Abstract:
This invention provides a circuit and a method for generating a low-level current using semiconductor charge pumping. The invention provides a means of generating a range of current sources by varying the frequency of a repetitive voltage pulse input signal. Also, this invention utilizes one or many MOSFET devices in order to produce higher levels of current. The current source embodiments of this invention generate very stable current sources with high input impedances.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a circuit and a method for generating a low-level current using semiconductor charge pumping.  
         [0003]     More particularly this invention relates to a means of generating a range of current sources by varying the frequency of a repetitive voltage pulse input signal.  
         [0004]     Also, this invention relates to utilizing one or many MOSFET devices in order to produce higher levels of current.  
         [0005]     In addition, this invention relates to the ability to generate very stable current sources with high input impedances.  
         [0006]     2. Description of Related Art  
         [0007]     The prior art related to this invention includes various high input impedance low level current sources. Typically, these current sources are custom designed monolithic circuits. They are not modular or are they scalable from the lowest current level produced by a single device to high current levels produced by hundreds of device.  
         [0008]     U.S. Pat. No. 6,285,243 B1 (Mercier et al.) “High Voltage Charge Pump Circuit” describes a circuit which transfers a voltage signal in an output stage without signal level degradation. By-pass techniques are used to avoid semiconductor damage or breakdown.  
         [0009]     U.S. Pat. No. 6,326,839 B2 (Proebsting) “Apparatus for Translating a Voltage” discloses a low voltage current source is used for translating voltage levels using a charge pumping mechanism.  
         [0010]     U.S. Pat. No. 6,323,721 B1 (Proebsting) “Substrate Voltage Detector” discloses low voltage current source circuit, which generates low voltage signals for powering a variable frequency oscillator.  
         [0011]     U.S. Pat. 5,561,385 (Choi) “Internal Voltage Generator for Semiconductor Device” discloses an internal voltage generator for a semiconductor device, for generating an internal voltage within the device.  
         [0012]     U.S. Pat. No. 6,278,315 (Kim) “High Voltage Generating Circuit and Method for Generating a Signal Maintaining High Voltage and Current Characteristics Therewith” discloses a high voltage generating circuit, which is used to generate a high voltage with a high current. This circuit is used for on-chip programming and erasing of electrically erasable programmable read only memory EEPROM or flash memory.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     It is the objective of this invention to provide a circuit and a method for generating a low-level current using semiconductor charge pumping.  
         [0014]     It is further an object of this invention to provide a means of generating a range of current sources by varying the frequency of a repetitive voltage pulse input signal.  
         [0015]     Also, it is further an object of this invention to utilizing one or many MOSFET devices in order to produce higher levels of current.  
         [0016]     In addition, it is further an object of this invention to generate very stable current sources with high input impedances.  
         [0017]     The objects of this invention are achieved by a frequency-controlled low-level current source based on charge pumping and using a single metal-oxide semiconductor field effect transistor, MOSFET, a voltage pulse generator attached to a gate of the MOSFET, a ground connected to a drain of the MOSFET, a ground connected to a source of the MOSFET, and an output current produced at a substrate terminal of the MOSFET. The frequency-controlled current source of this invention has a MOSFET device whose source region of the MOSFET is made of n+ semiconductor material. The drain of the MOSFET is made of n+ type semiconductor material. The circuit of this invention has a substrate of the MOSFET which is made of p type semiconductor material. The frequency-controlled current source MOSFET has claim  1  wherein a gate which is made of is made of poly-silicon. The current source has a voltage pulse applied to the MOSFET gate by the voltage pulse generator. The frequency-controlled current source&#39;s gate voltage pulse causes a charge inversion in to the p type substrate which results in negative charge accumulation in the p-type substrate located between said n+ MOSFET drain and said n+ MOSFET source. This negative charge accumulation is caused by the flow of electrons from the MOSFET source and MOSFET drain where some electrons are trapped in interface states. When the MOSFETgate pulse goes to its low state the mobile electrons, those that are not trapped in the interface states, will return to said MOSFET source and drain. The non-mobile trapped or electrons will recombine with the holes in the p-substrate. The trapped, recombined electrons results in a net flow of negative charge into the substrate. This net flow of negative charge is the resultant charge pump current which is generated by this low-level current source of this invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  shows a cross-sectional view of the single MOSFET device embodiment of the current source of this invention.  
         [0019]      FIG. 2  shows a plot of charge pump output current vs. the Vbase voltage of the input voltage pulse applied to the gate of the MOSFET device.  
         [0020]      FIG. 3  contains a plot of charge pump output current vs. frequency of the gate input repetitive pulse voltage applied.  
         [0021]      FIG. 4  shows the system level connection of the single MOSFET device current source of this invention.  
         [0022]      FIG. 5  shows the multi-MOSFET device embodiment of this invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIG. 1  shows a cross-sectional view of the main embodiment of this invention. A metal oxide semiconductor field effect transistor MOSFET  111  is shown. The gate  160  is made of poly-silicon. The source  180  is connected to ground  145 . The drain  170  is also connected to ground  145 . Both the source  115  and drain  125  are made of n+ semiconductor material. The substrate  190  is made of p-type silicon material. The substrate is connected  135  to a DC current meter  150 , which is used to test the output current  120  produced by this change pump MOSFET.  
         [0024]     The gate  160  is connected to the input line  110 . A repetitive voltage pulse signal  112  is applied to the input  110 . This pulse signal has a base voltage level Vbase  140  and a voltage amplitude delta equal to delta VA  130 .  
         [0025]     Charge pumping in MOSFETs is a well-known phenomenon that is related to the recombination process at the SiO2/Si interface  190  involving the interface states as shown in  FIG. 1 . For a MOSFET with the connections shown in  FIG. 1 , when the transistor is pulsed into inversion, the p-type silicon surface  190  becomes deeply depleted and electrons will flow from the source  115  and drain  125  regions into the channel  190  where some of them will be captured by the interface states. When the gate pulse  112  is driving the surface back to accumulation, the mobile charges flow back to the source  115  and drain  125  but the charges trapped in the interface states will recombine with the majority carriers (i.e. holes for p-Si substrate) from the substrate  190  and give rise to a net flow of negative charge into the substrate  135 . The charge Qss which will recombine is given by 
 
 Qss= ( q* 2) S D P  
 
         [0026]     Where (q*2) is the electron charge squared in coulombs squared, S is the channel area of the MOSFET (cm2), D it is the mean interface state density over the energy range swept through by the Fermi level and P is the total sweep of the surface potential. When applying repetitive pulses to the gate with frequency, f, this charge Qss will give rise to a steady-state current in the substrate  135 . This current  150  is the so-called charge pumping current, and it is given by 
 
 Icp=f Qss=f ( q* 2) S D P  
 
 where f is the frequency of the repetitive input gate voltage signal. 
 
         [0027]     The charge pumping current can be observed with different pulse shapes (square, triangle or other pulse shapes). For square pulses, if the amplitude of the pulses is kept constant but the pulse base level Vbase is varied from inversion to accumulation, the charge pumping current  210  will vary with the Vbase  220  is shown in  FIG. 2 . In the saturation region  230  with Vt−dVa&lt;Vbase&lt;Vfb  240 ,  250 , where Vt, dVa and Vfb represent the threshold voltage, the pulse amplitude, and the flat band voltage, respectively, the charge pumping current is a constant  230  and is determined by the Icp equation (2) above.  
         [0028]     The current source of this invention has its output current  310  proportional to the frequency  320  input pulses. The frequency  330  dependence of the charge pumping current at different V base within the saturation region is shown in  FIG. 3 . A good linear frequency dependence was observed up to the frequency of 2.5 MHz (at higher frequencies  340  there was a slight departure because those “slower” interface states were not able to response quickly enough). The charge pumping current can serve as a low-level DC current source (for example a DC current 0.1 uA). As the charge pumping current is proportional to the frequency of the input pulses, the output current can be easily controlled through the frequency.  
         [0029]     Another feature of the current source of this invention is that it is insensitive to the drift of pulse voltage. As can be seen in  FIG. 2 , the charge pumping current was essentially independent of the Vbase in the saturation region  230 . If the pulse base level is within the saturation region, then the output current will be insensitive to a small drift of the pulse base level or top level. It is possible to have a desired saturation region through a proper selection of the values of the Vt, the Vfb and the dVa. The Vt and the Vfb depend on the substrate doping, gate materials, the fixed charges in the gate oxide, as well as the thickness of the gate oxide, and they can be controlled during the device manufacturing. To maintain a constant level of interface state, the pulse base level as well as the top level must be below the threshold voltage for the Fowler-Nordheim tunneling (7V for a 50 A thick gate oxide).  
         [0030]     The current source circuit of this invention has an extremely high input resistance, as the gate oxide is an excellent isolator, the charge pumping current source will have an extremely high input resistance.  
         [0031]     As can be seen in  FIG. 4 , for low-level output current with current less than 0.1 uA, the frequency-controlled charge-pumping (FCCP) DC current source  410  can be fabricated using a single transistor, with the resistive load  430  connected in series with the substrate terminal  420  of the transistor. As is illustrated in  FIG. 5 , if higher output current is desired, a two-dimensional array  530 ,  540  of FCCP can be made up of hundreds of transistors. All of the gate terminals are tied together and connected to a single pulse generator  510 . The output current  520  is the linear sum of the charge-pumping currents contributed from all of the substrate terminals.  
         [0032]     The advantage of this invention is the possibility of creating a range of low-level current sources. The range goes from a single MOSFET device to an array of MOSFET devices connected in parallel to create a higher level of current. In addition, the stability of the current source with respect to variations in the drift of the input pulse voltage is an important advantage. In addition, another advantage is the ability to control the output current by varying the frequency of the input pulse signal. The high input resistance of this current source is also an important feature of this invention.  
         [0033]     While this invention has been particularly shown and described with Reference to the preferred embodiments thereof, it will be understood by those Skilled in the art that various changes in form and details may be made without Departing from the spirit and scope of this invention.