Charge storage in the base of an MTL structure is evaluated using two-dimensional simulation. The results are compared to those predicted by the method of injection modeling as developed by H.H. Berger. The results show that injection modeling yields pessimistic estimates of the stored base charge for a homogeneous base device. For the more important case of a graded base structure, injection modeling yields estimates which are very close to those computed in the two-dimensional simulation. © 1976.