|Year : 2009 | Volume
| Issue : 4 | Page : 169-172
Design and Realization of Electrostatic Discharge Protected DC to 2GHz GaAs Monolithic Microwave Integrated Circuits SP4T Switch for TDMA Communication Applications
Shib Shankar Singh
Department of Design, Gallium Arsenide Enabling Technology Center (GAETEC), Vignankancha Post, Hyderabad - 500 069, India
|Date of Web Publication||23-Sep-2009|
Shib Shankar Singh
Department of Design, Gallium Arsenide Enabling Technology Center (GAETEC), Vignankancha Post, Hyderabad - 500 069
| Abstract|| |
This paper discusses the small signal simulated and measured results of a DC to 2GHz monolithic GaAs SP4T switch which has been designed and realized. The measured 1 dB compression point of SP4T switch at different frequency range has also been presented. The DC to 2GHz switch has been designed using series-shunt FETs topology. This topology has worked over very large bandwidth. In this topology series MESFETs allows the SP4T switch to operate down to DC. The SP4T switch has been achieved better than 1.1dB of insertion loss, better than 32 dB of input to "Off" port isolation and better than 16 dB of the input and output return loss @2GHz. The performance of the SP4T switch has been measured on an FR4 PCB eval board. The on-chip higher value of ballast resistor has been used for ESD protection. The switch has designed using 0.7 mm Ion-Implanted recessed gates MESFET technology and used in switching matrices for TDMA communication system. The major advantages of the solid-state integrated switch are low power consumption, small size and lightweight. These fea tures of solid state switch are more attractive for communication applications. The chip dimension is 1.2 × 1.2 mm.
Keywords: MESFET, Series-Shunt topology and Monolithic microwave integrated circuits , SP4T and switch.
|How to cite this article:|
Singh SS. Design and Realization of Electrostatic Discharge Protected DC to 2GHz GaAs Monolithic Microwave Integrated Circuits SP4T Switch for TDMA Communication Applications. IETE J Res 2009;55:169-72
|How to cite this URL:|
Singh SS. Design and Realization of Electrostatic Discharge Protected DC to 2GHz GaAs Monolithic Microwave Integrated Circuits SP4T Switch for TDMA Communication Applications. IETE J Res [serial online] 2009 [cited 2013 May 21];55:169-72. Available from: http://www.jr.ietejournals.org/text.asp?2009/55/4/169/55987
| 1. Introduction|| |
A switch plays an important role in controlling the RF signal and is one of the important components in communication and radar systems ,, . The switching circuit is one of the most suitable circuits for monolithic integration and has many applications. Therefore it requires a large production technology capability. Switches have been used in applications such as transmit-receive module for phase array antennas, basic switching blocks for phase shifters and phase modulators, switching matrices for TDMA communication system, multiple access for test and measurement equipment, switching blocks for highly reliable system based on the redundancy concept, multiple-switching oscillators for military application and multiple switching spot-beam coverage of territory by communication satellites. The main advantage of the integrated switch is very low DC power consumption compared to PIN diode based switch designs This paper discusses a DC- 2GHz MMIC SP4T switch composed of series, shunt MESFETs switching elements and a transmission line. This topology has worked over a very large bandwidth. In this topology series MESFETs allows the SP4T switch to operate down to DC  .
| 2. Switching Elements and Its Model|| |
It has been well documented , that MESFET has been used as a switching element and integrated ana logue switches can easily be designed using any metal semi con ductor field effect transistor (FET) based process. The switching MESFETs have generally been known as Cold MESFET because drain bias has not been applied. The gate bias has only been applied in switch mode operation and the gate acts as a control terminal. The bidirectional RF signal has flows between the drain and source terminal. In MESFETs the control voltages are essentially isolated from the main signal path and hence very low current is required. The bias control is very straightforward in the MESFETs-based switch. The chip size is also critical as the switches are price sensitive in communication. The switch ing MESFET, in switching mode, acts like a passive devise source and drain elec trodes can be interchanged for connecting the other passive components while gen erating a layout. The switching MESFET behaves in a passive manner, is dominantly resistive in "On" state and dominantly capacitive while in "Off" state. The electrical equivalent circuit of switching MESFET is shown in [Figure 1]a. For RF characterization of switching MESFETs, S-parameters are measured under zero volts and below pinch-off volt ('On' and 'Off' states) bias conditions applied to the gate. In the "On" and "Off" state, the gate is biased at 0 and beyond pinch-off voltage correspond ingly. The "On" state can be modeled by DC resistance between source and drain terminal as shown in [Figure 1]b. Simple capaci tor can model the "Off" state. The simplified MESFET model is indicated in [Figure 1]c.
| 3. Circuit Design|| |
The schematic of DC to 2GHz SP4T is shown in [Figure 2]a. In this schematic, there is one input (IN) and four outputs (OUT1, OUT2, OUT3 and OUT4). One high impedance terminal is connected opposite to input for ESD protection. In this circuit the switching MESFETs are connected in series and shunt configuration in conjunction with a transmission line. The circuit has eight independently biased arms. The electrical equivalent circuit with the switch biased to pass signals between In and Out1 is shown in [Figure 2]b. The switching MESFETs are represented as their simplified equivalent circuits. The series MESFET (FET1) in OUT1 arm is in "On" state, and acts as resistor. The shunt MESFET (FET2) in OUT1 is in "Off" state, and acts as capacitance.
The switching series MESFETs (FET3, FET5 and FET7) in OUT2, OUT3 and OUT4 arms are in "Off" state, and act as capacitance. The switching shunt MESFETs (FET4, FET6 and FET8) in OUT2, OUT3 and OUT4 arms are in "On" state, and act as resistor. The transmission lines are inductive in nature and their lengths are small. The extra values of inductances are realized by bond wire, and the length of bond wire is approximately one mm. The diameter of bond wire is approximately one mil. There is no ground via used in this design, the off-chip grounding has been used.
| 4. Electrostatic Discharge and Its Protection Circuits|| |
ESD protection of GaAs based devices is critical for long-term reliability of final products. The GaAs based devices are very sensitive to electrostatic discharge (ESD) events, because GaAs has lower thermal conductivity and melting temperature than silicon. Therefore GaAs is very sensitive to lattice damage from high current ESD stress. The GaAs based SP4T is fabricated on a GaAs semi-insulting. The SP4T switch connected to external ports is susceptible to damaging ESD pulses from the operating environment and peripherals The dimension of SP4T switch is very small. The off-chip grounding has been used in SP4T. Due to compactness and off-chip grounding, SP4T switch is also sensitive to ESD. Due to ESD series MESFETs of SP4T switch will be affected and SP4T will not work as usual. Keeping in this view on-chip ESD protection is essential in this circuit. The on-chip ESD protection of the circuit has been provided by a ballast resistor. The small value of the resistor is not sufficient for ESD protection. Hence high value and high width (20 kilo-ohm) value of resistor have been used opposite the input terminal of SP4T to prevent from ESD. Proper ballast resistors can even out the ESD current distribution in multiple arms of SP4T switch during ESD stress. The ballast resistor is connected just opposite to the input terminal of switch. Hence ballast resistor is connected with every arm of SP4T switch and constantly adjusted to maintain an even current distribution in all arms of SP4T switch due to ESD stress.
| 5. Monolithic Microwave Integrated Circuits Fabrication|| |
The layout of DC to 2GHz SP4T switch is shown in [Figure 3]. The series switching MESFETs have four-gate finger and its dimension is 600μm, and the shunt MESFETs have two gate fingers and its dimension is 200μm. All the switching MESFETs have been biased by 5 Kohm at the gate terminal. The gate length of switching MESFETs is 0.7μm. This is a 10-mask process using Contact lithography. This process makes use of 3 levels of metallization, viz., Thermal evaporated Au-Ge/Ni/Au for Ohmic metal (MO), E-Beam evaporated Ti/Pt/Au for gate metal (MB) and Sputtered Ti/Au for top metal (MT). A wide range of resistors can be realized using the ion-implanted layer as well as Nichrome thin films layers. Silicon nitride and a sandwich of Silicon nitride and Polyimide form the dielectric layers for two types of MIM capacitors with bottom (MB) and top (MT) metals. Planar spiral inductors are formed using MT, the top metal and the bottom metal, MB, being used for under-passes. The starting wafer is 3" semi-insulating GaAs wafer of 600 mm thickness. The wafer is thinned down to 200±10 mm thickness and provided with backside gold metallization.
| 6. Measured and Simulated Performance|| |
The performance of SP4T switch has been measured on an FR4 PCB eval board. The measured and simulated small signal performance of DC to 2GHz switch is shown in [Figure 4]. The single quad off-chip TTL driver has been used to measure the performance of the switch. The measured insertion loss is between 0.55 to 1.11 dB, and measured input to "Off" port isolation is between 66.16 to 32.77 dB from dc to 2GHz. The measured input and output return loss (over four-output port) is better than 16.77 dB from dc to 2GHz. The effect of FR4 PCB eval board was not removed from the insertion loss, isolation and return loss measurement. The effect of FR4 PCB eval board was clearly shown in the return loss and insertion loss measurement. The small signal measured and simulated results have been well matched from dc to 2GHz except return loss and insertion loss of the switch due to the FR4 PCB eval board. This circuit has designed using Agilent ADS 2004A. The 1dBc Compression point has been measured from dc to 2GHz @ 0/-5v control and 0/-8v controls correspondingly. The measured 1dBc compression points of switches @ 0/-5V and 0/-8V control are shown in [Figure 5].
| 7. Summary|| |
An MMIC-based DC to 2GHz SP4T switch has been designed using 0.7 mm ion-implanted recessed gates MESFET technology. This switch has been designed using series-shunt FET topology, which is worked over a broadband frequency range. The on chip ballistic resistor has been used for ESD protection. The switch has been used for switching matrices of TDMA communication application and has achieved better than 1.1dB of insertion loss, better than 32 dB of isolation and better than 16 dB of input and output return loss.
| 8. Acknowledgment|| |
The author would like to thank Mr. A.V.S.K. Rao, Dr. G. Sai Sarvan, Mr. Mahadev Bhatt, Mr. A.K. Pandey, Mr. Sandeep Chaturvedi, Mr. Sangam Bhalke and their colleagues for fabrication, assembly and testing of the SP4T switch. The author expresses gratitude to Dr. R. Muralidharan, CEO (GAETEC) for reviewing the paper and his constant support and motivation during the project. The author would like to thank his family members for constant support and motivation.
Shib Shankar Singh received his Electronics Engineering degree from The Institution of Electronics and Telecommunication Engineers, Delhi, India. He completed M.Tech in Microwave Electronics from the Delhi University South Campus, Delhi, India, in 2001. In 2001, he joined Gallium Arsenide Enabling Technology Center (GAETEC), Hyderabad, India. Currently a Senior MMIC Design Engineer at GAETEC, he is engaged in the design of GaAs Monolithic Microwave Integrated Circuits. He has authored over ten publications at conferences. His work interest is high frequency designing of MMICs such as LNA, Power Amplifier, Solid State Switches, Digital Controlled Phase Shifter, Digital Controlled Attenuator, Voltage Variable Attenuator, Voltage Variable Phase Shifter etc.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]