  We derive simple formulas to explain terahertz (THz) emission from
semiconductor surfaces excited by a femtosecond (fs) laser. Femtosecond optical
pulses with energies larger than the bandgap create photocarriers which travel
and generate THz radiation, according to the time derivative of the
photocurrent. By assuming that only electrons traveling in an ultrafast time
scale, less than a few hundred fs, contribute to THz radiation, one can obtain
simple expressions for the emission originating from the photocarrier drift
accelerated with a built-in field or from the photocarrier diffusion. The
emission amplitude of the former is in proportion with electron mobility, the
Schottky-Barrier height, and the laser intensity and one of the latter with the
laser intensity and diffusion coefficient squared. We also discuss the formula
for emission from metal-insulator-semiconductor structures. The derived
expressions are useful in understanding the THz emission properties observed by
a laser THz emission microscope (LTEM), bringing LTEM into real applications in
the field of semiconductor research and development.
