The power gain is usually the parameter most limited by the frequency, since the devices normally are operated at the frequency-limited portion of the characteristics with a (theoretical) 10 dB/decade degradation.

The gain at RF can be described by the following relationship [3]:

where G ₀ is the zero-frequency gain ( ? or h _FE) and f _max is the maximum oscillation frequency, or the frequency where the power gain is equal to 1.

A plot of h _FE versus G is shown in Figure 3 for different f _max values at f=1 GHz. From this plot it can be concluded that a high f _max and a not too low ? are detrimental for a good RF power gain. If ? is higher than required for a constant RF gain, the device will be sensitive to parasitic oscillations, and will have a decreased BV _CEO. High ? also usually means high base resistance, which will lower the f _max and consequently the RF power gain. An approximate expression for f _max is:

where f _T is the transition frequency, (or the frequency where the current gain is equal to 1), R _b is the base resistance and C _bc is the collector-base capacitance. f _T is a function of delay and charge times in the vertical C/B/E structure of the transistor. It is dominated by the base...