Abstract:
A backward diode including a heterostructure consisting of a first layer of InAs and second layer of GaSb or InGaSb with an interface layer consisting of an aluminum antimonide compound is presented. It is also disclosed that the presence of AlSb in the interface enhances the highly desirable characteristic of nonlinear current-voltage (I-V) curve near zero bias. The backward diode is useful in radio frequency detection and mixing. The interface layer may be one or more layers in thickness, and may also have a continuously graded AlGaSb layer with a varying Al concentration in order to enhance the nonlinear I-V curve characteristic near zero bias.

Description:
TECHNICAL FIELD 
     This invention relates in general to semiconductor devices, and in particular to backward diodes useful in radio frequency detection and mixing. 
     BACKGROUND OF THE INVENTION 
     The tunnel diode is a well-known semiconductor device that conventionally includes two regions of heavily doped semiconductor material of opposite conductivity types, separated by a relatively thin junction which permits charge carriers to tunnel through upon the application of a suitable operating potential to the semiconductor regions. The p and n regions of tunnel diodes are so heavily doped that they are degenerate. At equilibrium, a portion of the valence band in the p region of the diode is empty and a portion of the conduction band in the n region is filled. 
     A slight forward bias brings some levels of the filled portion of the conduction band of the n region into energetic alignment with empty levels of the valence band of the p region. In this situation, quantum-mechanical tunneling allows electrons to flow from the n region to the p region, giving a positive current that first increases with increasing bias. When the filled part of the conduction band of the n region is maximally aligned with the empty part of the valence band of the p region, the current flow is maximized. Subsequently, the current decreases with increasing forward bias, and approaches a minimum value when the filled part of the conduction band of the n region lies opposite the energy gap of the p region. When a yet larger forward bias occurs, electrons and holes are injected over the barrier between the p and n regions, resulting in a rapid increase in current for increasing forward bias. Thus, the current-voltage has a negative differential conductance part in the forward region of the characteristic. 
     Use of a heterostructure consisting of adjoining regions of GaSb 1−y As y  and In 1−x Ga x As interfaced with a tunneling junction is described in U.S. Pat. No. 4,198,644 entitled, “Tunnel Diode” issued to Leo Esaki on Apr. 15, 1980. The heterostructure presented in the Esaki patent discloses first and second layers of Group III-V compound semiconductor alloys wherein the first layer is an alloy including a first Group III material and a first Group V material, and the second layer is an alloy including a second Group III material different from the first Group III material and a second Group V material different from the first Group V material, and wherein the valence band of the first alloy is closer to the conduction band of the second alloy than it is to the valence band of the second alloy. The preferred embodiment identified In as the first Group III material, As as the first Group V material, Ga as the second Group III material, and Sb as the second Group V material. 
     Also, U.S. Pat. No. 4,371,884 entitled “InAs-GaSb Tunnel Diode”, also issued to Leo Esaki provides for a tunnel diode requiring no heavy doping, and which the process of molecular beam epitaxy can readily fabricate. The &#39;884 tunnel diode heterostructure comprises first and second accumulation regions of relatively lightly doped group III-V compounds specifically consisting of In 1−x Ga x As and GaSb 1−y As y , where concentrations expressed in terms of x and y are preferably zero but less than 0.3, and where the improvement consists of an interface of a relatively thin layer of a quaternary compound whose constituent materials are those of the adjoining regions. This interface provides a tunneling junction as opposed to an ohmic junction between contiguous regions of InAs and GaSb. 
     An object of the present invention is to provide a new and useful improvement in tunneling diodes in order to expand their application to higher bandwidths, with greater dynamic range and greater sensitivity for radio frequency detection. In particular, the present invention is designed to provide a high degree of non-linearity near zero bias. This is in contrast to patents discussed above, which are designed to provide a negative resistance region for non-zero bias. 
     The present invention uses AlSb and AlGaSb layers to control the curvature of the current voltage (I-V) curve and current density through the device, thus decreasing the forward current while allowing the tunneling current in the negative bias direction to be relatively large and unaffected. The desirable characteristic of this design is to provide a highly nonlinear portion of the I-V curve near zero bias, which is greatly improved by the presence of the AlGaSb layers. 
     SUMMARY OF THE PRESENT INVENTION 
     According to the invention there is provided a high-speed semiconductor device that exhibits an interband tunneling characteristic. The present invention comprises two semiconductor regions having mutually different compositions from one another and separated by a thin interface layer through which tunneling occurs. 
     The semiconductor regions exhibit gaps that are shifted in mutually opposite directions and the interface layer is amply thin to allow for electron transfer via tunneling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an band-edge diagram illustrative of a conventional implementation of a backward or Esaki diode; 
     FIG. 2 is a qualitative example of the current-voltage characteristic diagram typically exhibited by the backward diode version of the Esaki diode; 
     FIG. 3 is a band-edge diagram illustrative of the InAs/AlSb/GaSb heterostructure backward diode system of an embodiment of the present invention; 
     FIG. 4 is a band-edge diagram illustrative of a variation of the embodiment of the present invention shown in FIG. 3, wherein InGaSb is substituted for GaSb; 
     FIG. 5 is a band-edge diagram illustrative of another variation of the embodiment of the present invention shown in FIG. 3, wherein two types of barriers are present, namely AlSb and AlGaSb; 
     FIG. 6 is a band-edge diagram of an embodiment of the present invention similar to that shown in FIG. 5, with an additional n-type InAs layer formed on the p-type GaSb layer on the right-hand side of the FIG. 5; 
     FIG. 7 is a band-edge diagram illustrative of another variation of the embodiment of the present invention shown in FIG. 3, wherein a single AlGaSb barrier with an adjustable valence band is present; and 
     FIG. 8 is a band-edge diagram illustrative of another variation of the embodiment of the present invention shown in FIG. 5, depicting the use of a continuously graded the barrier. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides improved backward diode structures, which may be tailored to a variety of applications. The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments. Thus, the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
     FIG. 1 presents a band-edge diagram associated with the conventional heavily doped p-n junction in Ge or another semiconductor. The heavy doping causes the energy bands to bend such that electrons in the n-type side can tunnel through the relatively thin band-bending region into the p-type side. If the doping is very heavy on both sides, a negative differential resistance peak in the current-voltage (I-V) curve is produced for positive bias as electrons tunnel from the n-side  100  to the holes in the p-side  102 . Both the n-type and p-type layers are typically formed of Ge. The arrows on the left and right sides of the FIG. 104 indicate the direction of the shift of the conduction band edge  106  and the valence band edge  108  with positive bias. For sufficient positive bias, the electron energies are too high for tunneling into the hole states, and negative differential resistance results. For negative bias, if the doping is high, the band-bending region  110  is short, and the electrons from the p-side  102  at energies below the Fermi level  112  can tunnel (from left to right) into the n-side  100  above the Fermi level  112 . The resulting current can be large, and increases exponentially with reverse bias. If the doping of the p-side  102  doping is not too large, the Fermi level  112  will be close to the valence band edge  108  on the p-type side  102 . In this state, there will not be many hole states for the electrons to tunnel into with forward bias. The peak current of the negative resistance current-voltage curve will be small, while the tunneling current in the negative bias direction is relatively unaffected and large, as shown in FIG.  2 . This is the backward diode version of the Esaki diode. The desirable characteristic is the highly nonlinear current-voltage curve characteristic near zero bias  200 , which makes it useful for mixing and detecting radio frequency signals. 
     The present invention achieves improved behavior using an InAs/AlSb/GaSb heterostructure system, an example band-edge diagram for which is depicted in FIG.  3 . This structure exhibits very little non-linearity at zero bias. The presence of an AlSb layer  300  provides greater design flexibility than the exclusive use of InAs and GaSb layers ( 302  and  304 , respectively). The width of the AlSb layer  300  can be designed to provide tailorable tunneling between the InAs layer  302  and the GaSb layer  304 . This is in contrast with the InAs/GaSb heterolayer system, which has a unique tunneling condition. For purposes of illustration simplicity, no band-bending is demonstrated such as that shown in FIG. 1, although it will generally be present in actual application. The Fermi level  306  is above the edge of the GaSb valence band edge  310  in the p-type GaSb layer  304 . The electron transport for forward bias (left to right in the figure) will be desirably small due to the lack of holes in the p-doped GaSb side  304  to tunnel into. However, the electron transport for backward bias (right to left in the figure) will also be small for small bias, because of the small density of holes in the GaSb side for carrying the current. 
     If the Fermi level is between the conduction band edge  308  of the InAs layer  312  and the GaSb valence band edge  310 , the forward and backward currents are approximately proportional to the bias voltage over a range, and thus the non-linearity is small. 
     A band-edge diagram of a variation of the embodiment of FIG. 3 is shown in FIG. 4, where InGaSb is substituted for GaSb. The edge of the InGaSb valence band  400  is raised by a small amount relative to that of the edge of the GaSb valence band  310  shown in FIG. 3, allowing electrons at the top of the InGaSb valence band  400  access to the energy range above the Fermi level  402  in the InAs layer  404  when negatively biased. The Fermi level  402  in the InGaSb layer  400  is preferably tailored close to the InGaSb valence band edge  400  to minimize forward current. Still, the density of holes in the InGaSb side may be too small to allow for sufficient backward current. 
     In FIG.  5  and FIG. 6 two further modifications are depicted. Two types of barriers, AlSb  500  and AlGaSb  502 , are depicted in FIG.  5 . The addition of Ga to provide AlGaSb decreases the barrier band gap and the valence band discontinuity with GaSb. The thickness of the AlSb barrier  500  is adjustable to control the overall current due to tunneling, while the AlGaSb barrier  502  contributes additional blocking to the forward current, i.e. electrons tunneling from the InAs layer  506  through to the GaSb layer  508 . However, the AlGaSb barrier does not significantly block the (beneficial) backward tunneling of electrons from the AlGaSb barrier valence band  510  as they are “underneath” the AlGaSb barrier  502  and can flow into available states above the Fermi level  512  in the InAs region  506 . As the Fermi level can now be significantly below the GaSb valence band edge, there will be sufficient holes to carry the backward current. In FIG. 6 there is shown an additional n-type InAs cladding layer  600  on the p-type GaSb layer  602  on the right. There is no barrier at the right side interface  604 , thus providing a convenient transition back to an InAs n-type contact on the right. 
     FIGS. 7 and 8 depict two additional embodiments. FIG. 7 demonstrates a single AlGaSb barrier  700  with an adjustable valence band edge  702 . The valence band edge of the AlGaSb barrier  702  is close to the Fermi level  704 . The presence of this barrier blocks electrons from the InAs layer  706  from tunneling through the large valence band barrier and reaching the GaSb layer  708  with forward bias. This variation can trade off higher currents for possibly lower nonlinearity since it lacks the AlSb barrier present in the embodiments shown in FIGS. 5 and 6. FIG. 8 depicts an embodiment of the present invention with a continuously graded AlGaSb barrier layer  800 , combining a higher Al concentration in the AlGaSb layer  800  near the interface with the InAs layer  802  and a lower Al concentration in the AlGaSb layer  800  near the interface with the GaSb layer  804 . This condition will produce an enhanced nonlinear increase in the backward current with negative bias, as the width of the effective triangular barrier that the valence band electrons must tunnel through decreases with bias.