Switching characteristics of BJT are shown in figure. Due to the presence of internal capacitances BJT cannot be turned instantly. Initially when base emitter voltage is applied in CE configuration leakage current flows. The collection current begins to rise after some delay time and reaches at its steady state value. the time from which collector current starts rising to its steady state value known as rise time. BJT gets turned on in and collector emitter voltages falls from its maximum value forward voltage drop.
Till the forward base current is maintained the BJT remains in on condition. As the base emitter voltage is removed at a time t1 does not change for a time ( there its is storage time in which the charges are removed from the base). After this time Ic begins to fall and collector voltage starts rising for a time tf ( fall time ) . in fall time decreases to leakage current and voltage rises to vcc. Hence the turn off time of BJT is t off = ts + tf. In conduction period of BJT a steady state current flows and forward voltage drop appears across the collection emitter junctions.