Abstract:
The present invention aims to provide a display screen where pixels are addressed by a scanning LASER. The screen performs as a photo-amplifier circuit, producing light output at the region being illuminated by the LASER. This illumination produces electron-hole pairs forming two small currents, one of which subsequently results in a much larger electron or hole current from a specific region of the photo-amplifier. This larger current reaches an emitting region where recombination with other electrons or holes produces light. The duration of the light output is increased up to a frame period or more by increasing the duration of the larger current using various materials having properties that prolong recombination of electrons and holes in a specific device region. In another instance, a feedback effect is utilized by using the incident output light, which may be filtered, replacing the scanning LASER that has left the pixel.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a monolithic display device, scanned by a laser in presence of an applied electric field, comprised of a light-sensing area and a light-emitting area such that an amplification of carriers within the device causes an emission of light. This emission of light is made to persist a frame period or more using carrier blocking materials, light-emitted feedback loop and materials with trap energy levels. 
   Prior art of the invention would involve projection type displays vastly different from the present invention, as these do not utilize a scanning laser to energize pixels on the display screen. Other prior art would involve display which use laser as the light source but not to energize pixels such as U.S. Pat. No. 6,636,274 and U.S. Pat. No. 6,594,090. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to a display device utilizing a scanning LASER for addressing the area where it is intended to induce light output by the display. The display operates as a NPN or PNP photo-amplifying and light-emitting device. The device produces light output only from the areas where the LASER scans it. The principal ideas of the invention are: to address the display elements with an infrared laser, to amplify the light output and to prolong the light output of the pixel up to a frame period or more. The proposed photo-amplifying and light-emitting device described below, achieves this. 
   The NPN device operates in the presence of an external electric field across it, which biases its two P-N junctions as reverse biased and forward biased. As the LASER strikes the device on its P-type photosensitive region, charged pairs of electrons and holes are produced and are pulled apart by the cumulative electric field present at the reverse bias region. The holes thus produced reduce the forward bias barrier allowing for a large amount of holes to cross into an emissive N-type region. The emissive region allows this large influx of holes to recombine, with electrons coming due to the external electric field, and emit light. In another embodiment of the invention there is an electron blocker barrier at the forward biased junction that blocks the incoming electrons, due to the external field, to cross over the lowered forward bias barrier and retains them in the emissive region. A similar function is performed by a hole blocker, which retains the in coming holes in the flux by blocking the flow to go out of the emissive region in the external circuit. This retention of electrons and holes allows more time for recombination. 
   In another embodiment, a high work function metal is used, in the fabrication of the photo-amplifying and light-emitting device, forming a Schottky contact, with the non-emitting, N-type region creating a PNPN device. As the LASER strikes the P-type photosensitive region charged pairs are produced with those at the reverse bias P-N region flowing in opposite directions due to the cumulative electric field present. In this case, the electrons flow towards the Schottky contact and accumulate there. This charge accumulation lowers the potential barrier at the Schottky contact and allows holes to cross and flow into the emissive region to recombine with electrons and emit light. The electron and hole blocker techniques as are also incorporated as described before. After some time the dissipation of accumulated charge at or near the Schottky contact resets it thereby switching off the light output. 
   In another embodiment, the N-type material forming the Schottky contact is a material M 1 . The properties of the M 1  allow for a slower dissipation of charge in the region of the Schottky contact such that it enables a continuous light output by the pixel for a frame period or more even after the scan. In yet another embodiment, the M 1  material is replaced by M 3  material having an extra trap energy level. Electrons arriving from the reverse bias region get trapped quickly and take longer to dissipate. This action allows continuous light output by the pixel as before. 
   In a further embodiment, there is a thin layer of a material M 2  within the Schottky contact such that it affects the charge dissipation at or near the contact as in the case of material M 1  with a similar result. The N-type material replaced earlier by M 1  is used to form the Schottky contact is this case. In yet another embodiment, the M 2  material is replaced by M 4  material having an extra trap energy level for the arriving electrons, from the reverse bias region, which get trapped quickly and take longer to dissipate at or near the Schottky contact. This action again allows continuous light output by the pixel as before. In another embodiment, the high work function metal forming the Schottky contact is any high work function metal or a P-type semiconductor. In case of a semiconductor, the Schottky contact is then replaced by a P-N junction. 
   In another embodiment, in presence of an external electric field as before, a low work function metal is used in the fabrication of a PNP photo-amplifying and light emitting device, to form a Schottky contact with a P-type region made of a P 2  photosensitive material. At the other end, P 3  material forms a forward bias P-N junction. As the LASER strikes the P 2  region, charged electron-hole pairs are produced and those pulled apart the reverse bias P-N region flowing in opposite directions as in the PNPN configuration. In this case, the holes flow towards and lower potential of the Schottky contact and allow electrons, from the low work function metal, to cross the potential barrier and flow into an N-type emissive region to recombine with holes and emit light. After a while, the dissipation of charge at or near the Schottky contact resets it, switching off the light output as before. Adding a material P 1 , with specific properties, to P 2  to form the Schottky contact allows the charge dissipation at or near the Schottky contact to take longer, as required by the invention, enabling the pixel to be lighted continuously for a frame period or more. In another embodiment, the hole-blocker technique is also employed by adding a hole-blocker material in the PNP assembly. In another embodiment the P 2  material or P 2  along with P 1  material is replaced by a material P 4  having an extra trap energy level for the arriving electrons, from the reverse bias region, which get trapped quickly and take longer to dissipate. This action also allows continuous light output by the pixel as before. In yet another embodiment, a thin layer of a material P 5  is placed within the Schottky contact having an extra trap energy level for the arriving electrons, from the reverse bias region, which get trapped quickly and take longer to dissipate. This action, again, allows continuous light output by the pixel as before. The P-type material P 2  is used to form the Schottky contact in this case. In another embodiment, the low work function metal forming the Schottky contact is any low work function metal or a N-type semiconductor material. Again, the Schottky contact is replaced by a P-N junction in case of a semiconductor. 
   In another embodiment, applicable to the NPN and the PNP assemblies, the P-type photosensitive region is only sensitive to infrared light and the scanning LASER is therefore infrared. However, the emissive region being only a visible light emitter inhibits any feedback effects as the emitted light becomes incident on the photosensitive region. In still a further embodiment, the P-type photosensitive region is also photosensitive to visible light. This allows a feedback effect to take place whereby the emitted visible light incident on the photosensitive material produces charged pairs just like the incident LASER. This feedback effect enables the LASER addressed area, functioning like a pixel; to remain lighted for a frame period or more or until the external electric field is switched off. In another embodiment, applicable to NPN assembly, we replace the function of the P-type photosensitive region by a N-type photosensitive material forming the Schottky barrier and with the feedback mechanism, just described, being active. In the PNP case, the N-type photosensitive layer forms the reverse bias junction and is placed alongside the N-type emissive material. In the NPN case, a regular P-type material is used in place of the previous P-type photosensitive region. In another embodiment, with a visible light N-type emitter, the photosensitive region is sensitive to visible light and the photosensitive area of the device is either a P-type or a N-type material as in previous embodiments. In addition, there is a filter placed as such that the emitted visible light coming from the display area and other ambient light is filtered, without affecting the LASER, for a select narrow band of visible light to strike the photosensitive area and provide a feedback effect. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an NPN device with an external electric field across it and an incident LASER initiating a current amplification and light emission process. 
       FIG. 2  illustrates an NPN device with an external electric field across it and an incident LASER initiating a current amplification and light emission process with an electron and hole blocker arrangement. 
       FIG. 3  illustrates an NPN device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with an electron and hole blocker arrangement. 
       FIG. 4  illustrates an NPN device, with a Schottky contact and N-type material M 1 , with an external electric field across it and an incident LASER initiating a current amplification and light emission process with an electron and hole blocker arrangement. 
       FIG. 5  illustrates an NPN device, with a Schottky contact containing thin layer N-type material M 2 , with an external electric field across it and an incident LASER initiating a current amplification and light emission process with an electron and hole blocker arrangement. 
       FIG. 6  illustrates an NPN device, with a Schottky contact and N-type material M 3  with trap energy level, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with an electron and hole blocker arrangement. 
       FIG. 7  illustrates an NPN device, with a Schottky contact containing N-type material M 4  with trap energy level, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with an electron and hole blocker arrangement. 
       FIG. 8  illustrates an NPN device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with emitted light incident on P-type photosensitive region for feedback loop. 
       FIG. 9  illustrates an NPN device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with emitted light incident on N-type photosensitive region for feedback loop along with an electron and hole blocker arrangement. 
       FIG. 10  illustrates an NPN device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with emitted light incident on P-type photosensitive region, through a filter, for feedback loop. 
       FIG. 11  illustrates an NPN device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with emitted light incident on N-type photosensitive region, through a filter, for feedback loop. 
       FIG. 12  illustrates an PNP device with N-type materials P 1  and P 2 , with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process. 
       FIG. 13  illustrates an PNP device with N-type materials P 1  and P 2 , with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process along with a hole blocker arrangement. 
       FIG. 14  illustrates an PNP device with N-type material P 4  with a trap energy level, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process along with a hole blocker arrangement. 
       FIG. 15  illustrates an PNP device, with a Schottky contact containing thin layer of N-type material P 4  with a trap energy level, with an external electric field across it and an incident LASER initiating a current amplification and light emission process along with a hole blocker arrangement. 
       FIG. 16  illustrates an PNP device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with emitted light incident on P-type photosensitive region for feedback loop. 
       FIG. 17  illustrates an PNP device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with emitted light incident on N-type photosensitive region for feedback loop along with a hole blocker arrangement. 
       FIG. 18  illustrates an PNP device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with emitted light incident on P-type photosensitive region, through a filter, for feedback loop. 
       FIG. 19  illustrates an NPN device, with a Schottky contact, with an external electric field across it and an incident LASER initiating a current amplification and light emission process with emitted light incident on N-type photosensitive region, through a filter, for feedback loop. 
       FIG. 20  illustrates an NPN device forming display screen being addressed with a LASER on a small part of it that acts like a pixel. 
       FIG. 21  illustrates an PNP device forming display screen being addressed with a LASER on a small part of it that acts like a pixel. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   To facilitate description any numeral identifying an element in one figure will represent the same element in any other figure. 
     FIG. 20  and  FIG. 21  represent the functioning of the proposed display screen  34 . Light is emitted  49  from the screen  34  only from the area illuminated by the laser. This area performs as a photo-amplifying and light-emitting device  48  or  50  as will be described in detail in the following. As the light emission occurs only from a restricted area, at a certain time this creates the equivalent of an addressed pixel. By scanning the LASER  10  across the screen  34 , we can generate and address all pixels on the screen. 
   In case of a first embodiment, with reference to  FIG. 1  and  FIG. 12 , we have NPN and PNP photo-amplifying and light-emitting devices respectively. Both devices use negatively charged electrons and positively charged holes as carriers of current. Both devices in  FIG. 1  and  FIG. 12  have an external electric field  1  across it and in this state; there is no current flowing across either device. The direction of the external electric field  1  implies that region  4 , for both cases, is reverse biased. In the case of reverse biasing, an external voltage is applied to a P-N junction such that the potential at a P-type semiconductor is made negative with respect to the potential at the N-type. This increases the potential barrier at the junction of the two types of semiconductors and it increases the width of a depletion layer at the P-N junction. Thus, the effect of the external electric field  1  on  FIG. 1  and  FIG. 12  is such that it reverse biases the region  4 . This reverse bias causes the internal potential barrier, created at the P-N junction in the region  3 , for both cases, to become higher so that very little current flows across this P-N junction. 
   In the case of a NPN photo-amplifying and light-emitting device  2 , with reference to  FIG. 1 , there is a photosensitive region  6 , not available in a normal device. When a light source (LASER)  10  is incident, upon the photosensitive region  6 , it produces electron-hole pairs all over the region  6 . Carriers  11 , generated within the depletion region  3 , and approximately one diffusion length from the depletion region in the P-type material  6  are pulled apart by the combined influence of the external electric field  1  and the internal electric field  8  at the reverse bias junction  3 . As such the electrons are pulled and directed to go towards the N-type region  13  and the holes  12  get transported as a small current, which is proportional to the intensity of the incident light source  10 , to get accumulated at forward bias P-N junction in region  7 . This accumulation of holes lowers the forward bias potential barrier  9 . The lowering of the forward bias barrier  9  by the small hole current  12 , allows relatively a very large hole current  17  to flow. Once this potential barrier  9  is lowered, these holes  17  form a very large current as compared to the small current  12  generated by the incident light source  10  in the photosensitive region  6 , and flow into the N-type region  14 . The holes  17  are replenished by still more holes injected  16  at anode contact through the N-type region  18 . The larger hole current  17  is thus an amplification whereby a small hole current  12 , generated by the incident light source  10  under the influence of the external electric field  1  and the P-N junction field  8 , results in a much larger hole current  17  into the N-type region  14  in the forward bias region  5 . The holes arriving into the N-type region  14  recombine with electrons  15  injected through the cathode contact by the external electric field. The N-type region  14  is an emissive region where recombination of electrons and holes takes place as well as emission of light due to that recombination. Thus, the holes  17  recombine with electrons  15  and emit light from region  14 . 
   In another embodiment of the invention, with reference to  FIG. 2 , we have a NPN photo-amplifying and light-emitting device with an additional material which functions as an electron blocker barrier  19 . Here we have, initially, an identical process taking place as in the case of  FIG. 1  NPN photo-amplifying and light-emitting device i.e. the process in which the incident light source  10  strikes the photosensitive region  6  and from the resulting charged electron-hole pairs in the region  6  the ones  11  near the reverse bias junction  3  give rise to the small hole current  12  which in turn allows the accumulation of holes near the depletion region  7 . There again it follows the same sequence of events whereby the accumulation of holes due to the small hole current  12  lowers the internal potential barrier  9  and allows holes in the P-type region to flow in form of a much larger current  17  into the N-type region  14 . The larger hole current  17 , resulting from the incident light source (LASER)  10  at region  6 , goes into the N-type emitter region where these holes  17  are blocked by a hole-blocker  21 , allowing the holes  17  to accumulate in region  14 . Electrons  15 , injected through the cathode into the N-type region due to the external electric field  1 , are blocked  20  by the electron blocker barrier  19  and this gives the electrons and holes enough time to recombine. The electrons and holes then recombine allowing for light emission from the emissive N-type region  14 . 
   In another embodiment of the invention, with reference to  FIG. 3 , we have the equivalent of a PNPN assembly wherein a high work function metal  22 , specifically ITO or Indium Tin Oxide, which is a transparent conductive layer, forms a Schottky contact  23  with the N-type organic semiconductor or specifically tris(8-hydroxy-quinoline)aluminum (Alq 3 ) material. This Schottky contact  23 , to be referred as anode-contact or NPN-Schottky contact onwards, formation is equivalent to a P-N junction making the PNPN device. The external electric field  1  makes this contact  23  to be forward biased. The rest of assembly is identical as the NPN photo-amplifying and light emitting device with electron blocker  19  in  FIG. 2 . The P-type photosensitive organic semiconductor is made of TiOPc or specifically titanyl phthalocyanine, while the N-type emissive region  14  is made of Alq 3 , which is a green light emitter. The electron blocker  19  is made up of N,N′-diphenyl-N-N′-bis(1-naphtyl)-1-1′biphenyl-4,4″diamine (NPB). When a light source (LASER)  10  is incident, upon the photosensitive region  6 , it produces electron-hole pairs all over the region  6 . Carriers, generated within the depletion region  3 , and approximately one diffusion length from the depletion region in the P-type material  6  are pulled apart by the combined influence of the external electric field  1  and the internal electric field  8  at the reverse bias junction. This cumulative effect transports the holes in the form of a current  12  to the depletion region  7  where it may lower the barrier of the forward biased P-N junction. The electrons, however, travel in the opposite direction  13  towards the NPN-Schottky contact  23 . This small electron current leads to a charge build-up at the NPN-Schottky contact  23 . This charge accumulation will reduce the potential barrier for hole injection from the high work function metal anode. This barrier reduction allows the holes to be injected more easily from the anode, made of Indium tin oxide (ITO). The injected holes form a large current  24  that flows into the N-type emissive region  14 . This current is an amplification whereby a small electron current  13 , generated by the incident light source  10  under the influence of the external electric field  1  and the P-N junction field  8 , results in a much larger hole current  24  into the N-type region  14  in the forward bias region  5 . Meanwhile the electron blocker  19  blocks  20  the electrons  15 , injected through the cathode due to the external electric field  1 , from crossing the depletion region  7 . This process facilitates the recombination of holes  24  which are flowing into the emitter region  14  and electrons  15  injected at the cathode contact by giving them time to recombine and emit light from the emissive region  14 . 
   With reference to  FIG. 20 , by scanning the LASER  10  across the screen  34 , we can generate and address all pixels  48  on the screen  34 . With a million pixels and a frame period of 30 milliseconds, the LASER remains fixated on the pixel  48  for 30 nanoseconds before moving further. For the pixel  48  to continuously emit light for the whole frame period or more, there has to be a residual effect after the LASER leaves that pixel to move to another pixel. This residual effect ensures that the pixel remains lighted for the whole frame period or more, even after the LASER has left it. With reference to  FIG. 3 , the interval of time, starting when the LASER leaves the pixel and ending when the pixel stops emitting light, is determined by how long it takes for the electrons accumulated  13  at barrier potential, associated with the NPN-Schottky contact, to dissipate and reset the potential barrier height to its initial higher level as described before. As it is the requirement of the invention that the electrons  13  at or near the NPN-Schottky contact dissipate slowly, the objective is to increase this time. By increasing this time interval, as explained before, the light output of the pixel, in region  14 , remains continuous till the end of the frame period or more. 
   In another embodiment, with reference to  FIG. 4 , we have an identical assembly as that of  FIG. 3  with the difference that the N-type material on one side of NPN-Schottky contact  23  is a material M 1   25  instead of the Alq 3 . This material M 1  has properties such that the dissipation of the accumulated charge, due to trapped photo-generated electrons, through recombination or transfer to the anode, takes longer than for other materials, such as the Alq 3  material, at or near the NPN-Schottky contact  23 . This charge dissipation eventually resets the potential barrier to its original higher level at which holes are no more injected from the anode. When the LASER  10  strikes, the dissociation of the resulting exitons produce electron-hole charged pairs  11 . The holes  12 , under the combined influence of the external electric field  1  and the internal electric field on the depletion region  3 , go towards the depletion region  7  at the forward bias P-N junction. The electrons  13  move toward and accumulate at the NPN-Schottky contact  23  due to the existing potential barrier. Once the electrons accumulate at the barrier, they lower the potential of the potential barrier  23  allowing the injection of more holes from the anode. The holes current  24  flow towards the emissive region  14  to emit light by recombining with electrons  15 , being injected due to the external electric field  1 , in region  14 . The electrons  13  accumulating and lowering the potential barrier at the NPN-Schottky contact then start to dissipate through recombination or other mechanisms. Once all the accumulating electrons  13  are dissipated, the potential barrier  23  is restored to its original elevated level thereby stopping the hole current  24 . 
   In another embodiment, with reference to  FIG. 5 , we have again, an identical assembly as that of  FIG. 3  with the only difference that we have a thin layer  26  of the material M 2  at the NPN-Schottky contact  23 . This material M 2  has properties such that they affect the time interval it takes for the electrons  13  to dissipate and restore the potential barrier to its initial higher level. This dissipation, once complete after the LASER leaves the pixel, effectively shuts off the hole current  24  to the region  14  thereby switching off the light emission from region  14 . Therefore, as in a requirement of the invention, the time interval it takes for the completion of the dissipation process of electrons  13  at the NPN-Schottky contact, caused by the properties of material M 2 , is of a longer duration, than in case of the Alq 3  material. In such case, the pixel would remain at a continuous output of light from the emissive region  14 , for a frame period or more. 
   In another embodiment, with reference to  FIG. 6 , we have an identical construct as that of  FIG. 3  except that we replace the N-type material made of Alq 3  on one side of the NPN-Schottky contact  23  by a material M 3   27  which provides an extra trap energy level  28  for electrons  13  coming from region  3 . Likewise, as in the description of  FIG. 3 , the charge due to electrons  13  accumulating and lowering the potential barrier gradually starts to dissipate. Once all the charge dissipates, the potential barrier is restored to its original higher level thereby stopping the hole injection current  24  to flow into region  14  for emission of light. With the trap energy level  28 , within the material M 3 , the electrons get trapped quickly and remain trapped for a longer period of time at or near the NPN-Schottky contact  23  allowing for an even more gradual and slow dissipation of the accumulated charge  13 , than would be possible in a material without a trap as in the requirement of the invention. This trapping effect gives more time to restore the potential barrier&#39;s high potential and allows the hole current  24  to flow into the emissive region  14  for continuous light emission for one frame period or more. 
   In another embodiment, with reference to  FIG. 7 , we again have an identical construct as that of  FIG. 3  except that we place a thin layer of material M 4   29  at the NPN-Schottky contact which provides an extra band or a trap  30  for electrons  13  coming from region  3  in form of a photocurrent. Likewise, as in the description of  FIG. 3 , the electrical charge due to electrons  13  accumulating, and lowering the potential barrier, starts to dissipate. Once the charge due to the electron photocurrent  13 , is dissipated, the barrier is restored to its original higher level. This stops the holes, from the high work function metal anode, to overcome the restored barrier and flow as the hole current  24  into region  14  for emission of light. With the trap  30 , within the material M 4 , the electrons are trapped quickly and take a longer interval of time, as is the requirement of the invention, than is normal in a material without the trap to dissipate at or near the NPN-Schottky contact  23 . Thus the trap  30  allows for an even more gradual and slow dissipation of the trapped photo-electrons giving more time, to fully restore the potential barrier  23  and allows the hole current  24  to flow into the emissive region  14  for an extended interval for continuous light emission for a frame period or more. 
   In another embodiment of the present invention, with reference to figures  FIG. 3  to  FIG. 7 , the high work function metal forming the NPN-Schottky contact can be any high work function metal or P-type semiconductor. The NPN-Schottky junction will then be replaced by a P-N junction in case of a P-type semiconductor. The remaining assembly and operation for embodiments given by figures  FIG. 3  to  FIG. 7  would remain similar. 
   In another embodiment of the present invention, which may also be incorporated in the embodiments represented by figures starting from  FIG. 1  up to  FIG. 7 , with reference to  FIG. 7 , the photosensitive region  6  is sensitive only to infrared light such as the one which is incident on region  6  due to LASER  10 , the LASER  10  being only an infrared LASER. As the LASER  10  strikes the photosensitive region  6  charged electron-hole pairs  11  are produced all over region  6 . Likewise, as in the description of embodiments depicted by figures  FIG. 1  to  FIG. 7 , the production of the charged pairs  11  composed of electrons and holes subsequently leads to the emission of light from the emissive N-type region  14 . In this embodiment, the N-type emitter  14  emits only visible light. The emission of only visible light from region  14  ensures that when the emitted visible light falls on the photosensitive region  6 , it does not produce a feed back effect i.e. the incident visible light on region  6  from the emitter region  14 , does not produce any charged electron-hole pairs in region  6 . This is so, because the photosensitive region  6  is sensitive only to infrared light. As such, the production of charged electron-hole pairs  11  in region  6  is dependent only on the light due to the incident infrared LASER  10 . 
   In another embodiment of the present invention, which may also be incorporated in the embodiments represented by figures starting from  FIG. 1  up to  FIG. 7 , with reference to  FIG. 8 , the photosensitive region  6  is sensitive to the visible light in addition to light incident due to the infrared LASER  10 . In this embodiment, the N-type emitter  14  emits only visible light. As the LASER  10  strikes the photosensitive region  6  charged electron-hole pairs  11  are produced all over region  6 . Likewise, as in the description of embodiments depicted by figures  FIG. 1  to  FIG. 7 , the production of the charged pairs  11  composed of electrons and holes subsequently leads to the emission of visible light  31  from the emissive N-type region  14 . When the emitted visible light falls  32  on the photosensitive region  6 , it produces a feed back effect i.e. the incident visible light  32  on photosensitive region  6 , which is sensitive to the visible light from the emitter region  14 , produces charged electron-hole pairs in region  6 , in addition to the ones produced due to the incident LASER  10 . This light absorption subsequently prolongs the emission of visible light  31  from region  14  till the end of the frame period when the electric field  1  is switched off to discontinue emission of light from the N-type emissive region  14 . Thus the feedback loop, created due to the incident visible light  32  from the emissive region  14  on the photosensitive region  6 , enables the pixel to be continuously lighted till the end of frame period or more. 
   In another embodiment of the present invention, with respect to  FIG. 9 , we have a photosensitive region  6  as being sensitive to visible light to produce a feedback effect, as previously described in another embodiment, and in addition we can also have a N-type material  33  in a reverse bias region  4  replacing the P-type material as the photosensitive region  6 . In addition, in this embodiment, N-type emitter  14  emits only visible light. When the LASER  10  becomes incident on the N-type photosensitive material  33 , charged electron-hole pairs are produced all over region  6 . Within depletion region  3 , in reverse bias region  4 , and also approximately one diffusion length in addition to the depletion region in the N-type material  33 , the charged electron-hole pairs  11  are pulled apart by the combined influence of external electric field  1  and internal electric field  8  at the reverse bias junction. This cumulative effect transports the holes in the form of a current  12  to the depletion region  7  where it may lower the barrier of the forward biased P-N junction. The electrons travel in the opposite direction  13  towards the NPN-Schottky contact  23 . This small electron photocurrent accumulates at the interface and lowers the potential barrier height, which allows a large amount of holes to be injected at the high work function material anode. This enhancement of hole injection forms a large hole current  24  to flow into the N-type emissive region  14 . This leads up to the emission of visible light  31  from the N-type emissive region  14  as described in previous embodiments. The visible light being emitted  31  from the region  14  becomes incident  32  on the photosensitive N-type region  6  and produces the feed back loop, as described in a pervious embodiment, which allows the pixel to remain continuously lighted till the length of the frame period when the electric field  1  is switched off to discontinue emission of light from N-type emissive region  14 . 
   In another embodiment of the present invention, with respect to  FIG. 10 ,  FIG. 11  and  FIG. 20 , we have the photosensitive region  6  as being also sensitive to visible light to produce a feedback effect, as previously described in another embodiment, and in addition, we have a filter  35 , which allows only a select narrow band of frequencies of visible light to pass through it. This band is included in the band of frequencies of visible light being emitted by pixel at the N-type emissive region  14 . In addition, in this embodiment the N-type emitter  14  emits only visible light. The filter  35  is placed in a position such that it shields the photosensitive region  6 , which could either be a P-type or a N-type as described in previous embodiments, to avoid a feedback effect due to ambient light coming from the direction of display screen  34  ( FIG. 20 ). As the LASER  10  strikes the photosensitive region  6 , the N-type emissive region  6  emits visible light  31  subsequently, as described in the previous embodiments. The filter  35  allows the visible light being emitted  31 , by the pixel from N-type emissive region  14 , to pass through, except the visible light coming from the pixel that is composed of frequencies not passable through the filter  35 , and become incident  32  on the photosensitive region  6  and produce the feedback loop. The feedback effect allows the generation of charged electron-hole pairs to be produced in region  6  and thereby subsequently allowing the pixel, as described in the previous embodiments, to continue to emit light till the end of the frame period when the electric field  1  is switched off. The filter  35  does not inhibit the path of the LASER  10  incident on the photosensitive region  6 . 
   In another embodiment of the invention in a case of a PNP photo-amplifying and light-emitting device  36 , with reference to  FIG. 12 , there is a photosensitive region  6 , not available in a normal device. Also included in the assembly is a contact made from a low work function metal  40 , such as Aluminum, which forms a Schottky contact  41 , to be referred as cathode-contact or PNP-Schottky contact onwards, with the P-type semiconductor, composed of P-type material P 2  or P 2  with an additional material P 1  for a specific purpose to be explained shortly. The P-type semiconductor at the forward bias region  5  is composed of material P 3 . When a light source (LASER)  10  is incident, upon the photosensitive region  6 , it produces electron-hole pairs all over the region  6 . Carriers, within the depletion region  3 , in the reverse bias region  4 , and also approximately one diffusion length from the depletion region are pulled apart by the combined influence of the external electric field  1  and the internal electric field  8  at the reverse bias junction  3 . As such, the electrons are pulled and directed to go towards the N-type region  37 , and the holes  38  get transported as a small current, which is proportional to the intensity of the incident light source  10 , to get accumulated at the PNP-Schottky junction  41 . This accumulation of holes, at the PNP-Schottky junction, reduces the potential barrier  41 . The lowering of the potential barrier  41  by the small hole current  38 , allows relatively a very large electron current  39  to flow due to the injected electrons  15 , from the contact metal  40 . Once this potential barrier  41  is lowered, these electrons  39 , form a very large current as compared to the small hole current  38  generated by the incident light source  10  in the photosensitive region  6 , and flow into the N-type region  14 . This large electron current  39  is an amplification whereby a small hole current  38  generated by the incident light source  10  under the influence of the external electric field  1  and the P-N junction field  8 , results in a much larger electron current  39 . The electron current  39  goes into what constitutes the emissive layer of the device, composed of N-type organic semiconductor tris(8-hydroxy-quinoline)aluminum (Alq 3 ) material, called N-type emitter region  14 . The electrons  39  arriving into the N-type region  14  recombine with holes  16  injected at the anode contact, made of ITO material  22  or Indium Tin Oxide, due to the external electric field  1 . The ITO  22  is an efficient hole injector. This recombination, taking place in N-type emissive region  14 , results in the emission of light. 
   The embodiment in  FIG. 12  represents, with reference to  FIG. 21 , the functioning of a pixel in a display screen  34 . Light is emitted  49  from the screen  34  only from the areas illuminated by the laser creating the equivalent of an addressed pixel  50 . By scanning the LASER  10  across the screen  34 , we can generate and address a million pixels  50  on the screen. With a frame period of 30 milliseconds, the LASER remains fixated on the pixel  50  for 30 nanoseconds before moving further. If the pixel  50  were to continuously emit light for the whole frame period or more, there has to be a residual effect after the LASER leaves that pixel to move to another pixel. This residual effect ensures that the pixel remains lighted for the whole frame period or more, even after the LASER has left it. With reference to  FIG. 12 , the interval of time, starting when the LASER leaves the pixel and ending when the pixel stops emitting light, is determined by how long it takes for the holes  38  at the PNP-Schottky contact  41  to dissipate and reset the barrier potential to its initial higher level as described before. As it is the requirement of the invention that the holes  38  accumulated at the Schottky contact dissipate more slowly, the objective is to increase this time. Keeping the requirement of the invention in view and therefore to add more time to the interval, in which the holes  38  dissipate and thereby reset the barrier to its initial higher level, we add the P 1  P-type semiconductor along with P 2  type material. The properties of P 1  material allow for a slower dissipation of holes  38 , at or near the Schottky contact, as it is required by the invention. By increasing the time interval, for the charge dissipation, the light output of the pixel, in region  14 , remains continuous till the end of the frame period or more. 
   With reference to  FIG. 13 , we have a PNP photo-amplifying and light-emitting device with an additional material which functions like a hole-blocker  21 . Here we have, initially, an identical process taking place as in the case of  FIG. 12  NPNP photo-amplifying and light-emitting device i.e. the process in which the incident light source  10  strikes the photosensitive region  6  and from the resulting charged electron-hole pairs in the region  6  the ones  11  near the reverse bias depletion region  3  gives rise to the small hole current  38  which in turn allows the accumulation of holes at the PNP-Schottky contact  41 . There again it follows the same sequence of events whereby the accumulation of holes due to the small hole current  38  lowers the potential at the PNP-Schottky contact  41  and allows the injection of electrons from the contact metal  40  to flow in form of a much larger electron current  39  into the N-type region  14 . Thus, the larger electron current  39  resulting from the incident light source (LASER)  10  at region  6  goes into N-type emitter region  14 . The holes  16  coming into the region  14  due to the external electric field  1  are blocked by the hole-blocker  21 . This process allows holes to accumulate in region  14  and facilitate the subsequent recombination process. Electrons  39  and holes  16  then recombine allowing for light emission from the emissive N-type region  14 . 
   In another embodiment, with reference to  FIG. 14 , we have an identical construct as that of  FIG. 13  except that we replace the P-type material composed of material P 2  or material P 2  along with material P 1  on one side of the Schottky barrier by a material P 4   44  which provides an extra trap energy level  45  for holes  38  coming in form of a current. Likewise, as in the description of  FIG. 13 , electrical charge due to the holes  38 , accumulating and lowering the potential barrier, gradually starts to dissipate. Once all the charge dissipates, the potential barrier at the Schottky contact is restored to its original higher level thereby stopping the electron injection current  39  to flow into region  14  for emission of light. With the trap energy level  45 , within the material P 4 , the holes get trapped quickly and take longer to dissipate at or near the Schottky contact, as is the requirement of the invention. Thus, the trap energy level  45  allows for an even more gradual and slow dissipation of accumulating holes  38 , than would be possible in a material without a trap energy level, adding more time to restore the potential barrier and allows the electron current  39  to be injected and to flow into the emissive region  14  for continued light emission for an interval of a frame period or more. 
   In another embodiment, with reference to  FIG. 15 , we again have an identical construct as that of  FIG. 13  except that we place a thin layer of material P 5   46  at the Schottky contact which provides an extra trap level  47  for holes  38  coming in form of a photo-current. Likewise, as in the description of  FIG. 13 , the holes  38  accumulating and lowering the potential barrier start to dissipate. Once all the accumulated charge dissipates at or near the Schottky contact, the barrier potential is restored to its original higher level. This stops the injection of electrons, from the low work function metal contact, to overcome the restored potential barrier and flow as the electron current  39  into region  14  for emission of light. With the trap energy level  47 , within the material P 5 , the holes  38  get trapped quickly and take a longer interval of time, as in the requirement of the invention, than is normal in a material without the trap to dissipate at the Schottky contact. Thus, the trap energy level  47  allows for an even more gradual and slow dissipation of holes giving more time to fully restore the potential barrier and allow the injected electron current  39  to flow into the emissive region  14  for an extended interval for continued light emission for an interval of a frame period or more. 
   In another embodiment of the present invention, with reference to figures  FIG. 12  to  FIG. 15 , the low work function metal forming the PNP-Schottky contact can be any low work function metal or a N-type semiconductor. The PNP-Schottky contact will then be replaced by a P-N junction in case of a N-type semiconductor. The remaining assembly and operation for embodiments given by figures  FIG. 12  to  FIG. 15  would remain similar. 
   In another embodiment of the present invention, which may also be incorporated in the embodiments represented by figures starting from  FIG. 12  up to  FIG. 15 , with reference to  FIG. 15 , the photosensitive region  6  is sensitive only to infrared light such as the one which is incident on region  6  due to LASER  10 , the LASER  10  being only an infrared LASER. As the LASER  10  strikes the photosensitive region  6  charged electron-hole pairs  11  are produced all over region  6 . Likewise, as in the description of embodiments depicted by figures  FIG. 12  to  FIG. 15 , the production of the charged pairs  11  composed of electrons and holes subsequently leads to the emission of light from the emissive N-type region  14 . In this embodiment, the N-type emitter  14  emits only visible light. The emission of only visible light from region  14  ensures that when the emitted visible light falls on the photosensitive region  6 , it does not produce a feed back effect i.e. the incident visible light on region  6  from the emitter region  14 , does not produce any charged electron-hole pairs in region  6 . This is so, because the photosensitive region  6  is sensitive only to infrared light. As such, the production of charged electron-hole pairs  11  in region  6  is dependent only on the light due to the incident infrared LASER  10 . 
   In another embodiment of the present invention, which may also be incorporated in the embodiments represented by figures starting from  FIG. 12  up to  FIG. 15 , with reference to  FIG. 16 , the photosensitive region  6  is sensitive to the visible light in addition to light incident on region  6  due to LASER  10 . In this embodiment, the N-type emitter  14  emits only visible light. As the LASER  10  strikes the photosensitive region  6  charged electron-hole pairs  11  are produced all over region  6 . Likewise, as in the description of embodiments depicted by figures  FIG. 12  to  FIG. 15 , the production of the charged pairs  11  composed of electrons and holes subsequently leads to the emission of visible light  31  from the emissive N-type region  14 . When the emitted visible light falls  32  on the photosensitive region  6 , it produces a feed back effect i.e. the incident visible light  32  on photosensitive region  6 , which is sensitive to the visible light, from the emitter region  14  produces charged electron-hole pairs in region  6 , in addition to the ones produced due to the incident LASER  10 . This light absorption subsequently prolongs the emission of visible light  31  from region  14  till the end of the frame period when the electric field  1  is switched off to discontinue emission of light from the N-type emissive region  14 . Thus the feedback loop, created due to the incident visible light  32  from the emissive region  14  on the photosensitive region  6 , enables the pixel to be continuously lighted till the end of frame period or more. 
   In another embodiment of the present invention, with respect to  FIG. 17 , we have a photosensitive region  6  composed of an additional N-type material  33  alongside with the N-type Alq 3  emitter in a reverse bias region  4  forming a reverse biased P-N junction  3  with the P-type material. Thus, the N-type material  33  replaces the P-type material as the photosensitive region  6 . In addition, we have the photosensitive region  6  as being sensitive to visible light to produce a feedback effect, as previously described in another embodiment. Further, in this embodiment the N-type emitter  14  emits only visible light. When the LASER  10  becomes incident on the N-type photosensitive material  33 , charged electron-hole pairs are produced all over region  6 . Within depletion region  3 , in reverse bias region  4 , and also approximately one diffusion length in addition to the depletion region in the N-type material  33 , the charged electron-hole pairs  11  are pulled apart by the combined influence of external electric field  1  and internal electric field  8  at the reverse bias junction. This cumulative effect transports the electrons in the form of a current  37  to the depletion region  7  where it may lower the barrier of the forward biased P-N junction. The holes, however, travel in the opposite direction  38  towards the PNP-Schottky contact  41 . This small hole current accumulates holes at the interface and lowers the potential barrier height, which allows the injection of electrons from the low work function material. These electrons form a large current  39  that flows into the N-type emissive region  14 . This leads up to the emission of visible light  31  from the N-type emissive region  14  as described in previous embodiments. The visible light being emitted  31  from the region  14  becomes incident  32  on the photosensitive N-type region  6  and produces the feed back loop, as described in a pervious embodiment, which allows the pixel to remain continuously lighted till the length of the frame period when the electric field  1  is switched off to discontinue emission of light from N-type emissive region  14 . 
   In another embodiment of the PNP assembly, with respect to  FIG. 18 ,  FIG. 19  and  FIG. 21 , we have the photosensitive region  6  as being also sensitive to visible light to produce a feedback effect, as previously described in another embodiment, and in addition, we have a filter  35 , which allows only a select narrow band of frequencies of visible light to pass through it. This band is included in the band of frequencies of visible light being emitted by pixel at the N-type emissive region  14 . In addition, in this embodiment the N-type emitter  14  emits only visible light. The filter  35  is placed in a position such that it shields the photosensitive region  6 , which could either be a P-type or a N-type as described in previous embodiments, to avoid a feedback effect due to ambient light coming from the direction of display screen  34  ( FIG. 21 ). As the LASER  10  strikes the photosensitive region  6 , the N-type emissive region  6  emits visible light  31  subsequently, as described in the previous embodiments. The filter  35  allows the visible light being emitted  31 , by the pixel from N-type emissive region  14 , to pass through, except the visible light coming from the pixel that is composed of frequencies not passable through the filter  35 , and become incident  32  on the photosensitive region  6  and produce the feedback loop. The feedback effect allows the generation of charged electron-hole pairs to be produced in region  6  and thereby subsequently allowing the pixel, as described in the previous embodiments, to continue to emit light till the end of the frame period when the electric field  1  is switched off. The filter  35  does not inhibit the path of the LASER  10  incident on the photosensitive region  6 . 
   DRAWING LEGEND 
   
       
       
         
             1 . External electric field 
             2 . NPN device 
             3 . Reverse bias depletion region 
             4 . Reverse bias region 
             5 . Forward bias region 
             6 . P-type photosensitive region 
             7 . Forward bias depletion region 
             8 . Electric field in the depletion region  3   
             9 . Electric field in the depletion region  7   
             10 . Incident light source (LASER) 
             11 . Charged pair production 
             12 . Small hole current towards P-type region 
             13 . Electron current towards N-type region 
             14 . N-type Alq 3  light emission region 
             15 . Cathode injected electrons 
             16 . Anode injected holes 
             17 . Larger hole current going through the lowered barrier 
             18 . Replenishing of holes 
             19 . Electron blocker barrier 
             20 . Electrons getting blocked 
             21 . Hole blocker 
             22 . Indium Tin Oxide (ITO) metal 
             23 . NPN-Schottky contact 
             24 . Larger hole current towards N-type emissive region 
             25 . N-type material M 1  instead of Alq 3    
             26 . Thin layer of N-type material M 2   
             27 . N-type material M 3  having a trap energy level for electrons 
             28 . Trap energy level for electrons in N-type material M 3   
             29 . Thin layer of material M 4  with a trap energy level for electrons 
             30 . Trap energy level for electrons in thin layer of material M 4   
             31 . Emission of visible light 
             32 . Emitted visible light becoming incident on photosensitive region 
             33 . N-type photosensitive region 
             34 . Display screen 
             35 . Filter allowing only a narrow band of frequencies of visible light 
             36 . PNP device 
             37 . Small electron current towards N-type region 
             38 . Hole current towards PNP-Schottky contact region 
             39 . Larger electron current going through lowered barrier 
             40 . Low work function metal Aluminum 
             41 . PNP-Schottky contact 
             42 . P-type material P 2   
             43 . P-type material P 1   
             44 . P-type material P 4  with a trap energy level for holes 
             45 . Hole trap energy level for P 4  material 
             46 . Thin layer of P-type P 5  material 
             47 . Hole trap energy level for P 5  material 
             48 . Part of NPN device as a pixel 
             49 . Emission of light 
             50 . Part of PNP device as a pixel