Bipolar Transistor Test Structures for Extracting Minority Carrier Lifetime in IGBTs

Vertical PNP bipolar transistor test structures were fabricated and measured, attempting to electrically obtain information on carrier lifetime in the voltage-supporting base region of Insulated Gate Bipolar Transistors (IGBTs). Owing to the structural similarity, the test structures and functional IGBTs can be integrated on the same wafers, making it possible to directly correlate lifetime data and IGBT characteristics. To solve a problem of leaky backside PN junction, common base current gain of the test devices was measured without applying a reverse bias between the collector and base terminals, which suppressed the leakage to an acceptable level. A simple analytical formula to convert the current gain to hole lifetime in the N-type base region was proposed and used, that takes into account the existence of a commonly used N-buffer layer adjacent to the backside P-collector layer. The validity of the formula was confirmed using TCAD simulations. This method was applied to IGBT wafers with two different wafer thicknesses (i.e., base lengths): $120~{{\mu }}\text{m}$ and $360~{{\mu }}\text{m}$ . Consistent lifetime values extracted in spite of the largely different thicknesses supports the validity of the proposed lifetime estimation method.