284 Chapter 9
very high purity single crystal silicon. A few devices are still made in germanium, mainly
for replacement purposes, and some VHF components are made in gallium arsenide, but
these will not, in general, lie within the scope of this book. The fabrication techniques
may be based on the use of a completely undoped (intrinsic) slice of silicon, into which
carefully controlled quantities of impurities are diffused through an appropriate mask
pattern from both sides of the slice. These are described in the manufacturers ’ literature
as double diffused, triple diffused, and so on.
In a later technique, evolved by the Fairchild Instrument Corporation, all the diffusions
were made from one side of the slice. These devices were called planar and had,
normally, a better HF response and more precisely controlled characteristics than, for
example, equivalent double-diffused devices. In a further, more recent, technique,
also due to Fairchild, the silicon slice will have been made to grow a surface layer of
uniformly doped silicon on the exposed side (which will usually form the base region
of a transistor) and a single diffusion was then made into this doped layer to form the
emitter junction. This technique was called epitaxial and led to transistors with superior
characteristics, especially at HF. Since this is the least expensive BJT fabrication process,
it will normally be used wherever it is practicable, and if no process is specifi ed it may
reasonably be supposed to be a planar-epitaxial type.
In contrast to a thermionic valve, which is a voltage-controlled device, the BJT is a
current operated one. So while a change in the base voltage will result in a change in the
collector current, this has a very nonlinear relationship to the applied base voltage. In
comparison to this, the collector current changes with the input current to the base in a
relatively linear manner. Unfortunately, this linear relationship between Ic and Ib tends
to deteriorate at higher base current levels, as shown in Figure 9.1. This relationship
between base and collector currents is called the current gain, and for AC operation is
given the term hfe , and its nonlinearity is an obvious source of distortion when the device
is used as an amplifi er. Alternatively, one could regard this lack of linearity as a change
ofhFE (this term is used to defi ne the DC or LF characteristics of the device) as the base
current is changed. A further problem of a similar kind is the change inhfe as a function
of signal frequency, as shown in Figure 9.2.
However, as a current amplifi er (which generally implies operation from a high
impedance signal source) the behavior of a BJT is vastly more linear than when used as
a voltage amplifying stage, for which the input voltage/output current relationships are