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circuit to the pregion where they recombine with the newly created holes. In this way the en-

ergy of incident photons can be converted to electric energy. Diodes of this kind are widely used

to detect photons in such devices as light meters in cameras as well as to produce electric energy

from solar radiation.

p region

n region

I

Figure 10.32A semiconductor laser. Each dimension is less than a millimeter and its light output, as

in all lasers, is coherent. The junction between the pand nregions from which the light emerges is

only a few micrometers thick.

The only charge carriers shown in Fig. 10.30 were the electrons. Actually, of course,

what was said also applies to the holes, which act as positive charges and behave in

exactly the opposite way to add their current to the conventional current.

When a pmaterial joins an nmaterial, a depletion regionoccurs between them

instead of a sharp interface, as shown in the lower part of Fig. 10.30a. In this region

electrons from the donor levels of the nmaterial fill the holes of the acceptor levels of

the pmaterial, so that few charge carriers of either kind are present there. The width

of the depletion region depends on exactly how the diode is produced, and is typically

about 10^6 m.

Tunnel Diode

The pand nparts of a diode can be heavily doped to give the energy band structure

of Fig. 10.33a. The depletion region is very narrow, 10 ^8 m, and the bottom of the

nconduction band overlaps the top of the pvalence band. The large concentration of

impurities causes the donor levels to merge into the bottom of the nconduction band,

which moves the Fermi energy there upward into the band. Similarly the acceptor

levels merge into the top of the pvalence band, which lowers the Fermi energy below

the top of the band.

Because the depletion region is so narrow, only a few electron wavelengths across,

electrons can “tunnel” through the forbidden band there by the mechanism described

in Sec. 5.9. For this reason such a diode is called a tunnel diode.When no external

voltage is applied to the diode, electrons tunnel in both directions across the gap in

equal numbers and the Fermi energy is constant across the diode.

Figure 10.33bshows what happens when a small forward voltage is applied to

the diode. Now the filled lower part of the nconduction band is opposite the empty

upper part of the pvalence band, and the tunneling is from nto ponly. This gives

an electron current to the left, which corresponds to a conventional current to the

right.

When the external voltage is increased further, the two bands no longer overlap, as

in Fig. 10.33c. The tunnel current therefore ceases. From now on the diode behaves

exactly like the ordinary junction diode of Fig. 10.30. Figure 10.34 shows the voltage-

current characteristic curve of a tunnel diode.

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