Patent Abstract:
An indirectly induced tunnel emitter for a tunneling field effect transistor (TFET) structure includes an outer sheath that at least partially surrounds an elongated core element, the elongated core element formed from a first semiconductor material; an insulator layer disposed between the outer sheath and the core element; the outer sheath disposed at a location corresponding to a source region of the TFET structure; and a source contact that shorts the outer sheath to the core element; wherein the outer sheath is configured to introduce a carrier concentration in the source region of the core element sufficient for tunneling into a channel region of the TFET structure during an on state.

Full Description:
BACKGROUND 
     The present invention relates generally to semiconductor device structures and, more particularly, to an indirectly induced tunnel emitter for tunnel field effect transistor (TFET) devices. 
     Microelectronic devices are typically fabricated on semiconductor substrates as integrated circuits, which include complementary metal oxide semiconductor (CMOS) field effect transistors as one of the core elements thereof Over the years, the dimensions and operating voltages of CMOS transistors are continuously reduced, or scaled down, to obtain ever-higher performance and packaging density of the integrated circuits. 
     However, one of the problems resulting from the scaling down of CMOS transistors is that the overall power consumption of the devices keeps increasing. This is partly because leakage currents are increasing (e.g., due to short channel effects) and also because it becomes difficult to continue to decrease the supply voltage. The latter problem, in turn, is mainly due to the fact that the inverse subthreshold slope is limited to (minimally) about 60 millivolts (mV)/decade, such that switching the transistor from the OFF to the ON states requires a certain voltage variation, and therefore a minimum supply voltage. 
     Accordingly, tunnel field effect transistors (TFETs) have been touted as “successors” of metal oxide semiconductor field effect transistors (MOSFETs), because of the lack of short-channel effects and because the subthreshold slope can be less than 60 mV/decade, the physical limit of conventional MOSFETs, and thus potentially lower supply voltages may be used. On the other hand, TFETs typically suffer from low on-currents, which is a drawback related to the large resistance of the tunnel barrier. 
     SUMMARY 
     In an exemplary embodiment, an indirectly induced tunnel emitter for a tunneling field effect transistor (TFET) structure includes an outer sheath that at least partially surrounds an elongated core element, the elongated core element formed from a first semiconductor material; an insulator layer disposed between the outer sheath and the core element; the outer sheath disposed at a location corresponding to a source region of the TFET structure; and a source contact that shorts the outer sheath to the core element; wherein the outer sheath is configured to introduce a carrier concentration in the source region of the core element sufficient for tunneling into a channel region of the TFET structure during an on state. 
     In another embodiment, a method of forming an indirectly induced tunnel emitter for a tunneling field effect transistor (TFET) structure includes forming an elongated core element from a first semiconductor material; forming an insulator layer that at least partially surrounds the core element; forming an outer sheath that at least partially surrounds the insulator layer at a location corresponding to a source region of the TFET structure; and forming a source contact that shorts the outer sheath to the core element; wherein the outer sheath is configured to introduce a carrier concentration in the source region of the core element sufficient for tunneling into a channel region of the TFET structure during an on state. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  is a band diagram that illustrates electron tunneling across a P/N junction of a TFET; 
         FIG. 2  is a band diagram for a TFET device having a staggered band heterojunction in the “on” state; 
         FIG. 3  is a band diagram for a TFET device having a staggered band heterojunction in the “off” state; 
         FIG. 4  is a cut-away sectional view of a source region of a TFET device wherein holes are induced in a nanowire by a surrounding metal sheath that is separated from the nanowire by a thin insulator layer; and 
         FIG. 5(   a ) is a side cross-sectional views of a TFET structure having an Indirectly Induced Tunnel Emitter (IITE), in accordance with an exemplary embodiment of the invention; 
         FIG. 5(   b ) is an end cross-sectional view of the IITE, taken along the lines b-b of  FIG. 5(   a ); 
         FIG. 6  is a partial band diagram illustrating the valence bands for an exemplary n-channel TFET as shown in  FIGS. 5(   a ) and  5 ( b ); 
         FIG. 7(   a ) is another side cross-sectional view of the TFET structure of  FIG. 5(   a ); 
         FIG. 7(   b ) is a band diagram corresponding to the structure of  FIG. 7(   a ); and 
         FIG. 8  is a generic band diagram of a heterojunction tunneling emitter where the bands correspond to conduction band convention and are inverted with respect to  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     As indicated above, in recent years the TFET has generated much interest as a possible candidate used for low power electronics. Typically, in an n-channel TFET for example, electrons are injected from the top of the valence band in the source region of the device into the bottom of the conduction band in the channel of the device.  FIG. 1  is a band diagram that illustrates this process for a simple P/N junction, wherein the “P” side represents the source region and the “N” side represents the channel of a TFET. In the “on” state (as indicated by the darkened curves denoting the bands) electrons can tunnel from the valence band in the source to the conduction band in the channel. Applying an increasing negative gate voltage to a “partially on” state causes the tunneling distance to increase (indicated by the large dashed curves), and eventually the bands become uncrossed (indicated by the short dashed curves) shutting off the current. 
     One type of junction arrangement for a TFET device is what is known as a staggered band heterojunction line up, illustrated in the band diagrams of  FIGS. 2 and 3 . In this arrangement, the energy bands in the source and channel regions are offset from one another so as to allowing switching from the “on” state in  FIG. 2  to the “off” state in  FIG. 3  with much smaller longitudinal electric fields. 
     A primary objective of TFET use is to achieve switching from “on” to “off” over a much smaller voltage range than a conventional FET. This is realized because a conventional n-type source used in an NFET is replaced by a p-type tunneling source (also referred to herein as an “emitter”) where the top of the valence band cuts off the thermal tail of the Fermi function, which is present in the n-type source, allowing for an inverse sub-threshold slope S of smaller than 60 mV/dec at room temperature, where S=[d(log 10  I D )/dV G ] −1 , wherein I D  is the drain current and V G  is the gate voltage. 
     On the other hand, the band diagrams of  FIGS. 2 and 3  also illustrate several factors that serve to increase S and degrade the performance of the TFET. For example, in the “on” state depicted in  FIG. 2 , degeneracy in the source (region (a) in  FIG. 2 ) reduces the states available for tunneling, thereby reducing the “on” current. In addition, band bending (region (b) in  FIG. 2 ) increases the gate voltage needed to turn on the TFET. In the “off” state depicted in  FIG. 3 , band bending (regions (c) and (d) in  FIG. 3 ) increases the voltage swing required to turn off the TFET and leaves potential wells in the valence and conduction bands. Here, thermal tails can cause a reversion to the 60 mV/decade slope when tunneling from the wells, band-to-band transfer by multiphonon processes (region (e) of  FIG. 3 ), or band-to-band transfer via tunneling by gap states (region (f) of  FIG. 3 ). 
     Although the presence of a high dopant concentration in the source could reduce such band bending, the resulting disorder caused by the doping can induce gap states, and the high carrier concentration could in turn lead to excessive degeneracy. Thus, one possible solution to this problem is to use “electrostatic doping”, as illustrated in  FIG. 4 . More specifically,  FIG. 4  is a cut-away sectional view of a source region of a TFET device  400  where, in this example, holes are induced in a nanowire  402  by way of a surrounding metal sheath  404  that is separated from the nanowire  402  by a thin insulator layer  406 , similar to a gate conductor and gate dielectric layer of an FET. The proximity to the surrounding metal sheath  404  screens the electric field inside the nanowire  402 , thus obviating the need for a large hole concentration in the nanowire itself Here, a heavily doped section  408  of the nanowire  402 , remote from the tunnel injector (not shown in  FIG. 4 ), provides electrical contact to the TFET. While this solution solves some of the problems outlined above, it also creates others. For example, the TFET  400  of  FIG. 4  would need a separate electrical contact for the metal sheath  404 , complicating the design. In addition, the interface states at the insulator-nanowire boundary may provide additional tunneling paths, and metal-induced gap states may be induced by the close proximity of the sheath  404  to the channel. 
     Accordingly,  FIGS. 5(   a ) and  5 ( b ) are side and end cross-sectional views, respectively, of a TFET structure  500  having what is referred to herein as an Indirectly Induced Tunnel Emitter (IITE), in accordance with an exemplary embodiment of the invention. As is shown, the IITE includes a elongated core element  502  (e.g., a nanowire) formed from a first semiconductor material (S 1 ), an insulator layer  504  formed from a second semiconductor material (S 2 ) that surrounds the nanowire, the second semiconductor material (S 2 ) having a wider bandgap than the first semiconductor material (S 1 ), a doped outer semiconductor sheath  506  formed from a third semiconductor material (S 3 ) that surrounds the insulator  504 , and a source contact  508  formed from a fourth semiconductor material (S 4 ) that shorts the outer semiconductor sheath  506  to the core element  502 . 
     In an exemplary embodiment, the materials used for semiconductors S 1 -S 4  could all be epitaxially grown semiconductors forming heterojunctions at their interfaces. This could reduce or eliminate interface states, which represent a problem for TFET structures such as the one shown in  FIG. 4 . Because the outer sheath  506  is also a doped semiconductor (S 3 ), metal induced gap states (MIGS) are also eliminated. Further, the TFET structure shown in  FIGS. 5(   a ) and  5 ( b ) may be simplified by using the same semiconductor material for S 1 , S 3  and S 4 . 
     Although the exemplary embodiment depicted illustrates a concentric circular configuration for the core element, insulator and outer sheath, it is contemplated that other suitable geometries may be used. For example, the cross-sectional shapes of the individual element may be other shapes besides circular, such as elliptical, oval, square or rectangular, for example. Furthermore, while the illustrated embodiment depicts layers completely surrounding other layers (e.g., the insulator layer  504  surrounding the core element  502 ), it is also contemplated that an outer layer of the structure can partially surround an inner layer of the structure, such as an omega (Ω) shape, for example. 
     With respect to the elongated core element  502 , in addition to a nanowire structure element, the core element  502  could also be formed from other structures such as a semiconductor fin or a carbon nanotube, for example. 
     Referring now to  FIG. 6 , there is shown a partial band diagram  600  illustrating the valence bands for an exemplary n-channel TFET as shown in  FIGS. 5(   a ) and  5 ( b ), cutting across the circular cross section. As is shown, E 01  and E 03  are the ground state sub-band energies in regions  1  and  3 , respectively, and V S  is the Fermi energy (source voltage). The bandgaps of S 1 -S 3  are assumed to be wide enough so that the conduction bands in the emitter do not play a role in its operation. The band alignments and thickness of the layers are adjusted to achieve a configuration such that the ground-state energies E 01  and E 03  ensure that the holes in S 1  are barely degenerate while S 3  has a much larger hole concentration. Thus, the same screening advantages may be obtained as in the metal-sheathed TFET structure  400  of  FIG. 4 . 
     In order to use the same semiconductor material for S 1  and S 3  as mentioned above, the thickness of S 3  and diameter of S 1  are carefully adjusted so that the ground-state energies line up as shown in  FIG. 6 . The requirements for S 4  may be relaxed if the interfaces between S 1  and S 4  and S 3  and S 4  are heavily doped, in which case another embodiment may to use a metal in lieu of S 4 . It is even further contemplated that S 3  may be replaced with a metal sheath (as in  FIG. 4 ), so long as the work functions and band-offsets are adjusted to ensure a suitable hole concentration in S 1 . In yet another embodiment, S 3  may be coated with an additional metal layer (not shown) to improve screening. It should also be understood that the exemplary IITE embodiments disclosed herein are equally applicable for a complementary tunneling-hole injector by replacing all p-type semiconductors with n-type semiconductors, and ensuring a suitable conduction band line up as shown in  FIG. 6 , but inverted. 
     In summary, the above discussed disadvantages are addressed by the IITE embodiments. This is depicted schematically in  FIGS. 7(   a ) and  7 ( b ), wherein  FIG. 7(   a ) is another side cross-sectional view of the TFET structure of  FIG. 5(   a ), and  FIG. 7(   b ) is a band diagram corresponding to the structure of  FIG. 7(   a ). For one, the outer doped sheath (S 3 ) provides longitudinal screening and reduces band-bending. Secondly, the doping-induced states and degeneracy conditions in S 3  are isolated from the injector core (S 1 ) by S 2 . Thirdly, the semiconductor bandgap of S 3  minimizes metal induced gap states. In addition, epitaxial compatible materials S 1  -S 3  eliminates interface states due to a single crystal structure. The source contact layer S 4  eliminates the need for an extra external contact to the sheath. 
     Finally,  FIG. 8  is a generic band diagram  800  of a heterojunction tunneling emitter where the bands correspond to conduction band convention and are inverted with respect to  FIG. 6 . That is, the band diagram  800  is drawn in the radial direction and with energy of the charge carrier upwards, as is the convention for electrons in conduction bands (whereas for holes the convention is downwards, as shown in  FIG. 6 . 
     The inequalities given below apply to both electrons and holes with the understanding that “energy” may refer to either electron or hole energy for the relevant case. Here, E b1 , E b2  and E b3  are band-edge (conduction or valence band) energies, E 01  and E 03  are ground-state energies of the quantized sub-bands, and E F1  and E F3  the electron of hole Fermi energies. The diagram  800  is drawn in a flat-band condition, assuming a suitable voltage is applied between S 1  and S 3  and that band-bending induced by the charge itself, such as shown in  FIG. 6 , is neglected. In operation, S 1  is shorted to S 3  by S 4  and the Fermi levels, thus E F1  and E F3  will equalize. These simplifications and approximations are shown in order to clarify the conditions on S 1 , S 2  and S 3  to facilitate operability of the exemplary embodiment(s) described. Using the vacuum level E VAC  as a reference, the following conditions apply for the embodiments herein: 
     1. The band-edge energy of S 2  (E b2 ) is greater than those of S 1  and S 3  (E b1  and E b3 ), which is to say that the band discontinuities between S 2  and S 3  and S 1  and S 3  are positive. 
     2. The Fermi energy in S 3  (E F3 ) is higher than the Fermi energy in S 1  (E F1 ). This enables charge to flow from S 3  to S 1 , wherein this condition may be expressed by the following equation:
 
( E   F3 −E   03 )+( E   03   −E   b3 )−Δ E   b23 &gt;( E   F1   −E   01 )+(E 01   −E   b1 )−Δ E   b21    (Eq. 1)
 
     3. For a given band alignment of E b3  and E b 1 , and for given ground-state energies E 01  and E 03 , the doping in S 3  has to be sufficiently large to raise E F3  above E F1  in order to satisfy condition 2. 
     4. For a given band alignment of E b3  and E b1 , and for given doping in S 3 , the radius r 1  has to be sufficiently large to decrease E 01 , and the difference in radii, r 3 −r 2  sufficiently small to increase E 03 , in order to satisfy condition 2. 
     5. For radii r 1  and r 2 , and for a given doping in S 3 , the band edge energy E b3  must be sufficiently larger than E b1 , or when E b1  is greater than E b3  the difference must be sufficiently small, in order to satisfy condition 2. The conditions for S 4  ( FIG. 7(   a )) are not critical. S 4  must have heavy enough doping or a small enough bandgap to ensure a good ohmic contact with both S 1  and S 3 . S 1  and S 3  may also be doped adjacent to S 4  to ensure an ohmic contact. In this case, S 4  may be a metal. 
     In an exemplary embodiment, suitable selected semiconductor materials are as follows: InAs 0.8 P 0.2  for S 1 , InP for S 2  and InAs for S 3  and for S 4 . The radii of S 1 , S 2  and S 3  are 30, 40 and 50 nm respectively. S 1  and S 4  are doped with silicon to a concentration of 10 19  atoms/cm 3 , ensuring a good ohmic contact of S 3  to S 1  via S 4  and that Eq. 1 above is satisfied. That equation becomes:
 
 E   F3 +0.033)+(−0.033+0.173)−0.6533&gt;( E   F1 +0.044)+(−0.044+0.0744)−0.5544   (Eq. 2)
 
     This expression in turn reduces to:
 
 E   F3   &gt;E   F1 +0.0003 eV   (Eq. 3)
 
     Thus, substitution of the selected system parameters for the equation terms results in the condition, E F3 &gt;E F1 +0.0003 eV, which is satisfied with the chosen doping level in S 3 . Referring once again to  FIG. 7(   a ), the lengths of S 1 , S 2 , S 3  and S 4  are not critical but should be longer than about 10 nm to allow for the band bending shown in  FIG. 7(   b ), but also shorter than about 100 nm to minimize series resistance. 
     While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Technology Classification (CPC): 1