Abstract:
An ultra-low noise, high-gain interface pixel amplifier is provided with capability for single-photon readout of known photodetectors at high electrical bandwidths for diverse spectral bandpass from the x-ray to long IR bands. The detector charge modulates a source follower whose output is double sampled to remove correlated noise by a compact stage that also facilitates low-noise gain adjustment for a second gain stage of programmable amplification. Single-photon readout of photodetectors at high electrical bandwidths in small pixel areas is thereby facilitated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to photon detection, and more particularly to detector amplifiers for single photon read-out of semiconductor photodetectors in pixellated imaging arrays. 
     2. Description of the Related Art 
     Optical sensors transform incident radiant signals in the X-ray (λ&lt;0.001 μm), ultraviolet (λ=0.001-0.4 μm), visible (λ=0.4-0.8 μm), near infrared (IR) (λ=0.8-2 μm), shortwave IR (λ=2.0-2.5 μm), mid IR (λ=2.5-5 μm), and long IR (λ=5-20 μm) bands into electrical signals that are used for data collection, processing, storage and display, such as real-time video. Available conventional photodetectors such as photodiodes and photoconductors are inexpensive, exhibit bandwidths that support current video frame rates, are sensitive to wavelengths well into the long IR band, and exhibit a high degree of uniformity from pixel to pixel when used in an imaging array. However, these photodetectors have no gain, i.e. each incident photon generates, at most, a single electron. Thus, these imaging systems work well only in moderate to bright light conditions. At low light levels, they provide electrical signals that are too small to be read-out by conventional readout circuits. 
     In conditions of low ambient light, the standard photodetector is often replaced with an avalanche photodiode that provides significant gain such that conventional read-out circuits, such as charge coupled devices, i.e. CCDs, can read out the amplified signal at video frame rates with a high signal-to-noise ratio (SNR). The fabrication of avalanche photodiodes is much more difficult and expensive than standard photodetectors because they must simultaneously exhibit very high controlled gain and very low noise. Furthermore, currently available avalanche photodiodes exhibit relatively poor uniformity, are constrained to shorter wavelengths than standard photodetectors (0.7 μm), and have limited sensitivity due to their relatively low quantum efficiency. Imaging intensified systems use an array of avalanche photodiodes or micro-channel plates to drive respective display elements such as CCDs or phosphors, and have even lower wavelength capabilities (approximately 0.6 μm max) due to the limitations of the photodiode. 
     Chamberlain et al. “A Novel Wide Dynamic Range Silicon photodetector and Linear Imaging Array” IEEE Transactions on Electron Devices, Vol. ED-31, No. 2, February 1984, pp. 175-182, herein incorporated by reference, describes a gate modulation technique for single photon read-out of standard photodetectors with wide dynamic range. Chamberlain provides a high-gain current mirror that includes a load FET (Field Effect Transistor) whose gate is connected to its drain to ensure sub-threshold operation. The signal from the photodetector is injected into the load FET thereby producing a signal voltage at the gate of a gain FET with high transconductance. This signal modulates the gain FET&#39;s gate voltage, which is read out and reset via a FET switch. The key benefit of this approach is that a detecting dynamic range of more than 10 7  for each detector in the array is produced. Unfortunately, the circuit is highly sensitive to variations in the threshold voltage of the various transistors. The pixel-to-pixel V T  non-uniformity associated with standard silicon CMOS fabrication processes degrades the instantaneous dynamic range of the imaging array even as the circuit&#39;s logarithmic characteristic enhances each pixel&#39;s ability to operate over a much larger total dynamic range. 
     Although this specific gain modulation technique is useful for detecting signals across a broad spectral range, the front-end bandwidth severely restricts the imaging array&#39;s bandwidth. Specifically, the dominant RC time constant is the parallel combination of the photodetector&#39;s capacitance and the resistance of the load FET. In sub-threshold operation, the FETs transconductance is very low and, hence, its load resistance is very large, at ≧10 15  ohms; the minimum resulting RC time constant is on the order of tens of seconds. Chamberlain&#39;s gate modulation technique is thus only practically useful for imaging daylight scenes or static low-light-level scenes such as stars. Furthermore, to achieve large current gain, the load FET is typically quite small. As a result, the load FET exhibits substantial 1/f noise, which under low light conditions seriously degrades the performance of the imaging array. 
     U.S. Pat. No. 5,933,190 discloses a circuit having a first reading transistor 23 in series with the load transistor of Chamberlain to read-out the voltage across the load transistor rather than the other leg of the current mirror. While this configuration self-biases the detectors in the imaging array, and the usable dynamic range for each pixel is still at least 10 7 , the time constant is unchanged relative to Chamberlain&#39;s teaching. Further, the instantaneous dynamic range at a specific irradiance across an imaging array having pixels of such design is still highly sensitive to the threshold uniformity from transistor to transistor. The pixel-to-pixel V T  non-uniformity associated with standard silicon CMOS fabrication processes degrades the instantaneous dynamic range of the imaging array even as the circuit&#39;s logarithmic characteristic enhances each pixel&#39;s ability to operate over a much larger total dynamic range. Though the &#39;190 reference also teaches a method for reducing the non-uniformity by degrading the various transistors by applying a stressing over-voltage, this is definitely not a recommended procedure for a high-quality, long-life camera system. 
     U.S. Pat. No. 5,929,434 teaches an alternative current mirror configuration that suppresses the impact of the V T  non-uniformity via an alternative current mirror configuration that also reads the integrated current after an integration period rather than the instantaneous voltage. The preferred embodiment minimizes, to first order, the variations in threshold non-uniformity by subtracting the non-uniformity within each pixel. Unfortunately, the residual pixel-to-pixel variations still dominate the imager&#39;s fixed pattern noise irrespective of background flux, depending on the MOS fabrication technology. The magnitude of pattern noise can often be larger than the signal, so off-chip compensation of pixel-to-pixel non-uniformity is required. 
     The negative feedback amplifier, 16, disclosed in U.S. Pat. No. 5,929,434, significantly reduces the input impedance of the high-gain circuit and thereby enhances its bandwidth. In the case where the buffer amplifier is approximated to have infinite voltage gain and finite transconductance, the dominant pole is given by:          τ     B   -   L       =       C   f       g     m   Q1                                
     where C f  is the effective feedback capacitance of the buffer amplifier from its output to its input. Assuming a cascoded amplifier configuration, the gate-source capacitance of Q1 (FET 74) is dominant and C f  is set by the gate-to-source capacitance of the sub-threshold Q1 (FET 74). This is approximately given by the parasitic metal overlap capacitance. Assuming a minimum width transistor in 0.25 μm CMOS technology, for example, the minimum C f  will be approximately 0.2 fF for transistors having minimum width. The resulting time constant is on the order of tenths of a second. Though this facilitates single photon sensing at roughly video frame rates, additional improvements are needed to truly support single-photon imaging at frame rates higher than typically used for standard video. 
     U.S. Pat. No. 5,665,959 teaches yet another approach consisting of a digitized system wherein each pixel uses a pair of cascaded inverters with a sub-threshold transistor at its front-end to generate extremely high transimpedance. Since the small photosignal at backgrounds on the order of one electron translates to extremely high input impedance, the photosignal is effectively integrated onto the Miller capacitance of a first-stage inverter prior to being further amplified by a second stage inverter. A resulting charge-to-voltage conversion gain &gt;1 mV/e− is hence claimed. Nevertheless, the read noise of the charge-integrating first stage will limit the SNR for many practical cases since insufficient means are provided to band-limit the first amplifier&#39;s wideband noise. The read noise for the first stage can be approximated as similar to that of a charge integrator such that:          N     stage_      1       =       1   q              kTC   fb     ·         C   det     +     C   fb           C   L     +         C   fb     ·     C   det           C   fb     +     C   det                                          
     where k is Boltzmann&#39;s constant, T is the temperature, C fb  is the parasitic feedback capacitance of the first stage, C det  is the photodiode capacitance and C L  is the load capacitance at the amplifier&#39;s output. Assuming practical values consistent with the understanding of those skilled in the art, the detector capacitance is typically a minimum of 15 fF for the hybrid imager of the U.S. Pat. No. 5,665,959 preferred embodiment. Assuming a Miller capacitance for the first stage amplifier of 5 fF and a load capacitance of 350 fF (i.e., the storage capacitance C str1 ), then the minimum read noise for the first stage will be in the range of 6 to 7 e−; this is on top of the kT/C noise generated by opening transistor switch Q sw1  to perform the offset compensation of the composite two-stage amplifier. This performance is very good, but does not facilitate photon counting. Further, while the clocking of the two-stage amplifier facilitates large reductions in amplifier non-uniformity, this invention does not suppress the threshold variations of the load resistor at the front end. 
     U.S. Pat. No. 6,069,376 teaches a pixel amplifier with a speed switch suitable for still camera applications. This apparatus provides high-bandwidth signal integration with downstream gain, but its sensitivity is limited by the generation of reset noise at the storage element. Also, no facility is provided for maximizing the signal&#39;s dynamic range at the input to the amplifier. 
     SUMMARY OF THE INVENTION 
     In general, the present invention is a photodetector readout circuit, with extremely high sensitivity, capable of single-photon detection. A photodetector (preferably a photodiode) integrates a small-signal photocharge on the detector capacitance in response to incident photons, producing a photodetector output signal. A buffer amplifier is arranged to receive the photodetector output signal and to produce a buffered photodetector output signal. A coupling capacitor has a first terminal connected to the buffered output signal and a second terminal connected to a signal input of a signal amplifier. The coupling capacitor shifts a signal level at the input to the signal amplifier by an adjustable offset voltage. An electronic offset reset switch, connected to the coupling capacitor, allows resetting of the offset voltage, preferably just after reset of the photodiode to allow transient decay. The offset voltage is the reset noise (kTC) generated by resetting the detector capacitance. 
     To synchronize the start of image formation across a pixellated array, the reset is simultaneous across the entire array. Each pixel&#39;s offset voltage is clamped across each pixel&#39;s coupling capacitor by reading the specific detector&#39;s voltage, while simultaneously clamping the coupling capacitor to a specified voltage. When sampling of the photodiode signal begins, the actual signal is read relative to the offset voltage stored across the coupling capacitor. This effects correlated double sampling of the photogenerated signal, and eliminates the correlated noise generated by resetting (discharging) the photodetector capacitance. The clamping voltage is an adjustable voltage that also sets the quiescent operating point of the video signal amplifier above the threshold voltage of an integrating gain stage having common gate configuration and noise bandwidth set by a reset integrator. 
     The common gate amplifier provides large, adjustable current gain to further amplify the low-noise signal and integrate the boosted signal in a dedicated integration capacitor. At the end of a specified integration time, the integrated signal is sampled onto a second capacitor to synchronize the end of signal integration. Snapshot image capture is thus provided with very low noise referred back to the photodetector. The invention thereby improves transimpedance and dynamic range relative to prior solutions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 is a schematic diagram of a generalized pixel amplifier in accordance with the present invention; and 
     FIG. 2 is a schematic diagram of an alternative embodiment of the present invention in which the pixel amplifier enhances the instantaneous dynamic range and minimizes amplifier nonuniformity in exchange for lower transimpedance. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the basic principles of the present invention have been defined herein specifically to detector amplifier circuits for single photon read-out of semiconductor photodetectors in pixellated imaging arrays. Any and all such modifications, equivalents and alternatives are intended to fall within the spirit and scope of the present invention. 
     The present invention provides a high bandwidth, ultra low-noise pixel amplifier that is capable of single photon read-out of photodetectors in extremely low-light conditions, i.e. photon flux levels approaching zero photons per sampling period. This circuit can be used to effectively count incident photons on individual photodetectors, either in an imaging array as the front-end to a conventional video system or in high frame-rate wavefront sensors. One of the primary benefits of the approach is that the circuit can use off-the-shelf photodetectors such as photodiodes or photoconductors that have gain ≦1 rather than, for example, avalanche multiplication within the photodiode. Such known photodetectors with gain ≦1 are cheaper, more uniform, easier to fabricate, more reliable, less susceptible to excess noise mechanisms within the detector, and support a much broader range of the electromagnetic spectrum than avalanche photodiodes. 
     The generalized circuit in accordance with the present invention is shown in FIG.  1 . Before photodetection begins, enabling switch transistors M 1  and M 2  initializes the circuit to synchronize the subsequent start of signal integration across the pixellated array. In this initial state, any photo-generated charge on C pd  (which represents the capacitance of photodiode PD 1 ) is discharged (reset) and the detector voltage is set to V det −V det     —     rst . Further, the signal integration capacitor, C int , is set to V cell     —     rst . Transistors M 1  and M 2  are subsequently disabled to allow the associated voltage transients to decay. Switch transistor M 3  is then enabled to clamp C CDS  to V Gain . This process stores each pixel&#39;s reset noise across C CDS  and sets the starting quiescent voltage for the front-end amplifier. The front-end amplifier comprises a source follower buffer amplifier formed by transistors M 4  and M 5 , the correlated double sampling capacitor C CDS , and a switch transistor M 3 . By subsequently disabling switch transistor M 3 , the video signal established by the difference between the buffered photodetector signal and the clamped offset voltage subsequently modulates the gate of gain transistor M 6 . Transistor M 6  is a common gate amplifier that supplies an adjustable current to integration transistor C int . This current is adjusted by setting the difference between V Gain −V s  and the threshold voltage of M 6  since the source of M 6  (V s ) is preferably operated at, or near, 0V to minimize pickup of bias-induced noise from the source terminal of M 6 . V s  is thus preferably set at ground for basic operation. 
     The current integrated in C int  is therefore a gain-proportioned facsimile of the photo-generated signal originally applied to source follower amplifier transistor M 4 . At the end of the prescribed integration time, switch transistor M 7  is briefly enabled to store the signal voltage on the sample-and-hold capacitor C S/H . This signal voltage modulates the gate of a second source follower amplifier transistor M 8 . The final signal voltage is read on a row-by-row basis to produce the video signal by enabling the switch transistor M 9  via the Φ pixel  clock. The current sink for transistor M 8  is typically common to all the pixels in each column and shared in this manner to minimize the power dissipation and the demand on support circuitry. 
     The output of the source follower transistor M 4  is capacitively coupled by a series capacitor C CDS  initially, under control of a reset signal Φ CDS  applied to the gate of M 3  at the start of integration. The clamping and sampling facilitated in this manner effects correlated double sampling of the photogenerated signal. This signal is essentially free of circuit-induced noise except for the 1/f noise of transistor M 3 . The correlated noise generated by resetting the detector capacitance is thereby eliminated. By minimizing the capacitances of PD 1  and the gate of transistor M 4 , the basic transimpedance can be maximized to first order to minimize the required size of the capacitor C CDS . To facilitate sub-electron read noise, the value of C CDS  must, at a minimum, be at least several femtofarads for operation at room temperature (295 K). 
     The clamping circuit comprising the capacitor C CDS  and switch transistor M 3 , also effects a compact method for arbitrarily setting the minimum signal level at a quiescent operating point for compatibility with exercising either a portion of the available dynamic range, or the full dynamic range of the common gate amplifier. The clamping circuit thus provides both correlated double sampling and dynamic range management when the source terminal of transistor M 6  is either connected to an externally accessible pad to enable external adjustment or biased by an adjustable on-chip reference voltage. 
     Since the transimpedance established by the combined capacitance of the detector and amplifier transistor M 4  does not facilitate reading noise levels &lt;10 e− at typical video rates, the present invention uses the level-shifting stage in conjunction with the common gate amplifier to effect large overall transimpedance. For example, the combined total capacitance of the photodetector and the gate of MOSFET M 4  will practically be, at a minimum, ≧5 fF. The maximum photoconversion gain defined at the input to the compact amplifier is thus 32 μV/e−. Because the minimum read noise referred to the output needs to be from 250 μV to &gt;1 mV in practical video cameras, the ability to detect quanta requires additional gain of from 10 to 30, at a minimum. Optimally adjusting the gate-to-source voltage by appropriately adjusting V GAIN  provides this additional gain. The output of the low-noise pixel amplifier is read from the pixel by enabling Φ pixel  to supply the signal to the bus via the switch transistor M 9 . 
     FIG. 2 is a schematic circuit diagram of an alternative embodiment wherein the instantaneous dynamic range is increased from a maximum of about 10 bits to larger values depending on effective resistance of a switched-capacitor resistor placed at the source of common gate transistor M 10 . The programmable switched-capacitor resistor comprises transistor M 10  and capacitor C dr  and reduces the gain of common gate amplifier M 6  by adding a series resistance to the supply V s . This source resistance increases the dynamic range and reduces the impact of the threshold voltage nonuniformity of transistor M 10  in the pixellated array in exchange for lower overall transimpedance and adjustability. Alternatively, a high value fixed-resistor (0.5 to 50 MΩ) can replace transistor M 10  and capacitor C dr  if a specific dynamic range or amplifier transimpedance is needed. The required value for the effective series resistance is that it acts as a current source. In order to do this, it must have a higher impedance than transistor M 10 . Looking into its source terminal, the resistance of transistor M 10  is given by:          R   M10     =     1       g   m     +     g   d                                
     where g m  is the FET transconductance and g d  is the drain conductance. In weak inversion or subthreshold operation, the resistance is thus g m   −1 ; the resistance is g d   −1  in the linear region. 
     The ultra-low noise amplifiers of the present invention thus provide a total transimpedance that is approximately:          Z     T   ,   Amp       =         t   int         C   det     +     C   input         ·       q                 Δ                   V   sig       nkT     ·       C   int         C   det     +     C   input         ·     A   atten                              
     where t int  is the integration time, C int  is the integration capacitance, C det  is the detector capacitance, C input  is the combined capacitance of the source follower transistor and any other capacitances at this node, both stray and intentional, q is the electron charge, ΔV sig  is the integrated signal voltage programmed by tuning V gain −V s , n is the subthreshold ideality of transistor M 6 , k is Boltzmann&#39;s constant, T is the temperature, and A atten  is the attenuation facilitated by the series resistor. For the preferred embodiment, A atten =1. The compact amplifier&#39;s gain is thus adjustable to compensate for transimpedance degradations resulting from either short integration time or large detector capacitance. The amplifier&#39;s gain can also be very large since ΔV sig  can be much larger than the thermal voltage of transistor M 10  and capacitor C int  is often 10X to 100X larger than the sum (C det +C input ). Nevertheless, for those sensors having large detector capacitance the former can still effect gains exceeding 100X. 
     The attenuation factor in the preceding equation is defined:          A   atten     =       R   M10         R   M10     +     R   SCR                                
     where R SCR  is the effective resistance of the switched-capacitor resistor of the alternative embodiment. The preferred embodiment thus has no attenuation, i.e., A atten =1 and the alternative embodiment can have attenuations from 1 to greater than 100. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.