Radio Frequency Integrated Circuit Design - Webs

44 Radio Frequency Integrated Circuit Design
Figure 3.1 Planar bipolar transistor cross-section diagram.
formed inside the base region. The basic transistor action all takes place directly
under the emitter in the region shown with an oval. This can be called the
intrinsic transistor. The intrinsic transistor is connected through the diffusion
regions to the external contacts labeled e, b, and c. More details on advanced
bipolar structures, such as using SiGe heterojunction bipolar transistors (HBTs),
and double-poly self-aligned processes can be found in the literature [1, 2].
Note that although Si is the most common substrate for bipolar transistors, it
is not the only one; for example, GaAs HBTs are often used in the design of
cellular radio power amplifiers and other high-power amplifiers.
Figure 3.2 shows the transistor symbol and biasing sources. When the
transistor is being used as an amplifying device, the base-emitter junction is
forward biased while the collector-base junction is reverse biased, meaning the
collector is at a higher voltage than the base. This bias regime is known as the
forward active region. Electrons are injected from the emitter into the base
region. Because the base region is narrow, most electrons are swept into the
collector instead of going to the base contact. This is equivalent to conventional
(positive) current from collector to emitter. Some holes are back-injected into
the emitter and some electrons recombine in the base, resulting in a small base
current that is directly proportional to collector current ic = �i b . Thus, the
overall concept is that collector current is controlled by a small base current.
The collector current can also be related to the base-emitter voltage in this
region of operation by
Figure 3.2 Bipolar transistor symbol and bias supplies.