High electron mobility transistor and method of manufacture

Abstract: A high electron mobility transistor (50) includes a pair of charge screen layers (58b,64) disposed over a first one of active and charge donor layers (59) of the high electron mobility transistor (50). The two screen layers (58b,64) are patterned to provide a double recessed channel. A first charge screen layer (58b) disposed adjacent to the charge donor layer (58,59) is etched to provide a recess having a first length between source and drain electrodes (68a,68b), whereas a second charge screen layer (64) disposed over the first aforementioned charge screen layer (58b) and a portion of the aforementioned first charge screen layer (58b) are etched to provide a second, substantially longer length between source and drain electrodes (68a,68b). The gate electrode (70) is provided in the first aforementioned recess in Schottky barrier contact with the charge donor layer (58,59).

Description: This invention relates to a high electron mobility transistor comprising: a charge donor layer of a first Group III-V material with dopant atoms; a channel layer disposed adjacent to the charge donor layer and composed of an undoped second Group III-V material having a bandgap energy lower than the bandgap energy of the first Group III-V material; a pair of ohmic contacts disposed over first portions of the charge donor and channel layers; and a Schottky barrier control contact disposed over second portions of the charge donor and channel layers and located in a recess extending through at least a further layer of a third Group III-V material having a bandgap energy lower than the bandgap energy of the first Group III-V material and including dopant atoms. As is known in the art, there are several types of active devices used at microwave and millimetre wave frequencies to provide amplification of radio frequency signals. Commonly used semiconductor devices at these frequencies are field effect transistors, especially metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs). These last two types of transistor are produced from Group III-V materials such as gallium arsenide. What distinguishes a HEMT from a MESFET is that charge in a HEMT is transferred from a doped charge donor layer to an undoped channel layer whereas in a MESFET the charge donor layer and the channel layer are the same layer. The charge transport properties of the undoped channel layer in a HEMT are better than those of the doped channel layer of a MESFET type structure. Accordingly, due to the presence of an undoped channel layer HEMTs provide higher frequency operation than MESFETs. In a HEMT, the charge donor layer is generally a wide bandgap material, such as aluminium gallium arsenide, whereas the channel layer is a lower bandgap material, such as gallium arsenide or indium gallium arsenide. The channel layer is typically comprised of a material having better electron transport properties. It is to be noted that bandgap refers to the potential gap between valance and conduction bands of the semiconductor materials. In general, there are two types of HEMT structures. One type is simply referred to as a HEMT, whereas the other type is referred to as a pseudomorphic HEMT. The difference between the HEMT and the pseudomorphic HEMT is that in the pseudomorphic HEMT one or more of the layers incorporated into the HEMT structure is comprised of a material having a lattice constant which differs significantly from the lattice constants of the other materials of the device. Thus, due to resulting lattice constant mismatch, the crystal structure of the material providing the channel layer is strained. In a pseudomorphic HEMT, the undoped gallium arsenide channel layer is replaced by a channel layer comprised of a lower bandgap material, such as indium gallium arsenide. HEMT and pseudomorphic HEMT structures are used to provide amplification of high frequency microwave and millimetre wave signals. For low noise and high frequency applications of high electron mobility transistors, it is important to have a narrow recess disposed through the contact layers of the device and over the charge donor layer. That is, the recess opening is preferably only slightly longer than the gate length of the gate electrode disposed within the recess. This arrangement has provided HEMTs and pseudomorphic HEMTs that have relatively high frequency operating characteristics and relatively low noise figures. The nature of the recess in which the gate electrode is disposed is discussed in an article entitled "Effect of In Ga As Well Width on Low-Noise Performance in Al Ga As/In Ga As Pseudomorphic HEMT" by M. Itoh, K. Ohmuro, H. Nakamura, S. Nishi, and S. Takahashi, at pages 285 to 288, Japanese Journal of Applied Physics, 21st Conference on Solid State Devices and Materials, August 1989, Tokyo, and EP-A-0 367 411. For power applications in MESFETs, it is generally known that a recessed opening larger than the gate is necessary to provide the MESFET with relatively high gate to drain breakdown voltage characteristics. Returning to a HEMT, on the etched gallium aluminium arsenide surface which is generally the upper surface in most HEMT structures, there exists a large number of surface states. Such surface states also exist on the GaAs surface. Some authors have estimated the surface states to be as many as 10 14 cm -2 . These states most likely arise from gallium and aluminium oxides. It has been suggested that these states once occupied, increase the gate to drain breakdown voltage characteristic by capturing electrons and thus decreasing the electric field concentrated at the gate metal edge on the drain side of the transistor. The breakdown voltage characteristics of high electron mobility transistors limit their use to relatively low power, low noise applications. This follows since the output impedance of a HEMT is generally related to the drain bias level. Low breakdown voltage characteristics limit the operating drain voltage of a HEMT. For a given DC power level, it is general advantageous to bias a HEMT for high power applications at relatively high drain voltages and low drain current rather than vice versa. Biasing at high drain voltage provides a higher output impedance for the HEMT and therefore a more easy impedance match to a 50 ohm system characteristic impedance which is generally encountered in most applications. In particular, this match is more easily made over broad ranges of operating frequencies. Further, to provide high levels of RF voltage gain from such a device it is generally necessary to operate the device at a relatively high drain voltage DC bias. However, as indicated above, although it would be desirable to bias HEMTs at high breakdown voltage, such is generally not possible since the HEMTs have relatively low breakdown voltage characteristics. Therefore, high electron mobility transistors are used in relatively low power, low noise, applications, because the known high electron mobility transistors generally have relatively low gate to drain reverse breakdown voltage characteristics. This situation is undesirable since the high frequency characteristics of HEMTs and the relatively high gain of HEMTs in comparison to MESFETs, would otherwise be useful for higher power applications. We believe that breakdown voltage limitations in high electron mobility transistors are at least partially related to surface states or excess surface charge present in the charge donor layer or channel layer of the transistor. These surface states are the same surface states generally believed to alleviate the high electric field between the gate metal edge and the charge donor layer on the drain side of the device. Electrons can spatially tunnel from the metal gates to the nearby surface states in the semiconductor layers, such as the charge donor layer. However, as the gate to drain voltage increases, those surface states which are spatially removed from the gate electrode also become ionized. These trapped electrons thus provide a depletion region which is generally commensurate with the density of the surface states. The re-emission of these trapped charges requires a finite transit time which is generally much greater than the transit time of charge response under the gate electrode. Thus, the depletion region is essentially static relative to the input RF signal and as the RF signal increases in frequency the effects become more pronounced. Thus, the presence of the surface charges provide, in essence, a parasitic gate. The parasitic gate electrode thus acts as a slow gate which does not respond to high frequency signals as does the regular gate electrode of the HEMT. Accordingly, the presence of this gate electrode reduces the overall efficiency of the transistor. Further, we have also observed that the RF characteristics of a transistor can vary dramatically with quiescent bias voltages if surface states are present. The greater the drain gate voltage the more surface states are filled and the greater the large signal degradation that is provided. JP-A-57 026472 (Patent Abstracts of Japan, vol. 6, No. 91 (E-109), 28 May 1982) describes a field effect transistor in which the transistor has an insulating substrate, a buffer layer, an N type electron donor layer, a channel layer, and an electron shielding layer, in that order, with a control electrode and source and drain electrodes disposed on the free surface of the shielding layer. EP-A-0 183 146 describes a field effect transistor in which high transconductance is provided by a Dirac-delta doped monatomic layer that extends between source and drain regions. EP-A-0 367 411 describes a high electron mobility transistor of the kind defined hereinbefore at the beginning in which the transistor is composed of layers of Group III-V materials which are either undoped or uniformly doped. A channel layer of IN 0.53 Ga 0.47 As and a charge donor layer of n-type In 0.52 Al 0.48 As are used so that a two dimensional electron gas layer is established in the channel layer close to the channel layer-charge donor layer interface. For the purpose of producing HEMTs with sufficiently uniform and controllable characteristics for high density integration, at least one further layer of Group III-V material is included between the charge donor layer and a doped cap layer provided for establishing the ohmic contacts, the recess for the control electrode extending through the cap layer and the at least one further layer. In one example of this known HEMT, the following layers are provided in the order given, between the charge donor layer and an n-type GaAs cap layer: GaAs; n-type InGaAs; n-type GaAs; n-type InGaAs. In each example, the layer immediately above the charge donor layer is of a Group III-V material that does not contain Indium. Consequently dry etching can be used in the last step of forming the recess for the control electrode, which forms a Schottky barrier with the Indium-containing charge donor layer, and this last step can be controlled accurately. According to the present invention, a high electron mobility transistor of the kind defined hereinbefore at the beginning is characterised in that the charge donor layer has a sheet of dopant atoms disposed substantially close to the interface provided by the charge donor layer and the channel layer, the sheet having a concentration of dopant atoms confined to a few atomic layer thicknesses of the charge donor layer and in that the charge donor layer is relatively lightly doped in the layer between the sheet and the control contact. By providing a shield to screen the channel layer from surface charge, the parasitic effects associated with surface charges in the gate to drain region of the transistor are reduced thereby permitting higher breakdown voltage characteristics from the high electron mobility transistor without degrading the RF performance characteristics of the HEMT. By providing the dopant sheet, the dopant has been removed from the vicinity of the gate electrode which results in a reduction of avalanche breakdown effects between the gate and drain regions of the high electron mobility transistor. In a preferred embodiment of the present invention, the shielding means is a pair of charge screen layers comprised of relatively lightly doped materials disposed between the gate electrode and source and drain electrodes of the high electron mobility transistor. With such an arrangement, by providing charge screen layers disposed between gate and source and drain electrodes, each of the charge screen layers supply positive space charge that compensates for the negative surface states present in the active layer. The pair of screen layers are preferably patterned to provide a double recessed structure. The first charge screen layer disposed adjacent to the charge donor layer is etched to provide a recess having a first length between source and drain electrodes, whereas the second charge screen layer disposed over the first aforementioned charge screen layer is etched to provide a second, substantially longer length between source and drain electrodes. The gate electrode is provided in the first aforementioned recess in Schottky barrier contact with the charge donor layer. With this particular arrangement, by providing a double recessed structure, the structure reduces the effect of surface states between the gate electrode and the active layer of the high electron mobility transistor by further removing such regions having such states from the immediate vicinity of the gate. This arrangement thus provides HEMTs having relatively high gate-drain breakdown voltage characteristics. Further, however, the presence of the lightly doped charge screen layers permits the above device to exhibit good breakdown voltage levels at relatively high frequencies. Brief Description of the Drawings The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following detailed description of the drawings, in which: FIGs. 1A-1E are a series of cross-sectional views showing the steps in fabricating a high electron mobility transistor in accordance with the one aspect of the present invention, and FIG. 2 is a cross-sectional view showing a high electron mobility transistor in accordance with a further aspect of the present invention. Description of the Preferred Embodiments Referring now to FIG. 1A, a substrate 12, here comprised of semi-insulating gallium arsenide or other suitable semiconductor material is shown having a plurality layers disposed thereon. In particular, disposed over substrate 12 is a first layer 14, comprised of a superlattice buffer layer comprised of alternating layer pairs (not shown) of gallium arsenide and aluminum gallium arsenide, each one of said layers having a typical thickness of 50-100Å disposed to provide a superlattice as is known in the art. Disposed over superlattice 14 is a undoped layer 16 of, here, gallium arsenide. Disposed over layer 16 is a back layer 18 comprised of aluminum gallium arsenide which provides a large bandgap material on the back or bottom surface of a channel layer 20, as will be described shortly. Back layer 18 has a pair of regions 18a, 18b. Region 18a is a first region or spacer layer region of generally undoped wide bandgap material such as Al x Ga 1-x As where X is typically between 0.1 and 0.3, whereas region 18b is a second region of undoped, wide bandgap material here also AlGaAs. Such regions 18a, 18b have disposed therebetween a region of high dopant concentration 19, here commonly referred to as a dopant spike and which is preferably described as a substantially two-dimensional layer or sheet of dopant material having a dopant concentration in the typical range of 0.3X10 12 to 5X10 12 a/cm 2 with the range of 0.3X10 12 to 1.5X10 12 being preferred. Further, the dopant layer 19 is confined to several angstroms of thickness of composite layer 18 thus to provide a sheet having a thickness of a few atomic layers. Dopant layer 19 is spaced some 30-50Å by region 18a from the interface or heterojunction between region 18a and a layer 20 which provides the channel layer for the device to be fabricated. The channel layer 20 is comprised of a lower bandgap material, such as gallium arsenide or indium gallium arsenide or other suitable materials as is known. Layer 20 is undoped and provides a region of low impurity concentration and thus provides a region where carrier mobilities are relatively high. Disposed over channel layer 20 is a second wide bandgap material layer 22, having an undoped spacer region 22a having a typical thickness of 30Å to 50Å and a lightly doped screening region 22b which are separated by a dopant spike region 21, having a sheet doping concentration of typically 2X10 12 to 5X10 12 a/cm 2 and provides the charge donor region for the channel layer 20. Region 22b is lightly doped having a dopant concentration in the range of 1X10 17 to 5X10 17 a/cm 3 typically and a thickness typically of 200Å to 400Å and provides a first charge screen layer, as will be further described. Disposed over region 22b is a second charge screen layer 24 here comprised of a medium bandgap material such as gallium arsenide (GaAs) having a dopant concentration of 1X10 17 to 5X10 17 a/cm 3 . Layer 24 has a thickness typically in the range of 300Å to 400Å. Of course, for both layers 22 and 24 other thicknesses could be used. Disposed over layer 24 is a contact layer 26 comprised of a relatively highly doped layer (i.e. having a dopant concentration generally greater than 1X10 18 a/cm 3 or greater) of a Group III-V material such as gallium arsenide which is used to provide ohmic contact to source and drain electrodes, as will be described below. Referring now to FIG. 1B, layer 26 has disposed thereover a patterned photoresist layer 42 which defines regions for source and drain contacts. Layer 42 is patterned to provide regions or apertures 42a, 42b, as shown, and a layer of metal is introduced into apertures 42a, 42b and over portions of photoresist 42. Thereafter, the photoresist layer 42 is removed leaving behind electrodes 28a, 28b in the regions of apertures 42a, 42b, as shown. Such electrodes provide ohmic source and drain contacts to layer 26. Preferably the electrodes 28a, 28b are alloyed with layer 26 using techniques conventionally known to provide low resistivity ohmic type contacts to layer 26 and layers there below including underlying portion of the channel layer 20. Referring now to FIG. 1C, photoresist layer 42 (FIG. 1B) having been removed leaves behind source and drain contacts 28a, 28b, respectively as shown. A second photoresist layer 44 is deposited over and is patterned to provide an aperture 44a which, here, is patterned to provide a relatively wide aperture between source and drain contacts 28a, 28b, as shown. The structure 10 is brought in to contact with an etchant which will suitably etch away exposed portions of layer 26, as well as, the selective portion of underlying layer 24, as shown. This provides a first recess through layers 26 and 24, as shown, for eventual formation of a gate electrode as will be described in conjunction with FIG. 1E. A suitable etchant for layers 26 and 24 is a dilute solution of ammonia hydroxide and hydrogen peroxide which is held in contact with said layers for a selected period of time. Referring now to FIG. 1D, photoresist layer 44 (FIG. 1C) is removed and is replaced with a third photoresist layer 46 which is likewise patterned to, here, provide an aperture 46a in photoresist layer 46 generally disposed within the recess portion 25' of the layers 24 and 26. Aperture 46a exposes a selective underlying portion of layer 24. An etchant such as a dilute solution of ammonia hydroxide and hydrogen peroxide is brought into contact with the exposed portion of layer 24 to form a recess 23 within the aforementioned recess portion 25'. Here recess 23 is provided through layer 24 and through a selected portion of layer 22. This mask is also used to deposit a Schottky barrier metal (not shown) to provide a gate electrode in Schottky barrier contact with layer 22 as generally shown in FIG. 1E. The structure 10 shown in FIG. 1E includes a pair of charge donor layers 18, 22, disposed over and under channel layer 20. This is a so-called high breakdown high electron mobility transistor structure. Further, the structure described in conjunction with FIG. 1E includes a pair of charge screening layers 22b, 24 disposed between the contact layer 26 and the dopant spike or charge donor region 21. Here region 22b differs from the conventional charge donor layer of a typical high electron mobility transistor since it has a generally low dopant concentration except for the dopant spike region 21 disposed substantially adjacent the heterojunction provided between layers 20 and 22. The heterojunction is formed in said layers by disposing the spacer region of the wide bandgap material, such as aluminum gallium arsenide characteristic of spacer region 22a with the lower bandgap material, such as gallium arsenide or indium gallium arsenide used for layer 20. With this arrangement of the layer 22, a substantial portion of the charge is removed from the vicinity of the gate electrode 30 which is accomplished by the relatively low dopant concentration throughout most of the layer 22 except for the dopant spike regions 21. The presence of the double wide recessed gate electrode in the pair of charge screen regions or layers 22b, 24 serves to further reduce the influence of surface states or a resulting surface layer which exists on the surface of the semiconductor regions disposed between the gate electrode 30 and drain electrode 28b from providing parasitic gate effects in the channel layer 20. Furthermore, the lightly doped charge screen layers 22b and 24 further reduces the effect of surface states between the gate electrode and the channel layer 20. With each of the aforementioned arrangements therefore since the parasitic effects between the gate electrode and the channel layer are reduced, the resulting high electron mobility transistor can be operated at substantially higher drain bias voltage than conventionally known HEMTs. As also shown in conjunction with FIG. 1E, a ground plane conductor 11 is disposed over an opposing surface of substrate 12, as is conventionally provided. Referring now to FIG. 2, a high electron mobility transistor 50 having a single charge donor layer and having the structure as otherwise generally described in conjunction with FIG. 1 is shown. Here the transistor is fabricated using similar approaches generally described above. Thus, here the transistor 50 includes a substrate 52 having a ground plane 51 disposed over the first surface thereof and a buffer layer 54, here a superlattice type buffer layer, disposed over a second surface of substrate 52. Disposed over the superlattice buffer layer 54 is a channel layer 56 comprised of a relatively low or narrow bandgap material such as gallium arsenide or indium gallium arsenide. Disposed over channel layer 56 is a charge donor layer 58 comprised of a dopant spike region 59 having the general characteristics as discussed above in conjunction with FIG. 1E for regions 19 and 21. That is, dopant region 59 is generally characterized as a sheet or two-dimensional layer of dopant material having a dopant concentration of 2X10 12 to 5X10 12 a/cm 2 . Dopant region 59 is spaced from channel layer 56 by a spacer layer 58a (30Å to 50Å in thickness) of undoped wide bandgap material such as gallium aluminum arsenide. The remainder of layer 58, here region 58b (i.e. typically 200Å to 400Å), is, again, lightly doped gallium aluminum arsenide. Disposed over region 58b of layer 58 is a layer 64 comprised of lightly doped N-type gallium arsenide as for layer 24. Disposed over layer 64 is a relatively heavy doped layer of N-type gallium arsenide to provide a contact with source and drain electrodes as shown. Gate electrode 70 is disposed in a double recessed portion of layers 66 and 64, as also shown, and as generally described above in conjunction with FIG. 1E. The structure shown in FIG. 2 is a high electron mobility transistor having a single charge donor layer. The presence of the double wide recess, the dopant spike region 59, and the pair of charge screen regions or layers 58b and 64 provide individually and collectively improvements in breakdown voltage characteristics as generally described for the structure shown in FIG. 1E. Such structures could alternatively be used with so-called inverted high electron mobility transistors which have a charge donor layer under rather than over the channel layer as generally described above.

Claims: A high electron mobility transistor comprising: a charge donor layer (22) of a first Group III-V material with dopant atoms; a channel layer (20) disposed adjacent to the charge donor layer (22) and composed of an undoped second Group III-V material having a bandgap energy lower than the bandgap energy of the first Group III-V material; a pair of ohmic contacts (28a,28b) disposed over first portions of the charge donor and channel layers (22,20) ; and a Schottky barrier control contact (30) disposed over second portions of the charge donor and channel layers (22,20) and located in a recess (23') extending through at least a further layer (24) of a third Group III-V material having a bandgap energy lower than the bandgap energy of the first Group III-V material and including dopant atoms, characterised in that the charge donor layer (22) has a sheet (21) of dopant atoms disposed substantially close to the interface provided by the charge donor layer (22) and the channel layer (20), the sheet (21) having a concentration of dopant atoms confined to a few atomic layer thicknesses of the charge donor layer (22) and in that the charge donor layer (22) is relatively lightly doped in the layer (22b) between the sheet (21) and the control contact (30). A transistor according to claim 1, characterised in that the charge donor layer (22) is undoped in a region (22a) between the sheet (21) and the channel layer (20). A transistor according to claim 1, characterised in that the said sheet (21) has a dopant profile confined to a few tenths of nanometers thickness of the charge donor layer (22) and has a dopant concentration generally in the range of about 2.0x10 12 to 5x10 12 atoms per square centimetre; in that two spaced contact regions (26) comprised of said first Group III-V material having a dopant concentration greater than about 1x10 18 a/cm 3 are disposed over the said further layer (24); and in that the said ohmic contacts (28a,28b) are disposed in ohmic contact with said spaced contact regions (26). A transistor according to any preceding claim, characterised in that the said further layer (24) is comprised of relatively lightly doped Group III-V material having a doping concentration in the range 1x10 17 to 5x10 17 atoms per cubic centimetre. A transistor according to any preceding claim, characterised in that the charge donor layer (22) is comprised of aluminium gallium arsenide, and the channel layer (20) is comprised of gallium arsenide. A transistor according to any preceding claim, characterised in that the said first material is aluminium gallium arsenide, the said second material is indium gallium arsenide, and the said third material is gallium arsenide. A transistor according to any preceding claim, characterised in that the said recess (23',25') extends into the relatively lightly doped layer (22b) of the charge donor layer (22), the recess having a portion (25') of greater width in the said further layer (24) than the portion (23') extending into the relatively lightly doped layer (22b), and the Schottky barrier contact (30) being disposed on the relatively lightly doped layer (22b) of the charge donor layer (22) in the recess (23',25'). A transistor according to any preceding claim, characterised in that the channel layer (20) has the charge donor layer (22) on one face and a back layer (18) on the opposite face, the back layer (18) being comprised of the said first Group III-V material. A transistor according to claim 8, characterised in that the back layer (18) comprises: a first portion (18a) adjacent the channel layer (20) and of undoped first Group III-V material; an intermediate portion (19) adjacent the undoped first portion (18a) and comprising a dopant sheet of N-type dopant having a concentration of dopant atoms confined to a few atomic thicknesses of the back layer (18); and a third portion (18b) adjacent the dopant sheet and of undoped first Group III-V material. A transistor according claim 1, characterised in that the charge donor layer (22) has an undoped spacer region (22a) with a thickness of 3 to 5 nanometres adjacent to the channel layer (20), the relatively lightly doped layer (22b) is 20 to 40 nanometres in thickness and has a dopant concentration in the range 1x10 17 to 5x10 17 atoms per cubic centimetre, and the sheet (21) is in the form of a dopant spike with a sheet doping concentration of 2x10 12 to 5x10 12 atoms per square centimetre which separates the relatively lightly doped layer (22b) from the undoped spacer region (22a).

Inventors: HUANG JOHN C, JACKSON GORDON S, HUANG, JOHN C., JACKSON, GORDON S.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1992
Main classification: H01L29
IPC codes: H01L29, H01L21