n-type semiconductor. Using this plot, find the width of the forbid-
den band of the semiconductor and the activation energy of donor
levels.
6.210. The resistivity of an impurity-free semiconductor at room
temperature is p = 50 Q • cm. It becomes equal to pi = 40 Q• cm
when the semiconductor is illuminated with light, and t = 8 ms
after switching off the light source the resistivity becomes equal to
p, = 45 I2-cm. Find the mean lifetime of conduction electrons and
holes.
6.211. In Hall effect measurements a plate of width h = 10 mm
and length 1 = 50 mm made of p-type semiconductor was placed
in a magnetic field with induction B = 5.0 kG. A potential differ-
ence V = 10 V was applied across the edges of the plate. In this
case the Hall field is VH = 50 mV and resistivity p = 2.5 52•cm.
Find the concentration of holes and hole mobility.
6.212. In Hall effect measurements in a magnetic field with
induction B = 5.0 kG the transverse electric field strength in an
impurity-free germanium turned out to be rl = 10 times less than
the longitudinal electric field strength. Find the difference in the
mobilities of conduction electrons and holes in the given semicon-
ductor.
6.213. The Hall effect turned out to be not observable in a semi-
conductor whose conduction electron mobility was 7.1 = 2.0 times
that of the hole mobility. Find the ratio of hole and conduction
electron concentrations in that semiconductor.6.5. Radioactivity
- Fundamental law of radioactive decay:
N=Noe—xt. (6.5a)- Relation between the decay constant X., the mean lifetime T, and the
half-life T:
1 ln 2
T • - Specific activity is the activity of a unit mass df a radioisotope.
6.214. Knowing the decay constant X of a nucleus, find:
(a) the probability of decay of the nucleus during the time from 0
to t;
(b) the mean lifetime ti of the nucleus.
6.215. What fraction of the radioactive cobalt nuclei whose half-
life is 71.3 days decays during a month?
6.216. How many beta-particles are emitted during one hour by
1.0 p,g of Na 24 radionuclide whose half-life is 15 hours?
6.217. To investigate the beta-decay of Mg 23 radionuclide, a coun-
ter was activated at the moment t = 0. It registered Ni beta-parti-
cles by a moment t, = 2.0 s, and by a moment t 2 = 3t 1 the number
(6.5b)270