Figure 1. Photograph of W-band monolithic InGaAs PIN diode SPST switch
Graduate Student: Egor Alekseev
Professor Dimitris Pavlidis
Support: URI DAAL03-92-G-0109, MURI DAAH04-96-1-0001, and Daimler Benz
The SPST switch employs a section of high-impedance (Zo=85 Ohm) microstrip transmission line, which was connected to two 50-Ohm coplanar microwave probe pads and shunted in the middle by a InGaAs/InP PIN diode. When the diode is in the OFF-state, the total diode capacitance (Ctot = Coff + Cpar), the airbridge/via-hole inductance, and the sections of microstrip line form a bandpass filter at the design frequency. The switch uses a single shunt stub filter. Given the fact that pass frequency of the ON-state filter is sufficiently higher than the design frequency, the incoming signal is reflected back at the diode and only small portion of it, proportional to the voltage across the turned-on diode, propagates to the output port.
Figure 1. Photograph of W-band monolithic InGaAs PIN diode SPST switch
The monolithic PIN W-band SPST switches were fabricated using a technology specially developed for this purpose. Fig.1 shows a photograph of the monolithic chip, which is 1.5 x 0.6 mm2. The switches employed InGaAs PIN diodes fabricated on InP substrates. A bias network in distributed form is also shown in the photograph. It was integrated on-chip and does not require the use of bypass capacitors. The bias network consisted of a microstrip open radial stub with a quarter-wave impedance transformer and allowed decoupling of the DC biasing pad from the high frequency signal path. The size of the radial stub was optimized for maximum bandwidth. The SPST switches used one backside via hole for shunting the diode and four via holes to form coplanar-to-microstrip mode transitions at the microwave probe pads.
Figure 2. SEM of 10µm-thick InGaAs/InP PIN diode
The InGaAs PIN diodes were made from the following layers starting from the SI InP substrate: n+ (1µm, 1.4x1019 cm-3), i (1µm, ~1014 cm-3) and p+ (0.15µm, 1.4x1019 cm-3). The layers were grown using solid-source MBE. The growth rate was 0.7µm/hr, and a 380-Å undoped AlInAs buffer was used between the substrate and the diode layers. In order to obtain an abrupt doping profile from the n+ to i-region and to assure a low background doping in the i-InGaAs layer, the growth temperature was kept at a low value of about 450oC. The diodes were fabricated by employing wet etching to form 10µm-diameter mesas. Airbridges were used to connect the top p-ohmic metal contacts to the interconnect lines of the circuit. A scanning electron microscope photograph of a fabricated PIN diode is shown in Fig.2.
Figure 3. I-V characteristics of InGaAs/InP PIN diode
DC characterization of the PIN diodes showed a high reverse breakdown voltage of 17V and a low turn-on voltage of 0.36V (@ I = 10µA), as demonstrated in Fig.3. Discrete PIN diodes were characterized initially from DC to 26 GHz and later directly at W-band to verify their characteristics with higher precision. The measurement setup used for this purpose consisted of an HP 8510B network analyzer and a millimeter-wave waveguide test set with WR-10 waveguides and W-band coplanar probes for on-wafer characterization of the devices up to 110 GHz.
On-wafer characterization of the integrated InP-based monolithic PIN switch was conducted from 75 to 100 GHz using the setup described earlier. The switches demonstrated state-of-the art performance. Typical response characteristics in the ON and OFF states are shown in Fig.4.
Figure 4. W-band SPST switch S-parameters measured on-wafer.
For design and analysis purposes, the switches can be considered as a high-impedance microstrip line (Zo = 85 Ohm) shunted by an InGaAs PIN diode.When a diode is OFF, its impedance can be approximately modeled by a small depletion capacitance. In the OFF-state, its impedance is much higher than Zo, and the injected signal passes through the switch with only a small insertion loss. The latter was measured to be only 1.3 dB at 83 GHz for the switch presented in this work. When the diode is turned ON, its impedance may be modeled as a small resistance plus the inductance of the airbridge and the via hole providing the ground connection. If the total diode ON-state impedance is much smaller than Zo, the signal is shunted to ground and reflected back with a small reflection loss. This was measured to be as low as 0.8 dB in case of the SPST switches described in this paper. A small portion of the signal also leaks to the isolated port and was measured to correspond to an isolation of 25 dB. The minimum VSWR was 1.1, and its value was less than 2 over a 5.4-GHz bandwidth, demonstrating good matching in the OFF-state. Tests of the integrated radial stub bias network confirmed that its parasitics does not interfere with the switch performance.
