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Gallium arsenide HEMTs for low-noise GHz communications engineering
Gallium arsenide HEMTs for low-noise GHz communications engineering Heinrich D~imbkes Microwave Components, AEG, UIm Research Center, F.R.G. The demands of communication systems for low noise receiver front-ends has led to the development of new transistor structures based on GaAs material and fabricated by molecular beam epitaxy. This paper describes one such device, the high electron mobility transistor (HEMT). 1. Introduction Microwave technology has made its way into many areas of everyday life. Telephone and video signals are transmitted via radio links, millions of TV users and radio listeners already receive their programmes from direct-broadcasting satellites. Safe air traffic would be inconceivable without radar engineering and microwave communications. Be it in the navigation of ships on the high seas, in the communications systems of modern high-speed trains or in a live broadcast from a sports stadium, microwave technology has become an integral element of daily life in industrial society (Fig. 1).
Fig. 1 Microwave and mm-wave communications systems
The importance of monolithically integrated mm-wave circuits (M 3ICs) was recognised several years ago. They allow complete subsystems, e.g. a receiver circuit with mixer and IF amplifier, to be manufactured and built up on a single small chip, making the units extremely small, light and reliable. Two problems arise when using GHz communications lines: the natural attenuation caused by the atmosphere and the increasing flood of electromagnetic radiation. Both effects result in the demand to use more sensitive systems and to manage even with very low transmitter power. The latter requirement is of particular importance for military applications, where the locatability of own transmitters plays a role. For a receiver circuit, this means using amplifiers with the lowest possible noise and highest possible gain (Fig. 2). MICROELECTRONICS JOURNAL Vol. 20 No. 5 © 1989 Elsevier Science Publishers Ltd., England
1
Lownoise Highgain
L,ml~er ?re-~mpli/~er
k
xer
IF amolifier
Local o s c i l l a t o r Fig. 2 Receiver circuit A reduction in receiver noise improves the signal-to-noise ratio and increases the sensitivity of the overall system. This is the approach used at AEG in the development of new transistors and hetero-layer structures based on gallium arsenide.
2. Noise due to speed fluctuation The basis of the amplification of high-speed signals in active components is the charge carder current. As many charge carders as possible should pass through the component as fast as possible, being modulated only by the input signal (Fig. 3).
0
0 0
0
O 0
0 0
0 0
0
0
0
0
0 0
0 0
0 0
0
0
0
0 Gallium
Arsenic
0 Dopant
Fig. 3 Current flow through homogeneously doped GaAs
The main cause of high-frequency noise in microwave transistors is the fluctuation of the speed of the electrons in the channel. This is a result of the interaction between the charge carriers and the atoms of the crystal lattice. The free electrons in the conductive channel of a field-effect transistor normally originate from dopant atoms which are specifically incorporated into the host crystal during processing of the material. By releasing an electron, the dopant atom remains in its place in the lattice as a positively charged ion. When current is transported, the free electrons are accelerated by the external field and oriented, but they are also repeatedly attracted by the ionized donors. They collide with them, thus being decelerated and deflected. The effective forward speed is reduced and a wideband noise is superimposed on the pure signal. However, reducing the number of scattering centers also means reducing the carrier concentration and, consequently, the current, this being undesirable for the function of the component. This is a dilemma which cannot be resolved in a conventional semiconductor.
3. Haterostructures offer advantages Improved understanding of semiconductor physics and new methods for manufacturing ultra-thin monocrystalline layers have resulted in the development of a new class of components offering completely new possibilities: heterostructure components.
0
0
0
O
O
@ 0
0
0
@ 0
0
0
0
0
0
0
0
0
0
0
\
0
Fig. 4 Separation of the electrons from the dopant ions in the heterostmcture
What is the specific advantage of such a structure (Fig. 4)? Ifa material with a large energy bandgap, e.g. aluminium gallium arsenide (A1GaAs), is combined with a material having a small bandgap, in this case gallium arsenide (GaAs), an energy dip favoring the GaAs is formed at the interface. If n-doped AIGaAs is grown on undoped GaAs, the free electrons from the A1GaAs layer are transferred into the energetically more favorable potential dip in the undoped GaAs. A very thin layer of electrons is formed along the interface. With roughly 10 nm, the thickness of the potential well is so thin that movement of the electrons perpendicular to the interface is restricted and only discrete, i.e. quantized energy levels can be assumed (quantized Hall effect). This is referred to as the formation of a twodimensional electron gas. A very high electron density is obtained in this electron gas, even though no dopant ions are present: the free electrons are spatially separated from their donors! Consequently, they can move along the interface virtually without any collisions and reach very high effective speeds. If this effect is exploited by using the two-dimensional electron gas as the conductive channel of a field-effect transistor, transistors with very high cutoff frequencies and very low noise are achieved. Due to the extremely high mobility of the electrons in these structures, the transistors manufactured in this way are referred to as high electron mobility transistors (HEMT). This name has since become a proprietary designation of Fujitsu, while other manufacturers and institutions selected a variety of other designations to describe the same effect: selectively doped hetero transistor (SDHT); two-dimensional electron gas FET (TEGFET); modulation doped FET (MODFET).
4. A versatile tool: molecular beam epitaxy The manufacture of ultra-thin layer sequences with atomically sharp transitions necessary for the implementation of hetero transistors places extreme demands on crystal growth. The Ulm Research Center has for many years been carrying out pioneering work in the field of growing silicon-germanium (SiGe) heterostructures. This work shows that a different semiconductor can be grown abruptly on a monocrystalline starting substrate, it too being of monocrystalline form. The interface between one material and the next can be grown extremely sharp, fight down to atomically sharp transistions. The combination of
different materials in extremely thin layers leads to entirely new physical possibilities (bandgap engineering). The Ulm Research Center was able to produce mm-wave Impatt diodes, hetero field-effect transistors made of silicon-germanium structures and SiGe/Si hetero bipolar transistors as a result of this research. While this work focusses on the use of heterostructures on silicon, which will become necessary in the long term, work is now also in progress on gallium arsenide structures. This allows the available experience in the design of hetero-layer sequences to be combined with the immediately applicable advantages of gallium arsenide in super-high-frequency engineering A molecular beam epitaxy system for GaAs heterostructures has been available at the Ulm Research Center since August 1988. Figure 5 shows the typical structure of such a sysFluorescent screen Ga,a
Shutter Effusion cells Aluminum Gallium Indium
Arsenic Si I icon (dopant)
c~l
Fig. 5 Fua(Isumeatals~ucmre of the gallium amemidemolecular beam epi~L~ system
tern. The central element is an ultra-high vacuum chamber in which the material to be grown him the heated substrate in the form of a molecular beam. The molecular beam is generated by evaporating the species in question from exactly controlled oven cells. The desired material composition of the layer to be produced can be controlled by opening and closing the individual cells and varying the cell temperatures. The ultra-high vacuum (approx. 10 -'° mbar) has two effects on crystal growth: the semiconductor is produced in its pure form and can, due to the rapid change in the composition of the molecular beam, abruptly change from one material to the next at the atomic level. This means that the layers can be produced virtually as thin as desired, in extreme cases in sub-atomic layers (e.g. delta doping).
5. Multiple use of advantages: the multi-quantum-well transistor The use of low-scatter transport in the two-dimensional electron gas leads directly to the manufacture of hetero transistors of the kind shown in Fig. 6. Precisely this type of transistor has been manufactured and investigated in detail. Evaluation of the properties revealed that there is only space for a certain number of electrons in the two-dimensional electron gas. The maximum charge carrier concentration per unit area is limited to about 11)'2 electrons cm -2. This restricts the maximum achievable current through the transistor to approximately 180 - 200 mA with a gate width of 1 mm and a typical gate length of 1 [.tm.
Exploiting the possibilities of molecular beam epitaxy, it seemed logical to make multiple use of the advantageous properties of the two-dimensional electron gas: by periodically growing carefully dimensioned A1GaAs/GaAs layer sequences, several conductive channels are produced, one on top of the other, with a two-dimensional electron gas being
L
•
'.
L_ I
~-
J
GaAs
\
Semi-insulatingGaAssubstrate
Fig. 6 Fundamental structure of the GaAs/AIGaAs hetero FET with two-dimensional electron gas
,,.- V J-
\ /
Fig. 7 Layer sequence and energy band profile of a multi-quantum-well transistor
formed in each of them (Fig. 7). Transistors with this structure, which were developed in cooperation with the Research Institute of the German Bundespost, display considerable improvements in comparison with conventional structures: the maximum current was increased by 100%, this having an effect on the maximum output power. Furthermore, it was also possible to raise the cut-off frequency in comparison with simple HEMTs. The advantages ofhetero FETs in terms of cut-off frequency and, above all, noise are shown in Fig. 8. It can be clearly seen that the HEMTs have a far lower noise level than the conventional GaAs FETs. This means that powerful components are now available for use in G H z communications systems, as low-noise amplifiers, as oscillators in integrated circuits and as power amplifiers in the microwave and mm-wave range. Up to now, the research and development work has been carried out on components with a gate length of about 1 ~tm, structured by optical lithography. Cut-off frequencies of 50 to 60 G H z are achieved with these transistors, an excellent value by world standards. HEMTs with a gate length of 0.1 jam, which will have cut-off frequencies in excess of 200 GHz, are currently being developed for use in the mm-waveband using electron beam lithography.
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~'
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2 1
I
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Gate lengtn L ~tJm)
I
0,Z5
50
Frequency(GHz)
Fig. 8 Advantages of the hetero FET
6. Integration in circuits for GHz communications engineering In addition to better noise properties and high cut-off frequencies, the hetero FET displays an important advantage for all circuitry applications (Fig. 8): the minimum noise is not strongly dependent on the bias point. Furthermore, optimum noise matching is achieved near to the operating point for maximum gain. This behavior permits a high degree of flexibility in designing amplifiers. Thus, the hetero-FET is also an ideal candidate for integrated circuits. Therefore, in order to integrate these transistors into the current range of M 3 ICs, the technological processes were developed so as to allow easy integration. The combination of the hetero layer sequence with previously developed technologies results in a completely planar circuit with hetero-FETs (Fig. 9). As in the M 3IC process, the active components are Hetero FET Inductor Diode
Capacitor
Fig. 9 Schematic structure of an M3 IC with hetero FET, diode and passive elements
again insulated by implanting boron in the epitaxy layer. The passive elements are also produced in an identical fashion. One important prerequisite for designing hetero-FETs was that the specific properties of the hetero-FETs should not be impaired by the technological manufacturing processes. Here, too, the necessary performance has been demonstrated. Thus, a flexible technology is available for future low-noise components in the GHz range. The first circuits will specifically exploit the advantages of the new components in low-noise pre-amplifiers for use in the X-band (10 - 12 GHz). In addition, AEG is well on the way to developing a corresponding input amplifier for 60 GHz, thus demonstrating the way into monolithically integrated low-noise mm-wave engineering.
Fig. 1 Microwave and mm-wave communications systems
The importance of monolithically integrated mm-wave circuits (M 3ICs) was recognised several years ago. They allow complete subsystems, e.g. a receiver circuit with mixer and IF amplifier, to be manufactured and built up on a single small chip, making the units extremely small, light and reliable. Two problems arise when using GHz communications lines: the natural attenuation caused by the atmosphere and the increasing flood of electromagnetic radiation. Both effects result in the demand to use more sensitive systems and to manage even with very low transmitter power. The latter requirement is of particular importance for military applications, where the locatability of own transmitters plays a role. For a receiver circuit, this means using amplifiers with the lowest possible noise and highest possible gain (Fig. 2). MICROELECTRONICS JOURNAL Vol. 20 No. 5 © 1989 Elsevier Science Publishers Ltd., England
1
Lownoise Highgain
L,ml~er ?re-~mpli/~er
k
xer
IF amolifier
Local o s c i l l a t o r Fig. 2 Receiver circuit A reduction in receiver noise improves the signal-to-noise ratio and increases the sensitivity of the overall system. This is the approach used at AEG in the development of new transistors and hetero-layer structures based on gallium arsenide.
2. Noise due to speed fluctuation The basis of the amplification of high-speed signals in active components is the charge carder current. As many charge carders as possible should pass through the component as fast as possible, being modulated only by the input signal (Fig. 3).
0
0 0
0
O 0
0 0
0 0
0
0
0
0
0 0
0 0
0 0
0
0
0
0 Gallium
Arsenic
0 Dopant
Fig. 3 Current flow through homogeneously doped GaAs
The main cause of high-frequency noise in microwave transistors is the fluctuation of the speed of the electrons in the channel. This is a result of the interaction between the charge carriers and the atoms of the crystal lattice. The free electrons in the conductive channel of a field-effect transistor normally originate from dopant atoms which are specifically incorporated into the host crystal during processing of the material. By releasing an electron, the dopant atom remains in its place in the lattice as a positively charged ion. When current is transported, the free electrons are accelerated by the external field and oriented, but they are also repeatedly attracted by the ionized donors. They collide with them, thus being decelerated and deflected. The effective forward speed is reduced and a wideband noise is superimposed on the pure signal. However, reducing the number of scattering centers also means reducing the carrier concentration and, consequently, the current, this being undesirable for the function of the component. This is a dilemma which cannot be resolved in a conventional semiconductor.
3. Haterostructures offer advantages Improved understanding of semiconductor physics and new methods for manufacturing ultra-thin monocrystalline layers have resulted in the development of a new class of components offering completely new possibilities: heterostructure components.
0
0
0
O
O
@ 0
0
0
@ 0
0
0
0
0
0
0
0
0
0
0
\
0
Fig. 4 Separation of the electrons from the dopant ions in the heterostmcture
What is the specific advantage of such a structure (Fig. 4)? Ifa material with a large energy bandgap, e.g. aluminium gallium arsenide (A1GaAs), is combined with a material having a small bandgap, in this case gallium arsenide (GaAs), an energy dip favoring the GaAs is formed at the interface. If n-doped AIGaAs is grown on undoped GaAs, the free electrons from the A1GaAs layer are transferred into the energetically more favorable potential dip in the undoped GaAs. A very thin layer of electrons is formed along the interface. With roughly 10 nm, the thickness of the potential well is so thin that movement of the electrons perpendicular to the interface is restricted and only discrete, i.e. quantized energy levels can be assumed (quantized Hall effect). This is referred to as the formation of a twodimensional electron gas. A very high electron density is obtained in this electron gas, even though no dopant ions are present: the free electrons are spatially separated from their donors! Consequently, they can move along the interface virtually without any collisions and reach very high effective speeds. If this effect is exploited by using the two-dimensional electron gas as the conductive channel of a field-effect transistor, transistors with very high cutoff frequencies and very low noise are achieved. Due to the extremely high mobility of the electrons in these structures, the transistors manufactured in this way are referred to as high electron mobility transistors (HEMT). This name has since become a proprietary designation of Fujitsu, while other manufacturers and institutions selected a variety of other designations to describe the same effect: selectively doped hetero transistor (SDHT); two-dimensional electron gas FET (TEGFET); modulation doped FET (MODFET).
4. A versatile tool: molecular beam epitaxy The manufacture of ultra-thin layer sequences with atomically sharp transitions necessary for the implementation of hetero transistors places extreme demands on crystal growth. The Ulm Research Center has for many years been carrying out pioneering work in the field of growing silicon-germanium (SiGe) heterostructures. This work shows that a different semiconductor can be grown abruptly on a monocrystalline starting substrate, it too being of monocrystalline form. The interface between one material and the next can be grown extremely sharp, fight down to atomically sharp transistions. The combination of
different materials in extremely thin layers leads to entirely new physical possibilities (bandgap engineering). The Ulm Research Center was able to produce mm-wave Impatt diodes, hetero field-effect transistors made of silicon-germanium structures and SiGe/Si hetero bipolar transistors as a result of this research. While this work focusses on the use of heterostructures on silicon, which will become necessary in the long term, work is now also in progress on gallium arsenide structures. This allows the available experience in the design of hetero-layer sequences to be combined with the immediately applicable advantages of gallium arsenide in super-high-frequency engineering A molecular beam epitaxy system for GaAs heterostructures has been available at the Ulm Research Center since August 1988. Figure 5 shows the typical structure of such a sysFluorescent screen Ga,a
Shutter Effusion cells Aluminum Gallium Indium
Arsenic Si I icon (dopant)
c~l
Fig. 5 Fua(Isumeatals~ucmre of the gallium amemidemolecular beam epi~L~ system
tern. The central element is an ultra-high vacuum chamber in which the material to be grown him the heated substrate in the form of a molecular beam. The molecular beam is generated by evaporating the species in question from exactly controlled oven cells. The desired material composition of the layer to be produced can be controlled by opening and closing the individual cells and varying the cell temperatures. The ultra-high vacuum (approx. 10 -'° mbar) has two effects on crystal growth: the semiconductor is produced in its pure form and can, due to the rapid change in the composition of the molecular beam, abruptly change from one material to the next at the atomic level. This means that the layers can be produced virtually as thin as desired, in extreme cases in sub-atomic layers (e.g. delta doping).
5. Multiple use of advantages: the multi-quantum-well transistor The use of low-scatter transport in the two-dimensional electron gas leads directly to the manufacture of hetero transistors of the kind shown in Fig. 6. Precisely this type of transistor has been manufactured and investigated in detail. Evaluation of the properties revealed that there is only space for a certain number of electrons in the two-dimensional electron gas. The maximum charge carrier concentration per unit area is limited to about 11)'2 electrons cm -2. This restricts the maximum achievable current through the transistor to approximately 180 - 200 mA with a gate width of 1 mm and a typical gate length of 1 [.tm.
Exploiting the possibilities of molecular beam epitaxy, it seemed logical to make multiple use of the advantageous properties of the two-dimensional electron gas: by periodically growing carefully dimensioned A1GaAs/GaAs layer sequences, several conductive channels are produced, one on top of the other, with a two-dimensional electron gas being
L
•
'.
L_ I
~-
J
GaAs
\
Semi-insulatingGaAssubstrate
Fig. 6 Fundamental structure of the GaAs/AIGaAs hetero FET with two-dimensional electron gas
,,.- V J-
\ /
Fig. 7 Layer sequence and energy band profile of a multi-quantum-well transistor
formed in each of them (Fig. 7). Transistors with this structure, which were developed in cooperation with the Research Institute of the German Bundespost, display considerable improvements in comparison with conventional structures: the maximum current was increased by 100%, this having an effect on the maximum output power. Furthermore, it was also possible to raise the cut-off frequency in comparison with simple HEMTs. The advantages ofhetero FETs in terms of cut-off frequency and, above all, noise are shown in Fig. 8. It can be clearly seen that the HEMTs have a far lower noise level than the conventional GaAs FETs. This means that powerful components are now available for use in G H z communications systems, as low-noise amplifiers, as oscillators in integrated circuits and as power amplifiers in the microwave and mm-wave range. Up to now, the research and development work has been carried out on components with a gate length of about 1 ~tm, structured by optical lithography. Cut-off frequencies of 50 to 60 G H z are achieved with these transistors, an excellent value by world standards. HEMTs with a gate length of 0.1 jam, which will have cut-off frequencies in excess of 200 GHz, are currently being developed for use in the mm-waveband using electron beam lithography.
2;0 "20 t
/
I derer0 FET
200
~B0
MESFET
~O ~20
cl
S
ampl~
!00
c cr w.
6O
20:0
!
~'
HESFET
'1
2 1
I
0,5
0.]]
Gate lengtn L ~tJm)
I
0,Z5
50
Frequency(GHz)
Fig. 8 Advantages of the hetero FET
6. Integration in circuits for GHz communications engineering In addition to better noise properties and high cut-off frequencies, the hetero FET displays an important advantage for all circuitry applications (Fig. 8): the minimum noise is not strongly dependent on the bias point. Furthermore, optimum noise matching is achieved near to the operating point for maximum gain. This behavior permits a high degree of flexibility in designing amplifiers. Thus, the hetero-FET is also an ideal candidate for integrated circuits. Therefore, in order to integrate these transistors into the current range of M 3 ICs, the technological processes were developed so as to allow easy integration. The combination of the hetero layer sequence with previously developed technologies results in a completely planar circuit with hetero-FETs (Fig. 9). As in the M 3IC process, the active components are Hetero FET Inductor Diode
Capacitor
Fig. 9 Schematic structure of an M3 IC with hetero FET, diode and passive elements
again insulated by implanting boron in the epitaxy layer. The passive elements are also produced in an identical fashion. One important prerequisite for designing hetero-FETs was that the specific properties of the hetero-FETs should not be impaired by the technological manufacturing processes. Here, too, the necessary performance has been demonstrated. Thus, a flexible technology is available for future low-noise components in the GHz range. The first circuits will specifically exploit the advantages of the new components in low-noise pre-amplifiers for use in the X-band (10 - 12 GHz). In addition, AEG is well on the way to developing a corresponding input amplifier for 60 GHz, thus demonstrating the way into monolithically integrated low-noise mm-wave engineering.