Microwave bipolar transistors
For microwave applications, the silicon (Si) bipolar transistors dominate for frequency range from UHF to about S band (about 3 GHz). As the technology improves, the upper frequency limit for these devices is continuously being extended, and at the present time the devices are capable of producing useful power up to 22 GHz. The majority of bipolar transistors of current interest are fabricated from silicon, although GaAs devices offer prospects for improvements in operating frequency, in high temperatures, and in radiation hardness. The Si bipolar transistor is inexpensive, durable, integrative, and offers gain much higher than available with competing field-effect devices. It has moderate noise figure in RF amplifiers and 1/f noise characteristics that are about 10-20 dB superior to GaAs MESFETs. For these reasons, the Si bipolar transistors dominate in amplifier applications for the lower microwave frequencies and are often the devices of choice for local oscillators.
All microwave transistors are now planar in form and almost all are of the silicon n-p-n type. The geometry can be characterized as follows: (a) interdigitated, (b) overlay, and (c) matrix (also called mesh or emitter grid) as shown in Figure 1
Figure 1 Surface geometries of microwave power transistor
For high-frequency applications, the n-p-n structure is preferred because the electron mobility is much higher than the hole mobility . Figure 2 shows two schematic diagrams for a bipolar junction transistor (BJT): (a) the cross section of a discrete n-p-n planar BJT and (b) the cross section of a chip-type n-p-n integrated BJT.
Figure 2: Schematic diagramsof bipolarjunction transistors