HIGH POWER RF LDMOS TRANSISTORS FOR AVIONICS APPLICATIONS
This article describes the inherent advantages of using laterally diffused metal oxide semiconductor (LDMOS) amplifiers in high power avionics transponder applications. In comparing this technology to bipolars, key device features such as gain, linearity, switching, thermal issues and reduced parts count become clear.
The continuous growth of air traffic adds new safety and efficiency challenges, which impact the design of the transponder. Traditional ground-based air traffic control systems excel in managing incoming/outgoing flights but lack the real-time performance needed by airborne traffic collision avoidance systems (TCAS). Onboard every commercial and military aircraft, these transponders transmit and receive essential data such as forward speed, altitude and position coordinates to other aircraft occupying similar airspace. These data are disseminated by the transponder and provide the pilot with instructions to safely direct the flight path. In addition, flight crews depend on other safety features such as weather radar, distance measuring systems, navigation and communications.
As the number of aircraft safety tools increases, space becomes a premium within the electronics bay. System redundancy also detracts from available room. Because TCAS shares the same frequency band as other critical systems, integration of multibox units into one has become the most recent design criterion. Consolidation offers reduction in overall size and weight, enhancing installation and maintenance. A multibox concept also reduces the number of power supplies and related circuitry, improving overall power efficiencies. Using a single multifunction transponder will lower the costs of procurement, installation and maintenance.
With multicarrier operation, managing critical performance factors such as pulse shapes can be difficult. Pulse on/off transition times, linearity and compression come into play. Maximizing overall power-added efficiency and reducing surrounding ambient bay temperatures by using less DC current are also a concern. This article describes devices that greatly improve the performance of microwave power amplifiers employed in TCAS transponders using the latest LDMOS technology (LDMOST). Traditional common-base-configuration class C bipolar transistors vs. linear, higher gain, lateral structure FETs will be examined.
LDMOST was originally designed for use in GSM and PCS base station cellular sites. Minor optimizations in the technology have made the structure extremely suitable for avionics applications. The lineup discussed in this article features a 200 W output part using an all-gold metal system. Enhancements were made to the gates for this application as well as input and output matching over the 1030 to 1090 MHz frequency band.
Fig. 1 Typical output vs. input power for a 200 W bipolar transistor.
Fig. 2 Gain and efficiency vs. output power for an LDMOS power device at 150 mA quiescent current.
Fig. 3 Output power vs. modulation voltage, Vgs, for 7.95 W input power at 1030 MHz.
Up to now amplifiers were designed with bipolar devices, which posed a number of problems when designing the complete lineup after taking into account the previously mentioned requirements. The most significant of these requirements is the gain linearity of the output power over a large dynamic range. A typical plot of the output power vs. input power of a 200 W bipolar transistor, intended for use in an avionics application, is shown in Figure 1. It can be seen that the gain varies over the entire range of the input power. Obviously, using such a device has serious implications for the gain linearity of the entire lineup.
Fig. 4 Gain vs. output power for two values of quiescent current.
Fig. 5 Trace of input and output pulse showing insignificant switching deterioration.
A device using LDMOST was utilized to overcome this problem. With this technology, devices can be made that show excellent linearity over a large dynamic range. The gain and efficiency vs. output power for such a LDMOS device is shown in Figure 2. At 200 W, the device is still far from saturation with a dynamic range of more than 30 dB. Moreover, the device has a gain of 14 dB compared to the typical 8 dB of bipolar devices. This performance means that a considerable reduction in both the number of components and the PCB space can be achieved for the complete lineup. In addition, overall power-added efficiency is improved.
Additional advantages of LDMOST compared to bipolar technology include excellent thermal stability due to the negative temperature coefficient of the die technology. Thermal runaway is not an issue, and excellent ruggedness (SWR < 6) can be achieved due to the high breakdown voltage (80 V, typ). In addition, pulse shaping in LDMOST using the Vgs is relatively easy compared to bipolar devices. Figure 3 shows the easy gain control that is possible by simply modulating the Vgs of the device. The package does not contain toxic BeO. Removal of the BeO DC isolator, the source terminal of die that is attached directly to the heat spreader (flange), reduces thermal impedance.
As shown in Figure 4, the linearity of the device depends on the quiescent current. The two plots depict the gain at different quiescent current: 500 mA and 2 A. Setting a higher quiescent current means biasing the device more toward class A. Consequently, the maximum feasible efficiency will be lower. Such a bias condition is required only at low output power levels. At higher output power levels a class AB biasing is preferred. An optimum trade-off can be made between output power, linearity and efficiency simply by modulating the gate source voltage reciprocal to the required output power. By designing a device with a high power capability, no decrease in linearity will occur due to the device going into compression. In bipolar technology, fast switching times at high output powers are often difficult to achieve, a characteristic that is inherent in the bipolar structure.
Figure 5 shows that the rise time of the device is excellent while maintaining good gain linearity over a large dynamic range. It can be seen that the switching time has not increased significantly between the input signal and output signal through the device and that values of less than 50 ns are achieved at an output power of 52.4 dBm. Obviously the design of the biasing will have an influence on the behavior of the device. Good decoupling of low frequency components is essential.
RELIABILITY AND THERMAL BEHAVIOR
Fig. 6 Temperature distribution in an SOT502 package at t = 1.2 ms, h = 50%, Pdiss = 250 W and Ths = 20?C.
Fig. 7 Temperature profile in an SOT502 package at t = 1.2 ms, h = 50%, Pdiss = 250 W and Ths = 20?C.
Fig. 8 Wirebonding of a single LDMOST die.
One big advantage of LDMOST compared to bipolar is that the mode of operation is common source (source connected to ground), reducing parasitic source inductance and providing an excellent thermal interface. In bipolars the bulk of the material is the collector, which must be electrically isolated from ground (usually the heatsink) without degrading the thermal impedance. This configuration produces a better thermal behavior of the LDMOST compared to the bipolar.
Figures 6 and 7 show the results of thermal transient finite element method simulations for the die. In the simulations, a uniform distribution of power is illustrated under RF excitation conditions. This uniformity will be dominated by the quality and consistency of the assembly process of the device, which depends largely on the use of automated die attach and wire bonding equipment (minimizing lot-to-lot variation). The wire bonding of a single LDMOST die is shown in Figure 8.
Fig. 9 Cross section of a 1 GHz LDMOST device.
Fig. 10 A 200 W amplifier lineup.
Figure 9 shows the cross section of an LDMOST structure. The p+ via is used to ensure good electrical contact between the source contact and ground and eliminates the use of source bonding wires, which lowers the source inductance and increases the intrinsic device gain. A shielding is placed between the gate and drain to reduce the feedback capacitance. The die technology used in this 200 W device is based on 1 GHz technology optimized for the specific requirements of an avionics application that required enhanced gate periphery. The design is optimized to meet the required gain linearity over a large dynamic range. This optimization will improve both gain and intermodulation distortion performance as well as the RF stability of the device.
The MonoMOS structure - the separate active areas of a conventional high power RF die replaced by one highly integrated area - lends itself to easily incorporating internal input and output matching. MonoMOS uses metallized gates to reduce series resistance in order to increase power gain. The structure was designed for minimum Idq drift. Without any burn-in, the typical Idq drift is below 10 percent over 20 years. The gold top metallization, in combination with the gold bond wires, avoids intermetallic issues where the flexibility of gold compared to aluminum bond wires ensures excellent reliability under pulsed operation. The concept of this die has proven to be very reliable based on high volume production for base station products. In order to provide a cost-effective approach, the device uses a commercial, off-the-shelf, nonhermetic package.
The total lineup of a 200 W LDMOS amplifier is shown in Figure 10. The overall gain of 46 dB is achieved using three amplifier stages. A comparable bipolar lineup would consist of approximately six amplifier stages (depending on individual device gain) to achieve similar overall gain. Figure 11 shows the application circuit for the 200 W device used for the described characterization. This device contains internal input and output matching, which simplifies the design of the application circuit. The internal matching brings the device impedances to a level that is favorable in a manufacturing environment. Higher impedances create a situation that is less critical for component placement and substrate tolerances, enhancing final yield while eliminating rework.
Fig. 11 Application circuit of a 200 W LDMOS device for the 1030 to 1090 MHz band (er = 6.15).
It has been demonstrated that the use of the LDMOS transistor greatly improves the design of amplifiers for avionics applications. Transponders using LDMOST will enjoy gain linearity over a high dynamic range, easy gain control using the gate voltage and an almost zero switching time delay between input and output. Other features of this technology include better thermal control, nontoxic packaging and reduced parts count. The potential cost savings due to lower power supply costs, a gold metallization system, and automated die attach and wire bonding equipment ensure device consistency in high volume production. This consistency will enhance customer yield and reduce rework.