Transistors are the key components in a chip. The LM108 op amp uses NPN and PNP bipolar transistors, while many newer op amps use low-power CMOS transistors instead.
If you’ve studied electronics, you’ve probably seen a diagram of an NPN transistor like the one below, showing the collector (C), base (B), and emitter (E) of the transistor. A transistor is usually illustrated as a sandwich of P silicon in between two symmetric layers of N silicon; the N-P-N layers make an NPN transistor.
But it turns out that transistors on a chip look nothing like this, and the base often isn’t even in the middle!
The photo below shows an NPN transistor on a 741 op amp die.
The different brown and purple colors are regions of silicon that has been doped differently, forming N and P regions.
The whitish-yellow areas are the metal layer of the chip on top of the silicon—these form the wires connecting to the collector, emitter, and base.
Underneath the photo is a cross-section drawing showing approximately how the transistor is constructed. There’s a lot more than just the N-P-N sandwich, but if you look carefully at the vertical cross section below the ‘E’, you can find the N-P-N that forms the transistor. The emitter (E) wire is connected to N+ silicon. Below that is a P layer connected to the base contact (B). And below that is an N+ layer connected (indirectly) to the collector (C).
The innovative feature of the LM108 is the superbeta transistor, seen below. It has a much thinner base region below the emitter. This gives the superbeta transistor a much higher beta (i.e. amplification), but makes the transistor much more delicate: just 4 volts between the collector and emitter can “punch through” the thin base and destroy the transistor.