|RF switches: MEMS, PIN diode|
|Thursday, 16 November 2006|
RF switches come in a variety of configurations. The basic one is a single pole single throw (SPST), containing a single RF input and RF output. Others RF switches come in single pole multiple throw (SPMT) configurations. A typical RF switch contains two ports or outputs, which are separately controlled, that are fed from a single input port, and is referred to as a single pole double throw (SPDT) configuration. RF A/B switches are typically used to switch an input of an electronic receiver between two or more RF inputs. Such switches may be external of or internal to the receiver. External A/B switches usually comprise a housing having at least A and B inputs and an output that is coupled to an RF input of the receiver. Internal A/B switches perform essentially the same function of switching a receiver between two or more RF inputs. Internal A/B switches are usually electronic in nature and often respond to a remote control in order to select one of a plurality of inputs. Conventionally, RF switches consisting of solid state devices, such as diodes and field-effect transistors (FET) are used in communication systems applications. For very high frequencies of about 1 GHz, these diode and FET devices are typically fabricated using GaAs technology. Available solid state RF switches are generally made using galium arsenide (GaAs) processes. The GaAs and possibly the silcon/germanium processes are accepted as having higher frequency responses than silicon processes. GaAs chips can be made reasonably small and handle sufficient power with switch characteristics that have made these processes acceptable L-band switches. PIN diodes are semiconductor devices which can be made to operate at relatively high radio frequencies and which function essentially as switched resistors, having a high or low resistance value depending upon the value of the biasing characteristics. PIN diodes are typically fabricated in Si and GaAs. Often these devices are combined in series or shunt configurations to produce multipole-multithrow devices. Series configurations are used when minimum insertion loss is required over a broad frequency range. One of the more serious drawbacks of such switches is the necessity to provide a constant DC current. These devices typically require a relatively large reverse bias voltage to present a high impedance value, and draw a substantial forward current when forward biased to present a low impedance value. Moreover, using discrete PIN diodes increases both the size and the cost of the RF switch arrays. PIN diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value less than 20 dB. PIN diodes and transistors have a limited frequency response and typically only respond to frequencies below 20 GHz. In addition, the insertion losses and high isolation value for these switches vary depending on the frequency of the signal passing through the switches.
Recently, microelectronic mechanical (MEMS) technology has been used for the fabrication of RF switches. Microelectromechanical systems (MEMS) are miniature devices that are being manufactured in a wide variety of mechanical forms. MEMS devices are inherently both mechanical and electrical devices that are subject to wear and contamination and suffer from limited life times. A micro-electromechanical system (MEMS) is fabricated using semiconductor integrated circuit (IC) fabrication technology. The advent of microelectromechanical systems (MEMS) has allowed the creation of ultra-small switches. A variety of MEMS switches are in use in radar and communication systems, as well as other high frequency circuits for controlling RF signals. These MEMS switches are popular insofar as they can have a relatively high off impedance, with a low off capacitance, and a relatively low on impedance, with a high on capacitance, leading to desirable high cutoff frequencies and wide bandwidth operation. MEMS devices are small in size, and feature significant advantages in that their small size translates into a high electrical performance, since stray capacitance and inductance are virtually eliminated in such an electrically small structure as measured in wavelengths. MEMS switches have a small footprint, can operate at high RF voltages and are compatible with conventional integrated circuit fabrication techniques. The MEMS switches can be constructed as bi-stable devices and are switched by the application of an electrical voltage to an input terminal. Switches fabricated using MEMS technology normally include a substrate with one or more metal traces and control pads. A microelectronic mechanical switch (MEMS) may be constructed which uses electrostatic force to flex a thin membrane and thereby cause the switch to be opened or closed. Since an electrostatic force is used, the switch can be controlled using only a voltage and consumes virtually no power. This is an important advantage for portable communication systems, such as hand-held mobile phones or other wireless communication devices, where power consumption is recognized as a significant operating limitation. In the capacitive type MEMS switch, a dielectric layer is deposited on the first conductor in an area opposite the underside of the two-arm moveable bridge, with this area on the conductor acting as the pull down electrode. With this arrangement, the full pull down voltage appears across the dielectric layer resulting in a relatively high electric field across the dielectric.