Diffusion Length Measurement by Means of Ionizing Radiation

T h e minority-carrier diffusion length is an important q u a n t i t y characterizing the properties of a semiconductor material or device, particularly in any situation involving the transport of charge by minority carriers. It is related to the steady-state minority carrier lifetime through the expression L = Dr, in which D is the diffusion coefficient and r is the lifetime. A variety of methods have been employed to measure diffusion length in the steady state, or lifetime under transient conditions and are reviewed by Bemski. 1 The steady-state methods have a particular advantage in t h a t the measurement is uncomplicated by trapping effects. One particular steady-state method suggested by Gremmelmaier 2 involves the use of gamma radiation to generate excess carriers at a known rate in a pn junction device. The radiation-generated 1573 1574 T H E BELL SYSTEM TECHNICAL J O U R N A L , S E P T E M B E R 1962 short-circuit current m a y then be used to calculate diffusion lengths when the device geometry is known. It is the purpose of this paper to discuss, more generally, the use of penetrating ionizing radiations for diffusion-length measurement. Experimental results will he presented for energetic electrons and protons as well as gamma rays. It will be shown t h a t of the three, electrons lend themselves readily to an accurate absolute determination, with an accuracy of ±5 per cent having been achieved. Even though the method is quite general it will be treated in terms of one device, the silicon solar cell.