Development of an on-wafer test for rapid evaluation of doping spike carrier concentration levels in commercially manufactured GaAs Gunn diodes for automotive radar applications

At e2v Technologies, gallium arsenide Gunn diodes with hot electron injection, based on the heteroepitaxy of a step-graded AlxGa (1-x)As launcher, are commercially manufactured for 77GHz automotive adaptive cruise control (ACC) systems. Characterization of Gunn diode epitaxial material is problematic, especially the measurement of carrier concentration in the injector's thin (less than 10nm) n+ doping spike (around 10 18cm-3), which is key for efficient device operation and must be controlled to around ±2.5%. Currently the only methods to verify carrier concentration in this region are (1) growth and characterization of a verification-wafer immediately before the Gunn wafer growth, or (2) statistical analysis of final device characteristics. Neither method is ideal as (1) is accurate to only around ±15%, and (2) requires time-consuming and costly device fabrication before results can be fed back.

This paper discusses the development of on-wafer quasi-planar Gunn diode structures that allow fast, accurate evaluation of spike doping levels using pulsed-DC measurements, without the need for full device fabrication. This test has been successfully demonstrated and is currently being implemented as a wafer release test and as an MBE doping calibration check. Planning is underway to transfer the procedure to an on-wafer, in-process test to be carried out during initial frontside processing, thus leading to a significant reduction in characterization cycle time.