Elec Q2 Semiconductor Devices (M4) PDF

**SEMICONDUCTOR DEVICES (M4)‎ ‎** A **semiconductor device** - are electronic circuit components made from a material that is neither a good conductor nor a good insulator (hence semiconductor). **Electricity** is the flow of electrons Good conductors (copper) have easily released electron...

**SEMICONDUCTOR DEVICES (M4)‎ ‎** A **semiconductor device** - are electronic circuit components made from a material that is neither a good conductor nor a good insulator (hence semiconductor). **Electricity** is the flow of electrons Good conductors (copper) have easily released electrons that drift within the metal ∙ Under the influence of an electric field, electrons flow in a current - the magnitude of current depends on the magnitude of a voltage applied to the circuit, and the resistance in the path of the circuit - Current flow governed by Ohm's Law (V = IR) electron flow direction **Semiconductors** have a regular crystalline structure - for monocrystal, extends through the entire structure, - for polycrystal, structure is interrupted at irregular boundaries. **Monocrystal** has uniform 3-dimensional structure. **Atoms** occupy fixed positions relative to one another but are in constant vibration about equilibrium. Electrons circle the nucleus in **defined shells** - K(1) 2 electrons - L(2) 8 electrons - M(3) 18 electrons - N(4) 32 electrons ∙ Within each shell, electrons are further grouped into **subshells** - s 2 electrons - p 6 electrons - d 10 electrons - f 14 electrons electrons are assigned to shells and subshells from inside out - Si has 14 electrons: 2 K, 8 L, 4 M Silicon atoms have 4 electrons in outer shell - inner electrons are very closely bound to atom These electrons are shared with neighbor atoms on both sides to "fill" the shell - resulting structure is very stable  - electrons are fairly tightly bound - at room temperature, if a battery is applied, very little electric current flow Semiconductors (Si and Ge) have 4 electrons in their outer shell - 2 in the s subshell - 2 in the p subshell As the distance between atoms decreases the discrete subshells spread out into bands As the distance decreases further, the bands overlap and then separate - the subshell model doesn't hold anymore, and the electrons can be thought of as being part of the crystal, not part of the atom - 4 possible electrons in the lower band (valence band) - 4 possible electrons in the upper band (conduction band) - The space between the bands is the energy gap, or forbidden band This separation of the valence and conduction bands determines the electrical properties of the material - **Insulators** have a large energy gap - electrons can't jump from valence to conduction bands - no current flows - **Conductors** (metals) have a very small (or nonexistent) energy gap - electrons easily jump to conduction bands due to thermal excitation - current flows easily - **Semiconductors** have a moderate energy gap - only a few electrons can jump to the conduction band - » leaving "holes" - only a little current can flow Sometimes thermal energy is enough to cause an electron to jump from the valence band to the conduction band - produces a hole-electron pair Electrons also "fall" back out of the conduction band into the valence band, combining with a hole To make semiconductors better conductors, add impurities (dopants) to contribute extra electrons or extra holes - elements with 5 outer electrons contribute an extra electron to the lattice (donor dopant) - elements with 3 outer electrons accept an electron from the silicon (acceptor dopant) ***Phosphorus and arsenic are donor dopants*** - if phosphorus is introduced into the silicon lattice, there is an extra electron "free" to move around and contribute to electric current  - produces n-type silicon - phosphorus becomes a positive ion after giving up an electron Boron has 3 electrons in its outer shell, so it contributes a hole if it displaces a silicon atom - boron is an acceptor dopant - yields p-type silicon - boron becomes a negative ion after accepting an electron A **diode** is a semiconductor device that essentially acts as a one-way switch for current.  - It allows current to flow easily in one direction but severely restricts current from flowing in the opposite direction - change alternating current (ac) into pulsating direct current (dc). - also known as rectifiers -- P&N junction - Polarity- anode (+ lead)& cathode (- lead) -- Electrodes - Forward-biased -- current flow - Reversed-biased -- acts as insulator **Types of Diodes** 1. **Rectifier diodes**  - are a vital component in power supplies which they are used to convert AC voltage to DC voltage. 2. **Zener diodes** - are used for voltage regulation, preventing unwanted variations in DC supplies within a circuit.  3. **LEDs (light Emitting Diodes)** - are a particular type of diode that converts electrical energy into light. 4. **OLEDs (Organic Light Emitting Diode)** - Similar to an LED, however, an OLED has an emissive electroluminescent layer of film made up of organic molecules. - Light is emitted when an electrical current travels through the organic molecules  5. **AMOLEDs ( Active Matrix Organic Light Emitting Diodes)** - have full layers of the cathode, organic molecules, and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix. - They are efficient for large displays A **transistor** is a semiconductor device used to amplify or switch electronic signals and electrical power. - both conduct and insulate - 3 terminals (Base, Collector, & Emitter) - The first transistor was fabricated with germanium One of the most common uses for transistors in an electronic circuit is as simple switches. - It conducts current across the collector-emitter path only when a voltage is applied to the base. When no base voltage is present, the switch is off. When the base voltage is present, the switch is on. A transistor acts as an amplifier by raising the strength of a weak signal. - The DC bias voltage applied to the emitter base junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the signal. **Types of Transistors** 1. **Bipolar Junction Transistor (BJT)** - These are current-controlled devices and are of two types NPN and PNP. In NPN, the majority of the current is carried by electrons. In PNP, the majority of the current is carried by holes.  2. **Schottky Transistor** - When a transistor is combined with a Schottky diode, it is called a Schottky transistor. Introducing that type of diode prevents the transistors from saturating by the diversion of extreme input current. 3. **Field Effect Transistor (FET)**  - These are voltage-controlled and have high input impedance which causes little current to pass through them. 4. **Multiple-Emitter Transistor** - Here, the emitters are applied with input signals. It can reduce switching time and power dissipation.