A diode is a semiconductor device that allows current to flow in one direction only. It is made by joining p-type and n-type semiconductor materials, forming a p-n junction.

How a Diode Works

• When the p-side (positive) is connected to a higher voltage than the n-side (negative), the diode is forward biased. Current flows easily.
• When the n-side is connected to a higher voltage, the diode is reverse biased. Current is blocked (except for a tiny leakage).

This one-way behavior is called rectification—converting alternating current (AC) to direct current (DC).

Symbol and Circuit

The diode symbol shows a triangle pointing to a line, indicating the direction of conventional current flow.

Worked Example

A diode is connected in a circuit with a forward voltage drop of 0.7 V. If the supply voltage is 3.7 V and the resistor has resistance 100 Ω, calculate the current flowing through the circuit.

Solution:

The voltage across the resistor = Supply voltage − Diode voltage drop

$V_R = 3.7\ \text{V} - 0.7\ \text{V} = 3.0\ \text{V}$

Using Ohm's Law:

$I = \frac{V_R}{R} = \frac{3.0\ \text{V}}{100\ \Omega} = 0.03\ \text{A} = 30\ \text{mA}$

A transistor is a semiconductor device that can amplify signals or act as an electronic switch. The most common type is the bipolar junction transistor (BJT).

Types of Transistors

• n-p-n transistor: Has two n-type regions separated by a p-type region.
• p-n-p transistor: Has two p-type regions separated by an n-type region.

Terminals

A transistor has three terminals:

• Emitter (E): Emits charge carriers
• Base (B): Controls the current
• Collector (C): Collects charge carriers

Transistor as a Switch

A transistor can be turned ON or OFF by applying a small current to the base. When ON, a larger current flows from collector to emitter. This allows a small input signal to control a larger output—essential for digital logic and control systems.

Transistor as an Amplifier

In amplifier circuits, a small change in base current causes a large change in collector current. The ratio of output current to input current is the current gain (β or hFE).

An amplifier is a circuit that increases the amplitude of a signal (voltage or current). A simple amplifier uses a transistor together with resistors and capacitors.

Basic Single-Stage Amplifier

A single-stage amplifier contains one transistor that provides amplification. The three common configurations are:

1. Common-emitter (CE): Input at base, output from collector. Provides voltage gain and phase reversal.
2. Common-collector (CC): Input at base, output from emitter. Provides current gain (used for impedance matching).
3. Common-base (CB): Input at emitter, output from collector. Provides voltage gain with no phase reversal.

How Amplification Works

• A weak input signal modulates the base current.
• The transistor produces a proportionally larger collector current.
• The load resistor converts this larger current into a larger voltage at the output.

Gain

The voltage gain of an amplifier is given by:

$A_v = \frac{V_{\text{out}}}{V_{\text{in}}}$

Where $V_{\text{in}}$ is the input voltage and $V_{\text{out}}$ is the output voltage.

In Tanzania, simple electronic circuits using diodes and transistors are found in solar-powered phone chargers. The diode prevents the battery from feeding back into the solar panel at night, while the transistor acts as a switch to control charging. A small 12V solar panel can charge a phone through such a circuit, providing a practical solution in areas where grid electricity is unavailable.