326 Chapter 12
base material diffused into the wafer and an emitter dot
alloyed into the base region. A flat-topped peak or mesa
is etched to reduce the area of the collector at the base
junction. Mesa devices have large power-dissipation
capabilities and can be operated at very high frequen-
cies. Double-diffused epitaxial mesa transistors are
grown by the use of vapor deposition to build up a
crystal layer on a crystal wafer and will permit the
precise control of the physical and electrical dimensions
independently of the nature of the original wafer. This
technique is shown in Fig. 12-20F.
The planar transistor is a highly sophisticated
method of constructing transistors. A limited area
source is used for both the base diffusion and emitter
diffusion, which provides a very small active area, with
a large wire contact area. The advantage of the planar
construction is its high dissipation, lower leakage
current, and lower collector cut-off current, which
increases the stability and reliability. Planar construc-
tion is also used with several of the previously
discussed base designs. A double-diffused epitaxial
planar transistor is shown in Fig. 12-20G.
The field-effect transistor, or FET as it is commonly
known, was developed by the Bell Telephone Laborato-
ries in 1946, but it was not put to any practical use until
about 1964. The principal difference between a conven-
tional transistor and the FET is the transistor is a
current-controlled device, while the FET is voltage
controlled, similar to the vacuum tube. Conventional
transistors also have a low-input impedance, which may
at times complicate the circuit designer’s problems. The
FET has a high-input impedance with a low-output
impedance, much like a vacuum tube.
The basic principles of the FET operation can best be
explained by the simple mechanism of a pn junction.
The control mechanism is the creation and control of a
depletion layer, which is common to all reverse-biased
junctions. Atoms in the n region possess excess elec-
trons that are available for conduction, and the atoms in
the p region have excess holes that may also allow
current to flow. Reversing the voltage applied to the
junction and allowing time for stabilization, very little
current flows, but a rearrangement of the electrons and
holes will occur. The positively charged holes will be
drawn toward the negative terminals of the voltage
source, and the electrons, which are negative, will be
attracted to the positive terminal of the voltage source.
This results in a region being formed near the center of
the junction having a majority of the carriers removed
and therefore called the depletion regions.
Referring to Fig. 12-21A, a simple bar composed of
n-type semiconductor material has a nonrectifying
contacts at each end. The resistance between the two
end electrodes is(12-27)where,
P is the function of the material sensitivity,
L is the length of the bar,
W is the width,
T is the thickness.Varying one or more of the variables of the resis-
tance of the semiconductor changes the bar. Assume a
p-region in the form of a sheet is formed at the top of
the bar shown in Fig. 12-21B. A pn junction is formed
by diffusion, alloying, or epitaxial growth creating a
reverse voltage between the p and n-material producing
two depletion regions. Current in the n-material is
caused primarily by means of excess electrons. By
reducing the concentration of electrons or majority
carriers, the resistivity of the material is increased.
Removal of the excess electrons by means of the deple-Figure 12-20. Construction of various transistors.
A. Grown-junction transistor. B. Alloy-junction transistor.C CC CCB BB BE EE EE B Diffused E B
baseDiffused
baseDiffused
baseEpitaxial
layerEpitaxial
layerDiffused
emitterOriginal
waferCollector
original waferBaseEmitter
contactC. Drift-field transistor. D. Microalloy-diffused
transistor.E. Mesa transistor. F. Epitaxial mesa transistor.G. Double-diffused epitaxial planar transistor.Diffused
base
CR PL
WT= --------