High electron mobility transistors: mission accomplished for Voyager and Neptune encounter.
Voyager 2's rendezvous with Neptune in August of 1988 was a resounding success, an essentially flawless technical performance in communications between the 12-year old spacecraft and the 25-year old receiver antennas on Earth.
Part of that great communications success can be attributed to the smallest of the technical innovations, the high electron mobility transistors (HEMTs), which were used for the first time in the ground-based receivers for this mission, as shown in Figure 1. The use of HEMT-based amplifiers allowed low noise receiver performance, which was crucial to the encounter.
HEMT History for the Voyager and Neptune Encounter
Preparation for Voyager's flyby at Neptune began in the early 1980s when the Jet Propulsion Laboratory (JPL), which manages the Voyager missions for NASA, faced the great technical challenge of updating 12-year old technology to enhance the data return from the spacecraft over 2.7 billion miles away. JPL evaluated several options and new technologies at that time, and in 1984 the decision was made to adopt HEMT technology for improving the sensitivity of NASA's ground-based antenna network.
As the spacecraft-to-Earth distance increases and Voyager's transmitted power remains constant, the burden of communication has fallen on ground-based antennas. For the Neptune encounter, the 27 antennas of the National Radio Astronomy Observatory's (NRAO) very large array (VLA) in New Mexico were arrayed with JPL's deep space network (DSN) Goldstone complex in California to increase the array antenna's sensitivity.
To improve signal clarity, the VLA's 27 antennas were equipped with new 8.4 GHz HEMT amplifiers. In 1984, GE's Electronics Laboratory was selected for a cooperative program with JPL and NRAO to design and fabricate HEMT devices and amplifiers for the VLA upgrade; NRAO assumed responsibility for final fabrication and integration of the receivers in the VLA. At that time, HEMTs were a new technology and were evaluated against more established technologies, such as masers and field effect transistors (FETs).
Masers, FETs, or HEMTs?
Masers are the lowest noise amplifiers available and have been used on all DSN antennas since the early 1960s. However, masers are expensive and bulky, and require liquid helium temperatures for low noise operation. Because of these drawbacks, JPL had decided to use FET amplifiers instead, which require less cooling and have the ability to accept a higher system noise temperature as a tradeoff.
Then, during the definition phase of the project, laboratory models of HEMT devices appeared to be capable of significantly lower noise operation than FETs at cryogenic temperatures. Initial results, reported in 1983 by Thomson-CSF and Fujitsu, suggested that much of the system noise performance lost with the use of FETs instead of masers could be regained if the new HEMT could be developed in time for VLA implementation.
In 1983, it was estimated that it would cost $25 M to implement maser amplifiers in the VLA. The estimate for HEMT amplifiers was $10.5 M. In the final analysis, the HEMT amplifiers actually cost less than one-third of the projected cost for masers.
Since the cooperative program began, great strides have been made in improving HEMT performance. Using devices with 0.25 [micro] m gate lengths produced by E-beam lithography, key parameters of the device structure, such as the thicknesses and dopings of the thin semiconductor layers, were systematically optimized for cryogenic operation.
Figure 2 summarizes the evolution of performance improvement for both maser and HEMT amplifier systems over the past five years. The noise temperatures improved steadily until 1987 when they were nearly twice as low as the original program goal, from 8.5 [degrees] K in 1985 to 5.3 [degrees] K in 1987, when cooled to 15 [degrees] K at 8.4 GHz. After the device was optimized, 250 of the HEMTs were produced and delivered for integration into VLA amplifiers. 60 HEMT amplifiers, shown in Figure 1, were produced for the VLA (two per antenna and six spares). The amplifier package implemented for the VLA has measured 11 [degrees] K system noise temperature, close to that obtained with masers.
Figure 3 shows an example of noise temperature and gain response for a cryogenically-cooled three-stage 8.4 GHz amplifier designed with FETs. By substituting a HEMT for the FET in the first stage and without any further modification of the circuit, the amplifier's noise performance improves significantly from 22 [degrees] to 11 [degrees] K.
Moving to Higher Frequencies
A three-stage HEMT low noise amplifier, shown in Figure 4, demonstrated a 30 [degrees] K noise temperature (0.43 dB noise figure) with more than 26 dB of associated gain at a frequency of 32 GHz and 15 [degrees] K of environment temperature. Also, a four-stage HEMT LNA using 0.25 micron gate length HEMTs has exhibited a noise temperature of 40 [degrees] K with 26 dB of gain at 43 GHz.
Figure 5 presents the typical room temperature performance of a two-stage HEMT LNA. The LNA exhibits a flat noise response of less than 2 dB over a 10 GHz bandwidth, almost all of Ka-band. Figure 7 shows the noise performance upon cooling from 300 [degrees] K to 12 [degrees] K. Since HEMT LNAs require less cooling power to operate efficiently at a higher physical temperature, over 12 [degrees] K, they can be implemented for a lower cost.
Commercial and Military Satcom Systems
HEMTs have demonstrated the lowest noise figures to date at frequencies up to 94 GHz. C-and Ku-bands are being utilized for commercial communication satellites, and Q-, V- and W-bands for military communication satellites.
New military communication satellites are being designed using 44 GHz satellite receivers and 60 GHz satellite-to-satellite communication cross links. At 44 and 60 GHz, HEMT low noise amplifier technology results in a 1 to 2 dB reduction in noise figure over FET designs.
Beyond the Conventional GaAs HEMT
Further refinements and new material combinations in the basic HEMT GaAs material layers have led to two new devices, the GaAs-based and InP-based pseudomorphic HEMTs. Both of these HEMT devices, with similar InGaAs buffer layers but different material substrates, as shown in Figure 7, are state of the art in transistor technology and have shown the lowest noise performance to date and good power performance.
Also, these low noise HEMTs, based on DC life tests, exhibit a reliability comparable to that of a FET, with a mean time to failure of [10.sup.6] hours at 150 [degrees] C.
Despite the relative newness of this transistor technology, the application of HEMTs in radio astronomy, satellite communication, and radar systems is progressing rapidly.
We wish to express our thanks to the Jet Propulsion Laboratory, especially Jauvier Bautista, and JPL Highlights; Marian Pospieszalski of NRAO; and Milton Berkowitz of GE Astro-Space Division for contributions to this article. Also special acknowledgment is given to Alan Swanson, manager of the GE Advanced Materials & Devices Laboratory, for his support and to the entire GE HEMT/Voyager team, P.M. Smith, P.C. Chao and J.M. Ballingall. [Figures 1 to 7 Omitted]
K.H.G. Duh et al., "32 GHz Cryogenically Cooled HEMT Low Noise Amplifiers," IEEE Trans. Electron Devices, Vol. Ed-36, pp. 1528-1535. M. Pospieszalski, NRAO Internal Report. J. Eberhart, "Planetary Perks," Science News, Sept. 1988, Vol. 134, No. 11, pp. 161-176.
K.H. George Duh received his BS degree from the National Taiwan University, his MS from Syracuse University and his PhD in electrical engineering from the University of Minnesota. He joined GE's Electronic Laboratory in 1984, where he has been engaged in research and development of high performance HEMT devices and circuits for both room and low temperature applications. Currently, he is a principal staff engineer and program manager for various DoD contracts developing advanced devices and circuits. From 1983 to 1984, he was with Honeywell, where he worked on NASA 30 GHz monolithic receive module designs. Barbara Craig Adams received her BA from the University of Nebraska, Lincoln and her MA in creative writing from Syracuse University. Currently, she is a marketing communications specialist at GE's Electronics Laboratory. Her work involves coordination and development of contract proposals, as well as press relations and promotion of the laboratory's developing technologies, such as fiber optics, phased-array radars and HEMT development.
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|Title Annotation:||new transistor design was used in Voyager spacecraft ground tracking stations|
|Author:||Duh, K.H.G.; Adams, Barbara Craig|
|Date:||Aug 1, 1990|
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