Semiconductor processing is a time-consuming business. For example, on a commercial basis it is not unusual for over 50 hours of actual processing time to be used in converting a single wafer into a batch of tested but unpackaged chips. Since you are to perform three experiments/exercises in some 14 lab sessions, compromises must be made in order to save time. For this reason the diffusion intervals you use are much shorter than would be typical in commercial practice. Short diffusion times result in shallow junction depths. The figures shown below give some typical dimensions:
XjE, microns XjC, microns
---------------------------------
commercial 2 - 2.5 2.7 - 3.5
our lab 0.2 - 0.5 1.2 - 2
Shallow emitter junctions tend to give higher than normal values of RSE, the emitter sheet resistance. This, in turn, can act to lower the (BJT) transistor beta as may be seen by the approximate equation:
1/Beta= RSE/RBB + 1/2 (Wb/Lmb)² (1)
where RSE = emitter sheet resistance
RBB = buried base layer (between emitter and collector) sheet resistance
Wb = XjC - XjE = base width
Lmb = base minority carrier diffusion length.
For typical numbers in our lab, the first term in Equation (1) will be dominant; hence a high value of RSE due to a shallow XjE will result in a low beta.
Time restrictions also make it unfeasible for you to make standard integrated circuits. We cannot afford to carry out the steps involved in isolating the devices from each other. You will, however, make arrays of discrete devices covering nearly all devices that are used in ICs, namely:
Single Diffusion Double Diffusion
diodes BJTs
diffused resistors N-MOSTs
MOS capacitors
P-MOSTs
The most common form of isolation, so-called junction isolation (JI), requires a very deep isolation diffusion which extends from the wafer surface through the epitaxial layer (epi). Again, time limitations preclude such a long (up to several hours) diffusion. In summation, as you do the work in the microelectronics laboratory, learn the procedures and processes you use. For the most part, they are typical of commercial practice. But remember that diffusion cycles are short and Betas are low by normal standards.