High-Isolation W-band InP-based PIN Diode Monolithic Integrated Switches. Egor Alekseev, Dimitris Pavlidis, Delong Cui. Department of Electrical Engineering ...
1997 IEEE/Cornell Conference on Advanced Concepts in High-Speed Semiconductor Devices and Circuits August 4-6, 1997
Cornell University
High-Isolation W-band InP-based PIN Diode Monolithic Integrated Switches Egor Alekseev, Dimitris Pavlidis, Delong Cui Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109, USA Abstract This paper demonstrates the design of high-isolation millimeter-wave monolithic integrated switches using MOCVD-grown InGaAs/InP PIN diodes as switching devices. W-band single-pole single-throw switch using double shunt diode topology allowed significant improvement of isolation while keeping insertion loss low. The SPST switch used two InGaAs PIN diodes and demonstrated better than 35dB isolation and 1.8dB insertion loss at 92GHz. A comparative analysis of single and double diode switch characteristics is treated together with the design and fabrication details. The power handling capability of the switches is also discussed.
Introduction W-band radar systems are used in both military and commercial applications. Collisionavoidance automotive applications will profit from such systems by use of monolithic integrated components. Millimeter-wave monolithic transmitter-receiver modules will integrate such components, as high-power amplifiers, low-noise amplifiers, and high-power high-isolation switches on the same chip. InP- and GaAs-based high electron mobility transistors (HEMTs) operational up to W-band frequencies are being employed to develop such MMICs [1,2], but two-terminal devices play an important role in signal generation, mixing and switching. Millimeter-wave switching PIN diodes offer high power-handling capability, low insertion loss, and high switching cutoff frequencies as necessary for millimeter-wave transceiver switches and other amplitude and phase-control circuits [3]. Monolithic GaAs PIN diode switches for 35GHz [3] and 94GHz [4] have been demonstrated with 0.7dB insertion loss and 21dB isolation, and 1dB insertion loss and 30dB isolation respectively. While most of the millimeter-wave switches were developed using GaAs PIN diodes, monolithic integrated switches using InGaAs/InP PIN diodes have drawn less attention due to the less developed technology involved in their realization. An X-band transmit-receive switch was fabricated using InAlAs/InGaAs/InP HBT process. The InGaAs PIN diode switch demonstrated similar insertion loss and 10dB improvement in isolation due to lower series resistance, compared with the same GaAs circuit [5]. A key feature for these diodes is compatibility with InP-based high-frequency electronics for 77GHz automotive collision-avoidance and other W-band radar systems. InGaAs also has lower energy bandgap than GaAs and therefore demonstrates lower PIN diode turn-on voltages, lower DC power consumption, and lower series resistance. State-of-the-art performance for MBE-grown InGaAs PIN diode single-pole single-throw 83GHz switches has been demonstrated by the authors − 1.3dB insertion loss and 25dB isolation for a single shunt diode switch topology [6]. However, if higher isolation is required, a switch design with two shunt diodes placed one quarter-wavelength apart could be of interest at the cost of eventually increased insertion loss. This paper addresses both single- and double-diode MMIC
switches using MOCVD-grown InGaAs/InP PIN diodes. Since the insertion loss of InGaAs PIN diodes is small, the ON/OFF power transmission ratio can be maximized, as demonstrated by the work described in this paper. Understanding of PIN diode characteristics in the presence of high-power signals is important since a decrease of high-frequency impedance of the OFF-state PIN diode due to self-biasing effect can degrade the switch performance. Studies of the switch isolation were therefore performed as a function of the large-signal power level and are also reported. Design and Fabrication of High-Isolation SPST Switch The operation of the single-pole single-throw switch arm with a single shunt diode (shown in Figure 1a) can be understood by treating the diode as an impedance block. In the ON-state, the diode impedance ZON is low and smaller than the characteristic impedance Z0 of the transmission line. A signal injected to the input port of the switch will therefore be shunted to the ground through the diode. The OFF-state diode impedance ZOFF is higher or comparable to Z0, and the signal is not consequently reflected from the diode and is transmitted to the output port. Two most important performance measures of a SPST switch are its isolation (when the diode is in the ON-state and the switch is not transmitting) and insertion loss (when the diode is in the OFFstate and the switch is transmitting). These properties are analyzed in the paper as obtained for a single-pole single-throw switch employing two InGaAs/InP PIN diodes, the schematics of which is shown in Figure 1b. High-impedance 95Ω quarter-wave transmission line sections were used in both, the single and the double diode switch, in order to compensate for non-idealities of Z ON and ZOFF. Such design permits the adjustment of the impedances presented at the input port to obtain the best insertion loss-isolation trade-off. The main factor determining the switch isolation is a small, but finite voltage amplitude across the airbridge inductance LAB, diode ON-state resistance RON, and via hole inductance LVIA. This results in a secondary power wave that propagates to the output port and decreases the isolation. In our conventional SPST switch with a single shunt diode the isolation was limited to ~23-25dB. The integrated coplanar-to-microstrip transition losses and the signal leakage across the OFF-state diode capacitance COFF are the most important factors determining the insertion loss in our SPST switch. Total insertion loss including losses in two integrated coplanar-tomicrostrip transitions is typically 1.2-1.4dB for our conventional SPST switch.
Figure 1. Schematics of W-band shunt InGaAs PIN diodes switches.
Figure 2. Photograph of a 94GHz double-shunt InGaAs PIN diode SPST switch. The high-isolation SPST switch design of this work (see schematics in Figure 1b and photograph in Figure 2) employs a second shunt diode with the same biasing polarity to further shunt the secondary power wave resulting from the finite ON-state diode resistance and inductance. In the photograph one distinguishes the input and output coplanar-to-microstrip transitions where the use of InP via-hole technology was made. The two PIN diodes and two radial stubs used for biasing are also shown. Simulations using HP EEsof Libra showed better than 40dB isolation for this double-diode design. However high-isolation properties are also accompanied by a larger insertion loss, due to the presence of the second diode in the signal path. Under the assumption of low insertion losses the addition of the second diode in a high-isolation SPST switch design should double the value of the insertion loss. The InGaAs PIN diode SPST switches were fabricated on semi-insulating InP substrate. The PIN diode layers were grown at the University of Michigan by MOCVD and were, starting from the substrate:n+(1µm,1.5x1019cm-3),i (1µm,3-5x1015cm-3),p+(0.15µm,1.5x1019cm-3). The diodes were fabricated on 5• m-diameter mesas by wet etching, and the Au electroplated airbridges were used to connect the diodes to the rest of the circuit. The scanning electron microscope image of a fabricated diode is shown in Fig.3. The diodes demonstrated low turn-on and high reverse breakdown voltages of 0.48V and -23.5V respectively as determined at 10µA current. The wafers were then thinned down to 100µm, and backside-via holes were etched using an inhouse developed technique, which has been described previously[6]. The single-diode SPST chip was 1.1mm x 0.8mm large, while the high-isolation switch measured 1.7mm x 0.8mm.
Figure 3. SEM Photograph of a fabricated InGaAs/InP PIN Diode
Analysis of PIN Diode and Monolithic Switch Characteristics The performance of the SPST monolithic integrated InGaAs PIN diode switches was measured using on-wafer S-parameter testing at 70-105GHz and shown in Figures 3 and 4 for conventional and high-isolation SPST switches respectively. The switch insertion loss is characterized by S21 when the diodes are in the OFF-state (VD=-5V). When the diodes are in the ON-state (VD=0.7-0.8V, ID=4-5mA per diode) S21 determines the switch isolation. S11 measures reflection (ON-state) or return loss (OFF-state) at the input port. Coplanar waveguide standards were used for network analyzer calibration and results, presented in Figures 4 and 5, include losses in the two on-chip integrated coplanar-to-microstrip transitions. The transition losses were estimated by comparing the single- and double-diode switch characteristics, since these two topologies employ identical diode and biasing cells, and were found to be 0.4dB. W-band on-wafer S-parameter measurements showed for a single-diode 87GHz InGaAs PIN diode SPST switch a low 0.9dB insertion loss when the diode was in the OFF-state, and high isolation of 24dB when the diode was in the ON-state (after correction for coplanar-to-microstrip transition losses). The high isolation SPST switch with two InGaAs PIN diodes, developed in this work, demonstrated an insertion loss of 1.8dB at 92GHz, while its isolation was improved significantly and was in excess of 35dB. This is to the best of our knowledge, the highest reported isolation value for a W-band monolithic switch. The reflection loss for single-diode SPST and double-diode SPST switch was as low as 0.85dB and 1.05dB respectively. The return loss was less than 10dB inside of 5GHz-bandwidth for single-diode switch and 3GHz-bandwidth for double-diode switch. The bandwidth of both switch designs is determined by the frequencyselective radial stubs used for biasing the diodes. It should, however, be emphasized that load conditions on the biasing pads did not at all interfere with the switch performance.
Figure 4. S-parameters of W-band single-shunt InGaAs PIN diode SPST Switch
Figure 5. S-parameters of W-band double-shunt InGaAs PIN diode SPST Switch
The improvement in ON/OFF power transmission ratio achieved by using a high-isolation switch with two rather than one shunt diode is better demonstrated in Figure 6. The isolation of the high-isolation double-diode SPST switch is shown here to be almost double the isolation of the conventional single-diode SPST switch, while the degradation in the insertion loss is minimal due to the low ON-state resistance of the InGaAs PIN diodes. The isolation is saturated at a low DC power consumption level around 5mA per diode due to the parasitic airbridge and via hole inductances. This state-of-the-art performance for W-band monolithic integrated switches was achieved using InGaAs PIN diodes with low DC power consumption and employing InP-based technology compatible with high-frequency InP-based electronics.
Figure 6. Transfer function (Isolation/Insertion Loss) for single-shunt and double-shunt switches
Insertion Loss=(Output Power - Input Power), dB
0 0V -0.1V
-5
-0.5V -1V
-10 -2V
-3V
-4V
-15
-20 0
-6V -8V
5
10 15 20 Input Power, dBm
25
30
Figure 7. Isolation of InGaAs PIN diode switches vs. input power level
The power handling characteristics of the InGaAs PIN diodes were analyzed and allowed to determine the input power levels at which the diode transfer characteristics are compressed by 1dB. The OFF-state power dependence is of most importance because it is subject to self-biasing, which causes performance degradation. A series InGaAs PIN diode SPST switch in the OFFstate was analyzed by on-wafer power measurements at 10GHz at different bias points. The experimentally determined isolation compression as the function of the input power and reverse bias voltage is shown in Figure 7. As the input power level is increased a larger negative bias is needed to be applied to the diode to preserve its isolation characteristics. Moreover, at given input power level, the degradation of isolation from small-signal operation characteristics is reduced by increasing the bias across the diode. Assuming that for a large reverse bias and small compression levels the voltage swing at the diode can be estimated from the input power level as PIN=VIN2/ ROFF, one can plot the resulting linear relationship between the input power level of compression (in dBm) and the applied bias and use the characteristics obtained in this way for calculating by extrapolating a figure of merit for PIN switch power handling capability. The 1dB isolation compression point was found in this case to be ~18dBm for reverse bias of -5V and ~27dBm for reverse bias of -10V. Conclusions Single-pole single-throw W-band monolithic integrated switches have been realized using one and two InGaAs/InP PIN diodes. Conventional SPST switch with a single diode showed 24dB isolation, while high-isolation SPST switch with two diodes demonstrated isolation larger than 35dB. The single-diode switch had a low insertion loss of 0.9dB and the double-diode switch − 1.8dB at the operation frequency. These results indicate suitability of the SPDT switch with two shunt diodes for applications where high-isolation transceiver switch characteristics are necessary.
Acknowledgments This work is supported by URI (Contract No. DAAL03-92-G-0109), MURI (DAAH04-96-10001), and Daimler Benz AG. References [1] A.Colquhoun and L.P.Schmidt, “MMICs for automotive and traffic applications”, 1992 GaAs IC Symposium, pp.3-6 [2] J.-E.Mueller, A.Bangert et. al., “A GaAs HEMT MMIC chip set for automotive radar systems fabricated by optical stepper lithography”, 1996 GaAs IC Symposium, pp.189-192 [3] D.Teeter, R.Wohlert, B.Cole, G.Jackson, E.Tong, P.Saledas, M.Adlerstein, M.Schindler, S.Shanfield, “Ka-Band GaAs HBT PIN diode switches and phase shifters”, 1994 MTT-S Symposium Digest, pp. 451-454 [4] J.Putnam, M.Fukuda, P.Staecker, Y-H.Yun, “A 94 GHz monolithic switch with a vertical PIN diode structure”, IEEE 1994 GaAs IC Symposium, pp. 333-336. [5] K.W.Kobayashi, L.T.Tran, S.Bui, J.R.Velebir, A.K.Oki, D.C.Streit, “Low power consumption InAlAs-InGaAs-InP HBT SPDT PIN diode X-band switch”, IEEE Microwave and Guided Wave Letters, 1993, vol. 3, NO. 10, pp. 384-386. [6] E.Alekseev, D.Pavlidis, J.Dickmann, T.Hackbarth, “W-band InGaAs/InP PIN diode monolithic integrated switches”, 1996 GaAs IC Symposium, pp. 285-288
