OCVD carrier lifetime in P C NN C diode structures with axial carrier lifetime gradient V. Benda a, * , M. Cernik b , V. Papez a a Department of Electrotechnology, Czech Technical University in Prague, Technicka 2, 166 27 Praha 6, Czech Republic b Department of Mechatronics, Technical University Liberec, Liberec, Czech Republic Available online 23 November 2005 Abstract The OCVD (open circuit voltage decay) method is the generally used method for the determining of carrier lifetime in the structures of semiconductor devices. This paper is focused on power diode (P C NN C ) structures, in which is realised a carrier lifetime gradient to influence the current and voltage waveforms during the reverse recovery process. A theoretical analysis of the general features of voltage decay courses in OCVD measurements on diode structures with an axial carrier lifetime gradient in the diode base is presented. Some results obtained from both simulations and experimental measurements are discussed in the paper. q 2005 Elsevier Ltd. All rights reserved. Keywords: OCVD method; Carrier lifetime gradient; Power diode structures 1. Introduction The OCVD (open circuit voltage decay) method is a commonly used technique for carrier lifetime measurements in the structures of semiconductor devices. The measure- ment is relatively very simple: a diode part of a device is forward biased and then the circuit is opened. OCVD carrier lifetime t eff is determined from the slope of the voltage decay [1] t eff ZK kT e dV dt   K1 : (1) It depends on carrier lifetime t in the base of the diode structure, injection level, and on several more parameters. The simple formula (1) assumes a low injection level, uniform carrier lifetime distribution across the diode base region and disregards space charge effects. Nevertheless, the method is used generally. This paper focuses on interpretation results of OCVD measurements for diode structures with an axial carrier lifetime gradient in the diode base. 2. The OCVD method In a simple approximation, the voltage drop across a P C NN C diode structure V(t) is given by V ðtÞ Z V P ðtÞ C V N ðtÞ K ð w 0 Eðx; tÞdx; (2) where V P is the voltage drop at the P C N junction, V N is the voltage drop at the NN C junction and E is a local electric field in the base of diode structure Eðx; tÞ Z J ðx; tÞ eðm n C m p ÞDnðx; tÞ K kT ðm n Km p Þ eðm n C m p ÞDnðx; tÞ ! dDnðx; tÞ dx : (3) The junction voltages can be derived using the Boltzmann relations p N ð0; tÞ N C A Z exp Ke½4 P KV P ðtÞ kT   ; (4a) n N ðw; tÞ N C D Z exp Ke½4 N KV N ðtÞ kT   ; (4b) 4 P Z ðkT =eÞlnðN C A N D =n 2 i Þ, 4 N Z ðkT =eÞlnðN D N C N =n 2 i Þ are the built-in voltages at P C N and NN C junctions. Microelectronics Journal 37 (2006) 217–222 www.elsevier.com/locate/mejo 0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.09.027 * Corresponding author. Tel.: C420 224352163; fax: C420 224353949. E-mail address: benda@fel.cvut.cz (V. Benda).