From memory only, sketch the common-base BJT transistor configuration (for \(n p n\) and \(p n p\) ) and indicate the polarity of the applied bias and resulting current directions.

The Bipolar Junction Transistor (BJT) is a semiconductor device with three terminals: the emitter, the base, and the collector. It comes in two types: NPN and PNP. The primary function of a BJT is to amplify current, which it achieves by controlling the movement of charge carriers through a process called injection and recombination.

When a small current enters the base terminal, it controls a larger current flow between the emitter and collector terminals. This is possible due to the sandwiching of n-type and p-type semiconductor materials. BJTs operate in three modes: active, saturation, and cutoff. Most commonly, they are used in the active mode for amplification, where the emitter-base junction is forward biased, and the collector-base junction is reverse biased. This biasing creates a thin depletion region at the junctions, allowing for the control of electron (in NPN) or hole (in PNP) flow.

NPN and PNP are the two types of BJTs, both consisting of three layers of semiconductor material with alternating doping types. The NPN transistor has an n-type emitter, a p-type base, and an n-type collector. In contrast, the PNP transistor has a p-type emitter, an n-type base, and a p-type collector.

NPN transistors are more common because electrons (which are the charge carriers in NPN types) have higher mobility than holes (the charge carriers in PNP types). This means that NPN transistors can switch faster and are more efficient in many electronic circuits. In an NPN transistor, the current flows from the collector to the emitter when positively biased, whereas in a PNP transistor, it flows from the emitter to the collector when negatively biased.

Biasing in transistors is a fundamental concept that involves applying external voltages to the P-N junctions of the BJT for it to work correctly. Biasing sets the transistor’s operating point and determines its mode of operation.

For proper functioning, the emitter-base junction should be forward biased, while the collector-base junction should be reverse biased in active mode. Forward biasing a junction means that a positive voltage is applied across it, allowing charge carriers to overcome the barrier potential and flow. Reverse biasing, on the other hand, increases the width of the depletion region and prevents charge carrier flow, which is crucial for controlling the transistor's output characteristics.

In semiconductor devices like BJTs, there are three fundamental configurations: common-base, common-emitter, and common-collector. Each configuration has unique input and output characteristics and is chosen based on the desired amplification properties.

The common-base configuration, referenced in the exercise, is where the base is the terminal common to both input and output circuits, often providing low input resistance and high output resistance. It's usually used in high-frequency amplifiers as it has a wide bandwidth. On the other hand, the common-emitter configuration is well known for providing high current and voltage gain and is widely used in audio amplifiers. Lastly, the common-collector (or emitter-follower) arrangement offers high input impedance, making it suitable for impedance matching applications.