ICSE2002 Proc. 2002, Penang, Malaysia

MMIC Development for Local Multipoint Distribution Service (LMDS)

\ Azzemi Ariffm, S a l i Jaafar and Suhandi Bujang Telekom Research & Development Sdn. Bhd.

UKM-MTDC, 43600 Bangi, Selangor, MALAYSIA Email: azzemi@mdhn.net.my

Abstract: This paper covers a few Monolithic Microwave Integrated Circuit (MMIC) design using the Gallium Arsenide (Gds) pseudomotphic high electron mobiliw transistor @-HEMP technologv to be used in Local Multipoint Distribution Service (LMDS) system Three different sections describe in brief the design approach and methodology involved in each RF circuit design especially in stabilizing circuitry and meeting the small and large signal specifications.

I. INTRODUCTION

There are a few digital wireless communication modes all over the world. A digital transceiver

.can be divided into two main sections where the first one controk digital data processing along with maintaining air linkage through some specific protocols. The second one is analog RF portion which responsible to control and maintain a wireless communication over a specific band spectrum. The fxst portion which is also known as the baseband or CORE data processing is relatively similar from one application to another, which differs according to their modulation scheme.

A number of wireless systems were introduced due to the specific nature of its usage. Second Generation QG) or Global System for Mobile communication (GSM) was first introduced to cater for voice traffic rather than data to both static as well as mobile users. It requires only a bandwidth of 200 lcHz for its voice communication. The small channel spacing would make it impossible to have high data rate communication in a range of megabits per second. LMDS which operates at wider bandwidth can easily satisfy the demand for high data rate transfer. However, it is designed to serve static rather than mobile users.

LMDS or Local Multipoint Communication Service (LMCS), as the technology is known in Canada, is a wireless, tweway broadband technology designed to allow network integrations and communication service providers to bring a wide range of high-value, quality services to houses and businesses. The services that use LMDS technology include high- speed Internet access, reaktime multimedia file transfer, Personal Communication System (PCS) backhaul, remote access to corporate local area networks (LANs), interactive video, videeon- demand (VOD), video conferencing, digital radio, work at home, telemedicine and telephony among other potential applications [I].

The need to serve mobile and static users with effective and high data rate transfers has converge the current mobile communication into third generation mobile (3'3). 3G system or platform shares almost the same technology developed by Qualcomm known as Code Division Multiple Access (CDMA). 3G promises to increase current data rate transfer to mobile as well as static users up to megabits per second. The data rate is highly dependant on the air (RF) interface as all other interfaces are either one to one microwave link or cable connected which are immune to outside interfering factors. RF link will become effective through proper spectrum management which is control by Cell Resource Management (CRM) through application specific protocols and also through proper transceiver hardware design.

The requirement for mobile communication is to integrate all RF transceiver portion into a single chip. The recently matured technology based on p-HEMT GaAs has made it possible to such a small transceiver chip that would meet the specification for wireless communication. Four RF designs such as low noise amplifier (LNA), power amplifier driver, power amplifier and mixer are presented in the following sections. They were designed specifically for LMDS system.

0-7803-7578-5102/$17.00 02002 IEEE

308

ICSE2002 Proc. 2002, Penang, Malaysia

In the United States, LMDS employs 1.3 GHz of RF Spechum to transmit voice, video and fast data signals, to and from, homes and businesses. The 1.3 GHz can be used to cany digital data at speeds in excess of 1 Gbps. Canada has 3 GHz of spechum set aside for LMDS and is actively setting up systems around the counhy. Many other developing countries see this technology as a way to bypass the expensive implementation of cable or fiber optics. LMDS uses low powered, high frequency (25-3 1 GHz) signals over a short distance. LMDS systems are cellular and the cells are typically spaced 4 5 kilometers apart.

Specifically, the application is for point to multtpoint LMDS at 28 GHz. The LMDS system in a cellular network consisting of low power transmitters operating in 1 GHz bandwidth, provide transmission to the subscribers, as shown in Fig. 1 [2]. This paper describes the design and performance of a GaAs p-HEMT MMIC manufactired by GEC-Marconi Material Technology Ltd (GMMT) which designed specifically for LMDS application

because of its suitability for both high frequency transistors and low loss passive components. The different technologies are used for MMIC such as MESFET, HEMT (conventional and pseudomorphic) and HBT. The process stam with the pseudomorphic High Electron Mobility Transistor [HEMTI [3] wafer which consists of a semi-insdating GaAs subsbate with MBE grown layers of GaAs and AlGaAs and a heavily doped GaAs cap layer to reduce contact resistance. A GaAs wafer with ive epitaxial layer of GaAs, InGaAs, AIGaAs, AlGaAs n+ and GaAs n+ was used as a starting material. The H40 process was used where the PHEMT was formed on mesa structure and 0.2 pm log mushroom profile gate was defined by using direct-write electron beam lithography which positioned at 0.8 pm from the source electrode. This source-drain separation is 3 pm. All capacitors are Metakhsulator-Metal (MIM) structure with nitride and polyimide as dielectric material. Two types of resistors are n i c h m e thin film and mesa structure were used. The ground connections were made to the back of the wafer througb via hole. The chip surface was fully protected with silicon nitride passivation layer. The wafer was thinned to 100

Eit. I . L o ~ : . ~ l l i ~ o i ~ ; I , ~ i s l r i b u ~ ~ n ~ ~ ~ ~ ~ ~ W . W $ p M c m . pm before back metallization.

11. TECHNOLOGY The advantages of using MMIC ’ for communication systems, as opposed to hybrid

The technology used to manufacture the MMIC solutions with discrete transistors, are that they W circuit designs are based. on the well are low cost, high performance, small in physical established PHEMT (H40) MMIC process at size, low weight and have high reliability. GMMT Foundry. A monolithic microwave Multiple functions can also be incorporated on integrated circuit (MMIC) is a microwave circuit one MMIC and reducing the number of in which the active and passive components are fabricated on the same semiconductor substrate. The frequency of operation can range from IGHz to well over 100 GHz, and a number of different technologies and circuit approaches can be used. Gallium arsenide (GaAs) has been used extensively in the development of MMIC

components [4].

111. LOW NOISE AMPLIFIER

309

1CSE2002 Proc. 2002, Penang, Malaysia

The Low Noise Amplifier (LNA) circuit has three stages of gain with reactive matching to achieve low noise figure and good input match. Each stage design is using a 2 x 6 0 ~ gate width pHEMT in a single through-via configuration. All the stages were designed independently for 50R input and output impedances and then brought together to form the complete LNA. The first two stages were design for low noise and good input match and exhibited the typical negative gain slope across the frequency band. The last stage was designed for a good output match and a positive gain slope over the same frequency band, resulting in the complete LNA having the gain flamess i3dB over the frequency bandwidth.

The inductive matching networks are realied using narrow transmission lines. Stabilization resistors have been used in the networks on the gate and drain side of the transistor to ensure unconditional stability. Those in the gate networks which located at the end of grounded inductive networks acts to ensure low frequency stability; while those in the drain networks shunts the drain of the transistors and improve the high frequency stability. Spiral inductors have been used in the drain network as a mean providing high impedance points for injecting the drain bias [5]. Although each gate and drain contact has a separate bond pad they may be linked off-chip. The circuit operates at a drain bias of Vd,=+2.5V to V,,, VdZ and V, and a gate bias to V,,, V,, and V,, to set for L=50% Iorr on the respective drain supply (gate bias - - 0.4V).

The LNA was also designed for a high process yield, which a vital aspect in maintaining the MMIC as a low cost solution. To achieve this, MIM capacitors with their tightly controlled but nevertheless finite process spread, were not used for critical matching elements but were used almost exclusively as low impedance DC blocks. In this way, the RF performance spread was minimized by appropriate circuit design [6]. A layout size of the LMDS LNA is 0.8 mm x 1.5

The simulated performance of the S- parameters for a typical 26-30 GHz LNA is shown in Figure 2. It can be seen that the circuit covers a number of the operating bands and therefore can be used as a generic gain block in a variety of systems. Over the frequency range 26- 30 GHq the circuit exhibited an W gain

mm.

typically 25dB at 28GHz, nput match of -15dB and an output match better than -25dB.

w.0 I, D

1. 0

l S 0

( S . 0

I.0

I m -6 0

- 0 . 0

4 I . D

4. D

d. 0

-aI 0

Figure 2. S-parameter of the LNA

. m

..* ,o

z e

z .O

I S

3 0

Figure 3. Noise figure of the LNA

The noise figure (NF) performance of the MMIC is also been simulated. The plot of N versus frequency for the LNA circuit is shown in Figure 3. The result shows the circuit has a typical NF of 2.3dB at 28GHz and exhibited a maximum value of 2.5dB across the band. This is the performance d the monolithic chip as it would look when wire-bonded into a typical alumina or soft board circuit, based on RFOW measurements.

It is also apparent that the performance is not degraded much if examined over the whole frequency ranges 26 to 30 GHz. Thus the target specification is achieved.

A Monte Carlo yield analysis method is used in determining the tolerances of the pHEMT devices in the circuit design. Figures 4 and 5 show the results of the acceptable values in 20 design trials for both S-parameter and noise figure responses respectively.

310

ICSE2002 Proc. 2002, Penang, Malaysia

I I I .m

r..qY.nry 9.0 cu lp l"

Figure 4. S-parameter tolerance of the LNA

I I I

2.5

Fig. 6 . Simulated S-parameters, 2630 GHz PAD

This MMIC has a higher gain as well as lower noise figure performance than previously reported p-HEMT MMIC [ 5 ] , and it is unconditionally stable under all possible operating conditions. The advantage of this chip

is its high gain, low noise figure, good matching over a very wide frequency band, and the small chip size. This chip will therefore satisfy the requirements of the most proposed LMDS systems.

111. POWER AMPLIFIER DRIVER

The power amplifier driver (PAD) has been designed using MMIC CAD Series IV on PC, which includes the GMMT H40 @-HEMT) Process Smart Library. The PAD consists of five gain stages, which all the stages we% designed in-ndently for 50Q input and output impedances and then brought together to form thegcomplete PAD. The first three stages were desrigned for high gain with negative slope and g o d input match across the target specification frequency band. The last two stages were designed for a positive gain slope and good ou@t match over the same frequency band, resulting in the complete PAD having a very flat gain. The 4x150 p n gate width pHEMT were u s d for the fourth and fifth gain stages to a c b v e the desired output power as stated in the d d g n specification.

sThe stability factor, K is one of the most imwrtant parameters in designing an amplifiers, its value will determine the occurrence of oscfilation. Therefore, a particular care was taken to design each stage for unconditional stability at bot$ very low and very high frequency, as well as aver the specified operating frequency range. AlPthe stages used shunt inductance elements, whkh is connected between the pHEMT source contacts and the grounding vias (source peaking) to bbtain a good input matching. The gate and drah stabilization resistors, together with source p e h g were used to provide unconditional stability over the DC to 50 GHz frequency band.

rThe simulated RF performance of the PAD is s h m in figure 6, which the PAD exhibits a gain of p3dE3, less than -13dB input retum loss and - 1 IdB output return loss over the frequency 26 to 30 GHz, respectively. The output power at 28 GHz and IdB gain compression is +2ldBm, at a dram bias of 4 9 .

f The tolerance analysis is also performed on the PAD to determine the variation of the RF performances after the fabrication process completed. The analysis was done by replacing the linear foundry transistor and capacitor models with the foundry tolerance models. The tolerance analysis results as

31 1

ICSE2002 Proc. 2002, Penang, Malaysia

- F E= .r'. E"'

m.0 - "-m-

m.0

> L a

0..

.?Le

Y..

depicted in Fig. 7 shows that most of the p m Derfomances are well in the desien The h t stage was designed to increase the small

p F' F= t. 0

>. 0

0.1

*I

-,.o

-*. 0

4, 0

-** .e

- specification requirement.

Fig. 7. Tolerance analysis, 26-30 GHz PAD

Iv . POWER AMPLIFIER

The Power Amplifier (PA) circuit is initially designed for small signal matching circuits; with S-parameters for input and output match and gain response. Then, the non-linear models were placed into the circuit. The output matching circuit at the last stage was optimized to achieve the peak output performance. The circuit has three stages of gain and splitters in between where each stage used 4x150 pm gates width p- HEMT in a dual throughvia Pi configuration transistors. The choice of device size is determined by a compromise between using a large size to obtain maxi" output power whilst still maintaining a viable level of available gain from the device. At the last stage, output combiners were used to combine all the stages. All the stages of the amplifiers were designed

signal and good input matching The last stage was designed for a good output matching and a good 1 dB power compression at centre frequency. All stages were taken care in designing for unconditional stability at both very low and very high kequencies range as well as over the specified operating frequency range. Series resistor and capacitor elements were included at the gate and drain of the FET transistor to improve the in-hand stahility; raising the stability K factor above 1.1,

Simulations were done in a commercially available linear and non-linear simulator (Libra Series IV version with GMMT H40 Process Smart Library) which uses the Harmonic Balance method. The simulation included the effect of the input and output bond wires of 0.2 nH. A WOW pad was used for the input and output pads.

This 4.5 x 2.2 mm* MMIC chip exhibits a simulated result of small signal gain of 16&, input match of -lSdB and an output match of - 17dB (worst case over the hand 2 6 3 0 GHz) and output power of +25dBm at 1 dB gain compression under large signal conditions at a drain bias of +4SV and -0.W at gate. The simulated performance of the S-parameters for a typical 26-30 GHz PA is shown in Fig. 8 and 9 respectively. In Fig. 8 it shows gain compression under large signal conditions at 28 GHzwhere as at Fig. 9 it shows a small signal gain, input and output matching and stahility of the PA over the hand 2 6 3 0 GHz.

This MMIC is unconditionally stable under all possible operating conditions. This design will therefore satisfy the requirements of most proposed LMDS system.

Figure 8. Simulated power compression, 26-30 GHz PA

312

Figure 9. Simulated S-parameters 26-30 GHz PA

ICSE2002 Proc. 2002, Penang, Malaysia

A tolerance analysis simulation is performed to determine the variatbn of the fabrication process. This is done to ensure the PA design meets all the foundry tolerance parameter. A Monte Carlo yield analysis method was done by replacing the linear foundry transistor (pHEh4T devices) and capacitor with the foundry tolerance models in the circuit design.

I. --- Fig. IO. S parameter Qlerance analysis, 2 6

30 GHz PA

The design seems to be tolerant to the expected process variations. Figure 10 shows the results of the acceptable values in 20 design trials for PA Sparameter.It gave the best input and output match when used with bondwire inductances of 0.2 nH. The three stages of the amplifier appeared to be stable to 50 GHz. The amplifier is capable of producing an output power of 25 dBm at 1 dB compression with the drain biased at 4.N.

VI. CONCLUSION

As wireless communication becomes more integrated into OUT daily life, demand for higher data rate transfer increases tremendously. LMDS is one of the modes of communication for such high data rate "fer which cater for static users in concentrated area such as office environment, housing area and etc. Recent improvement in p-HEMT Gallium Arsenide (GaAs) fabrication and more accurate method of small signal parameter extraction have made it very attractive for millimeter wave RF circuit design. This paper covers a portion of a complete RF transceiver unit such as low noise amplifier (LNA), power amplifier driver, power amplifier and mixer. The integralpart of the above design is a p-

HEW GaAs based transistor which has a usable fequency ranges from 1 to 100GHz. Another obvious aspect of the technology is the inherently small size circuitry that enables more RF circuitry to be integrated into a single integrated circuit.

VII. REFERENCES

h n o : / / w m u . m " l l m d s . h r m MIA Com Frequencies, 1995. R. A. Davies, D. J. Wamer, R. S. Smith, D. R. Brambley, R. H. Bennet, D. M. Nugent, G. Green, R. H. Wallis, A. P. Long - Pseudomorphic HEMTs for Low Noise Millimetre Wave Transistor and Circuits, University of leeds, 2 f h March 1991, IEE Digest 19912/068. Marsh, S. P., Long, A. P., Edwards, G. D., Buck, B. I., Peniket, N. A., Geen, M. W. and Wadsworth, S. D. " GaAs MMIC Technology for IT and Wireless Communications", Proc. Of IEEE international Conference on Semiconductor Electronics, Bangi, November 1998. A P Long, G D Edwards, S P Marsh, I R Cleverly and A W Deam, "Single and Multifunction HEMT MMICs for Microwave and Millimeter Wave Communication Systems", Microwave and Communication Technologies Conference Proceeding, pp20 -25, October 98 S P Marsh, "GaAs Monolithic LNA for VSAT Applications", GEC-Marconi Materials Technology Ltd., Ref No I147/SM/CW. 1996

313