Semiconductor Devices PDF

# Semiconductor Devices ## P-N Junction - When a P-type semiconductor is suitably joined to an N-type semiconductor, a junction is formed. This junction is called P-N junction. - A representation of the junction: | P-type | N-type | |---|---| | Hole (positive charge) | Electron (negative...

# Semiconductor Devices ## P-N Junction - When a P-type semiconductor is suitably joined to an N-type semiconductor, a junction is formed. This junction is called P-N junction. - A representation of the junction: | P-type | N-type | |---|---| | Hole (positive charge) | Electron (negative charge) | - In the junction, a **depletion region** forms due to the diffusion of electrons and holes. - This depletion region is a thin layer around the junction that is depleted of free charge carriers. - **Potential barrier** forms across the junction in the depletion region, preventing further diffusion of electrons and holes. ## P-N Junction Diode - A bipolar junction diode, with anode (positive terminal) and cathode (negative terminal). - The anode refers to the P-type region, and the cathode refers to the N-type region. - A diagram of the P-N junction diode: **P** - type | **N** - type | **Anode** | **Cathode** |---|---|---|---| | Diagram of a simple diode (a) | | | ## Biasing of P-N Junction Diode - Applying a potential difference across a P-N junction is called **biasing**. - Two types of biasing: - **Forward Biasing**: Connecting the positive terminal of the battery to the P-side and the negative terminal to the N-side. - **Reverse Biasing**: Connecting the negative terminal of the battery to the P-side and the positive terminal to the N-side. ### Forward Biasing - When a diode is forward biased: - The width of the depletion layer **decreases**. - The barrier potential **decreases**. - Majority charge carriers result in the **flow of current**. - The diode offers **very low resistance** (called forward resistance). ### Reverse Biasing - When a diode is reverse biased: - The width of the depletion layer **increases**. - The barrier potential **increases**. - The flow of **negligible current** is due to **minority charge carriers**. - The diode offers **very high resistance** (called reverse resistance). ## Characteristics of a Junction Diode - The **characteristics** of a junction diode are graphs which show the relationship between bias voltage and current. ### Forward Bias Characteristics - The graph shows the variation of circuit current with forward biasing voltage. ### Reverse Bias Characteristics - The graph shows that with the increase in reverse biasing voltage, the reverse current remains very small. - But at further increasing reverse bias voltage, a stage appears at which current rises gradually. - The voltage where the reverse current suddenly increases sharply is called **breakdown voltage**. ## Applications of Diode - **Rectifiers** or **Power Diodes** in DC power supplies - **Zener Diode** in Voltage Regulating Circuits - **Signal Diode** in Communication Circuits - **Varactor Diode** in Radio and TV receivers - A **Switch** in logic circuits used in computers ## Rectification - The process of conversion of alternating current (AC) into direct current (DC) is called **rectification**. - **Rectifier** is the device used for rectification. - Types of Rectifiers: - **Half wave rectifier** - **Full wave rectifier** ### Half wave Rectifier - Converts only half cycle of AC into DC. - A half wave rectifier circuit diagram: **Vin (AC input)** | **RL (Load Resistor)** | **DC Output, Vout** |---|---|---| | Diagram of circuit diagram: | | - When the input voltage is positive, the diode is forward biased, and current flows through the load resistor. - The diode is reverse biased during the negative half cycle, and hence no current flows. ### Full Wave Rectifier - Converts full cycle of AC into DC. - Two types of full wave rectifiers: - **Centre Tapped Full Wave Rectifier** - **Bridge Rectifier** #### Centre Tapped Full Wave Rectifier - A centre tapped full wave rectifier circuit diagram: **Vin (AC input)** | **RL (Load Resistor)** | **D1 (diode)** | **D2 (diode)** |---|---|---|---| | Diagram of circuit diagram: | | - During positive half cycle, the diode D1 is forward biased, and D2 is reverse biased. - During negative half cycle, D2 is forward biased, and D1 is reverse biased. - Current flows through load resistor RL in both half cycles, resulting in output DC current. #### Bridge Rectifier - A bridge rectifier circuit diagram: **Vin (AC input)** | **RL (Load Resistor)** | **D1 (diode)** | **D2 (diode)** |**D3 (diode)** | **D4 (diode)** |---|---|---|---|---|---| | Diagram of circuit diagram: | | - During the positive half cycle, diodes D1 and D3 are forward biased, and D2 and D4 are reverse biased. - During the negative half cycle, diodes D2 and D4 are forward biased, and D1 and D3 are reverse biased. - Current flows through load resistor RL in both half cycles, resulting in output DC current. ## Filter Circuit - A filter circuit is used to convert pulsating DC into steady DC. - The circuit is connected between the rectifier and the load. ### Types of Filter Circuits - **Inductor Filter** - **Capacitor Filter** - **LC Filter** - **π Filter ** ## Reverse Break Down - A phenomenon in a PN-junction diode that occurs when it is reverse biased. - A very small amount of current called reverse saturation current flows, due to minority charge carriers across the junction. - As the reverse voltage increases, the current rises abruptly. - The voltage at which the abrupt rise in current occurs is called **breakdown voltage**. ### Types of Break Down - **Zener Break Down**: Occurs in heavily doped semiconductor diodes because electric fields are very strong in the depletion region. As the reverse voltage increases, electrons are pulled out of the covalent bond, resulting in high current flow. - **Avalanche Break Down**: Occurs when a reverse voltage is applied across the diode, and the electric field is strong enough to accelerate electrons and cause them to collide with other atoms, creating free electrons and holes. This leads to the multiplication of charge carriers and a high current flow. ## Zener Diode - A special PN-junction diode that's heavily doped and works in reverse bias. - The doping level is very high compared to a normal PN-junction diode. ## Logic Gates - Electronic circuits that make logic decisions. - Types of logic gates: - **Simple Logic Gates** - **Compound Logic Gates** ### Simple Logic Gates #### OR Gate - A logic gate that produces an output of 1 if at least one of its inputs is 1. - **Truth Table**: | A | B | Y | |---|---|---| | 0 | 0 | 0 | | 0 | 1 | 1 | | 1 | 0 | 1 | | 1 | 1 | 1 | - **Boolean expression**: Y=A+B - **Logic symbol**: diagram of an OR gate. #### AND Gate - A logic gate that produces an output of 1 only if all of its inputs are 1. - **Truth Table**: | A | B | Y | |---|---|---| | 0 | 0 | 0 | | 0 | 1 | 0 | | 1 | 0 | 0 | | 1 | 1 | 1 | - **Boolean expression**: Y=A.B - **Logic symbol**: diagram of an AND gate. #### NOT Gate - A logic gate that inverts its input. - **Truth Table**: | A | Y | |---|---| | 0 | 1 | | 1 | 0 | - **Boolean expression**: Y=A' - **Logic symbol**: diagram of a NOT gate. ### Compound Logic Gates - Logic gates formed by combining two or more basic logic gates. #### NOR Gate - A logic gate that produces an output of 1 only if all its inputs are 0. - **Truth Table**: | A | B | Y | |---|---|---| | 0 | 0 | 1 | | 0 | 1 | 0 | | 1 | 0 | 0 | | 1 | 1 | 0 | - **Boolean expression**: Y=(A+B)' - **Logic symbol**: diagram of a NOR gate. #### NAND Gate - A logic gate that produces an output of 0 only if all its inputs are 1. - **Truth Table**: | A | B | Y | |---|---|---| | 0 | 0 | 1 | | 0 | 1 | 1 | | 1 | 0 | 1 | | 1 | 1 | 0 | - **Boolean expression**: Y=(AB)' - **Logic symbol**: diagram of a NAND gate. ## NAND Gate as a Universal Gate - The NAND gate can be used to produce other logic gates. - **NOT Gate**: Diagram of NAND gate with input A connected to input B. - **AND Gate**: Diagram of two NAND gates connected in series. - **OR Gate**: Diagram of two NAND gates connected in parallel.