From memory, sketch the transistor symbol for a \(p n p\) and an \(n p n\) transistor, and then insert the conventional flow direction for each current.

PNP transistors are a type of bipolar junction transistor (BJT) that consist of a layer of n-type semiconductor sandwiched between two layers of p-type semiconductor. The key to identifying a PNP transistor in a circuit diagram is to look for its symbol: a circle with three legs, representing the base (B), collector (C), and emitter (E), and most distinctively, an arrow on the emitter leg pointing inward, towards the base.

The operation of a PNP transistor is characterized by the flow of positive charge carriers—holes—moving from the emitter (P-type) to the collector (N-type) when a suitable voltage is applied. This convention is embodied in the 'current arrow' in the symbol, which indicates the direction of conventional current flow (from positive to negative).

To use a PNP transistor effectively in circuits, such as in amplifiers or switches, remember that it 'turns on' or conducts when its base is at a lower potential compared to its emitter, which is typically connected to a positive voltage.

In contrast to PNP transistors, NPN transistors are constructed of two n-type semiconductor materials sandwiching a p-type material. On their schematic symbol, they similarly feature three legs for the base, collector, and emitter. However, the most notable difference is the direction of the arrow on the emitter leg, which points outward, away from the base.

With NPN transistors, conventional current flows from the collector to the emitter, due to the movement of electrons from the N-type emitter to the N-type collector through the P-type base. When a small current flows from the base to the emitter, it allows a larger current to flow from the collector to the emitter.

This type of transistor is commonly used in circuits where current needs to be sunk, meaning it flows into the transistor from a positive voltage source. NPN transistors are widely used because they tend to have faster switching speeds and higher efficiency.

The concept of conventional current flow is crucial to understanding how electrical circuits are analyzed and designed. It is defined as the flow of positive charge from a higher potential (positive voltage) to a lower potential (negative voltage). This convention was established before the discovery of the electron and its negative charge. In reality, electrons, which carry a negative charge, are the actual charge carriers in a conductor and they move from negative to positive potential.

In diagrams and schematics, current direction is usually indicated by arrows. For transistors, these arrows also signify the type of transistor (np or pnp), as well as the direction of current flow for a given polarity of applied voltage. For instance, in a PNP transistor, the arrow in the symbol indicates current entering the emitter, while in an NPN transistor, it shows current exiting the emitter. Even though electrons move in the opposite direction to conventional current, the conventional current flow is still used for consistency and ease of understanding in electronic schematics.

Semiconductor devices are the backbone of modern electronics, found in everything from simple diodes to complex microchips. They are made from materials that have electrical conductivity between that of a conductor and an insulator. The most common semiconductor material is silicon, which can be doped with impurities to alter its electrical properties, creating either an excess of electrons (n-type) or holes (p-type).

Transistors, including both NPN and PNP types, are semiconductor devices that can amplify signals, act as switches, and perform logic operations in digital circuits. Their function hinges on the behavior of the electron and hole carriers within the semiconductor material. Thanks to this controlled manipulation of charge carriers, transistors are integral parts of integrated circuits (ICs) and have a vast range of applications in electronic systems.

The use of semiconductor devices is so pervasive because they can be miniaturized, have low power consumption, and have the ability to control electron flow precisely, making them essential for the computing and digital communication age.