Atoms, elements and matter

• Matter occupies space, has mass, and exists as solid, liquid, or gas.
• Elements are pure substances that cannot be chemically broken down, such as iron, gold, silver, and copper.
• Compounds are formed from the chemical combination of two or more elements, examples are water (hydrogen + oxygen) and table salt (sodium + chlorine).
• There are 118 known elements.
• All substances are made up of one or more elements.
• Atoms are the smallest particle of an element that retains the element's properties.
• Atoms contain protons, neutrons, and electrons.
• Protons are positively charged and are located in the nucleus.
• Neutrons have no charge and are located in the nucleus.
• Electrons are negatively charged and orbit the nucleus in shells.
• The nucleus contains protons and neutrons.
• Electrons orbit the nucleus in specific energy levels (K, L, M, N, etc.).
• The K shell can hold up to 2 electrons.
• The L shell can hold up to 8 electrons.
• The M shell can hold up to 18 electrons.
• The N shell can hold up to 32 electrons.
• The valence shell is the outermost shell.
• The valence shell determines an atom’s chemical properties and reactivity.
• The atomic number is the number of protons in the nucleus.
• The number of protons in the nucleus is unique to each element.
• Atomic weight is the total number of protons and neutrons in the nucleus.
• The periodic table organizes elements by atomic number and properties.
• Elements are grouped into periods (rows) and groups (columns) based on similar chemical properties.
• Conductors have free electrons, allowing electricity to flow easily such as copper and silver.
• Insulators have full valence shells that resist the flow of electricity such as rubber and glass.
• Semiconductors have conductivity between conductors and insulators such as silicon and germanium.
• A neutral atom has an equal number of protons and electrons.
• A negative ion gains one or more electrons.
• Gaining one or more electrons results in the atom becoming negatively charged.
• A positive ion loses one or more electrons.
• Losing one or more electrons results in the atom becoming positively charged.

PN Junction Diode

• A PN junction diode is formed by joining P-type and N-type semiconductor materials.
• PN junction diodes allow current to flow in one direction only (forward bias).
• The depletion region is the area at the junction where electrons and holes are depleted.
• The depletion region creates a barrier to current flow.
• The barrier voltage is the voltage required to overcome the depletion region.
• The barrier voltage is typically 0.7V for silicon and 0.3V for germanium.

Diode Biasing

• Forward bias is where positive voltage is applied to the P-side and negative to the N-side; current flows.
• Reverse bias is where positive voltage is applied to the N-side and negative to the P-side.
• When reverse biased, no current flows except for a small leakage current.

Diode Characteristics

• An I-V graph shows the relationship between current (I) and voltage (V) for a diode.
• With forward bias, current increases rapidly after the barrier voltage is exceeded.
• With reverse bias, minimal current flows until the breakdown voltage is reached.
• Peak Inverse Voltage (PIV) reflects the maximum reverse voltage a diode can handle without breaking down.

Testing Diodes

• Use a Digital Multimeter (DMM) to test diodes.
• The DMM will show low resistance with forward bias (diode conducts).
• The DMM will show high resistance with reverse bias (diode does not conduct).
• A diode check function measures the forward voltage drop (e.g., 0.548V for a silicon diode).

Special Diodes

• A Zener diode operates in reverse bias.
• Zener diodes are used for voltage regulation.
• Breakdown Voltage (Vz) is the voltage at which the Zener diode begins to conduct in reverse bias.
• A varactor diode is used in tuning circuits.
• Its capacitance varies with reverse bias voltage.
• A Light Emitting Diode (LED) emits light when forward-biased.
• The color an LED emits depends on the bandgap energy of the material.
• A seven-segment LED display is used to display numbers and characters.
• It consists of seven individual LEDs arranged in a specific pattern.

Transistor Basics

• A transistor is a three-layer semiconductor device used for amplification or switching.
• Bipolar Junction Transistors (BJTs) come in two types, NPN and PNP.
• Field Effect Transistors (FETs) are voltage-controlled devices.

Transistor Construction

• Transistors are made of three layers, the emitter, base, and collector.
• Transistors contain two PN junctions, emitter-base (forward-biased) and base-collector (reverse-biased).
• Transistors are packaged to protect the crystal and allow connections to the three terminals.

Transistor Biasing

• In NPN transistors, positive voltage is applied to the base (forward bias); electrons flow from the emitter to the collector.
• In PNP transistors, negative voltage is applied to the base (forward bias); holes flow from the emitter to the collector.

Transistor Operation

• A transistor as an amplifier allows a small input current to control a larger output current.
• Gain (β) is the ratio of collector current (Ic) to base current (Ib).
• A transistor as a switch can be turned ON (saturation) or OFF (cutoff) by controlling the base current.

Transistor Testing

• Use an ohmmeter or transistor tester to check the junctions.
• It is important to ensure resistance changes when the leads are reversed.
• To test, measure resistance between emitter-base, base-collector, and emitter-collector in both directions.

Effects of Temperature

• Temperature changes can affect transistor performance.
• Cooling systems, such as heat sinks and fans, are often used to stabilize temperature.

Key Connections Between Topics

• How electrons behave in atoms (valence shells) helps explain how semiconductors work in diodes and transistors.
• Diodes are the building blocks for transistors, which are more complex semiconductor devices.
• Conductors, insulators, and semiconductors are key to understanding how diodes and transistors control current flow.
• The PN junction is the foundation of both diodes and transistors.
• In diodes, the PN junction allows one-way current flow.
• In transistors, the PN junction controls current amplification.