A general overview of IC fabrication has
been presented so far and more detail about
transistor, diode, resistor, and capacitor circuit
functions will now be offered. Figure 13-9
shows one way to fabricate an NPN junction
transistor. A P type substrate is shown. An N
layer is diffused into the substrate to form the
collector of the transistor. N means that more
than the average number of impurity atoms enter the crystal. This is called heavy doping and it
serves to lower the resistance of the collector.
An N layer is then formed over the substrate
using an epitaxial process. Epitaxy is the controlled growth on a crystalline substrate of a
crystalline layer, called an epilayer. The epilayer
exactly duplicates the properties and crystal
structure of the substrate. The epilayer is oxidized and exposed through a photomask. After
developing, a P type impurity such as boron is
diffused into the windows until the substrate is
reached. This electrically isolates an entire
region on the N type epilayer. This is called the
isolation diffusion and allows separate electrical
functions to exist in a single layer.
Refer again to Fig. 13-9. Again, photolithography opens up a window and a P-type impurity
can be diffused in to form the base of the transistor. Later, an N-type diffusion will form the
emitter. Polarity reversals by repeated diffusions would eventually saturate the crystal so
their number is usually limited to three. Since
emitters are normally heavily doped in any
case, the process is designed so that the emitter
diffusion is the last one.
A general overview of IC fabrication hasbeen presented so far and more detail abouttransistor, diode, resistor, and capacitor circuitfunctions will now be offered. Figure 13-9shows one way to fabricate an NPN junctiontransistor. A P type substrate is shown. An N layer is diffused into the substrate to form thecollector of the transistor. N means that morethan the average number of impurity atoms enter the crystal. This is called heavy doping and itserves to lower the resistance of the collector.An N layer is then formed over the substrateusing an epitaxial process. Epitaxy is the controlled growth on a crystalline substrate of acrystalline layer, called an epilayer. The epilayerexactly duplicates the properties and crystalstructure of the substrate. The epilayer is oxidized and exposed through a photomask. Afterdeveloping, a P type impurity such as boron isdiffused into the windows until the substrate isreached. This electrically isolates an entireregion on the N type epilayer. This is called theisolation diffusion and allows separate electricalfunctions to exist in a single layer.Refer again to Fig. 13-9. Again, photolithography opens up a window and a P-type impuritycan be diffused in to form the base of the transistor. Later, an N-type diffusion will form theemitter. Polarity reversals by repeated diffusions would eventually saturate the crystal sotheir number is usually limited to three. Sinceemitters are normally heavily doped in anycase, the process is designed so that the emitterdiffusion is the last one.
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