In the Fall of 1970, Chester Lee Balestra, a graduate student working
in Harry Gates' laboratory at the Massachusetts Institute of Technolog
y, observed a strong photovoltaic signal on CdS surfaces illuminated w
ith a sub-bandgap light. Part of the photovoltage spectrum disappeared
for surfaces cleaved in ultra-high vacuum and reappeared at higher am
bient pressure, implying the involvement of surface states. A correspo
nding technique, ''surface photovoltage spectroscopy'', was developed
and used for studying a wide range of surface-related phenomena in sem
iconductors. Two decades later, the spin-off techniques are used on si
licon IC fabrication lines for instantaneous, non-contact detection of
metal contaminants with an astounding sensitivity of one part per qua
drillion.