GB1592373A - Photodetector - Google Patents

Description

(54) PHOTODETECTOR (71) We, INTERNATIONAL BUSINESS MACHINES CORPORATION, a Corporation organized and existing under the laws of the State of New York in the United States of America, of Armonk, New York 10504, United States of America do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to semiconductor photodetectors.

According to the present invention there is provided a photodetector comprising a semiconductor substrate of one conductivity type; a first region of semiconductor material of opposite conductivity type disposed in the substrate surface to form a visible light sensitive photoelement region; a second region of semiconductor material of opposite conductivity type disposed in the substrate surface and spaced from the first region; a first insulated gate electrode disposed over the substrate surface between the first and second region and over the second region; a third region of semiconductor material of opposite conductivity type disposed in the substrate surface and spaced from the second region on the opposite side thereof to the first region, the third region forming a portion of a charge transfer shift register in the substrate; and a second insulated gate electrode disposed over the substrate surface between the second and third regions and over the third region; the first gate electrode, the second region, and the region of the substrate between the first and second regions acting in response to a first bias signal applied to the first gate electrode during an integration time period to activate the photo-element region to produce photocarriers in response to visible light directed on the photoelement region and to transfer substantially all of the photocarriers so produced during the integration time period to the second region for storage therein for maintaining a given width of the depletion layer of the photoelement region substantially constant during said integration time and thereby providing continuously a self-resetting voltage level on the photoelement region of the substrate between the second and third region acting in response to a second bias signal applied to the second gate electrode at the end of the integration time period for transferring the stored photocarriers out of the second region and into the third region of the shift register.

The invention will be described, by way of example, with reference to Figures 3 to 8 of the accompanying drawings after a brief description of examples of the prior art with reference to Figures 1 and 2 of the accompanying drawings.

In the drawings, FIGURE 1 shows a prior art photoelectric device; FIGURE 2 shows a prior art photodiode device; FIGURES 3 and 4 show in plan and in cross-section respectively an embodiment of a photodetector according to the present invention; FIGURE 5 shows the equivalent circuit of the photodetector of FIGURES 3 and 4; FIGURES 6 and 7 show diagrammatically the surface potentials of the photodetector of Figures 3 and 4 under different operating conditions; FIGURE 8 shows waveforms applied in operation to the photodetector of Figures 3 and 4.

Fig. 1 shows a semiconductor substrate 10 composed, for example, of n-type silicon, an insulating layer 12 composed, for example, of silicon dioxide, and a conductive gate plate 14 composed, for example, of polysilicon which is connected to a source of operating voltage.

When an appropriate polarity operating voltage Vg is applied to plate 14 a depletion region 16 is formed and upon the incidence of radiant energy into the depletion region minority charge carriers are generated which collect and are stored at the surface of the depletion region 16.

Fig. 2 shows a photodiode including a semiconductor substrate 18, which may be composed of p-type silicon, an insulating layer 20 which may be composed of silicon dioxide, and an type conductivity region 22. An operating voltage Vd is set on the n+ region 22 and a depletion region 24 is created associated with the n+ conductivity region 22. As in the structure of Fig. 1, radiation directed onto the structure of Fig. 2 produces photogenerated minority charge carriers which collect and are stored at the conductivity region 22.

Ideally, the collected photogenerated charge should be directly proportional (linearly related) to the integral of the radiation (light intensity) at any given wavelength over the scanning range. However, it has been found that in arrangements such as are illustrated in Figs. 1 and 2, the collected photogenerated charge is approximately linear at low light levels, but becomes non-linear as the level of illumination increases. These non-linearities occur well before saturation is reached.

The aforesaid non-linearities are believed to occur due to the reduction in the width of the depletion layer (Win Fig. 1) as a result of the collection of photogenerated charge during the integration time.

Turning now to Fig. 3, there is shown a diode photodetector array and a read-out Bucket-Brigade Device (BBD) shift register. The structure of Fig. 3 includes a substrate 18 and insulator layer 20 with n+ diffusion regions and BBD shift registers to be described. A cross section of this structure is shown in Fig. 4. An equivalent circuit is shown in Fig. 5.

In Fig. 3, the photodetector array has n +I diffusions 26 in region A-B.

A barrier region B-C is proximate to the photodetector region A-B and a storage region C-D including N + diffusions 28 is located on the other side of the barrier gate region B-C. A transfer region D-E is provided to transfer charge from the storage region C-D to ri+3 diffusions of an output shift register F-G formed in the substrate 18. A first insulated gate extended substantially from B to D and a second insulated gate extends substantially from D to F.

The operation of the structure of FIG. 3 is as follows. At the beginning of the integration time period (see FIG. 8) the voltage level on the N+ diffusions 26 (i.e. the photodiodes) is set by the barrier region B-C to the surface potential of the barrier region which is designated (TI - VT) where Q)TI is the voltage on the first insulated gate and VT is the threshold voltage.

This internal potential setting of the photodiode diffusions 26 is achieved by draining out charge carriers from the n+ region into the storage region C-D and in turn into the shift register F-G via transfer gate D-E. The storage regions (i.e. the n+ diffusion 28 of the region C-D) are set, during the integration period, to (TI + P)T2 - VT) by the transfer region D-E.

As the diffusions'of the photodiode regions 26 lose charge carriers (electrons), their potential moves toward a more positive level with respect to the substrate 18. When the potential of the photodiodes 26 moves to the potential set thereon (TI - VT), the draining of the carriers (electrons) ceases. An important feature' is that the potential setting of the photodiodes 26 is determined and governed by the barrier region B-C.

The surface potential during light integration of the region defined from A to G through a cross section taken through line 4 in FIG. 3 are shown in the cross section illustration of FIG.

6. Under visible light illumination conditions the photodiodes 26 collect photogenerated electrons which are continuously drained out of the n+l region 26 as shown in FIG. 6 and stored in the storage region C-D. Thus, FIG. 6 illustrates that the electrons generated in the n+l region 26 and its vicinity are collected by the potential well created in the regions A-B and that these electrons are continuously drained from the n+1 region, collected and stored in the potential well C-D of the n+2 regions 28. It should be noted that even during the time when the photo diodes 26 are collecting the photogenerated electrons, the potential of the photodiodes 26 is continuously being self-set to (TI - VT). Thus, even during the integration collection period, the potential of the photodiodes 26 is self-clamped to TI - VT).

As a result of this the depletion layer width of the n+ photodetector regions remains constant during the light integration period. Constant depletion layer width during integration results in a linear photocharge output. Thus the photogenerated collected charge is a linear function of the light-intensity-(watts/cm2)-times-the-integration-time (seconds).

At the end of the integration period the photogenerated charge is transferred from the storage region C-D into the read-out shift register in the region F-G by the application of shift pulse T2 to the second insulated gate in a conventional manner. The surface potential for the crossection defined from A to G through line 4 during charge transfer, is shown in Fig. 7.

Although the photodetector n+l may collect and store charge during the charge-transfer period, the linearity is not affected, since the transfer time is very short relative to the integration period as illustrated in Fig. 8. Typical practical values of integration time to transfer time ratio is 1000 to 1.

Waveforms and their time interrelationship during integration and transfer are shown in Fig. 8.

To summarise, Figs. 3 through 8 illustrated and described the operation of a novel photodetector structure with a diode array and a read-out BBD circuit wherein charge carriers are drained (i.e. transferred) and stored in order to maintain the depletion layer width of the photodetector regions constant in order to provide linear photocharge output.

WHAT WE CLAIM IS: 1. A photodetector comprising a semiconductor substrate of one conductivity type; a first region of semiconductor material of opposite conductivity type disposed in the substrate surface to form a visible light sensitive photoelement region; a second region of semiconductor material of opposite conductivity type disposed in the substrate surface and spaced from the first region; a first insulated gate electrode disposed over the substrate surface between the first and second region and over the second region; a third region of semiconductor material of opposite conductivity type disposed in the substrate surface and spaced from the second region on the opposite side thereof to the first region, the third region forming a portion of a charge transfer shift register in the substrate; and a second insulated gate electrode disposed over the substrate surface between the second and third regions and over the third region; the first gate electrode, the second region, and the region of the substrate between the first and second regions acting in response to a first bias signal applied to the first gate electrode during an integration time period to activate the photoelement region to produce photocarriers in response to visible light directed on the photoelement region and to transfer substantially all of the photocarriers so produced during the integration time period to the second region for storage therein for maintaining a given width of the depletion layer of the photoelement region substantially constant during said integration time and thereby providing continuously a self-resetting voltage level on the photoelement region during the integration time period; and the second gate electrode, the third region, and the region of the substrate between the second and third region acting in response to a second bias signal applied to the second gate electrode at the end of the integration time period for transferring the stored photocarriers out of the second region and into the third region of the shift register.

2. A photodetector substantially as hereinbefore described with reference to and as shown in Figures 3 to 8 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. Fig. 8. To summarise, Figs. 3 through 8 illustrated and described the operation of a novel photodetector structure with a diode array and a read-out BBD circuit wherein charge carriers are drained (i.e. transferred) and stored in order to maintain the depletion layer width of the photodetector regions constant in order to provide linear photocharge output. WHAT WE CLAIM IS:

1. A photodetector comprising a semiconductor substrate of one conductivity type; a first region of semiconductor material of opposite conductivity type disposed in the substrate surface to form a visible light sensitive photoelement region; a second region of semiconductor material of opposite conductivity type disposed in the substrate surface and spaced from the first region; a first insulated gate electrode disposed over the substrate surface between the first and second region and over the second region; a third region of semiconductor material of opposite conductivity type disposed in the substrate surface and spaced from the second region on the opposite side thereof to the first region, the third region forming a portion of a charge transfer shift register in the substrate; and a second insulated gate electrode disposed over the substrate surface between the second and third regions and over the third region; the first gate electrode, the second region, and the region of the substrate between the first and second regions acting in response to a first bias signal applied to the first gate electrode during an integration time period to activate the photoelement region to produce photocarriers in response to visible light directed on the photoelement region and to transfer substantially all of the photocarriers so produced during the integration time period to the second region for storage therein for maintaining a given width of the depletion layer of the photoelement region substantially constant during said integration time and thereby providing continuously a self-resetting voltage level on the photoelement region during the integration time period; and the second gate electrode, the third region, and the region of the substrate between the second and third region acting in response to a second bias signal applied to the second gate electrode at the end of the integration time period for transferring the stored photocarriers out of the second region and into the third region of the shift register.

2. A photodetector substantially as hereinbefore described with reference to and as shown in Figures 3 to 8 of the accompanying drawings.