Low Loss mm-Wave Monolithic SP4Ts

Many of the emerging broadband wireless access systems operate at mm-wave frequencies where large allocations of spectrum are available. GaAs Monolithic Microwave Integrated Circuits (MMICs) offer a means of realising low cost, high performance, high volume, reproducible mm-wave components. This paper details two Single Pole 4 Throw (SP4T) switch MMICs covering 24 to 34GHz and 34 to 45GHz. The measured insertion loss of the switches is 1.1dB ± 0.3dB and 1dB ± 0.3dB respectively. The two die were fabricated on Triquint Semiconductor Texas’ (TQT) commercially available GaAs Vertical PIN diode (VPIN) process. Good agreement between measured and modelled performance was achieved. Introduction High order multi-way mm-wave switches will allow a single mm-wave transceiver to be routed to one of a number of independent antennas. This will allow the development of low cost nodes for “mesh-network” [1] broadband, wireless access systems. This paper details two SP4Ts fabricated on a commercially available GaAs PIN diode process. They cover the frequency range 24 to 45GHz in two bands. The VPIN process A PIN diode takes its name from its structure; it comprises a region of high resistivity Intrinsic material sandwiched between a region of P-type semiconductor and N-type semiconductor. When the PIN diode is forward biased, charge carriers are injected into the I region lowering its resistance. Thus at RF and microwave frequencies a PIN diode behaves as a current controlled resistor. They are optimised to achieve wide resistance range, good linearity, low distortion, low drive current and high power handling capability. These properties mean that PIN diodes can be configured to make excellent RF/microwave switches and also find applications in variable attenuators and phase shifters. Figure 1 shows an equivalent electrical circuit model for a PIN diode at RF/microwave frequencies. A model for a discrete, packaged diode would also need to incorporate appropriate packaging parasitics. In the case of the monolithic PIN diodes used here, the feed structure was also modelled to ensure accurate simulated performance to mmwave frequencies. The forward current through the diode controls its resistance Rj. For zero or reverse bias, with no current flowing, the resistance Rj is high, in the region of several kΩ. As the forward bias current through the diode is increased, the charge carriers injected into the I region reduce the value of Rj to values as low as one or two ohms, depending on the diode structure.