Patent Publication Number: US-8120066-B2

Title: Single voltage supply pseudomorphic high electron mobility transistor (PHEMT) power device and process for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is a U.S. national stage application under 35 U.S.C. §371 of PCT Application No. PCT/IT2006/000705, filed Oct. 4, 2006, the entirety of which is incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to a power device, and more particularly to a pseudomorphic high electron mobility transistor (PHEMT) power device and to a process for manufacturing the same. 
     BACKGROUND ART 
     As is known, pseudomorphic high electron mobility transistors (PHEMTs) are extensively used in wireless communication systems for switching, power and low noise amplifier applications. These transistors find wide market acceptance because of their high RF gain and power added efficiency (PAE) and low noise figure (NF). The excellent properties of these transistors also make them attractive for use in satellite communication systems including direct broadcast satellite television (DBS-TV) and global satellite communication systems. PHEMT technology is also used in high-speed analog and digital IC&#39;s such as 2.5-10 Gb/s lightwave communication systems. The higher frequency response of PHEMTs are currently finding use in millimeter wave communications (40 Gb/s) and radar systems. 
     The increasing market demand for RF power devices with higher and higher performance for wireless communication systems, radar detection, satellite and electronic warfare systems has led the electronics industry to extend the operating frequencies of available power devices and technologies. To increase operating frequencies of power devices up to the millimeter wave range, several key technological features have been proposed including an optimized PHEMT epitaxial layer structure, an optimized device layout of the unit cell for specific applications, and thin-film technology development adequate for high frequency functions. In particular, the most significant requirement set for the epitaxial layer structure is associated with the condition of ensuring that free electrons in the conductive channel are physically separated from ionized donors. This solution allows for a significant reduction in ionized impurity scattering, enhancing electron mobility and therefore marked performance improvements over conventional metal-semiconductor field effect transistors (MESFET). 
     Additionally, pseudomorphic high electron mobility transistors PHEMTs usually require a negative gate voltage bias to operate (see for example U.S. 2002/024057 A1). Therefore, as compared to heterojunction bipolar transistors (HBTs) that use a positive gate voltage, the introduction of such a negative voltage disadvantageously requires a dedicated voltage supply circuit that increases chip complexity and manufacturing costs. 
     A PHEMT power device capable of operating with a single voltage supply is for example disclosed in U.S. Pat. No. 6,593,603. The PHEMT power device includes an epitaxial substrate including a GaAs buffer layer, an AlGaAs/GaAs superlattice layer, an undoped AlGaAs layer, a first doped silicon layer, a first spacer, an InGaAs electron transit layer, a second spacer, a second doped silicon layer having a different doping concentration from the first doped silicon layer, a lightly doped AlGaAs layer, and an undoped GaAs cap layer stacked sequentially on a semi-insulating GaAs substrate; a source electrode and a drain electrode formed on and in ohmic contact with the undoped GaAs cap layer; and a gate electrode formed on the lightly doped AlGaAs layer to extend through the undoped GaAs cap layer. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The Applicant has noticed that in order to provide a PHEMT power device operable with a single voltage supply (the drain supply voltage only), it is necessary to optimize the pinch-off voltage that permits to achieve class A operating conditions (drain-to-source current equal to half of the maximum achievable one) while maintaining the gate grounded without any bias. 
     In particular, the Applicant has noticed that an optimized PHEMT power device having excellent linearity and power-added efficiency, high breakdown voltage, and capable of operating with a single voltage supply should meet the following requirements: a low knee voltage, a high and uniform transconductance, and a low source-gate capacitance. 
     In this respect, the Applicant has noticed that even if the PHEMT power device disclosed in U.S. Pat. No. 6,593,603 is operable with a single voltage supply, its performance proves however to be satisfactory only in the C frequency band (4 to 6 GHz), while becomes quite unsatisfactory in the X frequency band (8.0 to 12.0 GHz), and the PHEMT power device is completely unusable in the Ka (K-above) frequency band (18 to 40 GHz). 
     Therefore, the Applicant has noticed that in order to provide a PHEMT power device operable with a single voltage supply and having also excellent performance in the X frequency band and the Ka frequency band, it is necessary to further optimize the structure of the dedicated epitaxial substrate and, consequently, the device manufacturing process disclosed in U.S. Pat. No. 6,593,603. 
     The main objective of present invention is therefore to provide a PHEMT power device operable with a single voltage supply, suitable for a satisfactory use in high-frequency digital wireless communications, in particular which may be used to amplify signals up to 40 GHz, i.e., for example C frequency band, X frequency band and Ka frequency band, and having a higher linearity, breakdown voltage and power-added efficiency than the PHEMTs in the prior art. 
     Another objective of present invention is to provide a process for manufacturing this PHEMT power device. 
     These objectives are achieved by the present invention in that it relates to a PHEMT power device and to a process for manufacturing the same, as defined in the appended claims. 
     The present invention achieves the aforementioned objective by exploiting advances in epitaxial growth technologies of III-V semiconductor heterojunctions, which advances ensure the possibility of engineering the band structure, doping type and level in different regions of semiconductor devices, and by adopting new semiconductor materials with superior electrical characteristics, such as wide band-gap semiconductor, and by properly designing the PHEMT structure. Use of these additional degrees of freedom provides the opportunity of developing new devices with considerable improvement in RF performance. In particular, the present invention achieves the aforementioned objective by introducing an optimized epilayer sequence and a double recessed gate geometry. More in detail, the present invention achieves the aforementioned objective by providing a pseudomorphic high electron mobility transistor (PHEMT) power device including: 
     a semi-insulating substrate; 
     an epitaxial substrate formed on the semi-insulating substrate; the epitaxial substrate including a buffer layer, a superlattice layer, a first electron supply layer, a first spacer layer, an electron transit layer, a second spacer layer, a second electron supply layer, a Schottky layer, and a contact layer sequentially stacked on the semi-insulating substrate; 
     source and drain electrodes formed on, and in ohmic contact with the contact layer; and 
     a gate electrode formed on the Schottky layer to extend through the contact layer; 
     the PHEMT power device being characterized in that the contact layer includes: 
     a lightly doped contact layer formed on the Schottky layer; 
     a highly doped contact layer formed on the lightly doped contact layer and having a doping concentration higher than the lightly doped contact layer; 
     the PHEMT power device being further characterized by: 
     a wide recess formed to penetrate the highly doped contact layer so as to expose a surface of the lightly doped contact layer; and 
     a narrow recess formed in the wide recess to penetrate the lightly doped contact layer so as to expose a surface of the Schottky layer; 
     wherein the gate electrode is formed in the narrow recess and in Schottky contact with the Schottky layer to extend from the exposed surface of the Schottky layer through the lightly and highly doped contact layers; and 
     the source and drain electrodes are formed on, and in ohmic contact with the highly doped contact layer outside the wide recess so that the wide recess is arranged between the source and drain electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, preferred embodiments, which are intended purely by way of example and are not to be construed as limiting, will now be described with reference to the attached drawings, wherein: 
         FIG. 1  is a sectional view of a PHEMT power device according to an embodiment of the present invention; 
         FIG. 2  is a sectional view of a PHEMT power device according to another embodiment of the present invention; and 
         FIG. 3  is a sectional view of the PHEMT power device according to a different embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein and defined in the attached claims. 
       FIG. 1  is a sectional view of a PHEMT power device according to an embodiment of the present invention. 
     The PHEMT power device  1  includes a III-V substrate  2 , e.g. a semi-insulating GaAs substrate, a doped epitaxial substrate  3  formed on the semi-insulating GaAs substrate  2 , and source, drain and gate electrodes  4 ,  5  and  6  formed on the epitaxial substrate  3 . 
     In particular, epitaxial substrate  3  comprises an undoped GaAs buffer layer  10 ; an undoped AlGaAs/GaAs superlattice layer  11 ; an undoped wide-bandgap AlGaAs layer  12 ; a first ultra thin doped silicon (pulse) electron supply (donor) layer  13 , a first spacer layer  14 , an undoped narrow-bandgap InGaAs electron transit (channel) layer  15 , a second spacer layer  16 , a second ultra thin silicon doped (pulse) electron supply (donor) layer  17  with a different doping concentration than the first electron supply layer  13 , an undoped or lightly doped wide-bandgap Schottky AlGaAs layer  18 , and a GaAs (ohmic) contact (cap) layer  19  sequentially stacked on the semi-insulating GaAs substrate  2 . 
     In order to lower ohmic contact resistance between GaAs contact layer  19  and source and drain electrodes  4 ,  5 , GaAs contact layer  19  is doped and includes a lower lightly doped GaAs contact layer  20  and an upper highly doped GaAs contact layer  21 . In particular, doped GaAs contact layer  19  is approximately 100 nm thick, lightly doped GaAs contact layer  20  is approximately 30 nm thick and is formed to have a doping concentration of approximately 3·10 17  cm −3 , and highly doped GaAs contact layer  21  is approximately 70 nm thick and is formed to have a doping concentration of approximately 3.5·10 18  cm −3 . 
     Undoped AlGaAs layer  12  is approximately 4 nm thick and contains aluminum (Al) approximately ranging from 22% to 24% by mole ratio, InGaAs electron transit layer  15  is approximately 13-15 nm thick and contains indium (In) ranging approximately from 15% to 20% by mole ratio; and lightly doped AlGaAs layer  18  is approximately 30 nm thick, contains aluminum (Al) approximately ranging from 22% to 24% by mole ratio, and is formed to have a doping concentration ranging approximately from 1.0·10 17  to 3.0·10 17  cm −3 . 
     In order to improve transconductance linearity, the first electron supply layer  13  has a lower doping concentration than the second electron supply layer  17 . Additionally, in order to have a doping level in the InGaAs electron transit layer  15  approximately ranging from 1.7·10 12  to 2.7·10 12  cm −2 , first electron supply layer  13  is formed to have a doping level of approximately 1·10 12  cm −2 , and second electron supply layer  17  is formed to have a doping level of approximately 5·10 12  cm −2 . Additionally, the first and the second electron supply layers  13 ,  17  are each approximately 0.5 nm thick. 
     To obtain Ohmic contact an Au/Ge/Ni/Au metallization is deposited on the highly doped GaAs contact layer  21  and subjected to rapid thermal annealing (RTA) to form source and drain electrodes  4 ,  5  in ohmic contact with the highly doped GaAs contact layer  21 . 
     In order to separate active areas of individual PHEMT power devices, epitaxial substrate  3  may be mesa-etched or, preferably, ion implanted using Deuterium or Fluorine high energy ions down to the AlGaAs/GaAs superlattice layer  11 . 
     After these two steps in order to increase the breakdown voltage and maintain the knee voltage low, while preserving high transconductance and high PHEMT gain, thereby improving the PHEMT power characteristics, doped GaAs contact layer  19  is recess etched to form a double recess structure  22  including an upper wide recess  23  formed in the highly doped GaAs contact layer  21  and a lower narrow recess  24  formed in lightly doped GaAs contact layer  20  and partly in lightly doped AlGaAs layer  18 . In particular, to form double recess structure  22 , highly doped GaAs contact layer  21  between source and drain electrodes  4 ,  5  is etched to form wide recess  23 , and then lightly doped GaAs contact layer  20  is selectively wet etched inside the wide recess  23  down to lightly doped AlGaAs layer  18  to form narrow recess  24 . In particular, selective etching is carried out using a ph-controlled and temperature-controlled citric acid and hydrogen peroxide solution. Additionally, each one of the two etchings are performed by patterning lightly and highly doped GaAs contact layers  20 ,  21  with positive i-line photoresists. Presence of the lightly doped AlGaAs layer  18  ensures etching uniformity even in wet etching performed to form the double recess during the manufacture of the PHEMT power device  1 . 
     After formation of double recess  22 , gate electrode  6  is then formed in contact with the AlGaAs Schottky layer  18  by evaporating titanium (Ti) and aluminium (Al) on an exposed surface of the AlGaAs Schottky layer  18 . In particular, gate electrode  6  extends centrally in double recess  22  from AlGaAs Schottky layer  18  and through both lightly and highly doped lower GaAs contact layers  20 ,  21  and protrudes outside double recess  22 . 
     A protective insulating layer  25 , for example made of silicon nitride (Si 3 N 4 ), is then formed, for example by Plasma Enhanced Chemical Vapour Deposition (PECVD), on a surface of the highly doped GaAs contact layer  21  exposed by source, drain and gate electrodes  4 ,  5 ,  6 . 
     A Ti/Pt/Au base metal layer  26  is then formed, for example by evaporation, and then layered on source and drain electrodes  4 ,  5 , and an Au layer  27  is then formed, for example by electroplating, on the base metal layer  26 . Base metal layer  26  defines an interdiffusion barrier that isolates ohmic contact and prevents gold from diffusing from Au-plated layer  27  to source and gate electrodes  4 ,  5 . 
     Source and drain electrodes  4 ,  5  are then connected to respective source and drain pads (not shown) via Au-plated airbridges (not shown). 
     Finally, semi-insulating GaAs substrate  2  is thinned down from an initial thickness of approximately 650 μm to a final thickness ranging approximately from 50 to 120 μm, and then the PHEMT  1  is back-etched to form a via-hole  28 , which is metallized, including a surrounding area, with a thick Au layer to extend from thinned semi-insulating GaAs substrate  2  up to source electrode  4  for source pad interconnection, in order to minimize the parasitic source-to-ground inductances, and heat sink provision. 
       FIG. 2  is a sectional view of a PHEMT power device similar to that shown in  FIG. 1 , where same reference numerals designate same elements. In particular, PHEMT power device shown in  FIG. 2 , designated by 1′, differs from PHEMT power device  1  shown in  FIG. 1  in that gate electrode  6  is generally T-shaped with a lower leg portion  6   a  formed of Ti and an upper head portion  6   b  formed of Al. More in detail, leg portion  6   a  of gate electrode  6  has an height substantially equal to depth of double recess  22 , so that head portion  6   b  of gate electrode  6  protrudes outside double recess  22 . More in detail, the T shape is obtained by dry-etching the Ti of the gate metal stack in a suitable Fluorine/Oxygen plasma chemistry in low-damage isotropic plasma-etch reactor. The Fluorine gas carrier could be CF 4 , or CHF 3  or SF 6 . The oxygen carrier gas could be O 2 . 
     The Plasma is created by an RF discharge in a Plasma Etch parallel plate configuration. Each plate of the chamber is heated, for example in the range 100 to 200° C., the chamber pressure is in the range 200 to 1000 mTorr. RF power discharge should be lowered to reduce as much as possible surface damage caused by ion and electron bombardment. When a Plasma Etch parallel plate configuration is utilised for this gate etch the RF power discharge must be lower than 50 Watt. Furthermore, better results are achievable if equipments able to ensure cold plasma are introduced (ECR—Electron Cyclotron Resonance, ICP—Inductively Coupled Plasma). 
     In such a process, the Al of the head portion  6   b  of the gate electrode  6  is used as a mask layer because the Al becomes fluorinated and remains un-etched by the Fluorine plasma. So, the head portion  6   b  of the gate electrode  6 , made of Al, stays un-etched, while the leg portion  6   a  of the of the gate electrode  6 , made of Ti and lying on the AlGaAs Schottky layer  18 , becomes laterally etched. In this way a gate length down to 0.15 μm can be obtained with a quick and low-cost manufacturing process, keeping a low gate finger resistance. 
       FIG. 3  is a sectional view of a PHEMT power device similar to those shown in  FIGS. 1 and 2 , where same reference numerals designate same elements. In particular, PHEMT power device shown in  FIG. 3 , designated by 1″, differs from PHEMT power devices shown in  FIGS. 1 and 2  in that it is provided with a field plate  29  connected to the gate electrode  6  and extending toward the drain electrode  5  without overlapping either the highly doped contact layer  21  or the drain electrode ( 5 ). In particular, the field plate  29  is formed as a gate extension  6   c  coplanar with highly doped GaAs contact layer  21 , vertically overlapping part of protective insulating layer  25  on the lightly doped GaAs contact layer  20 , and ending at a distance of 1.0 μm from highly doped lower GaAs contact layer  21 . The protective insulating layer  25  made of silicon nitride (Si 3 N 4 ) is ultra-thin (less than 50 nm), and is deposited by PECVD so optimized to reach a high dielectric constant of the Si 3 N 4 , and to control the GaAs/Si 3 N 4  interface states. 
     In the PHEMT power devices shown in  FIGS. 1 ,  2 , and  3  different GaAs and AlGaAs layers are provided, but the heterojunction of greatest interest is that between silicon doped AlGaAs layer  12  and  18  and undoped InGaAs layer  15 . In fact, due to the higher bandgap of AlGaAs layer compared to the adjacent InGaAs layer, free electrons diffuse into the InGaAs layer and form a two dimensional electron gas (2-DEG) at the heterointerface, where they remain confined in a very thin sheet, and transport properties of the 2-DEG are much better than those of free electrons in a conventional MESFET. 
     AlGaAs layer  18  is the donor layer, and in the final PHEMT power device this layer should be depleted from Schottky gate  6  to AlGaAs/GaAs interface to eliminate any effect of parallel conduction in AlGaAs otherwise detrimental to PHEMT performance. During heterostructure design, it must be considered that increasing the doping level in the donor layer reduces the breakdown voltage of the PHEMT. To overcome this difficulty while maintaining a high 2-DEG sheet carrier concentration, it is possible to use a pulsed doped, δ doped AlGaAs layer. This solution makes use of a Si monolayer, approximately 0.5 nm thick, with a very high doping level. 
     As for thickness of spacer layers  14 ,  16 , even thought free electrons are physically separated from donors, close proximity produces an electrostatic interaction, known as Coulomb Scattering. This effect is reduced by separating the 2-DEG from the AlGaAs donor layer by means of a thin spacer layer of undoped AlGaAs. It is worthwhile to mention that noise performance of a PHEMT is strictly connected to spacer thickness. 
     As for GaAs buffer layer  10 , because of drain to source electric field, part of electrons can be injected into GaAs buffer layer  10 . This phenomenon induces a gain reduction as a result of increasing drain output conductance. In the PHEMTs, electron injection into the GaAs buffer layer  10  is controlled by means of AlGaAs/GaAs superlattice layer  11  into GaAs buffer layer  10 , that at the same time does not change channel quality. 
     Described herebelow are results of characteristic measurements carried out on a PHEMT power device manufactured according to the present invention and with a 0.5-μm-gate length, 0.25-μm-gate length and with a field-plate structure. All the mentioned results are referred to a 100-μm-unit gate width and for 1-mm-total gate width. In particular, as far as transconductance and saturation current are concerned, the PHEMT power device with different gate length according to the present invention has a pinch-off voltage approximately from −0.8V to −0.6V, a saturation current ranging from 100 to 300 mA/mm at a gate voltage of 0V, and a maximum saturation current ranging from 300 to 600 mA/mm at a gate voltage of 0.8V. In addition, transconductance is nearly consistent in the range of 250-350 mS/mm at a gate voltage ranging from 0 to 1V. As a result, the PHEMT power device exhibits improved linearity. As far as current-voltage characteristic is concerned, the PHEMT power device according to the present invention exhibits a low knee voltage of about 1V and a off-state breakdown voltage of about 18V without a field-plate structure and of 40V with a field-plate structure, while maximum current remained unaffected. This result explains that a high gate drain breakdown voltage can be obtained keeping low ohmic contact resistance. As far as power characteristics are concerned, the PHEMT power device according to the present invention has an output power up to 29 dBm/mm with an associated power gain up to 12 dB, which power characteristics have been determined by a load-pull method at a typical value of 1 dB of gain compression up to a frequency of 10 GHz if the gate length is conveniently adapted. Introducing the field-plate structure, power performance are additionally increased achieving an output power up to 32 dBm/mm and an associated power gain up to 15 dB for C band applications. Finally, as far as high RF amplifier applications are concerned, the PHEMT power device according to the present invention may be used to amplify signals up to 40 GHz by appropriately varying the gate length. 
     The advantages of the present invention are evident from the foregoing. In particular, in the PHEMT power device according to the present invention, the formation of a doped GaAs contact layer  19 , particularly made up of a lower lightly doped lower GaAs contact layer  20  and an upper highly doped lower GaAs contact layer  21  in ohmic contact with the source and drain electrodes allows the contact resistance therebetween to be lowered, thus improving voltage characteristics of the PHEMT. 
     Additionally, the formation of a double recess structure  22  made up of an upper wide recess  23  formed in upper highly doped GaAs contact layer  21  and of a lower narrow recess  24  formed in lower lightly doped GaAs contact layer  20  allows for the ohmic contact to be placed on highly doped GaAs contact layer  21 , thus improving the overall power characteristics of the PHEMT, in particular significantly increasing the breakdown voltage while maintaining a low knee voltage, and improving linearity and power-added efficiency. 
     Moreover, the combination of the double recess structure  22  with the lightly and highly doped GaAs contact layers  20 ,  21  allows the PHEMT power device to operate with a single voltage supply, without any need for a negative bias voltage to the gate contact. 
     Further, the PHEMT of the present invention allows the Class A amplifier requirements to be fulfilled with the gate contact pad directly connected to ground. 
     Yet further, the doped silicon layer  13  with a lower doping concentration than the doped silicon layer  17  makes the transconductance of the PHEMT power device invariant with respect to gate-to-source voltages, while the lightly doped AlGaAs layer  18  ensures etching uniformity in wet recess etching performed to manufacture the PHEMT power device, thus making the overall manufacturing process for the PHEMT power device easy and hence improving the productivity. 
     Finally, a T-gate structure allows gate-to-source capacitance and gate finger resistance to be significantly reduced, allowing the PHEMT to achieve operating frequencies up to 40 GHz. 
     Finally, numerous modifications and variants can be made to the present invention, all falling within the scope of the invention, as defined in the appended claims. 
     For example, the undoped wide-bandgap AlGaAs layer and the first ultra thin doped silicon electron supply layer  13  may be replaced with a single uniformly doped AlGaAs electron supply layer.