Patent Publication Number: US-2010109050-A1

Title: Field effect transistor with independently biased gates

Description:
RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 11/406,838, entitled “FIELD EFFECT TRANSISTOR WITH INDEPENDENTLY BIASED GATES,” filed on Apr. 18, 2006, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF INVENTION 
     Field effect transistors (FETs) are semiconductor devices used in a wide variety of electronics applications. A FET has three terminals: a source, a drain and a gate. During operation of the FET, current flows between source and drain terminals through a channel region. The gate electrode, positioned between the source and the drain, enables the current through the FET to be controlled based on the strength of the signal applied to the gate. The signal and bias present at the gate, source and drain determines the electric field profile in the channel region between the source and the drain. The performance of the FET, e.g., factors such as current gain, carrier mobility, and transconductance (g m ), are determined by the profile of the electric field in the channel region. 
     In conventional FETs, the strength of the electric field varies over the length of the channel, being typically weaker near the source and stronger near the drain (in depletion mode). A non-uniform field can lead to decreased performance of the FET, because electrons near the source are accelerated slowly due to the relatively weak field in this region. Electrons near the drain may acquire too much energy due to the relatively strong field in this region, possibly causing damage to a gate insulator. An excessively strong electric field in one region can cause mobility degradation, hot electrons and impact ionization, and can generate gate leakage.  FIG. 1A  illustrates an example of a conventional metal semiconductor field effect transistor (MESFET) having a source  104 , drain  106  and gate  108  formed on a substrate  102 .  FIG. 1B  shows a curve  110  that illustrates an example of the magnitude of the electric field E in the channel region while the MESFET is in depletion mode. In this example, the magnitude of the electric field is relatively weak near the source and relatively strong near the drain. A non-uniform electric field, such as that illustrated by curve  110 , can lead to decreased carrier mobility, non-linearity and non-constant transconductance. 
     Various techniques have been used to mitigate the effect of the non-uniform electric field, such as using a lightly-doped drain, delta doping of the channel, or using one or more field plates behind the gate. However, these methods lack flexibility to tailor the field in response to a range of operational voltages on the gate of the FET. Furthermore, no known field effect transistor provides constant transconductance. 
     SUMMARY OF INVENTION 
     Embodiments of the invention relate to tailoring the electric field in the channel region of a FET by appropriately positioning and biasing at least two gates of the FET. In accordance with the invention, each of the gates may be biased independently. For example, the same AC voltage may be applied to each gate, but each gate may biased at a different DC voltage. In one aspect of the invention, the electric field may be tailored by positioning one of the gates closer to the channel than the other gate. Using a FET with independently biased gates may enable providing a uniform electric field in the channel region of the FET. Furthermore, such a device may exhibit substantially constant transconductance, a high degree of linearity, and high breakdown, i.e., Early voltage. Constant transconductance provides a high degree of linearity, which can be particularly beneficial for amplifier applications in which linearity across a broad range of gate voltages is desirable. 
     One embodiment of the invention relates to a field effect transistor that operates with substantially constant transconductance. 
     Another embodiment of the invention relates to a field effect transistor that includes a channel region, a first gate biased at a first DC voltage, and a second gate biased at a second DC voltage and positioned farther from the channel region than the first gate. 
     A further embodiment of the invention relates to a field effect transistor that includes a source, a drain and a semiconductor region. The field effect transistor also includes a first gate biased at a first DC voltage and contacting the semiconductor region. The field effect transistor further includes a second gate biased at a second DC voltage lower than the first DC voltage. The second gate is separated from the semiconductor region by an insulating region. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
         FIG. 1A  is a cross-section of a conventional MESFET; 
         FIG. 1B  is a diagram showing a curve that illustrates the magnitude of the electric field in the channel region of the MESFET illustrated in  FIG. 1A ; 
         FIG. 2  is a cross-section of a pseudomorphic high-electron-mobility transistor (pHEMT) according to one embodiment of the invention; 
         FIG. 3  is a diagram illustrating an example of a cross-section of a pHEMT having three gates, according to another embodiment of the invention; 
         FIG. 4  is a cross-section of a metal-oxide-semiconductor field effect transistor (MOSFET), according to another embodiment of the invention; 
         FIG. 5  is a cross-section of a pHEMT, including an overlapping gate structure, according to another embodiment of the invention; and 
         FIG. 6  is a diagram illustrating examples of curves that represent the transconductance of FETs for a range of gate voltages. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with some embodiments of the invention, a FET having at least two independently biased gates enables tailoring the electric field in the channel region of the FET. One of the gates may be positioned closer to the channel than the other gate. Using such a configuration, the electric field may be tailored so that it is substantially uniform in the channel region, which can improve the performance of the FET. For example, the FET can achieve substantially constant transconductance. Embodiments of the invention may be useful in a variety of amplifiers, mixers, switches or any other suitable circuits. 
       FIG. 2  is a cross-section of a pseudomorphic high-electron-mobility transistor (pHEMT) according to one embodiment of the invention. In this embodiment, pHEMT  200  includes a source  204 , drain  208 , a first gate  205  and a second gate  206 . Source  204 , drain  208 , first gate  205  and second gate  206  may be metallizations formed of any suitable material, e.g., a metal such as aluminum. Source  204 , drain  208 , first gate  205  and second gate  206  may be separated by an insulating region  210  which may be any suitable insulating material such as silicon nitride or silicon dioxide. Insulating region  210  may include one material or a combination of materials, as the invention is not limited in this respect. In this embodiment, pHEMT  200  includes a first semiconductor layer  212 , e.g., gallium arsenide (GaAs), a second semiconductor layer  214 , e.g., aluminum gallium arsenide (AlGaAs), a channel region  216 , e.g., indium gallium arsenide (InGaAs), and a substrate  218 . The materials described herein are provided merely by way of illustration, as the invention is not limited to any particular types of materials. 
     First gate  205  may form a Schottky contact with first semiconductor layer  212 , and may be closer to source  204  than second gate  206 . Second gate  206  may be separated from first semiconductor layer  212  by insulating region  210 , and may be closer to drain  208  than first gate  205 . In this embodiment, second gate  206  may be wider than first gate  205 . First gate  205  and second gate  206  may be separated by a portion of insulating region  210 . In some embodiments, second gate  206  may be separated from first semiconductor layer  212  by a distance of less than 1000 angstroms, e.g., 100 angstroms. Positioning second gate  206  within 1000 angstroms of first semiconductor layer  212  enhances the control of the electric field in the channel region. 
     During operation of pHEMT  200 , electrons may be conducted through InGaAs channel region  216  between the source  204  and drain  208  in response to a gate signal applied to first gate  205  and second gate  206 . In accordance with the invention, the first gate  205  and the second gate  206  may be biased at different DC levels. In particular, the gates may be biased such that the DC voltage of first gate  205  is greater than the voltage of second gate  206  (V g1, DC &gt;V g2, DC ). Such a biasing configuration may be used for depletion mode FETs, however, for enhancement mode FETs the biasing configuration may be the opposite (V g1, DC &lt;V g2, DC ). 
     First gate  205  and second gate  206  may have the same AC voltage applied thereto for modulating the conduction of pHEMT  200 . Alternatively, the AC signal may be applied to the two gates in a magnitude ratio that is the same as the ratio of their respective bias voltages, to enhance linearity. However, providing the same AC voltage to both first gate  205  and second gate  206  can reduce the effect of a parasitic capacitance that couples first gate  205  to second gate  206 . However, the same AC voltage need not necessarily be applied to both first gate  205  and second gate  206 . For example, an AC voltage be applied to only one of the two gates. Such a mode of operation may simplify the design of a circuit that supplies the AC voltage, and may be suitable for relatively low frequencies of operation. An AC signal may be applied to one or more gates in any suitable way, as the invention is not limited in this respect. 
     The Applicants have appreciated that using at least two independently biased gates in such a configuration enables tailoring the electric field in channel region  216  so that it is substantially uniform. As a result, such a FET may exhibit substantially constant transconductance over a wider range of gate voltages than was previously possible. Furthermore, linearity of the FET and electron mobility may be improved. 
     As discussed above,  FIG. 2  illustrates an example of a pHEMT  200  that has two gates  205  and  206 . However, more than two gates may be used, as the invention is not limited in this respect. Providing a transistor with more than two gates may facilitate achieving a uniform electric field in the channel region. For example, three or more gates may be used, and each gate may be biased independently of the other gates. 
       FIG. 3  is a cross-section of a pHEMT  300  having three gates, according to another embodiment of the invention. In this embodiment, pHEMT  300  includes three gates  205 ,  306  and  307 , each of which may be biased at different DC voltages. For example, the gates may be biased such that the voltage of first gate  205  is greater than the voltage of second gate  306 , and the voltage of second gate  306  is greater than that of third gate  307  (V g1, DC &gt;V g2, DC &gt;V g3, DC ). Such a biasing configuration may be used for depletion mode FETs, however, for enhancement mode FETs the biasing configuration may be the opposite (V g1, DC &lt;V g2, DC &lt;V g3, DC ). 
     Each of the three gates  205 ,  306  and  307  may have the same AC control signal applied thereto for modulating pHEMT  300 . However, the same AC voltage need not necessarily be applied to each gate. For example, an AC voltage be applied to only one or two of the three gates. Such a mode of operation may simplify the design of a circuit that supplies the AC voltage, and may be suitable for relatively low frequencies of operation. An AC signal may be applied to one or more gates in any suitable way, as the invention is not limited in this respect. Furthermore, embodiments of the invention may have any suitable number of gates. 
     The invention is not limited as to the particular materials used for the various regions of the FET. The semiconductor regions may be any suitable semiconductor regions, such as silicon, germanium, gallium arsenide, gallium nitride, etc., as the invention is not limited in this respect. Furthermore, the gate, source and drain metallizations may be formed of any suitable conductive material, e.g., a metal such as aluminum. As discussed above, the insulating regions may be formed of any suitable insulating material, e.g., silicon nitride or silicon dioxide, or a combination of materials. 
       FIGS. 2 and 3  illustrate examples of pHEMTs according to some embodiments of the invention. However, it should be appreciated the present invention is not limited to pHEMTs, but may be applied to any suitable type of FET, e.g., MOSFETs or MESFETs. Accordingly, another embodiment of the invention will now be described that illustrates how aspects of the invention may be implemented in metal-oxide-semiconductor (MOS) technology. 
       FIG. 4  is a cross-section of a metal-oxide-semiconductor field effect transistor  400  (MOSFET), according to another embodiment of the invention. MOSFET  400  may include a source  204 , drain  208 , first gate  405 , second gate  406 , insulating region  210 , N doped semiconductor regions  412  and  413 , and P doped semiconductor regions  414  and  415 . Semiconductor regions  412 - 415  may be regions of any suitable semiconductor material, e.g., silicon. Source  204 , drain  208 , first gate  405  and second gate  406  may be metallizations formed of any suitable material, e.g., polysilicon, or a metal such as aluminum. Source  204 , drain  208 , first gate  405  and second gate  406  may be separated by an insulating region  210  which may be any suitable insulating material, such as silicon nitride or silicon dioxide. Although  FIG. 4  illustrates an example of an N-channel MOSFET, it should be appreciated that the invention is not limited in this respect, as aspects of the invention may be implemented in a P-channel MOSFET or any other suitable FET. 
     First gate  405  and second gate  406  may be separated from semiconductor regions  412 - 415  by insulation region  210 . First gate  405  may be positioned closer to source  204  than second gate  406 , which may be positioned closer to drain  208  than first gate  405 . In this embodiment, first gate  405  is positioned closer to the channel region of MOSFET  400 , e.g., a portion of semiconductor region  414 , than second gate  406 . Positioning first gate  405  closer to the region  414  than second gate  406  allows shaping the electric field in the channel region so that is substantially uniform. 
       FIG. 5  is a cross-section of a pHEMT  500  according to another embodiment of the invention. In this embodiment, second gate  506  includes a portion that is above first gate  205  and a portion that is on one side of first gate  205 . Such an overlapping gate structure controls the parasitic capacitance between first gate  205  and second gate  506  by controlling the thickness of insulating region  210  between first gate  205  and second gate  506 . The thickness of the insulating region  210  between first gate  205  and second gate  506  may be, for example, approximately 100 angstroms. As discussed above, first gate  205  may be biased at a higher DC voltage than second gate  506 , yet both first gate  205  and second gate  506  may have the same AC voltage applied thereto for modulating pHEMT  500 . As one example, first gate  205  may be biased approximately 0.6 volts higher than second gate  506 . 
     By way of example,  FIG. 5  illustrates dimensions of various features of pHEMT  500  according to one embodiment of the invention. In this embodiment, the first gate  205  and the second gate  506  may be positioned approximately 1.5 μm from source  204  in the lateral dimension, e.g., parallel to a wafer surface. A portion of the second gate may be positioned approximately 1.5 μm from drain  208  in the lateral dimension. First gate  205  may extend approximately 0.5 μm in the lateral dimension, and second gate  506  may extend approximately 2.0 μm in the lateral dimension. These dimensions are provided merely by way of example, and are not intended to be limiting. One of ordinary skill in the art would appreciate that the FET may be of any suitable size, and that the dimensions may be scaled accordingly. However, different dimensions and/or relative sizes between dimensions may be used, as the invention is not limited in this respect. For larger devices, the difference between the DC bias voltages on first gate  205  and second gate  506  may be increased to maintain a uniform electric field in the channel region. 
     As discussed above, some embodiments of the invention are directed to a FET that provides substantially constant transconductance. As used herein, transconductance is the ratio of the change in drain current to the change in gate-source voltage of the FET. Substantially constant transconductance means that the transconductance of the FET does not vary substantially over a range of gate voltages. Furthermore, the transconductance can be substantially constant across a range of frequencies. 
       FIG. 6  is a diagram illustrating examples of curves  601  and  602  that represent the transconductance of two different FETs for a range of gate voltages. Curve  601  represents the transconductance for a conventional FET that uses a field plate in an attempt to provide a uniform electric field in the channel region. Curve  601  illustrates that the transconductance of this conventional FET reaches nearly 300 mS/mm, before dropping rapidly at approximately 0.3 V of gate bias. Such a non-constant transconductance may lead to non-linearity, e.g., some voltages being amplified at a higher level than other voltages. 
     Curve  602  illustrates the modeled transconductance that may be achieved for a FET according to the embodiment of the invention illustrated in  FIG. 5 . Curve  602  shows that the transconductance of the FET does not vary significantly (e.g., less than 25 mS/mm peak-to-peak) over the range of −0.6 V to 1.0 V gate voltage. Such a substantially constant transconductance provides improved performance and substantially linear operation of the FET over this range of operating voltages on the gate. 
     In the above-described embodiments, gates were positioned and biased to increase the electrical field near the source and decrease the electrical field near the drain, so that the electrical field would be substantially uniform throughout the channel. However, in some circumstances, the electric field may be large near the source and small near the drain. Such a situation may arise, for example, for a MESFET in enhancement mode. In such a situation, obtaining a uniform electric field may require decreasing the electric field near the source and increasing the electric field near the drain. To achieve a uniform electric field in such a scenario, the orientation of the above-described embodiments may be altered such that the positions of the drain and the source are reversed, for example. 
     Having thus described several aspects of at least one embodiment of the invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.