Abstract:
A semiconductor device provides a metal contact, a DNA layer, wherein the metal layer and the DNA layer are adapted to form a Schottky barrier junction there between, and a conductive contact with the DNA layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of U.S. Provisional Application Ser. No. 60/917,358, filed May 11, 2007 and incorporated herein by reference in its entirety. 
    
    
     STATEMENT OF US GOVERNMENT INTEREST 
     This invention is made partially with U.S. Government support from the Army Research Office under a contract No. W911NF-06-C-0014. The U.S. Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to Bio-electronics, and, more particularly, to DNA-based electronic devices, such as diodes and photodetectors. 
     BACKGROUND OF THE INVENTION 
     As the miniaturization of silicon-based electronic devices continues to approach physical, technical and economic limits, the electronics industry faces the new challenge of identifying materials that can replace silicon to overcome these limits, making much denser and ultra-miniaturized electronic devices. DNA is a promising candidate material drawing widespread interest in studies of its electrical properties, particularly as to whether it is a conductive wire, a semiconductor, an insulator, or even a superconductor. Through an improved understanding of DNA electrical properties, DNA-based electronics could form the basis for molecular scale electronics, extending well beyond the silicon-based electronics limitations. 
     DNA, the building block of life, has been the center of biological research and industries for five decades. Just after the famous DNA double helix structure was discovered, scientists found that DNA was semi-conductive. In the past few years, the observation of DNA&#39;s conductivity properties brought renewed attention in the search for new materials for next generation nano-technologies and microelectronics. The question surfaced as to whether DNA could be also the building block for molecular electronics and play a pivotal role in the future of modern electronics. More recently, the interests in the DNA conductivity have been greatly increased and more investigations have been carried out. However, the results are confusing and contradictory, so far showing that the DNA conductivity could be an insulator, semiconductor, conductor or a proximate-induced superconductor. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, a semiconductor device, comprises a metal contact, a DNA layer, wherein the metal layer and the DNA layer are adapted to form a Schottky barrier junction there between, and a conductive contact with the DNA layer. The metal contact may include gold, aluminum, copper or chromium or other metals. The DNA layer may include salmon sperm DNA. The DNA layer may be either n- or p-type semiconductor by doping. The n- or p-type DNA semiconductor materials could form p-n junction in making new DNA-based semiconductor devices. The conductive contact may be adapted to allow light energy to pass to the DNA layer. The conductive contact may be coated with indium tin oxide. 
     In another embodiment, a photodiode, comprises a metal contact, a DNA layer, wherein the metal layer and the DNA layer are adapted to form a Schottky barrier junction there between, and a conductive contact with the DNA layer. The metal contact may include gold, aluminum, copper or chromium. The DNA layer may include salmon sperm DNA. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present invention is illustratively shown and described in reference to the accompanying drawings, in which: 
         FIG. 1A  is an energy band diagram of metal (gold)-semiconductor (DNA) contacts before contacting; 
         FIG. 1B  is an energy band diagram of metal (gold)-semiconductor (DNA) contacts after contacting and using an n-type DNA semiconductor; 
         FIG. 2A  is a pictorial of the absorption spectrum measurements of DNA+ITO+glass and ITO+glass; 
         FIG. 2B  is a pictorial of the absorption spectrum measurements of DNA and ITO; 
         FIG. 3A  is a DNA-metal Schottky diode schematic diagram; 
         FIG. 3B  is an unwired DNA-metal Schottky diode device; 
         FIG. 3C  is an unwired DNA-metal Schottky diode device; 
         FIG. 4  is DNA-metal Schottky diode current-voltage measurement setup; 
         FIG. 5  is a current-voltage characteristics graph of chromium-DNA based Schottky diode; 
         FIG. 6A  is a time versus photocurrent graph illustrating the Measurement Setup (Inlet) and Typical Photocurrent Response of an Au-DNA device; and 
         FIG. 6B  is a time versus photocurrent graph illustrating Typical Photocurrent Response of an Cu-DNA device; and 
         FIG. 7  depicts the Square Root of Photocurrent Vs. Incident Photon Energy in Gold-DNA Schottky Barrier. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to native DNA-based Schottky diodes made by metal-DNA contacts. The novel Schottky diodes demonstrate excellent current-voltage characteristics with low forward threshold voltage and adequately large reverse breakdown voltage needed for a practical range in low-voltage operation. In addition, there are observed and demonstrated photoelectric effects (PE) in DNA-Gold Schottky diodes. Based on carefully controlled PE measurements, the DNA-Gold Schottky barrier height is measured in one embodiment measured as ˜1.52 eV. 
     The invention described herein also relates to photoelectric effects under visible and near-infrared illumination and diode-type rectifying current-voltage (I-V) characteristics in DNA-metal contact devices. These devices are constructed with DNA film sandwiched between gold, and transparent conductive indium tin oxide (ITO) electrodes. Other metals such as aluminum, chromium, and copper may also be used. In the embodiments described herein, DNA is used to make Schottky diodes through carefully formed contact with metals. 
     Prior to continuing the detailed description of the invention, a paper co-authored by the inventor, De Yu Zang, Proceedings SPIE Vol. 6470 8 Feb. 2007 “Photoelectrical effect and current-voltage characteristics in DNA-metal Schottky barriers”, is incorporated herein by reference. 
     Nonmetal-metal contact is generally not ohmic. In such a non-ohmic contact, the current response to the applied voltage is normally not linear. If the nonmetal material is a semiconductor, its contact with a metal could form a Schottky barrier if the interfaces are pure and clean. The Schottky barrier is a potential barrier arising from stable space charges in the semiconductor alone without the presence of a chemical layer. 
       FIGS. 1A and 1B  are the schematic diagrams of energy bands  11 ,  12  in a metal (gold) and an n-type semiconductor, respectively, at conditions before and after contacting. Before contacting ( FIG. 1A ), the system is not at thermal equilibrium. After contacting ( FIG. 1B ), the charges  14  will flow from semiconductor to the metal and the electronic equilibrium is established Fermi levels  16  on both sides line up. Here, LUMO (lowest unoccupied molecular orbital), EF (Fermi level) and HOMO (highest occupied molecular orbital) are the conduction, Fermi and valence energy levels, respectively; q  18  is the electron charge; Φm  21  is the gold work function, which is ˜5.1 eV for gold and ΦB  22  is the Schottky barrier height, q  24  is the electron affinity measured from conduction band to the vacuum level. 
       FIGS. 2A and 2B  show the absorption spectrum measurements of DNA and ITO. In  FIG. 2A , the dashed, the solid lines are the absorption spectra  31  of DNA film on an ITO-coated glass slide;  32  ITO-glass slide and  34  glass slide, respectively. In  FIG. 2B , the dashed and solid lines are the absorption spectra of DNA (extracted)  36  and ITO (extracted)  38 , respectively. All the measurements show that DNA and ITO have absorption peaks at &lt;300 nm, which are in the ultraviolet wavelength range and their bandgaps must be greater than 4 eV. These indicate that visible (or infrared) light cannot release an electron in the DNA or ITO areas (no photocurrent under visible and near infrared illumination) since the photons at these wavelengths do not have enough energy. 
     The DNA-metal device  10  of  FIGS. 3A ,  3 B, and  3 C includes three layers: top, gold layer electrode  20 ; a bottom electrode  50  coated with ITO  40 ; and a thin DNA film  30 , which is sandwiched between the two electrodes  20 ,  50 . In a preferred embodiment, the thickness of the gold electrode  20  is ˜1000 Å, and the thickness of the ITO coated electrode is ˜700 Å. The thickness of DNA film  30  is 13-19 μm. The DNA film  30  is generally shining and smooth with a uniformity of 10% typically in the device working area. The DNA/metal electrode  20  area, which defines the device working area, is ˜0.691 cm 2 . 
     Any suitable DNA may be used. In the preferred embodiment a) Salmon sperm-based DNA with a molecule weight of 8-10 kbps having a purity is 94-95%; b) De-ionic water with resistance greater than 18 MΩ and pH value of ˜8.0, and c) gold with purity greater than 99.99%. Chromium, aluminum and copper may alternatively be used, provided that the purity is also greater than 99.99%. 
     In fabrication, the DNA is completely dissolved into de-ionized water with a weight ratio of 1:100. The DNA-water solution is filtered with a 1.2 μm filter and then is cast on a 1″×1″ ITO-coated glass substrate. The sample is immediately placed into an oven to dry. As soon as the DNA film is dried completely, the sample is transported into a sputtering machine to deposit a patterned thin gold layer on the DNA film  30  as the top electrode  20 . Finally, the top and bottom electrodes are connected with electric wires. 
     To improve the sensitivity of photoelectrical effect, the device is mostly treated under an “electric poling process” in an attempt to align DNA molecules along the electric field and create DNA permanent dipoles. The method of the electric poling is as follows. The device is placed in a nitrogen environment and an electric field is applied. The device is heated and allowed to cool down to room temperature. The electric field strength is monitored by measuring the electric current through the device. After electric poling, the device is stored at room temperature for at least one week for discharging before any measurements. 
     The metal sputtering processing in the fabrication is important to form the DNA-metal Schottky barriers. During the sputtering deposition, the metal ions in the plasma beams bombard the soft DNA surface and eventually clean it of any contamination accumulated in prior preparations. The thin metal layer is then intimately contacted with the clean DNA surface forming a Schottky barrier. 
       FIG. 4  shows a circuit schematic to measure the current-voltage behavior of device  10 . A voltage V  60  is applied between the ITO coated electrode  40  and the gold electrode  20 , with resistor  70  and ammeter  80  connected in series. 
     As a voltage  60  is applied on ITO electrode  40  and the gold electrode  20 , a current is generated in the circuit. If the voltage  60  is plus on the ITO electrode  40 , which is commonly called a forward bias, the current is increased exponentially as the voltage  60  is increased beyond a certain value (threshold voltage). However, if the voltage  60  applied on the ITO electrode  40  is negative, which is commonly called reverse bias, the current is quite limited as the voltage is increased (not exceed to a certain value, the so-called breakdown voltage). 
     The reason for such rectifying I-V characteristics is that the forward bias reduces the Schottky barrier allowing the electrons to more easily to overcome the barrier, whereas the reverse bias increases the barrier making the electron flow more difficult. 
       FIG. 5  is an example I-V measurement data showing typical rectifying I-V characteristics. Under a forward bias  42 , the current increases exponentially, while at reverse bias  44 , the current is quite limited when the reverse voltage is smaller than the breakdown voltage. The reverse current is increased exponentially when the reverse bias exceeds the breakdown voltage  46 . 
     The DNA Schottky diodes showed excellent current-voltage characteristics with low forward threshold voltage and sufficiently large reverse breakdown voltage essential to allow useful and wide-ranging applications. 
       FIG. 6A  shows the schematic for photoelectric effect tests on a DNA-metal device  100 , illuminated by light of photon energy hν  110 , producing photo-induced current A  120 . The device is not biased. The analog current  120  value connected to a computer for sampling and real time recordings. The sampling rate is 100 Hz. The light source can be monochromic such as a laser with proper wavelength, or a lamp with a high-pass optical filter. Launching a light beam on ITO-DNA side to pass through the glass substrate, ITO-DNA interface and DNA layer to reach gold layer, electrons could be excited. The excited electrons in the gold layer can be either: a) across over the barrier height into the conduction band in the DNA to generate current if the light energy hv&gt;q ΦB, or b) remain in the gold layer (release energy to heat) if the light energy hv&lt;q ΦB. 
     Transient current spikes at  232  and  252  occur when input light  110  is switched on  200  and then later switched off  210 . The current characteristics are described in the five time spans shown  220 ,  230 ,  240 ,  250 , and  260 , which are: Region A  220  before light on  200 ; Region B  230  immediately after light-on  200 ; Region C  240  is during continued illumination; Region D  250  immediately after Light-Off  210 ; and Region E  260  beyond signal relaxation after Light-Off  210 . 
       FIG. 6B  shows a similar curve for a Cu-DNA device under the same test conditions, except that the spikes occur in the opposite direction. Compared with PE in Au-DNA devices, the PE current direction in Cu-DNA was noticeably opposite with that in Au-DNA devices, as shown in  FIG. 6B . For Au-DNA devices, the photocurrent was from Au to ITO (or DNA), while the photocurrent was from ITO (or DNA) to Cu for Cu-DNA devices. To explain the opposite photoelectric current in Au-DNA and Cu-DNA devices, we suggest that electrochemical doping might take place when the devices were in the process of electric poling. The doping of Au ions into DNA induces an n-type DNA, while Cu ions cause a p-type DNA. 
       FIG. 7  summarizes the results of a series of photoelectric effect tests using six different light sources with various values of said photon energy hν  110  performed on both a Cu-DNA device (plot  270 ) and a Au-DNA device (plot  272 ). The square root of photo-response/photon is plotted versus said photon energy  110 , and the linear intercept point is where the photon energy  110  equals the Schottky barrier height of 1.7 for Cu-DNA device and 1.52 eV for the Au-DNA device. 
     Although the invention has been described with respect to various embodiments, it should be realized this invention is also capable of a wide variety of further and other embodiments.