wafers. Both the above‐bandgap steady‐state and the below‐bandgap transient photoconductivities of 
wafers are measured using a Ka‐band reflection‐type microwave setup in which there is no need to fabricate electrical contacts on 
wafers. A photoconductivity model adopted from the two‐energy‐level defect model for undoped LEC semi‐insulating 
is used to derive the relationship between the photoinduced microwave response and the concentration of shallow acceptors, mainly carbon. In the above‐ bandgap photoconductivity measurement, it is found that the steady‐state microwave response is inversely proportional to the square root of the shallow acceptor concentration. In the below‐bandgap photoconductivity measurement, the transient microwave response shows an initial fast decay and a second slower decay which has a Na‐related decay time constant. More than 30 undoped LEC 
bulk wafers were used in our measurements to establish the correlation. These wafers have a carbon concentration ranging from 
to 
, determined by LVM infrared absorption. Good agreements have been found between the shallow acceptor concentration obtained from the microwave photoconductivity measurements and the LVM carbon concentration.Source:IOPscience
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