal representing the light value of the pixel. The imaging circuit scans the photosensors to read out the electrical signals representing the light values of all pixels from an image.

As a result of the advances in semiconductor technology, single chip imaging circuits are known in the art. The most common single-chip imaging technology is the charge coupled device (CCD) camera. A CCD device operates by accumulating photo-generated charge in potential wells in a semiconductor substrate at or slightly under the surface of the substrate. The depth of these wells is controlled by the voltage on gate electrodes located just above the surface of the substrate. By manipulating the voltage on several of the gates, the accumulated charge can be shifted along the surface to a sensing point, where it is amplified into a signal which can be driven off of the chip.

Using modern MOS processing techniques, the transport of charge in a CCD structure can be accomplished with almost perfect efficiency at video rates; typically less than ten parts per million of the charge in a well is lost in each transfer. In a typical serial-scanned CCD imager, such as is used for video applications, the charge Is shifted out once per frame, and hence the total charge in each well is collected over the entire frame time, typically 1/60 second. Sensing technologies which operate in this manner, by accumulating charge over the entire time between scanout events, are called integrating photosensors.

Despite its technical achievements and wide spread commercial success, CCD technology has its limitations. Because the photo-generated charge is shifted directly, the gain of the device (electrons out per photon in) is always less than unity. Because the charge stored in each well is limited, the CCD imager has severe dynamic-range constraints. At the low end, the sensitivity is limited by how small a charge can be sensed above the noise of the readout amplifier. At the high end, the range is limited by the total charge that can be shifted from one well into the next.

To overcome these dynamic-range limitations of the CCD imager, a number of attempts have been made to use phototransistors to sense incoming light, such as seen in Analog VLSI and Neural Systems, by Carver A. Mead, Addison Wesley Publishing Co., 1989, at pp. 260-261. Phototransistors can have a gain of over 100 electrons per absorbed photon. In a typical application, the photocurrent from each phototransistor is fed into a device with exponential current-voltage characteristics, such as a pair of diode-connected MOS transistors. The output voltage of such a device is then a logarithmic function of the light level. These devices show much wider dynamic range than do CCD devices, but are not as sensitive at low light levels because they operate in instantaneous-current mode rather than in integrating mode.

Photons absorbed in the neighborhood of a phototransistor collector-base or emitter-base junction create electron-hole pairs that are collected by the nearest p-n junction. Minority carriers collected by either junction act as base current, and are multiplied by the current gain of the transistor to produce the collector current. The emitter current is the sum of the base current and the collector current. For this reason, the emitter current is usually used as the output of the sensor.

Prior art phototransistor sensors suffer from low-current limitations. The current-gain of the phototransistor sensor device is high at high photocurrents, where it is not needed, and is low at low photocurrents, where it would desirably be higher. Because of this behavior of phototransistors, they have not found widespread use in commercial scanned imagers.

It is an object of the present invention to provide an improved single-chip serial-scanned imager, which can exhibit higher sensitivity at lower light levels than current mode imagers, and in the same device operate over a dynamic range larger than that achievable with CCD imagers.

BRIEF DESCRIPTION OF THE INVENTION

According to a first aspect of the present invention, a bipolar phototransistor used as a pixel comprises both an integrating photosensor and a select device. The base terminal of the bipolar phototransistor is utilized as the select node for the pixel. A plurality of integrating photosensors according to the present invention may be placed in an array of rows and columns, wherein the base terminals of all bipolar phototransistors in a row are capacitively coupled to a common row-select line. The emitter of the bipolar phototransistor is the output node of the integrating photosensor. The output nodes of all integrating photosensors in a column are connected together to a column sense line.

The integrating photosensor array is coupled to plurality of sense amplifiers, with a sense amplifier input connected to the sense line output of each column of integrating photosensors. An integrating sense amplifier according to the present invention includes an amplifying element having an inverting input is connected to the sense line. An MOS balance transistor is connected between the inverting input and the output of the amplifying element and a capacitor, preferably a varactor, is also connected between the inverting input and output of the amplifying element. In addition, in order to further extend the dynamic range, an exponential feedback element can be connected between the output and the inverting input of the amplifying element.

In one embodiment of the invention, this feedback element includes a first P-channel feedback transistor having its source connected to the sense line and its gate and drain connected together to the source of a second P-channel feedback transistor. The gate of the second P-channel feedback transistor is connected to the output of the amplifying element, and its drain is connected to a fixed voltage source, such as ground. In another embodiment of the invention, this feedback element includes an N-channel MOS transistor having its source connected to the output of the amplifying element, its drain connected to the input of the amplifying element, and its gate connected to a source of fixed voltage.

The outputs of the sense amplifiers are connected to a scanning device through sample/hold circuits. The sample/hold circuits store the outputs of each of the sense amplifiers, so that their outputs can be scanned while the charge from the next row of pixels is being sensed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG 1 is a graph showing the measured current gain of a typical phototransistor sensor device as a function of its base current.

FIG. 2a is a schematic diagram of an integrating photosensor according to a presently-preferred embodiment of the invention.

FIG. 2b is a cross sectional view of an embodiment of the circuit of FIG. 2a implemented in an N-well CMOS process.

FIG. 3 is a prior art sense amplifier suitable for use in the present invention.

FIG. 4a is a schematic diagram of a presently-preferred sense amplifier for use in the present invention which provides signal compression.

FIG. 4b is a graph showing the capacitance of the varactor structure of the sense amplifier of FIG. 4a as a function of voltage.

FIG. 5 is a graph showing the output voltage of the sense amplifier of FIG. 4a as a function of light intensity.

FIG. 6a is a schematic diagram of an alternative sense amplifier for use in the present invention incorporating a first exponential feedback element.

FIG. 6b is a schematic diagram of an alternative sense amplifier for use in the present invention incorporating a second exponential feedback element.

FIG. 7a is a block diagram of an imaging array comprising bipolar transistor integrating photosensors according to a presently preferred embodiment of the invention.

FIG. 7b is a block diagram of an imaging array comprising integrating photosensors a presently preferred embodiment of the invention.

FIG. 7c is a timing diagram of typical control signal waveforms for controlling the operation of array of type shown in FIGS. 7a and 7b.

FIG. 8a is a cross-sectional view of a portion of an array of integrating photosensors according to a presently preferred embodiment of the invention implemented in an N-well CMOS process taken along an axis parallel to one row thereof in a position which bisects a row-select line of the array.

FIG. 8b is another cross-sectional view of the integrating photosensor array shown in FIG. 8a, taken along an axis parallel to a column thereof in a position which bisects a column sense line of the array.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The low-current limitation of a prior art phototransistor photosensor can be appreciated with reference to FIG. 1, which shows the measured current gain of a typical phototransistor sensor device as a function of its base current. It can be seen that the current-gain of the phototransistor photosensor device is high at high photocurrents, where it is not needed, and is low at low photocurrents, where it would desirably be higher. Because of this behavior, phototransistors have not found widespread use in commercial scanned imagers.

Referring now to FIG. 2a, a presently preferred pixel integrating photosensor circuit 10 suitable for use in a single-chip integrated circuit imaging array is shown. The photosensor circuit 10 comprises NPN phototransistor 12, having its collector connected to a source of positive voltage, shown as VDD in FIG. 2a. NPN phototransistor 12 preferably has a current gain larger than 100. A capacitor 14 is shown coupling the base of phototransistor 12 to a select line 16.

Coupling the base of phototransistor 12 to a select line 16 allows the phototransistor to also be used as a select device. The emitter of phototransistor is the output of the integrating photosensor and is connected to column sense line 18.

In a preferred implementation of the Invention, a plurality of photosensor circuits 10 are formed in an N-well in a standard N-well CMOS integrated circuit fabrication process, with the single addition of a p-type base diffusion step. Because the N-well diffusion covers the entire photosensor array, the individual pixel circuits can be very dense since there are no large well-to-well spacing distances which waste semiconductor substrate area.

Referring now to FIG. 2b, a cross section of a presently preferred embodiment of a photosensor circuit 10 in an N-well CMOS process is shown. The entire array of photosensors 10 may be fabricated in N-well 20 formed in semiconductor substrate 22. N-well 20 forms the collector of all phototransistors 12 in the entire photosensor array. The p-base regions 24 of the individual phototransistors 12 are formed by implantation into regions of N-well 20 through apertures in a thick oxide layer 26. A gate oxide layer 28 forms the dielectric for capacitor 14, whose plates comprise p-type base region 24 and row-select line 16, comprising a polysilicon layer. An N+ region 30 in p-type base region 24 forms the emitter of phototransistor 12. N+ region 30 is formed by implanting an n-type dopant through an aperture in row-select line 16 and gate oxide layer 28. An oxide layer 32 is deposited over row-select line 16, and a metal contact to sense line 18 formed in oxide layer 32, using conventional semiconductor processing techniques.

Those of ordinary skill in the art will recognize that the integrating photosensor of the present invention may easily be Integrated Into a standard CMOS fabrication process. The collectors of the phototransistors may be the n type substrate of a p-well process, the base may be separate p-type wells, and the emitter may be the N+ source drain diffusions used for N-Channel MOS devices. Alternatively, in an n-well process, the entire array is contained within a single n-well, and the step used to form the p-type base regions 24 is the only additional step necessary and is performed after field oxide formation and nitride strip or similar step. The gate oxide layer 28 and row select line 16 are formed during the steps which form the gate oxide and polysilicon gates of the MOS devices. The emitter 30 is formed during the n+ source/drain implant step and the sense line 18 is formed during the contact and metallization steps. All of these steps are already present in a standard CMOS fabrication process. While NPN phototransistors have been used to illustrate the operation of the present invention, those of ordinary skill in the art will recognize that PNP transistors could be employed by appropriately reversing the power supply polarities.

In a typical implementation in a one micron CMOS process, NPN phototransistor 12 may have a base region about 7 microns by 7 microns. The entire pixel may be 10 microns by 10 microns or less, allowing for a highly dense array and making it competitive with the high quality CCD imager circuits.

During normal circuit operation, row-select line 16 is held at a fixed voltage selected to reverse bias the base-emitter junction of phototransistor 12. Under this condition, the photocurrent integrates on capacitor 14. When it is desired to read out the state of a selected pixel, row-select line 16 is brought high, thereby forward biasing the base-emitter junction of phototransistor 12. Under this condition, the integrated photocurrent, multiplied by the current gain of phototransistor 12, flows from the emitter of phototransistor 12 onto column sense line 18.

Because the discharge occurs very rapidly, phototransistor 12 is operated at much higher currents than the photocurrent. At this higher current level, the current gain of phototransistor 12 is much higher than unity as shown in FIG. 1. The mode of operation contemplated by the present invention thereby allows many charge carriers to be delivered to sense line 18 for every photo-generated carrier collected by the base.

Referring now to FIG. 3, a typical integrating sense amplifier 50 suitable for sensing the charge delivered to sense line 18 by a single pixel is shown. Sense amplifier 50 includes an amplifying element 52, which may be an operational amplifier. The non-inverting input of amplifier 52 is connected to a source of reference voltage V.sub.ref and its inverting input is connected to sense line 18.

A P-channel balance transistor 54 is connected between the inverting input and the output of amplifier 52. Its gate is connected to a balance line 56. A capacitor 58 is also connected between the inverting input and output of amplifier 52.

Before row select line 16 is brought high to read a pixel value, amplifier 52 is brought into a known state by bringing the gate of p-channel balance transistor 56 low to turn it on, thereby effectively shorting the inverting input of amplifier 52 to its output and hence driving both input and output voltages to reference voltage V.sub.ref. The gate of P-channel balance transistor 56 is then brought high, thereby isolating the input sense line 18 from the output of amplifier 52. The pixel is then addressed by bringing its row select line high, thus delivering its stored charge multiplied by the current gain of its phototransistor onto sense line 18.

Amplifier 52 will respond to this charge by maintaining its input at a nearly constant level by the feedback action of capacitor 58. The output voltage of amplifier 52 after the pixel has delivered all of its charge to sense line 18 will be lower than its initial value by that charge divided by the value of capacitor 58. The output of the sense amplifiers will be captured by a sample/hold circuit so that it may be scanned out while the sense amplifiers are sensing charge from the next row of pixels.

The operation of the imaging array of the present invention at high light levels may be improved by using the sense amplifier shown in FIG. 4a. Sense amplifier 60 includes most of the elements of sense amplifier 50. These elements have been assigned the same reference numerals as the corresponding elements of FIG. 3. Thus, sense amplifier 60 includes an amplifying element 52, having its non-inverting input connected to a source of reference voltage V.sub.ref and its inverting input connected to sense line 18. P-channel balance transistor 54 is connected between the inverting input and the output of amplifier 52. Its gate is connected to a balance line 56.

However, in place of capacitor 58, a varactor structure is connected between the inverting input and output of amplifier 52 in the sense amplifier of FIG. 4a. Use of a varactor structure in place of capacitor 58 allows compression of the output signal over a wide dynamic range of input signals. As known in the art, and as illustrated in FIG. 4a, a varactor structure may comprise a large-area MOS transistor. The gate of the MOS transistor comprises one terminal of the varactor structure and the other terminal of the varactor structure comprises the source and drain of the MOS transistor connected together.

Either an N-channel MOS transistor or a P-channel MOS transistor may be used as the varactor according to the present invention. Because, in the presently illustrated application, a positive charge is to be sensed, if an N-channel MOS transistor is used as the varactor as indicated at reference numeral 62, its gate is connected to the input of amplifying element 52 and its source and drain are connected to the output of amplifying element 52. If a P-channel MOS transistor is used as the varactor as indicated at reference numeral 64 in phantom connection, its source and drain are connected to the input of amplifying element 52 and its gate is connected to the output of amplifying element 52. If a charge of the opposite sign is to be sensed, those of ordinary skill in the art will realize that the connections to the varactor will be reversed. If an application requires sensing charge of both polarities, two varactors connected in opposite directions may be used.

Referring now to FIG. 4b, a graph of varactor gate-to-source capacitance as a function of VGS, the voltage across the capacitor, it may be seen that the capacitance from one terminal of the varactor structure to its other terminal varies with the voltage across the varactor device. At low gate to source voltages, the capacitance C.sub.0 is small and is determined by the overlap of the gate with the source and drain diffusions. As the gate voltage approaches the threshold voltage V.sub.t of the MOS transistor, a channel begins to form under the gate, and the capacitance increases exponentially. Above the threshold voltage, the capacitance reaches the maximum value C.sub.OX set by the oxide capacitance of the entire area of the MOS transistor.

According to the present invention, the varactor is oriented in sense amplifier 60 such that the capacitance increases as the input charge increases. For this reason, the output voltage of the sense amplifier 60 per unit photosensor charge is a decreasing function of the total charge sensed. For example, for the NPN phototransistor 12 of FIG. 2a, a positive charge is sensed at the input of sense amplifier 60, causing the amplifier output to go negative from its balanced value. As the source of N-channel varactor 62 is made more negative than the gate, a channel is formed and the gate-to-source capacitance increases.

Referring now to FIG. 5, a typical graph of the output voltage of the sense amplifier 60 of FIG. 4a is shown as a function of light intensity. The voltage (Y) axis is calibrated in volts and the light-intensity (X) axis is calibrated approximately in mW/meter.sup.2. The region past the knee of the curve in the graph of FIG. 5 represents the region where the capacitance of the varactor is increasing towards the value of C.sub.OX. Varying the size of the varactor device will translate the curve along the X-axis. Reducing the minimum capacitance C.sub.0 will translate the lower portion of the curve to the left. The horizontal displacement between the upper and lower portions of the curve of FIG. 5 is equal to the ratio of C.sub.OX to C.sub.0.

As the level of light intensity is increased further, the upper portion of the curve of FIG. 5 will steepen to a slope of 1 on the log- scale of FIG. 5. To prevent this return to non-compressive behavior, the operation of the imaging array of the present invention at high light levels may be improved by using the sense amplifier shown in FIG. 6a. Sense amplifier 70 includes all of the elements of sense amplifier 60. These elements have been assigned the same reference numerals as the corresponding elements of FIG. 4a.

As may be seen from FIG. 6a, in addition to the elements of the sense amplifier circuit 60 of FIG. 4a, sense amplifier 70 includes an exponential feedback element 72. In a first embodiment, exponential feedback element 72 of sense amplifier 70 includes first and second P-channel feedback transistors 74 and 76. First P-channel feedback transistor 74 has its source connected to sense line 18, and its gate and drain connected together to the source of second P-channel feedback transistor 76. The gate of second P-channel feedback transistor 76 is connected to the output of amplifier 52, and its drain is connected to a fixed voltage source, shown as ground in FIG. 6a.

The effect of first and second P-channel feedback transistors 74 and 76 is to provide a current which increases exponentially as the output voltage of amplifier 52 becomes more negative. This exponential function has a very small slope for output voltage excursions less than approximately 1 volt, thereby allowing sense amplifier 70 to function as a normal charge-sensing amplifier at low light levels. However, at higher light levels, by virtue of the current through the exponential element, sense amplifier 70 becomes a current-sensing amplifier, and its output voltage decreases as a logarithmic function of the input current from the pixel.

Those of ordinary skill in the art will recognize that the sense amplifier circuit of FIG. 6a automatically acts as both an integrating detector at low light levels, and as a current-mode detector at high light levels. The dynamic range of an imaging array may be greatly increased by use of the sense amplifier circuit of FIG. 6a.

Referring now to FIG. 6b, an alternate embodiment of a sense amplifier with an exponential feedback element to improve its dynamic range is shown. The sense amplifier of FIG. 6b includes all of the elements of sense amplifier 70. These elements have been assigned the same reference numerals as the corresponding elements of FIG. 3. Thus, the sense amplifier of FIG. 6b includes an amplifying element 52, having its non-inverting input connected to a source of reference voltage V.sub.ref and its inverting input connected to sense line 18. P-channel balance transistor 54 is connected between the inverting input and the output of amplifier 52. Its gate is connected to a balance line 56. Varactor 62 is connected between the output and inverting input of amplifying element 52.

N-channel MbS transistor 78 is connected as an exponential feedback element with its source connected to the output of amplifying element 52 and its drain connected to the inverting input of amplifying element 52. Its gate is connected to a source of fixed voltage. V.sub.Exp Ref. As more positive charge is sensed by amplifying element 52, its output becomes more negative. As the output decreases below V.sub.Exp Ref, the current through MOS transistor 78 begins to increase exponentially with decreasing output voltage. The amplifier then becomes a current-sensing amplifier with an output voltage logarithmic in input voltage. The magnitude of the fixed voltage source V.sub.Exp ref should be chosen such that the logarithmic compression occurs just as the varactor has achieved its maximum capacitance C.sub.OX.

Referring now to FIG. 7a, an integrating imaging array according to a presently preferred embodiment of the invention is illustrated. The array 80 of FIG. 7a is shown including twelve integrating photosensors according to the present invention arranged into three rows and four columns. Those of ordinary skill in the art will readily observe that the embodiment shown in FIG. 7a is illustrative and not limiting in nature, and that an array of any size is contemplated by the present invention, limited only by the inherent density and chip size limitations of CMOS technology.

Row 82 of array 80 Includes photosensors 10a-10d. Row 84 of array 80 includes photosensors 10e-10h. Row 86 of array 80 includes photosensors 10i-10 l. In FIG. 7a, photosensors 10a-10l are illustrated as the bipolar integrating photosensors of FIG. 2a. Those of ordinary skill in the art will recognize that other integrating photosensor elements may be used in place of the integrating photosensor element of FIG. 2a.

First row-select line 16-1 is connected to one plate of capacitors 14a-14d. Second row-select line 16-2 is connected to one plate of capacitors 14e-14h. Third row-select line 16-3 is connected to one plate of capacitors 14i-14l. First sense line 18-1 is connected to the emitters of phototransistors 12a, 12e, and 12i of the first column in the array 80. Second sense line 18-2 is connected to the emitters of phototransistors 12b, 12f, and 12j of the second column in the array 80. Third sense line 18-3 is connected to the emitters of phototransistors 12c, 12g, and 12k of the third column in the array 80. Fourth sense line 18-4 is connected to the emitters of phototransistors 12d, 12h, and 12l of the fourth column in the array 80.

First sense amplifier 60-1 is connected to first sense line 18-1. Second sense amplifier 60-2 is connected to second sense line 18-2. Third sense amplifier 60-3 is connected to third sense line 18-3. Fourth sense amplifier 60-4 is connected to fourth sense line 18-4. As will be apparent from the discussion accompanying FIG. 2a, a row of pixels from array 80 may be read by pulsing balance line 88, followed by bringing the desired one of row lines 16-1 through 16-4 to a high logic level. Sense amplifiers 60-1 through 60-4 may be any of the sense amplifiers disclosed herein, or may be any other appropriate sense amplifier which those of ordinary skill in the art will be readily able to select.

In a presently preferred embodiment, a vertical scanning circuit 90 is adapted to select only one of the row select lines 18 at any given time. Circuits for performing this function are well known in the art.

According to a presently-preferred embodiment of the invention, the outputs of all of the sense amplifiers are captured by sample/hold circuits 92-1 through 92-4 so that they may be scanned out while the sense amplifiers are sensing charge from the next row of pixels. As presently preferred, sample/hold circuits 92-1 through 92-4 may comprise pass gates 94-1 through 94-4 and capacitors 96-1 through 96-4. The N-Channel and P-Channel MOS transistors comprising the pass gates are conventionally driven by complementary signals SAMPLE and SAMPLEI on lines 98 and 100 as is well known in the art. The advantage of employing the sample/hold circuits is that the sense amplifiers have an entire line time to settle, and the residual charge on each row of pixels is much less than if the amplifier outputs were scanned directly.

As those of ordinary skill in the art will readily appreciate, the timing of the operation of the array of FIG. 7a is straightforward. Vertical scanner 90 selects a one of the rows of pixels. After the settling time for the sensing of charge in the selected row of pixels has passed, a sampling pulse is used to activate the pass gates 94-1 through 94-4 for a time long enough to charge capacitors 96-1 through 96-4 with the voltage outputs of the sense amplifiers 60-1 through 60-4. Horizontal scanner is then used to scan the output values out of the array. Vertical scanner 90 selects another row and the data acquisition and scanning process repeats until the pixel data for the entire array or selected portion thereof has been read out by horizontal scanner 102.

In a true serial scanned imager, horizontal scanning circuitry 102 is adapted to select the signal from the output of one of sample/hold circuits 92-1 through 92-4 at a given time, to form the output 104 of the array. Exemplary scanning circuits suitable for use as vertical and horizontal scanning circuits 90 and 102 are described in Analog VLSI and Neural Systems, by Carver A. Mead, Addison Wesley Publishing Co., 1989, at pp. 263-267. If parallel outputs are desired, all amplifier outputs can be used directly, or subsets can be sequentially selected by horizontal scanning circuit 102.

Referring now to FIG. 7b, a block diagram of a more generalized version of an integrating imaging array according to the present invention is depicted. Integrating photosensors 106a-106l are shown as rectangular boxes. Those of ordinary skill in the art will readily understand that integrating photosensors 106a-106l may comprise phototransistors, photodiodes and other integrating photosensor elements. The array structure of the array of FIG. 7b is otherwise much the same as that of the array of FIG. 7a.

Switching elements 17a-17l act to connect the elements of the single row selected by row scanner 82 to the column lines 20-1 through 20-4. Numerous circuit elements, such as single MOS transistors, junction FETs, and bipolar transistors, etc., may be used as switching elements 16a-16l. In the preferred embodiment of the pixel shown in FIG. 2a, the bipolar phototransistor itself is also used as the switching element as was illustrated in FIG. 7a.

In addition, sense amplifiers 108-1 through 108-4 are also shown generically including amplifying elements 110. Persons of ordinary skill in the art will appreciate that sense amplifiers 108-1 to 108-4 (shown within dashed lines) may be any of the sense amplifiers disclosed in FIGS. 3, 4a, 6a, and 6b herein, or may be other appropriate sense amplifiers which those of ordinary skill in the art will be able to select. Although sense amplifiers 108-1 through 108-4 are shown including MOS balance transistors 54-1 through 54-4 and varactor elements 62-1 through 62-4, those of ordinary skill in the art will recognize that the compressive feedback functions may be omitted or can be achieved by other means.

The remaining elements of the integrating imaging array of FIG. 7b are the same as the corresponding elements of the integrating imaging array of FIG. 7a. The integrating imaging array of FIG. 7b functions in the manner described for the array of FIG. 7a. Additionally shown in FIG. 7b is timing control unit 106. The function of timing control unit 106 is to provide the control signals to vertical scanner 90, SAMPLE line 98 and SAMPLEI line 100, and horizontal scanner 102.

FIG. 7c is a timing diagram showing a typical timing sequence for the signals to row lines, vertical scanner 90, SAMPLE line 98 and SAMPLEI line 100, and horizontal scanner 102 during three illustrative row scan cycles necessary for the operation of a typical array like the ones of FIGS. 7a and 7b. Circuitry for providing the signals shown in FIG. 7c is conventional and design of such circuitry is well within the level of ordinary skill in the art. From FIG. 7c, it may be seen that the signals driving the row lines may come from a serial shift register or a typical one of N decoder circuit. The SAMPLE and SAMPLEI signals are easily derived from the row line drive signals, by, for example, a monostable multivibrator circuit. Note that the horizontal scanner control signal (H. Scan) is shown as a scan period during which either serial scanning, such as by analog multiplexer, or parallel scanning, such as by enabling pass gates, may take place during the period when the H. Scan signal is high. Selection of serial or parallel scanning is a matter of design choice.

Referring now to FIGS. 8a and 8b, cross-sectional views of an N-well CMOS embodiment of the integrating photosensor array of FIG. 7a using the integrating photosensor of FIGS. 2a and 2b are shown. FIG. 8a is oriented along an axis parallel to the rows of the array. The cross-section of FIG. 8a bisects a row-select line of the array. The axis of FIG. 8b is orthogonal to that of FIG. 8a, and thus runs parallel to a column of the array. The cross-section of FIG. 8b bisects a column sense line of the array. The portion of the array shown in FIGS. 8a and 8b includes three rows and three columns of integrating photosensors.

Those of ordinary skill in the art will readily understand that this embodiment is merely illustrative and that practical arrays fabricated according to the principles of the present invention are likely to be substantially larger. The practical limit on array size is largely a function of CMOS processing limitations. The reference numerals used in FIGS. 8a and 8b will be the same as the reference numerals used for the same elements shown in FIG. 2b and 7a.

In FIG. 8a, array 80 is shown to include an exemplary row 82 of integrating photosensors including photosensors 10a, 10b, and 10c, fabricated in n-well 20, formed in p-type substrate 22 using conventional processing techniques. Although FIG. 7a illustrates a row including four integrating photosensors, only three photosensors 10a, 10b, and 10c are shown for simplicity in row 82 of FIG. 8a. Integrating photosensors 10a, 10b, and 10c may be separated by conventionally formed field oxide regions 26, as is well known in the art. Those of ordinary skill in the art will Immediately recognize that region 20 could itself be an n-type semiconductor substrate instead of being an n-well in a p-type semiconductor substrate. This Is may be seen in FIGS. 8a and 8b if region 22 is ignored.

P-type base regions 24a, 24b, and 24c, formed in regions of n-well 20 between birds beaks or in apertures etched in a uniformly grown field oxide 26, comprise the base regions of the NPN bipolar transistors comprising integrating photosensors 10a, 10b, and 10c, respectively. The minimum inter-pixel distance is set by the inter-base spacing. The base doping Is chosen such that the base surface concentration is larger than the well-surface concentration but light enough so that the base-emitter reverse leakage characteristic is not degraded by incipient breakdown at the reverse bias voltage used for integrating the photo-generated charge.

N-region 20 forms the common collectors for bipolar transistor integrating photosensors 10a, 10b, and 10c, and N+ emitter regions 30a, 30b, and 30c, form the emitters of bipolar transistor integrating photosensors 10a, 10b, and 10c, respectively. Conductive row-select line 16-1 comprises a line of conductive material, such as polysilicon, metal silicide or any other conductive layer which may be employed as a gate in a CMOS process. Row-select line 16-1 line runs the length of the row 82 over field oxide regions 26 and gate oxide regions 28a, 28b, and 28c. Deposited oxide layer 32 covers row-select line 16-1. Column sense lines 18-1, 18-2, and 18-3 are shown in end view disposed over deposited oxide layer 32 and contacting emitters 30a, 30b, and 30c of integrating photosensors 10a, 10b and 10c, respectively.

FIG. 8b is a cross-sectional view taken along the first column of array 80 in a location which bisects metal column sense line 18-1 of FIG. 7a. Column sense line 18-1 is shown running the length of the column and contacting emitters 30a, 30e, and 30i of integrating photosensors 10a, 10e, and 10i.

While, in FIG. 8a, row-select line 16-1 appears to be discontinuous, those of ordinary skill in the art will recognize that this is because the axis of the cross section bisects apertures in row line 16-1 used to define the emitters 30a, 30b, and 30c of photosensors 10a, 10b, and 10c. Metal contact vias are used to connect emitters 30a, 30b, and 30c to their respective column sense lines. Examination of FIGS. 8a and 8b together reveals that the contact vias pass through continuous conductive row-select lines which are insulated from the metal column sense lines 18-1, 18-2, and 18-3 by deposited oxide 32 and from base regions 24a, 24b, and 24c by thin gate oxide regions 28a, 28b, 28c.

Examination of both FIGS. 8a and 8b also reveals that the emitter regions are substantially self aligned to the edges of the polysilicon layers forming row-select lines 16-1, 16-2, and 16-3. This is a result of the fact that the emitter regions are created during the self-aligned gate source/drain implant step simultaneously with the sources and drains of the MOS transistors on the integrated circuit. Those of ordinary skill in the art will realize that this provides the advantage of minimizing the emitter-to-row-select-line capacitance in the integrating photosensors.

While a presently-preferred embodiment of the invention has been disclosed, those of ordinary skill in the art will, from an examination of the within disclosure and drawings be able to configure other embodiments of the invention. For example, those of ordinary skill in the art will readily observe that the array of the present invention may be rearranged by a simple rotation to interchange the roles of its rows and its columns, or two such arrays can be interspersed on offset centers to form a hexagonal, rather than a square, array. The invention may be easily fabricated in a P-well CMOS process, and that, in such an embodiment, the voltage levels and power supplies would be reversed. These other embodiments are intended to fall within the scope of the present invention which is to be limited only by the scope of the appended claims.

Claims

1. A bipolar phototransistor pixel element disposed on a piece of semiconductor material of a first conductivity type comprising:

a collector comprising a region of said semiconductor material;a base comprising a doped region of a second conductivity type disposed within said collector region;

an emitter comprising a doped region of said first conductivity type disposed within said base region;

a first oxide layer disposed over said base;

a first electrically conductive material disposed over said first oxide layer to form a select node, wherein said base region, said first oxide layer and said first electrically conductive material form a capacitive element;

a second oxide layer disposed in an outer portion of an aperture above said emitter formed by said first oxide layer and said first electrically conductive material; and

and a second electrically conductive material disposed in an inner portion of said aperture to form a sense node.

2. The bipolar phototransistor pixel element of claim 1 wherein said piece of semiconductor material of said first conductivity type comprises a well region in a semiconductor substrate of said second conductivity type.

3. A bipolar phototransistor pixel element disposed on a piece of n-type semiconductor material comprising:

a collector comprising a region of said semiconductor material;

a base comprising a p-type doped region disposed within said collector region;

an emitter comprising an n-type doped region disposed within said base region;

a first oxide layer disposed over said base;

a first electrically conductive material disposed over said first oxide layer to form a select node, wherein said base region, said first oxide layer and said first electrically conductive material form a capacitive element;

a second oxide layer disposed in an outer portion of an aperture above said emitter formed by said first oxide layer and said electrically conductive material; and

and a second electrically conductive material disposed in an inner portion of said aperture to form a sense node.

4. The bipolar phototransistor pixel element of claim 3 wherein said n-type piece of semiconductor material comprises a well region in a p-type semiconductor substrate.