Electrical Conduction in Semiconductors
Discrete Semiconductor Devices and Circuits
A common conceptual model of electrons within atoms is the “planetary” model, with electrons depicted as orbiting satellites whirling around the “planet” of the nucleus. The physicist Ernest Rutherford is known as the inventor of this atomic model.
A major improvement over this conceptual model of the atom came from Niels Bohr, who introduced the idea that electrons inhabited “stationary states” around the nucleus of an atom, and could only assume a new state by way of a quantum leap: a sudden “jump” from one energy level to another.
What led Bohr to his radical proposal of “quantum leaps” as an alternative to Rutherford’s model? What experimental evidence led scientists to abandon the old planetary model of the atom, and how does this evidence relate to modern electronics?
In solitary atoms, electrons are free to inhabit only certain, discrete energy states. However, in solid materials where there are many atoms in close proximity to each other, bands of energy states form. Explain what it means for there to be an energy “band” in a solid material, and why these “bands” form.
Engineers and scientists often use energy band diagrams to graphically illustrate the energy levels of electrons in different substances. Electrons are shown as solid dots:
Based on these diagrams, answer the following questions:
- Which type of material is the best conductor of electricity, and why?
- Which type of material is the worst conductor of electricity, and why?
Sadly, many introductory textbooks oversimplify the definition of a semiconductor by declaring them to be substances whose atoms contain four valence-shell (outer level) electrons. Silicon and germanium are traditionally given as the two major semiconductor materials used.
However, there is more to a “semiconductor” than this simple definition. Take for instance the element carbon, which also has four valence electrons just like atoms of silicon and germanium. But not all forms of carbon are semiconducting: diamond is (at high temperatures), but graphite is not, and microscopic tubes known as “carbon nanotubes” may be made either conducting or semiconducting just by varying their diameter and “twist rate.”
Provide a more accurate definition of what makes a “semiconductor,” based on electron bands. Also, name some other semiconducting substances.
If a pure (“intrinsic”) semiconductor material is heated, the thermal energy liberates some valence-band electrons into the conduction band. The vacancies left behind in the valence band are called holes:
If an electrical voltage is applied across the heated semiconducting substance, with positive on the left and negative on the right, what will this do to the energy bands, and how will this affect both the electrons and the holes?
In perfectly pure (“intrinsic”) semiconductors, the only way charge carriers can exist is for valence electrons to “leap” into the conduction band with the application of sufficient energy, leaving a hole, or vacancy, behind in the valence band:
With sufficient thermal energy, these electron-hole pairs will form spontaneously. At room temperature, however, this activity is slight.
We may greatly enhance charge carrier formation by adding specific impurities to the semiconducting material. The energy states of atoms having different electron configurations do not precisely “blend” with the electron bands of the parent semiconductor crystal, causing additional energy levels to form.
Some types of impurities will cause extra donor electrons to lurk just beneath the main conduction band of the crystal. These types of impurities are called pentavalent, because they have 5 valence electrons per atom rather than 4 as the parent substance typically possesses:
Other types of impurities will cause vacant electron levels (acceptor “holes”) to form just above the main valence band of the crystal. These types of impurities are called trivalent, because they have 3 valence electrons per atom instead of 4:
Compare the ease of forming free (conduction-band) electrons in a semiconductor material having lots of “donor” electrons, against that of an intrinsic (pure) semiconductor material. Which type of material will be more electrically conductive?
Likewise, compare the ease of forming valence-band holes in a semiconductor material having lots of “acceptor” holes, against that of an intrinsic (pure) semiconductor material. Which type of material will be more electrically conductive?
What type of substance(s) must be added to an intrinsic semiconductor in order to produce “donor” electrons? When this is done, how do we denote this type of “doped” semiconducting substance?
Likewise, what type of substance(s) must be added to an intrinsic semiconductor in order to produce “acceptor” holes? When this is done, how to we denote this type of “doped” semiconducting substance?
A fascinating experiment carried out by J. R. Hayes and W. Shockley in the early 1950’s involved a bar of N-doped germanium with two metal point contacts labeled Ë” and “C,” for “Emitter” and “Collector,” respectively:
Upon actuating the switch, two distinct pulses were noted on the oscilloscope display:
With less drift voltage (Vdrift) applied across the length of the bar, the second pulse was seen to be further delayed and more diffused:
The instantaneous effect of the first pulse (precisely timed with the closure of the switch) is not the most interesting facet of this experiment. Rather, the second (delayed) pulse is. Explain what caused this second pulse, and why its shape depended on Vdrift.
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