CHAPTER 4. DIODE CIRCUITS65semiconductor has the valence band close to the conduction band – separated by about a1 eV gap. Conductors on the other hand have the conduction and valence bands overlapping.The interesting property of a semiconductor is that thermally excited electrons can movefrom the valence band to the conduction band and conduct current. Silicon and germaniumhave thermally excited electrons at room temperature and hence their common use in diodesand transistors.When an electron has been excited into the conduction band, the hole left behind in thevalence band is also free to move through the crystal. A quantum mechanical treatment ofthis eﬀect puts the hole on an approximately equal footing with the electron. Temperaturecauses the thermal generation of electron-hole pairs. One of the components of the pair willaddalittletothe majority charge carriers. The other component of the electron-hole pairwill become the minority charge carrier. Minority charge carriers limit ideal performanceand increase with increasing temperature.A common method for generating even more charge carriers in a semiconductor is bydoping. That is, replacing a few atoms of the base material with atoms of a diﬀerent element.These impurities will contribute an excess electron or hole which is loosely bound and hencecan be excited into the conduction band by thermal energy. In N-type semiconductors themajority of free charge carriers are negative, while in a P-type semiconductor the majorityare positive.4.2The PN Junction and the Diode EﬀectBy joining a P-type and N-type semiconductor together we can make a diode (ﬁgure 4.1)MajorityCarriersMajorityCarriersP−TYPEN−TYPEEDeplectionRegion++++++++−−−−−−−−0VGoodConductorGoodConductorPoorConductorSlopeis EElectronEnergyFigure 4.1: PN junction diode.