Abstract:
A cold electron emitter may include a heavily n+ doped wide band gap (WBG) substrate, a p-doped WBG region, and a low work function metallic layer (n + -p-M structure). A modification of this structure includes heavily p+ doped region between p region and M metallic layer (n + -p-p+-M structure). These structures make it possible to combine high current emission with stable (durable) operation. The high current density is possible because the p-doped (or p+ heavily doped) WBG region acts as a negative electron affinity material when in contact with low work function metals. The injection emitters with the n + -p-M and n + -p-p+-M structures are stable since the emitters make use of relatively low extracting electric field and are not affected by contamination and/or absorption from accelerated ions. In addition, the structures may be fabricated with current state-of-the-art technology.

Description:
RELATED APPLICATIONS 
     The following application of the common assignee, which is hereby incorporated by reference in its entirety, may contain some common disclosure and may relate to the present invention: 
     U.S. patent application Ser. No. 09/975,297, entitled “High-Current Avalanche-Tunneling And Injection-Tunneling Semiconductor-Dielectric-Metal Stable Cold Emitter Which Emulates The Negative Electron Affinity Mechanism Of Emission”. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to electron emitters. In particular, the invention relates generally to cold electron emitters of p-n cathode type. 
     BACKGROUND OF THE INVENTION 
     Electron emission technology exists in many forms today. Hot cathode ray tubes (CRT), where electrons are produces as a result of thermal emission from hot cathode heated by electrical current, are prevalent in many displays such as televisions (TV) and computer monitors. Electron emission also plays a critical role in devices such as x-ray machines and electron microscopes. Miniature cold cathodes may be used for integrated circuits and flat display units. In addition, high-current density emitted electrons may be used to sputter or melt some materials. 
     In general, two types of electron emitters exist—“hot” and “cold” cathode emitters. The “hot” cathodes are based on thermal electron emission from surface heated by electric current. The cold cathodes can be subdivided into two different types: type A and B. The emitters of type A are based on the field emission effect (field-emission cathodes). The emitters of type B are the p-n cathodes using the emission of non-equilibrium electrons generated by injection or avalanche electrical breakdown processes. 
     Both types of emitters have drawbacks which make them virtually impractical. For type A emitters (field emission type), one of the main drawbacks is their very short lifetime. For example, the type A emitters may be operational for just hours, and perhaps even as short as minutes. In the cold field-emission cathodes (type A), electrons are extracted from the surface of a metal electrode by a strong electric field in vacuum. The field cathodes have a short lifetime at large emitted currents, which are needed in recording devices and other applications. 
     With reference to FIG. 1A, operation of type A emitters will be described. FIG. 1A illustrates a typical energy diagram for a metallic surface illustrating a concept of a work function of a metal. As shown, a material, in this instance a metal, is on the left and a vacuum region is on the right. E F  represents a Fermi level of the metal. The work function of the metal Φ M  is the energy required to move a single electron from the Fermi level in the metal into vacuum. Thus, the work function Φ M  is the difference between Vac and E F . The work function Φ M  for metal is typically between 4-5 electron volts (eV). 
     In very strong external field the energy diagram changes, and it looks as a triangular potential barrier for the electrons (FIG. 1A, dashed line). When the external field F increases, the barrier width decreases and the tunneling probability for electrons rapidly increases. The transparency of such a barrier is          D   =     exp        [     -       4        Φ   M     3   /   2              2      m           3      qhF         ]         ,                          
     where F the electric field, q and m are the electron charge and mass. Transparency represents the probability of electron tunneling. For current densities j=1-100 A/cm 2  (amperes per square centimeter) the corresponding field would be F&gt;10 7  V/cm. 
     In such strong fields, the ions, which are always present in a vacuum region in actual devices, acquire the energy over 10 3  eV in the vacuum region on the order of one micron or larger. Ions with such strong energies collide with the emitter surface leading to absorption of the ions and erosion of the emitter surface. The ion absorption and erosion typically limits the lifetime of type A emitters to a few hours of operation or even to a few minutes. Damage to cathodes in systems with the fields of similar strength has been studied in great detail and is rather dramatic. 
     For type B emitters (injection/avalanche type), one of the main drawbacks is that the efficiency is very small. In other words, the ratio of emitted current to the total current in the circuit is very low, usually much less than 1%. The cathode of type B based either on p-n junctions, or semiconductor-metal (S-M) junction including TiO 2  or porous Si, or the avalanche electrical breakdown need an “internal” bias, applied to p-n junction or S-M junction. 
     Alternatively, there have been suggestions to use the electrical breakdown processes to manufacture the cold emitters from Si. These types of avalanche emitters are based on emission of very hot electrons (with energies of the order of a few electron volts) accelerated by very strong electric field in the avalanche regime. As a result, they also have a disadvantage that the emitted current density of the hot electrons is very small. 
     Attempts have been made to increase the current density by depositing cesium (Cs) on semiconductor surface to use a negative electron affinity (NEA) effect. FIG. 1B illustrates the concept of NEA. As shown, a material, a p-type semiconductor in this instance, is on the left and a vacuum region is on the right. E C  represents a conduction band of the metal. Note that the NEA effect corresponds to a situation when the bottom of the conduction band E C  lies above the vacuum level Vac. One earlier p-n cathode of this type combined a silicon, or gallium arsenide avalanche region, with cesium metallic layer from where the emission took place (GaAs/Cs or GaP/Cs structures). However, Cs is a very reactive and volatile element. Thus, the GaAs and GaP emitters with Cs are not stable at high current densities. 
     In short, cold emitters with both high current emission and stability were not possible with previous designs. 
     SUMMARY OF THE INVENTION 
     In one respect, an embodiment of a cold electron emitter may include an heavily doped n-type region (n+ region). The n+ region may be formed from wide band gap semiconductors. The electron emitter may also include a substrate below the n+ region. Indeed, the n+ region may be formed by doping the substrate with electron rich materials. In addition, the electron emitter may include a p region formed within or above the n+ region. The p region may be formed by counter doping the n+ region with electron poor materials. The thickness of the p region is preferred to be less than the diffusion length of the electrons in the p region. Also, the hole concentration level in the p region is preferred to be less than the electron concentration in the n+ region. The electron emitter may further include a metallic layer formed above the p region. The work function of the metallic layer is preferred to be less than the energy gap of the p region. In addition, the thickness of the metallic layer is preferred to be on the order of or less than the mean free path for electron energy. The electron emitter may still further include a heavily doped p region (p+ region) formed within the p region, for example, by delta-doping the p region. The electron emitter may yet further include n and p electrodes so that n+-p junction may be forward biased for operation, for example, to control the amount of current emitted from the device. The electron emitter may still yet further include an M electrode, with or without the p electrode. 
     In another respect, an embodiment of a method to fabricate an electron emitter may include forming an n+ region, for example, from doping a wide band gap substrate with electron rich materials. The method may also include forming a p region within the n+ region, for example, by counter doping the n+ region with electron poor materials. The thickness of the p region is preferred to be less than the diffusion length of the electrons in the p region. Also, the hole concentration level in the p region is preferred to be less than the electron concentration of the n+ region. The method may further include forming a metallic layer above the p region. The work function of the metallic layer is preferred to be less than the energy gap of the p region, and the thickness of the metallic layer is preferred to be of the order of or less than the mean free path for electron energy. The method may still further include forming a p+ region, for example, by delta doping the p region. The method may yet include forming n and p electrodes so that n+-p junction may be forward biased for operation. The method may yet further include forming an M electrode, with or without forming the p electrode, to control-the amount of current emitted from the current emitter. 
     The above disclosed embodiments may be capable of achieving certain aspects. For example, the electron emitter may produce high density of emitted electron current. Also, the lifetime of the emitter may be relatively high. Further, the emitter may be based on well-known wide-gap materials and fabrication methods there of and thus, little to no capital investment is required beyond that present in the current state-of-the-art. In addition, the detrimental effects of high vacuum field—cathode surface erosion, ion absorption at the emitter surface, etc.—may be avoided since the device does not require strong electric fields in vacuum region, which results in stable operation. Thus, stability and high current density may be combined in a single device. The absence of need to use high fields in vacuum region may significantly simplify packaging, which would not require a high vacuum. 
     In short, unlike the prior devices, at least some embodiments of the present invention allows for cold durable emitters with large emitted currents and large efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings, in which: 
     FIG. 1A is a graph of a typical energy diagram for a material surface illustrating a concept of a work function of the material; 
     FIG. 1B is a graph of an energy diagram illustrating a concept of a negative electron affinity of a semiconductor material; 
     FIGS. 2A-2F illustrate exemplary cross sections of various embodiments of a cold emitter according to an aspect of the present invention; 
     FIG. 3A illustrates an exemplary energy band diagram in equilibrium across the line II—II of the embodiment of the cold emitter shown in FIG. 2A; 
     FIG. 3B illustrates an exemplary energy band diagram in equilibrium across the line across the line II′—II′ of the embodiment of the cold emitter shown in FIG. 2B; and 
     FIG. 4 illustrates an exemplary energy band diagram under bias of the cold emitters of FIGS.  2 A- 2 F. 
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent however, to one of ordinary skill in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structure have not been described in detail so as not to unnecessarily obscure the present invention. 
     FIG. 2A illustrates an exemplary cross section of a first embodiment of a cold emitter  200  according to an aspect of the present invention. The cold emitter  200  may generally be characterized as having an n+-p-M structure due to the presence of a n+ region  220 , a p region  230 , and a metallic layer  240 . As shown in FIG. 2A, the cold emitter  200  may include a substrate  210  and the n+ region  220  formed above the substrate  210 . The n+ region  220  may be formed from a wide band gap (WBG) semiconductor. Examples of WBG semiconductors include GaP, GaN, AlGaN, and carbon such as diamond, amorphous Si, AlN, BN, SiC, ZnO, InP, and the like. One of ordinary skill in the arts would recognize that other materials may be used as suitable WBG semiconductors. The electron concentration n, in the n+ region  220  is preferably above 10 17 /cm 3 , optimally may be above 10 19  cm −3 . However, depending on the types of applications, the concentration levels may be adjusted. 
     Indeed, the substrate  210  and the n+ region  220  may be formed from the same WBG semiconductor. The n+ region  220  may then be formed by doping the WBG semiconductor with electron rich materials. Examples of the electron rich materials include nitrogen (N), phosphorous (P), arsenic (As), and antimony (Sb). Again, one of ordinary skill in the arts would recognize that other electron rich materials may be used. 
     The cold emitter  200  may also include the p region  230  formed within or above the n+ region  220 . The p region  230  may be formed, for example, by counter doping the n+ region  220  with electron poor materials. An example of such materials includes boron. One of ordinary skill will recognize that other electron poor materials may be used. The p region  230  may also be formed from entirely separate materials than the n+ region  220 . It is preferred that the n+ region  220  be formed from a wider band gap material than the p region  230 . 
     The hole concentration p p  level in the p region  230  preferably ranges substantially between 10 16 −10 18 /cm 3 , with optimal concentration of about 10 18  cm −3 . The range may vary depending on the type of applications. It is preferred that the hole concentration is less than the electron concentration in the n+ region, i.e. p p &lt;n n . The ratio may be varied as well depending on the types of application. Also, W is preferred to be less than L, where W represents the thickness of the of the p region  230  as shown in FIG.  2 A and where L represents diffusion length of the non-equilibrium electrons in the p region  230 , also shown in FIG.  2 A. The diffusion length L is typically 0.3 μm. 
     The cold emitter  200  may further include the metallic layer  240  formed above the p region  230 . The metallic layer  240  may be formed from standard electrode materials like Au, Pt, W, and may also be formed from low work function materials. Examples of low work function materials include LaB 6 , CeB 6 , Au, Al, Gd, Eu, EuO, and alloys thereof. Preferably, the thickness t of the metallic layer  240  is on the order of or less than the mean free path l ε  for electron energy. Typically, l ε  ranges from 2-5 nanometers (nm). Thus, the thickness should be in the range t&lt;2-5 nm. 
     The selection of the material for the metallic layer  240  depends on the n + -p contact voltage difference between n+ region  220  and the p region  230 . With reference to FIG. 3A, which illustrates an exemplary energy band diagram in equilibrium of the first embodiment of the cold emitter  200  of FIG. 2A, the criteria for the selection of the material for the metallic layer  240  is explained below. If the n + -p contact voltage difference is represented as V np , then the built-in potential in the junction may be represented qV np ≈E g  (see FIG. 3A) where q&gt;0 represents the elementary charge and E g  represents the energy gap between the conduction band energy E C  and valence band energy E V  of the p-region  230  as shown in FIG.  3 A. 
     Preferably, the work function Φ M  of the metallic layer  240  is such that Φ M &lt;qV np ≈E g . For example, the E g  of diamond is about 5.47 eV. Thus, if diamond is used as the basis for the p region  230 , then gold may be employed as the metallic layer  240  since the work function of gold Φ M  is 4.75 eV. Other materials have even lower E g , such as LaB 6  and CeB 6  which have work functions that is substantially near 2.5 eV. One of ordinary skill would recognize that other materials maybe suitable as metallic layer  240 , and the layer  240  may not be limited strictly to metals. 
     Referring back to FIG. 2A, the electron cold emitter  200  may still further include an n electrode  260  and a p electrode  270  formed above the n+ region  220 . The n electrode  260  may be electrically connected to the n+ region  220  and the p electrode  270  may be electrically connected to the p region  230 . The n and p electrodes,  260  and  270 , may be formed from metal or other conductive materials. Examples of conductive materials include Au, Ag, Al, W, Pt, Ir, Pd, etc. and alloys thereof. In addition, the electron emitter  200  may include dielectric  250  to insulate the n and p electrodes,  260  and  270 , respectively. 
     FIG. 3A illustrates an exemplary energy band diagram in equilibrium across the line across the line II—II of the first embodiment of the cold emitter  200  of FIG.  2 A. As shown, left side of FIG. 3A corresponds to the bottom portion of the line II—II (n+ region  220 ) and the right side corresponds to the top portion (vacuum). 
     As noted above, it is preferred that the work function Φ M  of the metallic layer  240  be less than the energy gap of the p region  230 , i.e. E g ≈qV np &gt;Φ M . Under this condition, the energy level in the p region  230  junction exceeds the work function Φ M  of the metallic layer  240  as shown in FIG.  3 A. Thus, the cold emitter  200  behaves as if it has the negative electron affinity, Φ&lt;0, since the energy of electrons in p region lies above the vacuum level Vac. 
     The operation of the cold emitter  200  will be described with reference to FIGS. 2A,  3 A, and  4 . At equilibrium, no electron emission takes place. This is because equilibrium electrons are absent in p-region and a depletion interfacial layer is formed at the p-M interface between the p region  230  and the metallic layer  240  as shown in FIG.  3 A. Near the p-M interface, i.e. at the depletion interfacial layer, electrons lose energy and are not emitted from the metallic layer  240  into vacuum. This is due to the drop-off in the conduction band energy E C  near the p-M interface, such that at the interface, the conduction band energy E C  is below the energy level of vacuum Vac as shown in FIG.  3 A. 
     Ideally, there would be no depletion interfacial layer, and this is shown by the dotted line near the p-M interface. Without the depletion interfacial layer at the p-M interface, the cold emitter  200  has the property of a NEA, meaning that the electrons injected into p region  230  would be emitted out of the cold emitter  200 , since their energy in the p region  230  would be higher than the Vac. 
     The cold emitter  200  operates when the n + -p junction at the interface between the n+ region  220  and the p region  230  is forward biased, i.e. there is a positive potential on the p region  230  with respect to the n+ region  220 . The biasing potential may be applied via the n and p electrodes,  260  and  270 , respectively. When the n+-p junction is forward biased, the electrons from the electron-rich n+ region  220  are injected into the p region  230 . When the thickness W of the p region  230  is less than the diffusion length L of the non-equilibrium electrons in the p region  230 , the electrons traverse the p region  230  and accumulate in the depletion interfacial layer. 
     This is an analogue of a transistor effect, in which the current through the base electrode (attached to p region  230 ) is determined by recombination rate of injected electrons with holes. The injected electrons accumulate in the depletion layer, where the hole concentration is very small, so that their recombination rate is very small. As a result, electrons accumulate in the depletion interfacial layer until their local quasi-Fermi level E F  rises above the vacuum level Vac, as shown in FIG.  4 . Consequently, the emission of the injected electron rapidly increases. In this instance, the emitted current is much larger than the recombination current in the base (similar to usual semiconductor transistor). This allows for very large currents to be emitted. The emitted electrons are accelerated by field in vacuum towards an anode electrode (not shown in figures). 
     FIG. 2B illustrates an exemplary cross section of a second embodiment of a cold emitter  200 - 1  according to an aspect of the present invention. The cold emitter  200 - 1  may be described as a variation on the cold emitter  200  of FIG. 2A, and may generally be characterized as an n+-p-p+-M structure due to the presence of a p+ region  235  in between the p region  230  and the metallic layer  240 . As shown in FIG. 2B, the cold emitter  200 - 1  includes all of the elements of the cold emitter  200  shown in FIG.  2 A. For sake of simplicity, elements common to both cold emitters  200  and  200 - 1  will not be described in detail. It suffices to note that the behavior and the characterizations of the common elements may be similar. 
     The cold emitter  200 - 1 , in addition to elements of the cold emitter  200 , may also include the p+ region  235  formed within the p region  230 . The highly doped p+ region  235 , which may be very thin, may be formed by delta doping the p region  230  further with electron poor materials. The delta-doping produces a large concentration of a dopant in very thin layer. The hole concentration level in the p+ region  235  is preferably about 10 20 −10 21 /cm 3 , in a layer of thickness less than 100 nm. Also, the thickness W (this time of the p region  330  and the p+ region  335  combined) is preferred to be less than the diffusion length of the non-equilibrium electrons. Note that the p electrode  270  may be electrically contacting the p+ region  235  in addition to the p region  230 . 
     At least one role of the p+ region  235  is explained with reference to FIG. 3B, which illustrates an exemplary energy band diagram in equilibrium of the cold emitter  200 - 1  of FIG.  3 A. It was discussed above that with regards to cold emitter  200  (first embodiment) as shown in FIG. 2A, a depletion interfacial layer forms at the p-M interface between the p region  230  and the metallic layer  240 , and that near the p-M interface electrons lose energy. 
     The presence of the p+ region  235  decreases the band bending at the interface, and drives the emitter  200 - 1  closer to the ideal emitter with NEA. As shown in FIG. 3B, the drop-off in the conduction band level energy E C  for the emitter  200 - 1  is smaller than the drop-off for the emitter  200  (compare with FIG.  3 A). With the decreasing of the band bending, the quasi-local Fermi level for injected electrons, accumulated next to the p + -M interface, moves closer to the ideal position, which improves the conditions for electron emission. 
     The operation of the cold emitter  200 - 1  is similar to the operation of the cold emitter  200  as shown in FIG.  4 . In other words, the cold emitter  200 - 1  operates when the n + -p junction at the interface between the n+ region  220  and the p region  230  (and the p+ region  235 ) is forward biased. In this instance, the less forward biasing is required due to the presence of the p+ region  235  and the corresponding lessening of the depletion interfacial layer at equilibrium. 
     FIG. 2C illustrates an exemplary cross section of a third embodiment of a cold emitter  200 - 2  according to an aspect of the present invention. The cold emitter  200 - 2  may also be described as a variation on the cold emitter  200  of FIG. 2A, and may generally be characterized as an n+-p-M structure like the cold emitter  200 . 
     As shown in FIG. 2C, the cold emitter  200 - 2  may include all of the elements of the cold emitter  200  shown in FIG. 2A, except that the cold emitter  200 - 2  may not include the p electrode  270 , but may include an M electrode  290  formed above and electrically contacting the metallic layer  240 . For sake of simplicity, elements common to both cold emitters  200  and  200 - 2  will not be described in detail. It suffices to note that the behavior and the characterizations of the common elements may be similar. 
     At least one role that the M electrode  290  may play is explained as follows. With regards to the cold emitter  200  (and  200 - 1 ), the emitters operate when the n+-p junction becomes forward biased. The biasing was provided through application of appropriate potential to the n and p electrodes,  260  and  270 , respectively (see FIGS.  2 A and  2 B). With the cold emitter  200 - 2 , the n+-p junction may become forward biased by applying appropriate potential to the n and M electrodes,  260  and  290 , respectively. One of the advantages of the cold emitter  200 - 2  is that the device may be fabricated more easily when compared to the cold emitter  200  for example. 
     The operation of the cold emitter  200 - 2  is similar to the cold emitters  200  and  200 - 1  and need not be discussed in detail. 
     FIG. 2D illustrates an exemplary cross section of a fourth embodiment of a cold emitter  200 - 3  according to an aspect of the present invention. Like cold emitters  200 - 1  and  200 - 2 , the cold emitter  200 - 3  may be described as a variation on the cold emitter  200  of FIG.  2 A. The cold emitter  200 - 3  may generally be characterized as an n+-p-M structure. As shown in FIG. 2D, the cold emitter  200 - 3  includes all of the elements of the cold emitter  200  shown in FIG.  2 A. For sake of simplicity, elements common to both cold emitters  200  and  200 - 3  will not be described in detail. It suffices to note that the behavior and the characterizations of the common elements may be similar. 
     The cold emitter  200 - 3 , in addition to the elements of the cold emitter  200 , includes an M electrode  290  formed above and electrically contacting the metallic layer  240  and a second insulating layer  280 , which insulates the M electrode  290 . In this instance, the forward biasing of the n+-p junction may be provided through applying potentials to the n and p electrodes,  260  and  270 , respectively, as before with the cold emitter  200 . 
     The general operation of the cold emitter  200 - 3  is similar to the cold emitters  200  and  200 - 1  and need not be discussed in detail. However, the M electrode  290  adds an additional controllability in the operation of the cold emitter  200 - 3 . In this instance, the metallic layer  240  may be used to control the amount of emitter current. This is very advantageous in applications requiring arrays with individually controlled emitters. The emission current can be controlled by biasing the potential on metallic layer  240  through the M electrode  290 . This closes and opens the emission current from the cold emitter  200 - 3 . 
     The individual variations noted with the second, third, and fourth embodiments (cold emitters  200 - 1 ,  200 - 2 , and  200 - 3 , respectively) may be combined to reap the benefits of individual variations in one device. As examples, FIGS. 2E and 2F FIG. 2D illustrate exemplary cross sections of fifth and sixth embodiments of a cold emitter,  200 - 12  and  200 - 13  according to other aspects of the present invention. 
     FIG. 2E illustrates an example of a combination of the cold emitters  200 - 1  and  200 - 2  (second and third embodiments, respectively). As shown, like the cold emitter  200 - 1 , the cold emitter  200 - 12  includes a p+ region  235 , and thus may be generally characterized as having an n+-p-p+-M structure. Also, like the cold emitter  200 - 2 , the cold emitter  200 - 12  lacks the p electrode  270 , but includes the M electrode  290 . 
     The cold emitter  200 - 12  allows the potential to be applied to the p region  230  via the metallic layer  240 . Also, due to the presence of the p+ region  235 , relatively less forward biasing may be required. 
     FIG. 2F illustrates an example of a combination of the cold emitters  200 - 1  and  200 - 3  (second and fourth embodiments, respectively). As shown, like the cold emitter  200 - 1 , the cold emitter  200 - 12  includes a p+ region  235 , and thus may be generally characterized as having an n+-p-p+-M structure. Also, like the cold emitter  200 - 3 , the cold emitter  200 - 13  includes the M electrode  290  and the second insulator  280 . 
     The cold emitter  200 - 13  allows the current amount to be controlled through appropriate biasing of the M electrode  290 . Also, due to the presence of the p+ region  235 , it is easier to fulfill the condition for NEA. 
     What has been described and illustrated herein is a preferred embodiment of the invention along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.