A novel non-destructive way of characterizing electrical uniformity of semi-insulating GaAs substrates is presented, and applied to Cr-doped HB and undoped LEC wafers. The method utilizes photoconductance at microwave frequencies. Photoconduction is shown to be n-type in both type of substrates, and its magnitude is related to effective lifetime of photo-generated electrons on the basis of a simple theory developed here. Photoconductance variation in Cr-doped substrates is less pronounced, and is related to Cr acceptor concentration. On the other hand, photoconductance variation is more pronounced in undoped LEC substrates with U,M or W-shaped distribution. The photoconductance variation in LEC substrates reflects concentration fluctuation of residual acceptor such as carbon. The photoconductance technique collects information concerning the acceptor distribution near the surface which has a direct effect on the threshold voltage of FETs.
- non-destructive characterization