Radio Frequency Integrated Circuit Design - Webs

Figure 10.42 High-Q matching circuit.
Power Amplifiers
and collector stripes (multiple fingers), as shown in Figure 10.43, as well as
multiple transistors distributed to reduce the concentration of heat and to reduce
the current density. However, the use of transistors with multiple fingers and
multiple transistors introduces the new concern of making sure each finger and
each transistor is treated the same as every other finger and transistor. This is
important in order to avoid local hot spots or thermal runaway and to make
sure that the connection to and from each transistor is exactly the same to avoid
mismatch of phase shifts. Power combining will be discussed further in Section
10.17, and thermal runaway is discussed further in Section 10.18.
Also, for such high currents, metal lines have to be made wide to avoid
problems with metal migration, as described in Section 5.6.
As for transistors, a large transistor cannot become too long or it will not be
able to handle its own current. With transistors, the current handling capability is
directly proportional to emitter area. Thus, as the emitter length is doubled,
the current is doubled. However, if line width stays the same, the maximum
current capability is the same. As an example, a transistor that has emitters that
are 40 �m long and has 1-�m-wide lines can only handle about 1 mA. However,
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