I’m lumping the MOSFET and BJT labs together for an interesting reason. Normally, the process for making these two devices is pretty different. While both will have an N, P, and another N region (for N-MOSFETs and NPN BJTs), MOSFETS usually have these three regions as adjacent rectangular areas, while BJTs have them as three concurrent cuboid regions.

However, we (or rather, our lab manual) screwed up. The process for making a MOSFET actually had us adding, then removing, the gate oxide. What this meant is we had direct contact to the N, P, and N regions – a very weak BJT.

Half of us ended up continuing with that, and half of us hadn’t etched the gate oxide, so were able to modify the process to create a MOSFET. Because both processes started from the same place, I’m going to include both in this lab.

Introduction

This lab creates both a MOS Field Effect Transistor (MOSFET) and a bi-junction transistor (BJT). The MOSFET is a device where the gate voltage with respect to the source pin voltage determines the drain-source conductance. The BJT is a device where the base-emitter voltage determines the collector current (it just so happens by coincidence that the base current is proportional to the collector current).

This lab utilizes techniques from the MOSCAP lab and the PN diode lab to produce three adjacent rectangular regions of N, P, and N-type silicon. For the BJT, contacts are created directly to these three regions via aluminum deposition, whereas for the MOSFET, a gate oxide is created before adding the contacts.

Procedure and Results

MOSFET/BJT Base

Completing the MOSFET

Completing the BJT