Abstract :
The development of a microwave power amplifier transistor requires a highly advanced technology to achieve the basic parameters which theory predicts will be needed. In addition, very sophisticated microwave techniques must be used in the package design and characterization. Transitions between the active device and external circuitry often dominate performance. Development was started with a 7 mil2, seven-finger transistor with an output power of 100 milliwatts at 2 GHz. With improved diffusion techniques rb1was lowered and ftraised. As a result, the power output was raised to 150 mw at 2 GHz. The next step in increasing the power output was to interconnect four of the 7 mil2transistors on 10 mil centers using evaporated contracts. While this produced more power, the scaling factor was poor; (i.e a 4:1 area increase raised the power only 2:1). The poor scaling was traced to contact and lead inductance losses. The problem then resolved itself to increasing the device area while still maintaining low thermal resistance, low inductance bonds, and minimum contact area. Results of tests on a single 25 mil2transistor indicated that a larger area device could be scaled successfully provided the problems of heat dissipation and lead inductance could be solved. For the final device these were at least partially resolved by making a long thin device with a base aspect ratio of 10:1. This transistor was able to deliver the required 2 watts peak power at 2.25 Gc with a device area of 90 mil2. Thus, power output was increased 13:1 over the prototype device with an area increase of 14:1. Because of high thermal resistance of the packages used, pulsed operation was required (10% duty cycle) at the two watt level. Thermal and electrical properties of a suitable microwave package, and measurement problems are also discussed.
