How do we find the values of junction capacitances? Well, we re fortunate if manufacturers data is available. For instance, for the 2N3904 transistor, capacitance data is available for reverse bias voltages in excess of 0.1 volts (i.e., junction voltages less than 0.1V) on both the emitter-base and collector-base junctions, as shown in Figure 10-6.

Figure 10-6: Manufacturer s datasheet ⁵ tabulation of junction capacitance of 2N3904 transistor.

What do we do if we need charge data for positive voltages on the base-emitter junction? An NPN transistor is cutoff for V _BE less than 0.4V or so. The datasheet doesn t give us capacitance values for positive V _BE. Fortunately, there is a little bit of physics that s available to us. The junction capacitance of a PN junction is given by:

C _jo is the junction capacitance at a junction voltage V _j = 0; V _BI is the built-in voltage characteristic of the junction and is given by:

where N _A and N _D are the doping levels on the P and N sides, and n _i is the intrinsic minority carrier concentration. A typical value for V _BI is 0.8V at normal doping levels. The factor m depends on the type of junction, and is 0.5 for an abrupt junction and 0.333 for a linearly graded junction.

A curve fit for the emitter-base junction capacitance for the 2N3904 transistor is given in...