High electron mobility transistors with Sb-based channels

[0001] This application claims the benefit of U.S Provisional Application No. 60/615,118 filed Sep. 30, 2004.

[0002] 1. Field of the Invention

[0003] This invention pertains to high electron mobility transistors (HEMTs) that employ advanced material designs.

[0004] 2. Description of Related Art

[0005] Future generations of microwave and millimeter-wave radar, communications, electronic warfare, smart weapons, and imaging systems will require higher precision, smaller size, increased bandwidth, lower operating voltages, and lower cost of production. To meet the demand for improved high-frequency performance, considerable effort within the last 10 years or so has focused on the development of gallium arsenide(GaAs)-based and indium phosphide(InP)-based HEMTs. As a result, a variety of HEMT analog and digital circuits have been fabricated which exhibit higher gain, efficiency, and switching speeds with lower power dissipation. The primary factors responsible for the improved HEMT performance have been the increase in the indium mole fraction in the indium-gallium-arsenide (InGaAs) channel and the increase in the conduction band offset at the 2 DEG interface. As a result of these improvements, InP-based HEMTs have distinct millimeter-wave performance advantages compared to GaAs-based HEMTs, and currently hold the record in current gain cutoff frequency response and noise figure for any three-terminal electronic semiconductor device.

[0006] In the longer term, HEMTs which use In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y for the barrier layer and InAs_ySb_1-y in the channel may be more attractive than InP-based HEMTs for some of the above applications due to the substantially improved material properties of these new material systems. Higher electron mobility and higher electron velocity may be achieved with a channel composed of InAs_ySb_1-y. The low electron effective mass in InAs_ySb_1-y gives this material a significant advantage in the room-temperature mobility which can be achieved for a given HEMT sheet charge density. InAs_ySb_1-y-based channel materials also have the substantial advantage of a higher electron peak velocity, i.e., 5×10⁷ cm/sec, for pure InSb, compared to the other semiconductors. The large conduction band discontinuity at the donor layer/channel interface enables the formation of a deep quantum well and the associated benefits of a large 2 DEG sheet charge density, good carrier confinement, and high modulation efficiency. These features should enable improved scaling of the current-gain cutoff frequency (f_T) as the gate length is reduced to the nanometer range. In addition to the increased high-frequency performance potential, InAs_ySb_1-y channel HEMTs are also attractive for applications requiring low-voltage operation. The higher electron mobility and velocity, lower threshold field and reduced access resistance capability enable the attainment of higher effective velocity at a significantly lower drain voltage.

[0007] Although improvements have been made in recent years, the material growth and fabrication technology for antimony-based HEMTs is relatively immature. For the case of AlSb/InAs HEMTs, the high reactivity of AlSb in air and the low valence-band offset of the AlSb/InAs heterojunction increase the complexity of the material growth and device design requirements. For HEMTs with InAs_ySb_1-y channels, barrier layers composed of In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y are required. HEMTs with InAs_ySb_1-y channels, however, are susceptible to significant charge control problems associated with impact ionization in the channel due to the narrow bandgaps of these materials. These effects become increasingly pronounced as the gate length is reduced due to the higher fields present, thus hindering the high-frequency performance of short-gate length HEMTs and limiting their operating voltage range. The combination of Sb-based materials in the channel offers unique opportunities to reduce impact ionization effects by using composite channel layer designs. What is disclosed herein are high electron mobility transistors that employ advanced material designs to increase operating speed and reduce power dissipation.

[0008] It is an object of this invention to provide transistors that employ advanced material designs to increase operating speeds.

[0009] It is another object of this invention to provide high electron mobility transistors that employ advanced material designs to reduce power dissipation.

[0010] It is another object of this invention to provide high electron mobility transistors that are based on antimony-containing materials.

[0011] It is another object of this invention to provide high electron mobility transistors that exhibit improved frequency performance while operating at lower bias voltage.

[0012] It is another object of this invention to provide high electron mobility transistors wherein layers of the transistors use In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y materials in the buffer and the barrier layers

[0013] It is another object of this invention to provide high electron mobility transistors wherein channel layers use InAs_ySb_1-y materials or InAs_ySb_1-y and a combination of Sb-based materials.

[0014] It is another object of this invention to provide transistors that produce low noise figures.

[0015] It is another object of this invention to provide high electron mobility transistors that are particularly suitable for high speed and low noise applications.

[0016] It is another object of this invention to provide transistors which can operate at switching speed of on the order of 500 GHz or higher at reduced power dissipation compared to comparable prior art devices.

[0017] These and other objects of this invention can be attained by high mobility transistors that utilize In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y in the barriers and InAs_ySb_1-y in the channel.

[0020] High electron mobility transistors of this invention employ advanced material layer designs that can increase operating speed to over 500 GHz and reduce power dissipation by a factor of at least two. The HEMTs utilize In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y for the barrier layer and InAs_ySb_1-y in the channel. As a result, the devices exhibit improved frequency performance while operating at lower bias voltage.

[0021] More specifically, the HEMTs of this invention include, as shown in FIG. 1, a semi-insulating GaAs or InP substrate 12, buffer layer 14 of undoped In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y disposed on the substrate, channel layer 16 through which electrons flow horizontally, as depicted in FIG. 1, of undoped InAs_ySb_1-y disposed on the buffer layer, first barrier layer 18 of undoped In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y disposed on the channel layer, silicon or tellurium-doped layer 20 of InAs_ySb_1-y, In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y disposed on the first barrier layer, second barrier layer 22 of undoped In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y disposed on the silicon or tellurium-doped layer, and cap layer 24 of undoped or n-type doped InAs_ySb_1-y disposed on the second barrier layer. In a preferred embodiment, the barrier layer is composed of the following materials with the indicated mol fractions of x and y as follows: In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y where x varies from 0 to 0.7 and y varies from 0 to 0.3. In a preferred embodiment, the channel layer is composed of the following materials with the indicated mole fractions of x and y as follows: InAs_ySb_1-y where x varies from 0 to 0.8. Thickness of the various layers is typically in the following ranges:

[0022]

	substrate 12,	500-25	μm
	buffer 14,	0.5-3	μm
	channel 16,	50-300	Å
	first barrier 18,	20-300	Å
	doped layer 20,	0-50	Å
	second barrier 22,	20-300	Å
	cap layer 24,	20-800	Å

The two barrier layers can be combined with the doped layer disposed somewhere therein, typically around the middle thereof. Although the barrier is not as important as the channel, in terms of electron transport which is the key here, and its bandgap should be wider than that of the channel. The barrier is extremely important in the scheme of these transistors and their growth must be such as to avoid forming traps and defects. Therefore, knowledge of the growth of the barrier and other layers is not trivial. It is the barrier and the channel layers that characterize HEMTs.

[0023] Function of the substrate is obviously to provide support for the layers disposed thereon. Commercially available substrates are GaAs or InP and their lattice constants are in the range of 5.7-5.9 Å.

[0024] Function of the buffer layer is to change lattice constant of the substrate to the lattice constant of the barrier or channel material and it is the buffer layer that sets the lattice constant of the materials disposed above it by choosing mole fractions of the first barrier, or the barrier in general. The lattice constant of the buffer layer is typically 6.1-6.3 Å.

[0025] Function of the channel layer is to conduct or transport the speeding electrons, which determines maximum operational speed of a HEMT.

[0026] Function of the barrier layer is to put a metal electrode on the HEMT which can gate the electrons flowing to the channel since if the gate were to be placed in the channel, there would not be an electrostatic barrier there. The barrier layer is needed to isolate the gate metal from the channel, which allows channel modulation through application of gate voltage.

[0027] Typically, dopants are needed in the barrier layer which contribute their electrons to the channel, which increases conductivity of the channel. So, in these types of transistors, there is typically a doped layer in the barrier, herein denoted as the first barrier. Dopants include silicon or tellurium ions. The silicon or tellurium ions provide electrons to form an n-type channel. High or heavy doping is considered to be doping on the order of 1×10¹⁹ e/ cm³ and low doping is on the order of 1×10¹⁶ e/cm³.

[0028] In reference to fabrication of the HEMT device, the HEMT material can be grown metamorphically on a semi-insulating GaAs or InP substrate by molecular beam epitaxy. After the growth of a thick In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y buffer layer to accommodate the lattice mismatch, a channel layer containing InAs_ySb_1-y or a combination of InAs_ySb_1-y and other Sb-based materials is grown next, followed by an In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y barrier layer and an InAs_ySb_1-y cap layer. The bandgaps and band alignments for these material combinations were obtained using data from 8-band k-p simulations. The channel can be modulation doped in the conventional way using donor atoms in the barrier layers, or it can be modulation doped through the use of a thin, on the order of 12 Å, InAs_ySb_1-y layer which is doped with silicon or tellurium ions. If required, a thin In_xAl_1-xAs layer can also be grown above the barrier layer to enable a gate recess etch to be performed.

[0029] For a basic disclosure on the general fabrication of a device of this nature, see Boos et al U.S. Pat. No. 5,798,540, the entire contents of which is incorporated herein by reference.

[0030] HEMTs with InAs_ySb_1-y channels potentially have very attractive advantages compared to conventional GaAs or InP-based HEMTs. The key feature of this new design is the use of In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y for the barrier layer and InAs_ySb_1-y for the channel. Higher mobility in the channel can be achieved using InAs_ySb_1-y material compared to InGaAs material. The higher mobility will result in lower source and drain resistance and improved charge transport. The lower ohmic contact resistance to the InAs_ySb_1-y channel due to the lower bandgap of these materials will reduce the source-drain low field resistance further and facilitate the formation of non-alloyed ohmic contacts. Low source and drain resistance is one of the most important requirements for high-speed operation in HEMTs. HEMT designs using In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y for the barrier layer and InAs_ySb_1-y for the channel can also be optimized for large conduction and valence band offsets by adjusting the material compositions. The preliminary band diagrams that show the composition of the HEMTs for the lattice constants of 6.2 Å and 6.3 Å are shown in FIG. 2. In addition to the increased high-speed performance potential, InAs_ySb_1-y channel HEMTs are also attractive for applications requiring low-collector voltage operation. The high peak velocity at low electric field of InAs_ySb_1-y enables the HEMT to operate at a lower drain voltage thus reducing power dissipation.

[0031] Other key features of the new HEMT design is the use of an optional subchannel layer composed of InAs_ySb_1-y, In_xGa_1-xSb or other Sb-based materials under the main channel to reduce impact ionization in the device. The addition of this layer improves the performance of the HEMTs, particularly for gate lengths in the deep-submicron regime. Composite channel designs have previously been used in InP-based HEMTs to increase breakdown voltage. Electrons are transferred from the main channel to the subchannel before gaining enough kinetic energy for impact ionization. Electrons can also undergo real space transfer to the subchannel. As a result, the extent of impact ionization in the channel is reduced. By being able to vary the alloy composition and thickness, a wide range of possibilities exist to reduce impact ionization in the device.

[0032] AlSb/InAs HEMTs are particularly susceptible to gate leakage current due to the type II staggered band lineup of the AlSb/InAs heterojunction. As a result, there exists only a minimal valence band barrier to limit hole flow into the adjacent barrier and buffer layers. AlSb/InAs HEMTs reported thus far suffer from high gate leakage current due to the relatively poor insulating quality of the thin barrier layer and the large number of holes that are present in the channel due to impact ionization at relatively low drain voltages. A substantial portion of these impact-ionization generated holes surmount or tunnel through the small gate barrier present and appear as additional gate leakage current. Reduction of the gate current is essential for low-noise performance. The microwave noise measured on previous AlSb/InAs HEMTs was dominated by the gate leakage current shot noise. By reducing the extent of gate leakage current resulting from holes generated by impact ionization, the noise performance can be improved. The use of In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y for the barrier layer and InAs_ySb_1-y for the channel will increase the valence band offset, thus reducing the gate leakage current.

[0033] The material growth and fabrication technology for HEMTs with an In_xAl_1-xAs_ySb_1-y, In_xAl_1-xP_ySb_1-y, or Ga_xAl_1-xAs_ySb_1-y barrier layer and InAs_ySb_1-y channel layers has been demonstrated. Better determinations of the appropriate compositions for the different layer structures are also required to optimize performance. HEMT performance with these improved material design features is expected to improve considerably compared to InP-based and GaAs-based HEMTs that are currently available. The unique combinations of these new material heterostructure designs will make InAs_ySb_1-y channel HEMTs attractive candidates in future technologies where high speed, gain, and efficiency at low bias voltage will be required.

[0034] While presently preferred embodiments have been shown of the novel high electron mobility transistor, and of the several modifications discussed, persons skilled in this art will readily appreciate that various additional changes and modifications can be made without departing from the spirit of the invention as defined and differentiated by the following claims.