Large scale testing of switching speeds of junction transistors

Abstract

This paper describes a method of rapidly evaluating the switching speed of transistors by means of a fully automatic tester suitable for factory use. A transistor may be plugged into the tester and after a brief settling time, a voltage proportional to the expected switching speed appears on a meter or permanent recording device.Because of the non-linear nature of transistors, the test is restricted by the necessity for making the measurement using a circuit configuration resembling that of the intended application.The direct measurement of switching speed is certainly the most accurate and straightforward method. However, when dealing with speeds of the order of 0.5 p sec.. it would be difficult to instrument an automatic testor using this method. Therefore, a roundabout slower speed method suitable for application in an automatic tester, but with some inherent inaccuracy, was developed.Instead of driving the transistor hard into saturation, the base current (in a common emitter configuration) is of sufficient amplitude to just drive vc(t) from cut-off to saturation. A slow transient resembling an exponential rise and traversing the entire active region results. The vc(t) transient is adjusted by servo means for a least error between it and a reference linear RC exponential rise. The expected switching time i s shown to be proportional to the product of the RC time constant and the base current drive, both adjusted a s described.A representative number of transistors of one type were measured by both direct and automatic meane and the accuracy of the proposed test was shown to be +-20%. Although such accuracy is not sufficient for all applications, it would be suitable for a coarse factory test. It has the advantage of quick and simple operation. 7