I. Introduction

A number of automotive radar applications operate in the W-band range. While the first HEMT-based 77GHz chipsets for automotive applications have been demonstrated [], monolithically integrated transmit-receive and steering-beam antenna switches are desired to reduce cost and extend the functionality of the radar systems. Traditionally most research and development work concentrated on GaAs PIN diode switches fabricated using microstrip technology. Microstrip InGaAs PIN diode SPST switches have been demonstrated by the authors and offered state-of-the-art performance while being compatible with InP-based high-frequency electronics []. Coplanar technology offers smaller parasitics and lower-cost processing, but until recent advances in electromagnetic simulators, it lacked good design models. Based on the new coplanar models, W-band coplanar GaAs PIN diode SP3T switches with 20dB isolation at 77GHz have been recently demonstrated []. Due to the lower bandgap, InGaAs PIN diodes offer lower ON-state resistance, which leads to reduced power consumption and improved isolation characteristics. In this work, we report and discuss the design and performance of 77GHz coplanar InGaAs PIN diode SPDT switches with a record isolation of over 43dB and small insertion loss of 1.35dB.

II. Design and Fabrication of SPDT Switches

Coplanar-waveguide technology was employed to design single-pole double-throw (SPDT) InGaAs PIN diode switches in order to reduce the fabrication cost and minimize the parasitics. The SPDT switch consists of one input and two output arms joined together with a coplanar-waveguide tee, as shown in Fig.1. Each output arm consists of two quarter-wavelength coplanar waveguide sections shunted in the middle by the InGaAs PIN diode. Two DC blocking capacitors were connected in series to the signal line between the diodes and the tee, thus allowing independent biasing of the diodes and DC isolation of the input port. Similar fabricated designs included on-chip biasing network and DC isolation of all three ports. The chip is shown in Fig.1 and its size was 2mm x 0.8mm.

The transition between the PIN diode and the coplanar waveguide has been optimized in order to reduce parasitic capacitance and thus improve the switch isolation characteristics (see Fig.2). An air-coplanar signal line was connected for this purpose to the p-type contact on the top of the PIN diode mesa while wide ground lines were used to access the n-type contacts on both sides of the mesa. The symmetrical shunt transition formed between the PIN diode and the coplanar waveguide allows realization of a high-impedance line segment due to the presence of air rather than dielectric and manifests very low parasitic inductance. The method of moments simulator was used to extract the parasitic inductance of the "low-inductance" transition and to compare it with conventional transitions employing shunt airbridges. The latter were assumed to have symmetric (ground-diode-ground shunt connection by airbridge) or asymmetric (diode-ground shunt connection by airbridge) topology.

The parasitic inductance of the "low-inductance" transition and conventional, symmetric and asymmetric shunt-airbridge transitions was 3.6pH, 11pH, and 96pH respectively, while the parasitic capacitance remained ~15fF. The reduction in parasitic inductance from 11pH to 3.6pH corresponds to a 10-dB increase in isolation for the switch using "low-inductance" transition compared with the switch with conventional symmetric shunt-airbridge transitions. Finally, the microwave simulator EEsof Libra was used to optimize the dimensions of all waveguides, discontinuities, and DC blocking capacitors for optimum performance at the design frequency. The W-band monolithic integrated SPDT transmit-receive switches employing InGaAs PIN diodes were fabricated on semi-insulating InP substrates.

III. Small-Signal Characteristics of SPDT Switches

Two-port on-wafer W-band S-parameter measurements of the switches were performed using three W-band Pico-probes and a HP8510B network analyzer with millimeter-wave option. The third port of the SPDT switch was connected to a W-band probe loaded with a waveguide termination. The return loss from the waveguide termination was larger than 20dB.

The switch is in practice employed with the antenna feeding the input port (Port 1) and a high-power amplifier (HPA) and low-noise amplifier (LNA) connected to the output ports (Ports 2 and 3 respectfully). A diode D1 is in this case employed in the arm of Port 2 while a diode D2 is employed in the arm of Port 3. The insertion loss was measured by the scattering parameter S₂₁ when the bias was set to obtain transmission conditions between Port 1 and Port 2. Under such circumstances diode D1 is off and diode D2 is on. The isolation between input and output was measured by S₁₂ when the bias conditions were reversed; under such circumstances, diode D1 is on and diode D2 is off. Scattering parameters S₁₁ and S₂₂ provided information on input and output matching, respectively, under both bias conditions.

The 77GHz InGaAs PIN SPDT switch demonstrated low insertion loss of less than 2dB over a 4GHz bandwidth between 74.2 and 78.2GHz with minimum loss in the range of 1.15dB to 1.35dB; 1.15dB for transmit direction (S₁₂^D1-OFF) and 1.35dB for receive direction (S₃₁^D2-OFF), as shown in Fig.3.

The isolation (S₁₂^D1-ON, S₃₁^D2-ON) between the input and the output ports was larger than 40dB over a 3GHz bandwidth between 75.5GHz and 78.5GHz for both directions, and exceeded 43dB at 77GHz as shown in Fig.4. The low insertion loss and exceptionally high isolation are due to the low ON-state resistance of InGaAs PIN diodes and a very low parasitic inductance of the PIN-to-CPW transition used in the switch design.

The leakage of the input signal to the output port is measured by crosstalk between the output ports. Output crosstalk (S₂₃) was characterized by measuring on-wafer scattering parameters between the two output ports and using a matched termination for the input port. The results are shown in Fig.5 under various diode bias conditions. When the switch is in transmit or receive mode, one diode is in the ON- and the other diode is in the OFF-state. Under these bias conditions, the crosstalk was larger than 30dB over the entire measured bandwidth. However, the bias conditions can be different during switching between the above modes and, thus, the output signal can leak in a more severe way to the input port and be "received".

The maximum value of crosstalk was -9dB and occurred when both diodes were in the OFF state. It is, therefore, important to implement in actual operation an appropriate switching sequence which avoids the "both diodes OFF" state.

IV. Power Characteristics of SPDT and SPST Switches

The power characteristics of the InGaAs PIN diode switches were also investigated in order to determine their power-handling capability. The characteristics of the InGaAs PIN diode SPDT switch as a function of the input power level are shown in Fig.6. When the OFF-state diode was biased close to turn-on voltage (V_D1=+0.3V), increasing the input power above +1dBm caused the diode to self-bias toward the ON-state, which resulted in an increased insertion loss. However, under normal OFF-state biasing conditions (V_D1<=0V), no degradation of insertion loss was observed for input power levels up to +11dBm; this input power level was limited by the source.

The large-signal system did not allow measuring isolation larger than 20dB. However, it was possible to study the effect of input power on the isolation by biasing the ON-state diode below turn-on voltage (V_D1=+0.4V). Increasing the input power turned the diode ON and improved the isolation.

A comparison between power-handling capability of SPDT and SPST switches fabricated with the same InGaAs PIN diode technology is shown in Fig.7, which depicts the percentage of change in insertion loss with the input power level. The OFF-state diodes in both switches were biased for this purpose at V_OFF=+0.2V, and the ON-state diode in the SPDT switch was biased at V_ON=+0.7V. While typical OFF-state bias is negative, a positive OFF-state bias was chosen so that a comparison could be measured (see Fig. 7).

The degradation of the insertion loss in the SPDT switch is smaller and occurs at a higher level of input power than in the SPST switch. An analysis of the switch showed that the power handling capability improved by 3dB compared with the SPST switch, due to the fact, that the input power injected in the SPDT switch is distributed between two rather than one diode. The OFF-state diode of the SPST switch was eventually self-biased and under increased input power level (>+1dBm) changes to ON-state operation resulting in insertion loss degradation. In the case of the SPDT switch when the power is increased beyond +4dBm, the OFF-state diode is also self-biased as in the SPST switch. The degradation of insertion loss is, however, smaller in this case due to the presence of the ON-state diode.

V. Conclusions

Overall, a 77GHz coplanar transceiver switch using InGaAs PIN diodes has been developed and demonstrated state-of-the-art insertion loss of 1.15dB and 1.35dB in transmit and receive modes, respectively. A record-high isolation value of over 43dB was demonstrated at 77GHz due to low ON-state resistance of InGaAs PIN diodes and low-inductance PIN-to-CPW transitions employed in the low-parasitic coplanar switch design.

Power-handling capability of the SPDT switch was improved by more than 3dB compared with the SPST switch. No degradation of performance was observed when input power level was increased to +11dBm if typical OFF-state bias was applied.

The combination of low-cost low-parasitics coplanar technology, excellent high-frequency performance, and substrate compatibility with InP-based electronics makes coplanar InGaAs PIN diode switches very attractive MMIC components for emerging automotive radar system applications.

Acknowledgments

This work is supported by MURI (DAAH04-96-1-0001) and Daimler Benz AG.