Semiconductors, Diodes, Transistors, and Magnetic Properties
Semiconductors
Semiconductors are neither conductors nor insulators. At room temperature, they are poor conductors but can conduct electricity with external power. Examples include silicon and germanium.
Doping
To improve conductivity, semiconductors undergo a process called doping. There are two types:
- P-Type: Dopants create “holes” where electrons are missing. Examples: Aluminum, boron, and gallium.
- N-Type: Dopants add extra electrons.
PN Junction and Polarization
A PN junction is formed by joining a P-type and an N-type semiconductor crystal. A diffusion current forms as electrons move from the N-type material to the P-type material, leaving positive ions, and holes move from the P-type material to the N-type material, leaving negative ions. This creates a potential barrier (Vc) that isolates the P-side from the N-side. This is called a diode.
Reverse Bias
Connecting the negative terminal of a battery to the P-side and the positive terminal to the N-side increases the potential barrier. The PN junction does not conduct current. Exceeding a certain reverse voltage will destroy the junction.
Forward Bias
Connecting the positive terminal of a battery to the P-side and the negative terminal to the N-side counteracts the potential barrier, allowing current to flow. The barrier decreases.
Diodes
A diode is an electronic component that allows current flow in only one direction. It has two terminals:
- Anode: The positive electrode (P-side).
- Cathode: The negative electrode (N-side).
LED (Light Emitting Diode)
LEDs release energy as electromagnetic radiation (visible or invisible). Infrared LEDs are used in remote controls and alarms. Visible LEDs are used as indicators and in displays.
Identifying LED Terminals
- Ohmmeter
- Longer pin (anode)
- Flat edge on the cathode side
- Markings at the base
LEDs typically have a threshold voltage between 1.8V and 2V and a maximum current of 20mA.
Seven-Segment Display
A seven-segment display uses LEDs arranged to display numbers from 0 to 9.
Transistors
Invented in 1942, transistors paved the way for integrated circuits (ICs), which are essential for computers and other electronic systems.
Transistor Structure
A transistor consists of a semiconductor crystal with either a P-zone between two N-zones (NPN) or an N-zone between two P-zones (PNP). Three terminals extend outwards: base (B), emitter (E), and collector (C).
Transistor Polarity
For a transistor to operate, the base-emitter junction must be forward biased, and the base-collector junction must be reverse biased. The current gain (beta) is the collector current divided by the base current, typically between 100 and 300.
Transistor Applications
Transistors enabled smaller and more energy-efficient electronic devices. They can function as switches and amplifiers.
Transistor as a Switch
A transistor acts as a closed switch when current is applied to the base and an open switch when no current is applied. A circuit with an LED can demonstrate this. Resistors limit current to protect the components. An adjustable resistor can vary the voltage and thus the LED brightness.
Magnetic Properties
Materials attracted by magnets, such as iron, cobalt, nickel, and steel, have magnetic properties. Magnets have a north and south pole. Like poles repel, and opposite poles attract.
Electromagnetism
An electric current generates a magnetic field. Passing current through a coil with a steel core creates an electromagnet.
Relays
A relay is an electrically controlled switch that allows a low-power circuit to control a high-power circuit. Relays are useful when small electronic switches cannot handle high currents.
Relay Types
- Normally Open (NO) or Single Pole Single Throw (SPST)
- Normally Closed (NC)
- Double Pole Double Throw (DPDT)
Relay Examples
- NO Relay: When the control input is closed, the relay activates, completing the high-power circuit.
- NC Relay: When the control input is open, the high-power circuit is complete. When the input is closed, the relay activates, opening the high-power circuit.
- DPDT Relay: Can be used to reverse the direction of a motor.