Band-to-band tunneling (BTBT) determines the on-current in tunnel FETs (TFETs). There is a need to review and recalibrate BTBT models used in TCAD tools, which were developed when BTBT was essentially a leakage phenomenon. Here, we consider the process of BTBT through staggered heterojunctions which find application in the design of TFETs having high on-currents. We use a simple 1-D system and compare the estimates of BTBT computed with a semi-classical WKB approach and that obtained from a solution of Schrödinger's equation by a wavefunction matching procedure. We show that the WKB method significantly overestimates the tunneling current through heterojunctions. © 2012 IEEE.