Abstract:
A transistor operated by changing the electrostatic potential of an island disposed between two tunnel junctions. The transistor has an island of material which has a band gap (e.g. semiconductor material). Source and drain contacts are provided. The transistor has a first tunnel junction barrier disposed between island and source, and a second tunnel junction barrier disposed between island and drain. The island is Ohmically isolated from other parts of the transistor as well as a substrate. A gate electrode is capacitively coupled to the island so that a voltage applied to the gate can change the potential of the island. The transistor has n- and p-type embodiments. In operation, applying a gate voltage lowers (e.g., for positive gate bias) or raises (e.g., for negative gate bias) the conduction band and valence band of the island. When the conduction band or valence band aligns with the Fermi energy of the source and drain, tunneling current can pass between the source, island and drain.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to solid state switching and amplification devices, i.e. transistors. More particularly, it relates to tunneling transistor devices having tunnel junctions. 
   BACKGROUND 
   Complementary metal oxide semiconductor (CMOS) devices such as MOSFET transistors are commonly used in high speed, highly integrated circuits. Integrated circuit manufacturers are constantly increasing the operating speed and decreasing the size of MOSFET transistors. Such improvements yield smaller, faster ICs with more functions at lower cost. 
   Various problems exist with scaling MOSFET devices below 0.1 microns, however. For example, with channel lengths less than 0.1 microns the required channel doping levels become very high. It is difficult to produce high doping levels with high uniformity over the surface of a wafer. Therefore, different MOSFETs manufactured on the same wafer will have very different characteristics if high doping levels are used. Also, capacitive coupling between drain and source regions of individual MOSFETs becomes significant. Problems also exist in mass producing such devices. 
   For these reasons, researchers have been investigating transistor devices based on the quantum behavior of electrons in very small devices. A number of such devices that exploit electron tunneling are known in the art. 
   For example, U.S. Pat. No. 5,705,827 to Baba et al. discloses a tunneling transistor device having an insulated gate. The transistor operation is provided by band bending in a current channel adjacent to the gate electrode, as in a MOSFET device. The drain electrode forms an Esaki tunnel junction with the current channel. 
   U.S. Pat. No. 4,675,711 to Harder et al. discloses a tunneling transistor using an insulated gate electrode disposed adjacent to a tunneling layer. The tunneling layer has a band gap energy different from that of semiconductor source and drain contacts. A voltage applied to the gate changes an energy barrier height of the tunneling layer, thereby controlling a tunnel current through the tunnel layer. The device must be operated at low temperature so that thermally excited carriers do not provide conduction through the tunnel layer. 
   U.S. Pat. No. 5,834,793 to Shibata discloses a tunneling MOSFET transistor device having an insulated gate contact. Adjacent to the gate contact is a short current channel. Source and drain contacts are separated from the current channel by dielectric tunnel barriers about 30 Angstroms thick. The device exhibits negative resistance characteristics due to discrete energy states in the current channel. 
   U.S. Pat. No. 5,291,274 to Tamura discloses a tunneling transistor. The transistor of Tamura has a middle layer high dielectric constant material disposed between two tunnel junctions. The middle layer is in direct contact with a gate electrode. Source and drain electrodes are provided in contact with the tunnel junctions. When a voltage is applied to the gate electrode, the electrical potential of the middle layer is changed, thereby allowing electrons to tunnel between source and drain. A problem with the device of Tamura is that current will flow to and from the gate electrode when the device is on. Therefore, the device of Tamura requires continuous gate current for continuous operation. This is highly undesirable in many applications. 
   In addition to the above, others have investigated the uses of single electron transistors having tunneling junctions. A single electron transistor has a very small metallic or semiconductor island disposed between two tunnel junctions having a high resistance. Source and drain contacts are made to the tunnel junctions. A gate electrode capacitively coupled to the island provides switching control. The island is made sufficiently small such that an energy required to charge the island with a single electron is greater than the thermal energy available to electrons in the source and drain contacts. The energy required to charge the island with a single electron is given by E C =e 2 /2C, where e is the charge of an electron, and C is the capacitance of the island. This energy requirement for charging the island is termed the Coulomb blockade. 
   In operation, a voltage applied to the gate electrode capacitively raises or lowers the potential of the island. When the island potential is lowered by a certain amount, electrons can tunnel through one tunnel junction onto the island, and tunnel through the other tunnel junction off of the island. In this way, current is allowed to flow through the island for certain values of gate voltage. The resistance of a single electron transistor exhibits oscillations as gate voltage changes monotonically. 
   Available thermal energy increases with temperature, of course, so a single electron transistor has a maximum temperature at which it can be operated. The maximum operating temperature is determined by the capacitance of the island, which is a function of the island size. For devices to operate at room temperature, the capacitance C must be less than about 10 Attofarads. Realizing such low capacitance requires that the island be very small (e.g., less than 10 nm on a side) and located relatively far from the source, drain and gate. It is very difficult to make a single electron transistor which operates at room temperature. 
   An important concern in the design of a single electron transistor is the resistance of the tunnel junctions. It is best for a single electron transistor to have tunnel junctions with relatively high resistances (i.e., much greater than a quantum resistance R q =h/2e 2 ≈26 KOhms, where h is Planck&#39;s constant). If the resistance of the tunnel junctions is too low, then the number of electrons on the island is not well defined. Operation of a single electron transistor requires that the tunnel junctions have sufficiently high resistances such that electron locations are well defined as being either in the island or outside the island. However, high tunnel junction resistance results in a high resistance between source and drain contacts, even in a fully ‘ON’ state. A high resistance limits the switching speed and increases the power consumption of the device. Therefore, single electron transistors are limited in their electrical characteristics and potential applications. 
   A distinguishing characteristic of single electron transistor devices is that the island can be made of semiconductor material or metal. The island does not need to be made of material having an electron energy band gap. 
   SUMMARY OF THE INVENTION 
   Disclosed herein is a transistor that includes a pair of tunnel junctions (or barriers), each having a resistance less than or equal to approximately a quantum resistance. The tunnel junctions are separated from one another by an island formed of a material having a non-uniform density of energy states (e.g., at least one region that contains available energy states adjacent to at least one region that does not contain any available energy states). The tunnel junctions are each disposed between a respective one of a pair of conductors (e.g., source and drain conductors) and the island, and a gate electrode is capacitively coupled to the island. 
   In some cases, the island may be formed of a semiconductor material, for example, silicon, germanium or any other semiconductor. In other cases, a superconductor may be used. The tunnel barriers may be formed of an oxide of the material from which the conductors (and/or the gate electrode) or the island is/are made or may be formed from a different material all together. In operation, a conduction path between the tunnel junctions may be formed by shifting the energy states of the island through the application of a potential to gate electrode. A current may then be passed through the conduction path via the source and drain electrodes. 
   In one embodiment, an apparatus for switching electrical current has an ohmically isolated island made of material (e.g., a semiconductor material such as silicon, gemanium, etc.) having a band gap. The island is sufficiently large such that electron energy levels within the island are preferably separated by less than 100 meV. The apparatus also has a source contact and a first tunnel junction barrier located between the source contact and the island. The first tunnel junction barrier has a thickness and cross sectional area selected such that a first tunnel junction formed by the interconnection of the source contact, the first tunnel junction barrier and the island has a resistance less than a quantum resistance, i.e., less than 26 KOhms. The apparatus also has a drain contact and a second tunnel junction barrier located between the drain contact and the island. The second tunnel junction barrier has a thickness and cross sectional area selected such that a second tunnel junction formed by the interconnection of the drain contact, the second tunnel junction barrier and the island also has a resistance less than the quantum resistance. The apparatus also has a gate electrode capacitively coupled to the island. 
   In some cases, the first and second tunnel junctions may have resistances less than 10 KOhms. Further, in other embodiments the first and second tunnel junctions may have resistances less than 1 KOhm or even less than 100 Ohms. 
   The first and second tunnel junction barriers may be made of an insulating material, such as silicon dioxide or aluminum oxide, and may be separated by a distance of approximately 0.2-2.0 microns. 
   Preferably, the apparatus includes an insulating layer disposed between the gate electrode and the island. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present transistor is illustrated by way of example, and not limitation, in the accompanying drawings, in which: 
       FIG. 1  shows a transistor structure according to an embodiment of the present invention; 
       FIG. 2  shows an energy band diagram of the device illustrated in  FIG. 1 , in a particular embodiment where the island is n-doped; 
       FIG. 3  illustrates a circuit for using the device shown in  FIG. 1 ; 
       FIG. 4  shows the energy band diagram of the device illustrated in  FIG. 2  with a potential applied between source and drain, and zero potential applied between gate and drain; 
       FIG. 5  shows the energy band diagram of the device of  FIG. 2  with a potential applied between source and drain sufficient for conduction; 
       FIG. 6  shows the energy band diagram of the device of  FIG. 2  with a positive potential applied to the gate with respect to the drain; 
       FIG. 7  shows a set of I-V (current-voltage) curves for an n-type device configured in accordance with the present invention; 
       FIG. 8  shows an embodiment of the present transistor in which the island is p-doped, i.e., a p-type device; and 
       FIG. 9  shows an energy band diagram for p-type device configured in accordance with the present invention with a negative gate voltage applied. 
   

   DETAILED DESCRIPTION 
   A switching device employing low resistance tunnel junctions is disclosed herein. 
   More specifically, a transistor-like device having a pair of tunnel junctions, each with a resistance less than or equal to approximately the quantum resistance (R q ≈h/2e 2 ), and being separated by an island formed of a material having a non-uniform density of energy states is proposed. The use of low resistance tunnel junctions is in contrast to the approach used in single electron transistors and the like. In essence, by eschewing the Coulomb blockade approach, the present circuit is able to operate at room temperatures without the severe size restrictions imposed on Coulomb blockade devices. Furthermore, the present circuit differs from resonant tunneling transistors (RTTs) and similar devices, which rely on quantum wells to set the energy scale of the device for its operation. Although the present device is discussed with reference to certain illustrated embodiments thereof, upon review of this specification those of ordinary skill in the art will recognize that the present circuit may be constructed in a number of ways and may find application in a variety of systems. Therefore, in the following description the illustrated embodiments should be regarded as exemplary only and should not be deemed to be limiting in scope. 
   More precisely, the present transistor includes an island made of material having a band gap. The island is preferably sufficiently large such that electron energy states thereinare separated by less than 100 meV (i.e., energy states in the valence or conduction band, not the band gap). Therefore, at room temperature, the valence and conduction bands of the island behave as continuous energy bands. The island may be regarded as a region that is not connected by Ohmic conduction paths to any other region of the transistor. Metallic leads may be used for source and drain electrodes, and a gate electrode may be capacitively coupled to the island. The tunnel junctions may be formed at the interconnections of tunnel junction barriers disposed between the island and the source and drain electrodes and these tunnel junction barriers may be formed of an insulating material. As indicated above, the tunnel junctions have a resistance less than a quantum resistance, e.g., less than 26 KOhms. This is possible because the present transistor does not rely on a Coulomb blockade to achieve switching behavior. 
     FIG. 1  shows one embodiment of the present transistor. An insulating layer  22  (e.g., SiO 2 ) of thickness  40  is disposed on a substrate  20 . The substrate may be made of an appropriate semiconductor material, silicon, for example. Thus, layer  22  may be grown by wet or dry oxidation as is common in the semiconductor processing arts. A gate electrode  24  is located between the substrate  20  and layer  22 . 
   An island  26  is located on top of the layer  22  and is aligned opposite the gate  24 , so that the gate and the island are capacitively coupled. The island can have a wide range of doping levels, including no doping at all. A source contact  28  and a drain contact  30  are provide at opposite sides of the island  26 , and a thin, insulating film  32  forms a first tunnel junction  34  between the source  28  and the island  26 . Film  32  also forms a second tunnel junction  36  between the drain  30  and island  26 . First tunnel junction  34  (i.e., the film  32  at the point of the first tunnel junction) has thickness  35 , and second tunnel junction  36  (i.e., the film  32  at the point of the second tunnel junction) has thickness  37 . Thicknesses  35 ,  37  are determined by the thickness of film  32 . Note, the film  32  may be formed from a material of which island  26  is made (e.g., an oxide thereof), of which source and drain contacts  28  and  30  are made (e.g., an oxide thereof) or of a different material all together. 
   The source contact  28  and drain contact  30  are preferably made of a metal such as aluminum, copper, gold, titanium or the like. Source and drain contacts made of metal are preferred because metals have higher carrier mobilities. Therefore, metal source and drain contacts provide superior high frequency performance and switching and low power characteristics (e.g., over contacts formed of other materials, such as semiconductors). 
   It is noted that the apparatus of  FIG. 1  is symmetrical; that is, source  28  and drain  30  are interchangeable and tunnel junctions  34  and  36  are also interchangeable. Most embodiments of the present transistor are symmetrical. However, in some embodiments of the present transistor, first and second junctions  34  and  36  are not identical, and, therefore, in these embodiments the apparatus is not symmetrical. 
   Film  32  is preferably very thin so that tunnel junctions  34  and  36  have relatively low resistances. For example, film  32  may be 1-40 Angstroms thick. Film  32  may be formed by a chemical vapor deposition (CVD) process, or by oxidizing the island material, for example. Of course, other manufacturing processes may be used, depending on the material of which film  32  is made. In the figure, film  32  is shown to cover the entire island  26 , however, in other embodiments film  32  may cover the island only in regions close to the tunnel junctions  34  and  36 . 
   Island  26  is made of a material having a band gap, such as silicon, germanium or any other semiconductor material. Island  26  can also be made of superconductor materials, which have a band gap when cooled below a critical temperature. Island  26  is not made of metal. Preferably, island  26  is made of doped (or undoped) semiconductor material. Thus, embodiments of present transistor include p-type and n-type devices having p- and n-doped semiconductor islands. 
   Tunnel junctions  34  and  36  each have a resistance less than the quantum resistance (e.g., approximately 26 KOhms). The resistance of the first tunnel junction  34  is determined by the thickness  35 , and a surface area of contact (i.e., the junction area) of film  32  between the source  28  and island  26 . The resistance of the second tunnel junction  36  is determined by the thickness  37 , and a surface area of contact of film  32  between the drain  30  and island  26 . The resistance of the tunnel junctions  34 ,  36  scales linearly with junction area (lower resistance for larger junction area), and exponentially with thickness (lower resistance for thinner junctions). The tables below provide exemplary (and approximate) thicknesses and junction areas for tunnel junctions having different resistances: 
   
     
       
             
             
             
           
             
           
             
             
             
           
             
           
             
             
             
           
             
           
             
             
             
           
         
             
                 
                 
             
             
                 
               Junction Area 
               Film Thickness 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               For 26 K-Ohm Tunnel Junctions 
             
           
        
         
             
                 
               50 nm × 50 nm 
               12 Angstroms 
             
             
                 
               100 nm × 100 nm 
               18 Angstroms 
             
             
                 
               200 nm × 200 nm 
               24 Angstroms 
             
           
        
         
             
               For 13 K-Ohm Tunnel Junctions 
             
           
        
         
             
                 
               50 nm × 50 nm 
                9 Angstroms 
             
             
                 
               100 nm × 100 nm 
               15 Angstroms 
             
             
                 
               200 nm × 200 nm 
               21 Angstroms 
             
           
        
         
             
               For 2.6 K-Ohm Tunnel Junctions 
             
           
        
         
             
                 
               50 nm × 50 nm 
                2 Angstroms 
             
             
                 
               100 nm × 100 nm 
                8 Angstroms 
             
             
                 
               200 nm × 200 nm 
               14 Angstroms 
             
             
                 
                 
             
           
        
       
     
   
   More preferably, the tunnel junctions  34  and  36  each have a resistance less than 10 KOhms, and most preferably less than 1000 Ohms. These resistance values are achieved by appropriately selecting the thickness and junction area of film  32  in the area of the tunnel junctions  34  and  36 . It will be apparent to one of ordinary skill in the art that many different combinations of junction thickness and junction area provide junction resistance less than the quantum resistance. 
   Gate  24  is capacitively coupled to island  26  through layer  22 . Thickness  40  is thick enough so that a resistance between gate  24  and island  26  is very high, such that it essentially draws no current. For example, this resistance may be on the order of 10 8  Ohms or greater, more preferably, on the order of 10 10 -10 12  Ohms, or greater. Because gate  24  and island  26  are only capacitively coupled, essentially no tunnel current or Ohmic current can flow between the gate  24  and island  26 . 
     FIG. 2  shows a schematic band diagram for an n-type device with no voltages applied to the source  28 , drain  30  or gate  24 . In this embodiment the island  26  is made of n-doped semiconductor material. Source  28  and drain  30  are metals and so have well defined Fermi energies  42   s  and  42   d , respectively. Island  26  has a Fermi energy  43 . Island  26  has bandgap  52 , which is on the order of 0.5-3 electron volts, for example. Tunnel junctions  34  and  36  (i.e., the tunnel junction barriers disposed between the source/drain and the island) are made of an insulating material and so have large band gaps  50  compared to island  26 . Also shown is an island conduction band  54 , and an island valence band  56 . Since island  26  is made of n-doped semiconductor material, valence band  56  is completely full, and conduction band  54  is partially full. Also, island Fermi energy  43  is relatively close to conduction band  54 , and donor levels  45  are present just below the conduction band edge. 
   Conduction band  54  and valence band  56  have many electron energy levels  58  indicated by horizontal lines. As is known in the art, a spacing between the energy levels  58  is dependent upon the size of the island  26  and the material comprising the island. In the present transistor, the island  26  is designed so that the energy levels  58  are separated in energy by less than about 100 meV, more preferably, less than 50 meV and most preferably less than 25 meV. This is preferred in the present transistor because it assures that, at room temperature, the valence and conduction bands behave as approximately continuous bands. This is because at room temperature (i.e., where T is approximately 300K) K b T˜25 meV, where K b  is Boltzmann&#39;s constant. In other words, if the energy levels  58  are spaced apart by less than 25-100 meV, electrons at room temperature have enough thermal energy to travel between energy levels  58 . 
     FIG. 3  shows an electrical schematic illustrating how (in one embodiment) the present transistor is used in an electrical circuit. Source  28 , drain  30 , island  26 , and tunnel junctions  34 ,  36  are indicated. Capacitor  60  represents capacitance between gate  24  and island  26 . A bias voltage supply V b    61  provides a voltage between source  28  and drain  30 . The bias supply can provide voltage of both polarities to the source and drain. A gate voltage supply V g    62  provides voltage between gate  24  and drain. Gate voltage supply  62  can provide both positive and negative voltage to gate  24  with respect to drain  30 . 
     FIG. 4  shows a band diagram of an n-type device while the bias supply  61  applies a small negative voltage to the source  28  with respect to the drain  30 . Gate voltage V g  is zero (i.e., gate  24  and drain  30  are at the same voltage). Voltage  55  across first tunnel junction  34  is not equal to voltage  57  across second tunnel junction  36  due in part to different junction capacitances. More generally, relative voltages across the tunnel junctions  34  and  36  depend upon the relative capacitances between source  28 , island  26 , drain  30  and gate  24 . Also, the different voltages across tunnel junctions  34  and  36  are due to the fact that gate  24  is at the same voltage as drain  30 . 
   Current does not tunnel between source  28  and drain  30  because the bottom edge of conduction band  54  is higher in energy than the source Fermi energy. Therefore, electrons at the source Fermi energy  42   s  cannot tunnel to energy levels  58  in the conduction band  54 . Also, electrons in the valence band  56  cannot tunnel to energy levels at the drain Fermi energy  42   d.    
     FIG. 5  shows a band diagram of the device while the bias supply  58  applies a bias voltage just sufficient to cause conduction. Again, gate voltage V g  is zero. The bias voltage applied in  FIG. 5  is greater than the bias voltage applied in  FIG. 4 . The bias voltage necessary for conduction (with no gate voltage applied) is the voltage which causes the source Fermi energy  42   s  to align with the conduction band  54 /or donor levels  45 . Electrons at the Fermi energy E f  in the source  28  tunnel  64  to the conduction band  54 , and then tunnel  66  from the conduction band to the drain. The electrons arrive in the drain as hot electrons above the drain Fermi energy  42   d . Again, voltages across tunnel junctions  34  and  36  are shown as unequal, possibly due to differences in relative capacitances, as well as the fact that gate  24  and drain  30  are at the same voltage. It is noted that voltages across junctions  34  and  36  can be equal or unequal in the present transistor. 
     FIG. 6  shows a bandgap diagram of the n-type device with a positive voltage applied to the gate  24  with respect to drain  30 . The conduction band  54  is lowered in energy so that it aligns with the source and drain Fermi energies  42   s  and  42   d . Therefore, when a small negative voltage is applied to source  28  with respect to drain  30 , electrons can tunnel from source  28 , to island  26 , to drain  30 . Alternatively, a negative voltage applied to drain  30  will cause electrons to tunnel from drain  30 , to island  26 , to source  28 . Therefore, a sufficiently positive bias applied to gate  24  with respect to the drain  30  allows the device to conduct current in both directions. 
   To summarize, in the case where the island  26  is made of n-doped semiconductor material, application of a positive gate voltage V g  reduces the bias voltage V b  necessary to allow conduction. Conversely, for n-doped devices, a negative gate voltage V g  increases the bias voltage V b  necessary to cause conduction. 
     FIG. 7  shows a plot of bias voltage (i.e., voltage between source  28  and drain  30 ) versus drain current for different values of gate voltage V g . The plot of  FIG. 7  is for a device with an n-doped semiconductor island  26 . V d  represents drain voltage, and V s  represents source voltage. A threshold bias voltage  70  is the bias voltage for which the source Fermi energy  42   s  is aligned with the bottom edge of the conduction band  54 . The energy band diagram of  FIG. 5  corresponds approximately to the threshold  70 . 
   A complementary threshold bias voltage  72  represents the bias voltage for cases where a negative voltage is applied to drain. The threshold bias  70  and complementary threshold bias  72  do not necessarily have the same voltage magnitude. Thresholds  70  and  72  are defined for zero gate voltage. 
   It is noted that the threshold bias voltages  70  and  72  depend in part upon the band gap  52  of the island  26 . If the band gap energy  52  is high (e.g., 4-5 electron volts), then the threshold bias voltages  70  and  72  will be relatively high. If the band gap energy is low (e.g. 0.2-1.5 electron volts), then the threshold bias voltages  70  and  72  will be relatively low. 
   Also, threshold bias voltages  70  and  72  depend upon the doping level of the island  26 . If the island is highly doped, then threshold bias voltages will be relatively low; if the island is lightly doped, then threshold bias voltages will be relatively high. 
   The threshold bias voltages  70  and  72  also depend upon the relative capacitances of tunnel junctions  34  and  36 . Consider, for example, a case when source  28  is negative with respect to drain  30  and first tunnel junction  34  has a relative low capacitance. A voltage applied between source  28  and drain  30  will mostly be across the first tunnel junction  34 . Therefore, only a relatively low voltage is required to align source Fermi energy  42   s  and conduction band  56 . That is, threshold voltage  70  will be relatively low. Complementary threshold voltage  72  will be relatively high. Most generally, differences between the first and second tunnel junction characteristics result in differences in threshold bias voltage  70  and complementary threshold bias voltage  72 . 
     FIG. 8  shows an embodiment of the present transistor in which the island is p-doped, i.e., a ‘p-type’ device. The conduction band  54  and valence band  56  are shifted up in energy compared to the device of  FIG. 2 , which has an n-doped island  26 . The p-doped island  26  in  FIG. 8  has acceptor states  78  slightly above the valence band edge. The p-type device will conduct between source  28  and drain  30  when the valence band  56  is aligned with the source Fermi energy  42   s  or drain Fermi energy  42   d.    
     FIG. 9  shows a p-type device with a negative gate voltage applied. The valence band  56  and acceptor states  78  are raised in energy and aligned with the source Fermi energy  42   s  and drain Fermi energy  42   d . When a negative voltage is applied to the source  28  with respect to drain  30 , electrons tunnel  80  between the source  28 , island  26  and drain  30 . Alternatively, a negative voltage is applied to drain  30  with respect to source  28 . Of course, it should be remembered that island  26  may be undoped. 
   It will be clear to one of ordinary skill in the art that the above embodiments may be altered in many ways without departing from the broader scope of the present invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.