The bipolar and MOSFET components of a symmetric IGBT
are shown in Fig. 7.16. The components between the emitter
(e), base (b), and collector (c) terminals correspond to the
bipolar transistor and those between gate (g), source (s), and
drain (d) are associated with MOSFET. The combination of
the drain-source and gate-drain depletion capacitances is
identical to the base-collector depletion capacitance, and
therefore they are shown for the MOSFET components. The
gate-oxide capacitance of the source overlap (Coxs) and source
metallization capacitance (Cm) form the gate-source capacitance
(Cgs). When the MOSFET is in its linear region the gateoxide
capacitance of the drain overlap (Coxd) forms the gatedrain
capacitance (Cgd ). In the saturation region of MOSFET
the equivalent series connection of gate-drain overlap oxide
capacitance and the depletion capacitance of the gate-drain
overlap (Cgdj) forms the gate-drain Miller capacitance. The
gate-drain depletion width and the drain-source depletion
width are voltage dependent, which has the same effect on
the corresponding capacitances.
The most important capacitance in IGBT is the capacitance
between the input terminal (g) and output terminal (a),
because the switching characteristics are affected by this feedback