Transistor Intrinsic Capacitance

Transistors have internal capacitances that are usually not wanted and act to reduce the gain at high frequencies. In BJT's these intrinsic capacitors come from the construction of the device itself and are usually associated with its internal PN junctions. In BJT's we usually label $C_{\mu}$ as the capacitance associated with the base-collector junction, and $C_{\pi}$the capacitance associated with the base-emitter junction. These capacitances are illustrated in Fig. 4.10.

  

Figure 4.10: Equivalent Circuit Showing BJT Intrinsic Capacitances

In the right side of the figure we have drawn the small signal equivalent circuit, which is usually very useful for analyzing the effect of this response.