Measurement of Capacitances on Semiconductors

Measurement of capacitances is slowly becoming a very important aspect in production testing. It can be measurement on a diode (Cj), on a MOSFET (Ciss, Crs, Coss) or on an IGBT (Cies, Coes, Cres). The requirement is not just simple measurement of capacitance, more often now also voltage bias is needed in combination with the measurement of capacitance. For this requirement, we have developed a universal LCR adapter for bias up to 600V. In combination with 1Mhz LCR bridge (like E4980AL-102 from Keysight) we are able to measure biased capacitances in few milliseconds (diode 10-20ms) on our Effitest platform in production.

Features

• Measurement with bias up to 600V
• Up to 1MHz (depends on used LCR bridge)
• Open & Short calibration on demand from our SW SCADUS
• Correction of cables connections
• Full capacitance measurement in few milliseconds
• Virtual oscilloscope

Measurement of Capacitances (C iss, C rss, C oss) on a MOSFET

[1] Fig. 1 - Capacitances on a MOSFET

[1] In a MOSFET, the gate is insulated by a thin silicon oxide. Therefore, a power MOSFET has capacitances between the gate-drain, gate-source and drain-source terminals as shown in the figure above.
Ciss is the input capacitance, Crss is the reverse transfer capacitance, and Coss is the output capacitance. Capacitances affect the switching performance of a MOSFET – Miller capacitance (plateau). The gate-drain capacitance Cgd and the gate-source capacitance Cgs are mainly determined by the structure of the gate electrode, while the drain-source capacitance Cds is determined by the capacitance of the vertical p-n junction.
For the power MOSFET, the input capacitance (Ciss=Cgd+Cgs), the output capacitance (Coss=Cds+Cgd) and the reverse transfer capacitance (Crss=Cgd) are important characteristics.Power MOSFET Electrical Characteristics.
Switching characteristics of a MOSFET mainly vary with the input capacitance Ciss and the output impedance of the drive circuit. Gate current flows from gate to source instantaneously to charge the input capacitance. Therefore, the lower the output impedance of the drive circuit, the faster the switching speed. Large input capacitance of a MOSFET causes a large power loss at light load. Ciss, Crss and Coss hardly vary with temperature.

References:

[1] Power MOSFET Electrical Characteristics Application Note, 2018-07-26, 2017-2018, Toshiba Electronic Devices & Storage Corporation