HEMT Technology - Impact on Computers and C o m m u n i c a t i o n s - I

by Masayuki Abe

Ultra low-noise HEMTs are already commercially available for analog applications in satellite communications, especially have given a great impact the DBS market, and are also being effectively applied in radio astronomy observation applications to discover new interstellar materials in the universe. In this review, the current status of HEMT technology and its impact on computers and communications are presented. Part 1 focuses on device advantages in the sub-micron dimensional range, a self-aligned HEMT LSI process and Part 2 next issue on HEMT LSI implemented in supercomputer and communication systems.

Figure 1." Fujitsu's GaAs AS1Cs are now produced on 4-inch wafers.

B uilding on its well-established MESFET GaAs ASIC technology, see Figure 1, high electron mobility transistor (HEMT) technology devel-

oped by Fujitsu has opened the door to new possibilities for high performance computer and communication applications [1-2]. The evolution of high-speed, low-power HEMT devices is the result of continuous technological progress utilizing the superior electronic properties due to the supermobility of GaAs/AIGaAs heterojunction struc- ture [3].

HEMTs are already commercially available in satellite communications. In particular, HEMT technology has given much impacts to the expansion of the broadcast satellite market, and is also being effectively applied in radio astronomy observation applications to discover new interstellar materials in the universe [4].

For LSI level complexity, a 64-kbit static RAM with an address access time of 1.2 ns [5] and a 45K-gate array with 35 ps logic delay [6] have been achieved at room temperature, both the fastest circuit operations ever reported. These complexities are a critical threshold to

make HEMT technology practical in future high-speed computer systems.

This paper first presents the high-speed performance of HEMT approaches, focusing on device structure in the submicrometer-dimensional range and the current status of the ultralow-noise HEMT to be applied broadcast satellite receiver and cryogenic amplifier for radio astronomy. Then the technological challenges facing the LSI fabrication and the HEMT LSI implemented in supercomputer system are reviewed.

Performance The HEMT has a performance advantage over conven- tional devices. During switching, the speed of the device is limited by both low-field mobility and saturation drift velocity. The low-field mobility routinely obtained is 8000 cm2/V.s at 300K and 40000 cm2/V.s at 77K. Saturated drift velocity measured with HEMT structure at room temperature has been reported to be 1.7 to 2.0 x 107 cm/s.

These superior transport properties in HEMT channels result in a high average current-gain cutoff frequency / * value for a given gate length. In Figure 2, the current-gain cut-off frequency f * versus gate length (Lg) summarizes the typical performance of experimental HEMTs, GaAs

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MESFETs reported so far. At iool/i lempcralurc. Ei~c values o f / ~ were 80 G H z for GaAs-based 0.25 tam gate HEMT, and 250 G H z for lnGaAs-based 0,15 ~tm-gate H E M T [7]. As shown in this figure, no significant variation in threshold voltage with gate length was observed in the range from 1.4-tam to 0.14 tam [8]. This lack of sensitivit~ indicates that reducing the geometry of HEMTs is an acceptable way to increase performance without shorl- channel effect problems. Therefore, existing HEMTs potential ly can allow a 0.15- tam gate LS! to be produced. The E -HEMT at 300K exhibits a transconduc- tance (gin) of 360 mS/mm for a 0.5- tam gate length and 500 mS/mm for a 0.15- jam gate length [8].

Ul t ra low-no ise H EMT Microwave HEMTs with low noise performance have already been commercially available in satellite commu- nications and are being effectively applied in radio astronomy observation applications: we require a device with high current-gain cutoff frequency / ~ at low drain current to achieve superior low noise performance. This is because the noise figure decreases with drain current and decrease with increase of / ~ value. Self-aligned GaAs- based HEMTs with a gate length of 0.25 tam were developed to achieve the ultralow noise performance [9, 10]. The cross-sectional view of a 0.25 lain gate H E M T is shown in Fig. 3.

Figure 4 shows the comparison of the noise figure and associated-gain performances between HEMTs and GaAs MESFETs. These HEMTs have a T-shaped gate structure in which the gate cross-section is large enough to reduce the gate-resistance. The gate-resistance of the completed device is measured to be 0.3 ohm. The unit gate width is a quarter of the total gate width 200 gm. The chip size is 0.42 x 0.35 mm 2.

And also a H E M T chip mounted on a 0.25 mm thick

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.~;ipphire ~,ubstrate exhibited the optimum noise ligure ol 0.54 dB with an associated gain of 12.1 dB at 12 GHz, which was the lowest noise figure up to date. The DBS systems with the frequency of 12 GHz has been popularly introduced to utilize the low noise devices. H E M T made a breakthrough of 1 dB noise []gure overcoming the GaAs MESFET technology. It takes 10 years from 2 dB to 0.6 dB. By using the front-end receiver with low noise HEMT, the size of parabolic antenna can be reduced 1¥om 70 to 30 cm to give an impact for low cost effectively expanding the I)BS market. The amount of production is over several million devices every month.

Low noise H E M T is also effectively applied in the radio astronomy observation at cryogenic operating conditions. A 2-stage 0.5 tam gate H E M T amplifier set up in the vacuum chamber of closed-cycle liquid helium refrigerator system, cooled down to 20K, achieved the minimum noise figure 1.1 dB with the gain of 13 dB between 22-24 GHz at 25.3K cooled conditions [11]. Comparing cooled para- metric amplifiers which have been used in the K-band high-gain receiver for radio astronomical observations H E M T amplifiers have better features of bandwidth, no spurious signals and stable operation despite mechanical shock. The investigating round of the Nobeyama Radio Observatory discovered a new interstellar molecule by analyzing feeble ram-wave signals at around 23.5 GHz from a dark nebula [12].

For higher frequency region, we also developed cryogenic 43 GHz-band tow noise amplifier for radio astronomy. The ambient temperature dependence of the gain and noise temperature are shown in Figure 5 [13]. As lowering the ambient temperature, noise temperature and gain are improved from 450K with 9.5 dB at room temperature to 65K with 13 dB at ambient temperature of 30K. The amplifier has guaranteed a gain of 10.5 dB, a noise temperature of 125K from 41.3 to 44.5 GHz, and a minimum noise temperature of 95K, including the effects

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of the dewar's input and output sections. For sufficient input and output impedance matching at cryogenic temperature, waveguide-type hybrids have been used for the amplifier's input and output sections. An enclosure was made of Super-Invar, having a small coefficient of thermal expansion same as the 20 GHz amplifier. The HEMT amplifier is enclosed in a vacuum chamber and fixed on a 30 K cold stage cooled by a closed-cycle helium refrigerator, and is continued to operate stably without problems relating to mechanical shock to observe the signal for a long term analysis, and promising to contribute the identification of birth of stars.

Very low-noise 0. llam gate InP-based HEMTs have been developed recently [14]. At 60 GHz, a noise figure of 0.8 dB with an associated gain of 8.9 dB was achieved. The device also exhibited a minimum noise figure of 1.2 dB with 7.2 dB associated gain at 94 GHz.

C h a l l e n g e s f o r LSI Figure 6 is a cross section of a typical self-aligned structure for E- and D-HEMTs forming an inverter for DCFL

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circuit configuration [3]. The basic epilayer structure consists of a 600-nm undoped GaAs layer, a 30-nm A102Ga0.sAs layer doped to 2x1018 cm -3 with Si, and 70- nm, GaAs top layer doped to 10 ~8 cm -3, grown on a SI GaAs substrate.

The low-field electron mobilities were 7200 cm~/V.s at 300K and 40000 cme/v.s at 77K. A thin Alo.aGao.TAS layer embedded in the top GaAs layer acts as a stopper against selective dry etching. Schottky contacts for the E- and D- HEMT gates are formed by depositing A1, with the Schottky gate contacts and the GaAs top layer for the ohmic contact being self-aligned to achieve high-speed performance. Ti/Pt/Au electrical connections running from the interconnecting metal to the device terminals are provided through contact holes etched in a silicon oxynitride cross-over insulator film deposited by PECVD.

A standard deviation in threshold voltage over a full 3-in wafer is around 10 mV for E- and D-HEMTs. The ratio of the standard deviation (10 mV) of threshold voltage to the logic voltage swing (0.8 V for DCFL) is 1.4%, indicating excellent controllability of epitaxial growth and the LSI fabrication process.

An important problem in fabricating HEMT LSI is achieving the requisite highly uniform epitaxial wafer growth technology with high throughput and large wafer size. Selectively doped GaAs/n-A1GaAs heterostructures were grown by MBE on three 3-inch GaAs substrates

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mounted on a substrate holder with a dialnetet of t90 mm [15]. The substrate temperature during grow, th was held al

660:'C. A uniformity of + 1% for both the thickness and carrier concentration of the AIGaAs layers is achieved to control the threshold voltage of HEMT characteristics.

Epitaxial material growth of HE'MT LSl-quali ty A1GaAs/GaAs heterostructures has also been performed by low-pressure metal organic vapour-phase epitaxy (MOVPE) technology. An internally developed barrel- type reactor has a load capacity of twelve 3-in wafers [16].

Wafer rotation resulted in extremely uniform epitaxial layers. The variation in both layer thickness and carrier concentration of a Si-doped AIGaAs layer is less than _+ 1% across a 3-in wafer. The wafer-to-wafer variations among the 12 wafers are + 1.1% for layer thickness and +_1.8% ['or carrier concentration. This indicates the viability of these technologies for realizing ICs with LSI," VLSI level complexities.

It is very important to maintain large enough noise margins for LSI to operate stably. Both high- and low-level noise margins larger than 200 mV are required for LSI circuit design at 300K. The supply voltage is 1 V, The unloaded delay time of 0.5 lam gate HEMT is 19 ps, fan-in and fan-out dependence are 4 ps/F.l, and 12 ps/F.O., and wiring delay is 24 ps/mm. The loaded delay time (F.I. - F.O. = 3, 1 = 1 mm) is 75 ps at 1 mW/gate. The average value of unloaded delay time for 0.28 lam gate HEMT was 13 ps with a standard deviation of 0.4 ps over a 3-in wafer.

Figure 7 compares the HEMT delay/power perfor- mances with other high-speed device technologies. As shown in this figure, the delay times of 15 ps with 0.2 mW/ gate for a 0.3 IJm gate HEMT [17] and 40 ps with 0.2 mW/ gate for a 0.7-1am-gate GaAs MESFET [18] are one order of magnitude smaller than that of bipolar ECL technology [19] under the same power dissipation conditions. A 45K- gate array with a 0.6-1am-gate HEMT has achieved delay times of 35 ps with 0.24-mW/gate DCFL [16], one-fourth smaller than Si bipolar ECL logic [20]. This gate array is compatible with the ECL gate array at both 1/O and macro

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GaAs MESFET static RA Ms [17].

levels. Four-level low-capacitance wirings are used for high-speed interconnection.

Figure 8 shows the address access time versus power dissipation of the H E M T static RAMs compared with CMOS, BiCMOS, ECL, and GaAs MESFET static RAMs [17]. An ECL compatible H E M T 64-kbit static RAM with address access time of 1.2 ns with 5.9 W at room temperature [5] is fabricated using 0.6-!am gate H E M T technology. This is four times faster than the fastest 64- kbit ECL static RAM [21]. The chip size has been reduced by refined design rules and processing. Figure 9 shows a microphotograph of the H E M T 64-kbit static RAM. The

Figure 9. Shows a microphotograph of the HEMT 64-kbit static. RAM.

circuit organization is 8192 words x 8 bit. The chip measures 7.4 x 6.5 mm 2 and contains 462000 E/D-HEMTs. A subnanosecond access time is simulated for a H E M T 64- kbit static RAM with 0.3-pro gate device technology.

Fuj i t su Labora to r i e s Ltd. 10-1 M o r i n o s a t o - W a k a m i y a , A t s u g i 243-01, J a p a n . Tel~fax: [81] 4 6 2 4 8 3 1 1 1 / 3672.

This review is based on paper given at the 7th International Microelectronics Conference (June 3-5th 1992, Pacifico Yokohama, Japan) and also IPRM-IV, Newport Rhode Island, MA, USA, April 1992. This feature will conclude in issue 6.

Acknowledgements The author would like to express his appreciation to Dr. H. Ishikawa and Dr. T. Mimura for their encouragement and support, and to his colleagues whose many contribu- tions have made possible the results described here.

References 1. M. Abe, T. Mimura, N. Yokoyama and H. lshikawa, "New

technology towards GaAs LSI/VLSI for computer applica- tions", vol. ED-29, pp. 1088-1093, 1982.

2. T. Mimura, S. Hiyamizu, T. Fujii and K. Nanbu, "A new field-effect transistor with selectively doped GaAs/n-AlGaAs heterojunction", Japan. J. Appl. Phys., vol. 19, pp. L225- L227, 1980.

3. M. Abe et al., "Recent advances in ultrahigh speed HEMT LSI technology", IEEE Trans. Electron Devices, vol. 36, pp. 2021- 203 I, 1989.

4. M. Abe and N. Yokoyama, "Semiconductor heterostructure devices", in Japanese Technology Review, Electronics, Vol.8, T. lkoma, Ed. New York: Gordon and Brech Science Publishers, 1989, pp. 1-48,

5. M. Suzuki et al., "A 1.2 ns HEMT 64kb SRAM", in ISSCC Dig. Tech. Papers, pp.48-49, 1991.

6. S. Notomi et al., "A 45k HEMT gate array with 35ps DCFL and 50ps BDCFL gates", in ISSCC Dig. Tech. papers, pp.152-153, 1991.

7. U.K. Mishra et al., "Novel high performance self-aligned 0.15 micron long T-gate AIInAs-GalnAs HEMTs", in IEDM Tech. Dig., pp. 101-104, 1989.

8. Y. Awano et al., "Short-channel effects in subquarter- micrometer-gate HEMTs; simulation and experiment", IEEE Trans. Electron Devices, vol. 36, pp. 2260-2266, 1989.

9. S. Asai et al., "Super low-noise HEMTs with a T-shaped structure", 1987 IEEE MTT-S Dig., pp. 1019-1022, 1987.

I0. I. Hanyu et al., "Super low-noise HEMTs with a T-shaped WSi gate", Electronics Letters, vol. 24, pp. 1327-1328, 1988.

11. N. Okubo et al., "Low-noise HEMT amplifier for satellite communication systems", FUJITSU, vol. 38, pp.25-30, 1987.

12. H,Suzuki et al., "Detection of the interstellar C6H radical", Astron. Soc. Japan, vol.38, 911, 1986.

13. T. Saito et al., "A cryogenic 43-GHz-band low-noise amplifier for radio astronomy", 1989 IEEE MTT-S Digest, pp. 853- 856, 1989.

14. K, H.G. Duh et al., "A super low-noise 0.1 ~tm T-gate InA1As-lnGaAs-lnP HEMT", IEEE Microwave and Guided- Wave Letters, vol. 1, pp. 114-116, 1991.

15. K. Kondo et al., "MBE as a production technology for HEMT LSIs", J. Cryst. Growth, vol. 95, pp.309-316, 1989.

16. J. Komeno et al., "Recent progress in MOVPE for HEMT LSIs", J.Cryst. Growth, vol. 105, pp. 30-34, 1990.