On the determination of minority carrier lifetime from the transient response of an MOS capacitor

Minority carrier lifetime greater than 10^-10seconds can be determined for silicon at room temperature by observing the transient response of the MOS capacitance after the application of a large depleting voltage. The waveform is exponential for heavily doped samples and nonexponential for lightly doped samples. The transition occurs when the oxide capacitance approximately equals the space-charge capacitance. Results are presented for a lightly doped bulk silicon wafer exhibiting an effective minority carrier lifetime of 7.6 microseconds and a thin silicon-on-sapphire film with a lifetime of 4.5 nanoseconds. The ratio of transient time constant to lifetime is typically 10⁵-10⁸at room temperature. Lower lifetime may be determined by cooling the sample. A graphical method is presented to rapidly extract lifetime from the transient response of lightly doped samples when the waveform is nonexponential.