A Monolithic High Power, High Linearity, Multi-Octave PIN Diode T/R Switch
By T. Boles, J. Brogle, R. Hubert, M/A-COM

I. Introduction
The use of PIN diodes for high power, greater than a few watts up through 100s of watts, switching of RF signals, especially when low distortion and high linearity are required, ranging from HF through mmW frequencies, has been a mainstay of the high frequency industry since the 1950s. These high power switching functions have almost exclusively been realized using discrete PIN diodes as the switching circuit elements. The typical switch designs employed include PIN diodes that are interconnected via standard printed circuit board technologies in shunt, series, and series-shunt configurations, both reflective and absorptive topologies, and have varied from single-pole-single-throw through multi-pole-multi-throw configurations.

In terms of the ability to handle high incident power in these hybrid switch realizations, two basic approaches can be employed. The first is to utilize an all–shunt design in which the only dissipated RF power in either the “on” or “off” state is in the metal interconnections on the PCB. However, employing this shunt design approach limits the high frequency bandwidth to approximately one octave. The second approach, which does not have this 2:1 bandwidth limitation, utilizes a series or series-shunt switch topology, but requires that the discrete series diodes in either approach, which are in a dissipative state when the diode is “on”, must be tied to a good heat sink on the PCB.

In attempting to follow the general semiconductor industry trend toward integration, a number of solutions to monolithic high power PIN diode based switches have been attempted since the 1970s. With the monolithic switch approaches, the designer has been forced into a choice of power handling and linearity over bandwidth.

For incident power levels greater than one watt, an all–shunt diode design, where the electrical ground also serves as the switch thermal ground, is required, with the concurrent <2:1 frequency bandwidth limitations. Similarly, as in the discrete PIN diode PCB switch approach, the only dissipated RF power in this monolithic configuration, in either the “on” or “off” state, is in the metallized transmission lines on the integrated component.

Alternately, when greater than an octave or multi-octave instantaneous frequency response is required, a PIN diode monolithic switch requires that a series or series-shunt topology be employed. In either of these configurations, the series diode must be electrically isolated, both from a DC and RF perspective, from the ground plane. In a typical monolithic PIN switch design, this DC and RF isolation is accomplished via various insulating materials, such as unfilled epoxies, silica filled epoxies, polyimide, BCB, glasses, or, in some instances, even air. While these materials all provide good electrical isolation properties, the series diode is essentially thermally open circuited, severely limiting the incident RF power handling of the switch.

In this paper, a monolithic PIN diode Transmit/Receive switch structure, the MASW-000822, based upon patented Heterolithic Microwave Integrated Circuit technology, capable of both multi-octave frequency performance, the ability to handle greater than 8 watts of incident RF power and able to simultaneously provide very low RF distortion and high linearity, will be presented.

II. Discussion
The development of HMIC, which is an acronym for Heterolithic Microwave Integrated Circuit, technology was initiated in the early 1990s to create a high frequency integration medium capable of producing high Q passive elements, inductors, capacitors, and controlled impedance transmission lines, as well as high performance microwave and mmW active components, specifically silicon PIN, Schottky, and varactor diodes. This integration is accomplished via a marriage of silicon, which produces the high performance active elements, and a borosilicate glass having a low high frequency loss tangent, which enables the creation of high Q, passive structures, resulting in a number of monolithic components aimed at various RF, microwave and mmW applications. Since HMIC combines silicon and glass at a waferscale level, it is a true MMIC technology that enables complex high frequency circuits to be fashioned using standard semiconductor fabrication techniques.

A cross section of HMIC technology as applied to both series and shunt configured PIN diodes is shown in Figure 1. As can be seen, a shunt diode is easily realized with the anode, cathode, and “I” region contained within a silicon pedestal. The cathode of the PIN diode serves as both an electrical contact to the ground plane and, since silicon has a thermal conductivity approximately one third that of gold, as a low thermal resistance heat spreader. In addition, HMIC can provide a simultaneous cathode contact on the top surface of the via, enabling more complex switch configurations to be realized.

The formation of the series diode in this technology is a bit more complex. The series diode is initially identical in formation to the shunt diode. Since the insulating low loss borosilicate glass is transparent, a front-to-back alignment is a relatively simple matter, allowing the local removal of a portion of the silicon cathode. The cavity created by this selective removal is then packed with a low dielectric constant, high field strength, silica filled epoxy. This epoxy works very well as a DC and RF isolation medium. The only limitation with this switch design approach lies with the silica filled epoxy used to isolate the series diodes from the RF ground plane. The silica filled material is essentially a thermal open, and the only heat sinking that is occurring in the series diode is a result of heat flow along the metallization structures to the nearest silicon via.

In order to address this restriction on the thermal behavior of the series PIN diodes in the HMIC structure, new offerings of electrically isolating but thermally conductive epoxies have been investigated. Several boron nitride, BN, filled epoxies have proved to be 20 to 100 times more thermally conductive than the silica filled versions from multiple vendors, while simultaneously maintaining the desired DC and RF properties of high field strength, low dielectric constant, and virtually identical RF performance.

III. Results
In addition to the design goals for the MASW-000822 PIN diode switch to both handle high incident power and multi-octave bandwidth, the requirements to operate the PIN diode switch from a single low voltage, <5.0 volts, positive supply, in a small size, and at the lowest cost possible were imposed. In order to simultaneously accomplish all of these goals, a common anode, series only geometry was chosen as the vehicle for the MASW-000822 switch design. A circuit schematic of this design approach is shown in Figure 2.

In keeping with the low cost requirement, the packaging medium chosen for this high power switch is a 3mm x 3mm PQFN surface mount plastic encapsulated package, which has a large central, thick copper ground paddle providing both good isolation and excellent heat sinking. The only drawback to this packaging approach initially was the length of the required input and output wirebonds, which degraded the return loss of the switch to as low as approximately 9 dB at 6 GHz. To remedy this problem, discrete capacitors were placed into the PQFN package adjacent to the input and output ports of the switch, forming a series L-shunt C-series L low pass matching structure. It can seen in Figure 3, a plot of Return Loss vs. Frequency for the MASW-000822, that the use of the low pass matching network has greatly improved the port matches to the switch and the return loss is now better than 25 dB through 6.0 GHz

In Figure 4, a plot of insertion loss and isolation for both the transmit and the receive arms of the MASW-000822 monolithic switch versus frequency are presented. It can be seen that, as expected from a series only design, this monolithic switch is capable of performing quite well from 500 MHz to 6.0 GHz over approximately 3.5 octaves of RF bandwidth.

In terms of power handling, the use of the BN filled epoxy provides a greatly improved capability over the previously used silica filled epoxies. As shown in Figure 5, an incident power at 3.2 GHz of +39.1 dBm, or 8.1 watts, is able to be supplied to the MASW-000822 switch before the maximum safe operating junction temperature of 175o C is reached at an ambient heat sink temperature of 25o C. As the RF incident power is increased further, a junction temperature of 225o C is reached at a drive power of +40 dBm, or a 10 watt drive level.

In terms of linearity, the monolithic HMIC MASW-000822 switch was evaluated using EVM, Error Vector Magnitude techniques. In WiMax, an OFDM (Orthogonal Frequency Division Multiplexing) modulation format is employed, which is a spread spectrum waveform that looks similar to a multi-tone signal. In this modulation scheme, a perfectly linear signal has ~ 0% EVM. As non-linearities are introduced, the EVM starts to increase. The change in EVM % represents the percent change in the vector signal. The EVM percentage then can be correlated to the Bit Error Rate in the system application.

The EVM % is plotted versus incident power for the MASW-000822 monolithic PIN diode switch and is shown in Figure 6. In this plot, it can be seen that a 1.0% EVM level is achieved at an incident power of +39 dBm.

IV. Conclusion
It has been demonstrated that a fully monolithic PIN diode based T/R switch, the MASW-000822, capable of simultaneously delivering multi-octave frequency performance, reliably handling incident power levels in excess of 8 watts, and providing excellent linearity and low signal distortion in complex modulation formats, was achieved utilizing a patented HMIC integration technology. This technology has the ability to produce low loss series PIN diodes that have both high electrical isolation from the ground plane and, by means of high thermal conductivity epoxies, a low thermal impedance to maintain a reliable peak junction temperature. Both of these features are critical to the excellent switch characteristics exhibited by this series only PIN diode switch.

References
1. J. Goodrich and C. Souchuns, “PIN diode and method for making same.” US Patent No.
    5,268,310.
2. J. Goodrich and C. Souchuns, “HMIC – A Fully Capable Silicon Microwave Integrated
    Circuit Process,” Microwave USA Conf., Tokyo, Sept., 1993.
3. M/A-COM Semiconductor Division, “HMIC: A Silicon Microwave Integrated Circuit Process,”
    Microwave Journal, Vol. 37, no. 1, pp. 136-138, 1994.
4. T. Boles and J. Goodrich, “Heterolithic Microwave Integrated Circuits,” US Patent No.
    6,114,716.

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