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Number 3140 Performing Very Low Frequency Capacitance-Voltage Application Note Measurements on High Impedance Devices Using the Series Model 4200-SCS Semiconductor Characterization System Introduction Capacitor Capacitance measurements on semiconductor devices are usually made using an AC technique with a bridge-type instrument. These AC instruments typically make capacitance and impedance Test Device Force HI Force HI measurements at frequencies ranging from megahertz down to possibly tens of hertz. However, even lower frequency SMU1 SMU2 A A capacitance measurements are often necessary to derive specific smu_src smu_sense test parameters of devices such as MOScaps, thin film transistors (TFTs), and MEMS structures. Low frequency C-V measurements are also used to characterize the slow trapping and de-trapping Force LO phenomenon in some materials. Instruments capable of making (Internally Connected) quasistatic (or almost DC) C-V measurements are often used for these low frequency impedance applications. However, the Model SMU1 with preamp: SMU2 with preamp: 4200-SCS Semiconductor Characterization System uses a new Outputs DCV with Measures AC current superimposed ACV and at 0V DC. narrow-band technique that takes advantage of the low current measures AC voltage. measurement capability of its integrated source measure unit (SMU) instruments to perform C-V measurements at specified low frequencies in the range of 10mHz to 10Hz. This new method is Figure 1. Connections for very low frequency C-V measurements. called the Very Low Frequency C-V (VLF C-V) Technique. Basically, while the voltage is forced, voltage and current The VLF C-V Technique makes it possible to measure very measurements are obtained simultaneously over several cycles. small capacitances at a precise low test frequency. This patent- The magnitude and phase of the DUT impedance is extracted pending, narrow-band sinusoidal technique allows for low from the discrete Fourier transform (DFT) of a ratio of the frequency C-V measurements of v