Semiconductor Diodes PDF

1. Band Theory : Electrons in a solid material exist in energy bands. The valence band and conduction band are two key energy bands that determine how electrons move in a material, affecting its electrical conductivity. A. Valence Band: The valence band is the highest range of electron energie...

1. Band Theory : Electrons in a solid material exist in energy bands. The valence band and conduction band are two key energy bands that determine how electrons move in a material, affecting its electrical conductivity. A. Valence Band: The valence band is the highest range of electron energies where electrons are normally present at absolute zero temperature. It contains the outermost electrons of atoms that are bonded in a solid, typically involved in chemical bonding. Electrons in the valence band are tightly bound to atoms and do not contribute significantly to electrical conductivity unless they gain enough energy to jump to the conduction band. B. Conduction Band: The conduction band is the energy range above the valence band. It is usually empty at absolute zero but can hold electrons that gain enough energy to jump from the valence band (via thermal energy, light, etc.). Electrons in the conduction band are free to move throughout the material, enabling electrical conductivity. C. band gap: The band gap is the energy difference between the top of the valence band and the bottom of the conduction band in a solid material. This gap determines whether a material behaves as a conductor, insulator, or semiconductor. Mme K.Djelid 1  2. Semiconductors: Semiconductors are materials that have electrical properties between those of conductors (like metals) and insulators (like glass). They play a crucial role in modern electronics, including transistors, diodes, solar cells, and integrated circuits. They have intermediate resistance, allowing controlled electron flow. Silicon (Si) and germanium (Ge) are common examples. The size of the band gap influences a material's color, transparency, and electronic properties. Mme K.Djelid 2 Electron-Hole Pairs: In semiconductors, when electrons gain enough energy to leave the valence band and enter the conduction band, they leave behind a hole in the valence band. This hole acts as a positive charge carrier, and both electrons and holes contribute to current flow in semiconductors. 3. Types of Semiconductors : Semiconductors are categorized into different types based on purity and doping methods. 1) Intrinsic Semiconductors:  An intrinsic semiconductor is entirely pure, containing no intentional impurities or doping agents. This purity is key to its natural properties.  In intrinsic semiconductors, free electrons and holes are created only through thermal excitation (when heat causes electrons to jump from the valence band to the conduction band). As temperature increases, more electrons gain enough energy to jump from the valence band to the conduction band, increasing the number of electron-hole pairs.  Intrinsic semiconductors have relatively low conductivity at room temperature. Conductivity increases with temperature because higher thermal energy excites more electrons into the conduction band.  The band gap in intrinsic semiconductors is small, typically around 1.1 eV for silicon and 0.67 eV for germanium. This means that with enough energy (e.g., heat), electrons can be excited across this gap.  In an intrinsic semiconductor, when a voltage is applied, both electrons (moving in the conduction band) and holes (moving in the valence band) contribute to current flow. Mme K.Djelid 3   The intrinsic carrier concentration (number of charge carriers per unit volume) is relatively low at room temperature, so intrinsic semiconductors alone are not practical for most electronic applications. 2) Extrinsic Semiconductors : Extrinsic semiconductors are semiconductors that have been intentionally doped with impurities to enhance their electrical conductivity. Unlike intrinsic semiconductors, which are pure, extrinsic semiconductors contain small amounts of other elements (dopants) that introduce additional charge carriers, either electrons or holes. Doping makes it easier to control the material's electrical properties, making extrinsic semiconductors ideal for most electronic devices. Doping is the process of adding a controlled amount of impurity atoms to a pure semiconductor to alter its conductivity. It increases the number of charge carriers (either electrons or holes), greatly enhancing conductivity compared to an intrinsic semiconductor. a) n-type Semiconductors : Doped with pentavalent elements, like phosphorus (P), arsenic (As), or antimony (Sb), which have five valence electrons. Extra electrons are added as the majority carriers in n-type semiconductors, while holes are the minority carriers. The fifth electron from each dopant atom is weakly bound and can easily move to the conduction band, providing additional free electrons for conduction. When an electric field is applied, free electrons move toward the positive terminal, creating an electric current. b) p-type Semiconductors : Doped with trivalent elements, like boron (B), aluminum (Al), or gallium (Ga), which have three valence electrons. “holes” are the majority carriers in p-type semiconductors, while electrons are the minority carriers. Each dopant atom creates a hole by lacking an electron in the valence band. Electrons from neighboring atoms can move to fill these holes. When voltage is applied, holes move toward the negative terminal as electrons jump to fill the holes, allowing the holes to "move" in the opposite direction. Mme K.Djelid 4  3) Compound Semiconductors : Composed of two or more elements, typically from groups III and V of the periodic table, like gallium arsenide (GaAs) or indium phosphide (InP). They often have properties like higher electron mobility, direct band gaps, or specific wavelength sensitivities that make them suitable for specialized applications. 4) Organic Semiconductors : Made of organic materials (carbon-based compounds) with semiconducting properties. Typically flexible and lightweight but have lower conductivity compared to inorganic semiconductors. 5) Amorphous Semiconductors: Non-crystalline semiconductors that lack a long-range atomic order, such as amorphous silicon. Usually have lower mobility due to their disordered structure, but they can still be useful in certain applications. 4. PN junction: p-n Junctions are created by joining p-type and n-type materials, forming a crucial structure in many semiconductor devices. At the junction, electrons from the n-type region diffuse into the p-type region and combine with holes, and vice versa. This movement is driven by the concentration gradient of electrons and holes between Mme K.Djelid 5 the two sides. As electrons and holes recombine near the junction, a depletion region forms. This is a zone with very few free charge carriers because they have recombined with opposite charges from the other side. In the depletion region, immobile positive ions are left on the n-side, and immobile negative ions are left on the p-side. These charged ions create an internal electric field across the junction. This electric field creates a potential difference, known as the built-in potential or barrier potential, across the depletion region. It typically measures around 0.7 volts for silicon and 0.3 volts for germanium. 5. Behavior of the p-n Junction Under Different Conditions: The behavior of a p-n junction changes significantly depending on the external voltage applied: a) Unbiased (Non-Polarized) p-n Junction: In the absence of an external voltage, the junction remains in equilibrium, with no net current flowing across it. The built-in electric field in the depletion region prevents free electrons and holes from crossing the junction. b) Forward Bias (Positive Voltage on p-type, Negative on n-type): Mme K.Djelid 6 When a positive voltage is applied to the p-type side and a negative voltage to the n-type side, it reduces the barrier potential, making it easier for electrons and holes to cross the junction. Electrons move from the n-type to the p-type region, and holes move from the p-type to the n-type region, allowing a current to flow across the junction. In forward bias, the p-n junction conducts electricity, and the device behaves as a diode, allowing current to flow in one direction. c) Reverse Bias (Negative Voltage on p-type, Positive on n-type): When a negative voltage is applied to the p-type and a positive voltage to the n-type, it increases the barrier potential, making the depletion region wider. No Significant Current Flow: The increased potential barrier prevents electrons and holes from crossing the junction, so only a small leakage current flows. Breakdown Voltage: If the reverse voltage exceeds a critical level, the junction can experience breakdown, allowing a large current to flow. This behavior is used in devices like Zener diodes. 6. Junction diode: A junction diode is a semiconductor device that allows current to flow in one direction while blocking it in the opposite direction. It is one of the most basic and widely used electronic components. The behavior of a junction diode depends on the direction of the applied voltage. The current-voltage (I- V) characteristics of a diode describe the relationship between the voltage applied across the diode and the resulting current flow. 7. Diode in DC: When a diode is used in a direct current (DC) circuit, its behavior is primarily determined by its biasing condition, which can be either forward bias or reverse bias. The diode's response to these conditions is different and affects how it conducts or blocks current. a) Forward Bias : The diode starts to conduct significant current once the applied voltage exceeds a certain threshold voltage:  For silicon diodes, this is typically around 0.7V.  For germanium diodes, it is around 0.3V. Mme K.Djelid 7 In a DC circuit, if the applied voltage is greater than the threshold voltage, the diode conducts current from the anode to the cathode. The current increases rapidly as the applied voltage increases, following an exponential relationship. The characteristic curve or V-I curve or voltage current graph shows the diode current and voltage relationship. The voltage applied is independent variable and is along x-axis and the current is a dependent variable and is on y-axis. The I-V characteristic curve of the diode is a nonlinear exponential curve based on the diode's properties. The load line is a straight line that represents the relationship between voltage and current for the rest of the circuit (typically a resistor in series with the diode). The intersection of these two lines gives the operating point of the diode. At point A the current is maximum and the voltage is zero. This point is called 'Saturation'. At point B the voltage is maximum and the current is zero. This point is called 'Cut off'. The intersection of the load line and characteristic curve will define the Operating / Q point of a network. Load line analysis helps determine the optimal resistor values and operating conditions for rectifier circuits. It assists in understanding how diodes respond to different input signals in clippers and clampers. In Zener diode circuits, the load line can be used to determine the regulation behavior and current limits. b) Reverse Bias : Under normal conditions, only a tiny leakage current (in the nanoampere to microampere range) flows due to minority carriers. This current is negligible in most practical applications. If the reverse voltage Mme K.Djelid 8 exceeds a critical value known as the breakdown voltage (specific to each diode type), the diode will start conducting a large current in the reverse direction. This is not desirable in regular diodes but is utilized in Zener diodes for voltage regulation. In a typical DC circuit, a reverse-biased diode acts like an open circuit, effectively blocking current flow. 8. Diode in AC: When a diode is used in an alternating current (AC) circuit, its behavior changes with the polarity of the input voltage. AC voltage alternates between positive and negative cycles, so the diode will be subjected to both forward bias and reverse bias conditions during each cycle.  In the positive half-cycle of the AC waveform, the diode allows current to flow through it once the voltage across the diode exceeds its threshold voltage and behaves like a closed switch.  In the negative half-cycle, the diode blocks current flow, acting like an open circuit. Mme K.Djelid 9 Due to the diode’s behavior in both half-cycles, it effectively rectifies the AC signal, allowing current to pass only during the positive half-cycle and blocking it during the negative half-cycle. This process is known as half-wave rectification. 9. Types of Diode Rectification in AC Circuits 1. Half-Wave Rectification: In half-wave rectification, a single diode is used in an AC circuit to allow only one half-cycle of the AC signal to pass. As a result, the output is a pulsating DC signal, with only positive half- cycles of the AC input appearing at the output. 2. Full-Wave Rectification: In full-wave rectification, typically two or four diodes are used in a configuration (such as a bridge rectifier) to allow both the positive and negative half-cycles of an AC signal to be converted to DC. During each half-cycle, diodes are selectively forward or reverse biased to ensure current flows in the same direction through the load. Full-wave rectification provides a more stable DC output with higher efficiency and is commonly used in power supplies. Mme K.Djelid 10  10.Diode Applications in AC Circuits: a) Clipping Circuit: Clipping circuits use diodes to limit the voltage of an AC signal to a certain level, protecting components from high voltage. In a simple clipper, a diode is placed in parallel with the load. When the input voltage exceeds the diode's threshold (forward voltage), the diode conducts and "clips" the voltage to prevent it from going higher. b) Clamping Circuits Clamping circuits shift the entire waveform of an AC signal up or down to ensure it stays within a specific voltage range. A diode, capacitor, and resistor are used to change the DC level of the waveform without altering its shape. c) Voltage Doublers Diodes are used in voltage doubler circuits, where AC input is rectified and then combined in a way to produce an output voltage that is approximately twice the peak input voltage. 11.The Zener Diode: The Zener diode is a special type of diode designed to operate in reverse bias (when the cathode is more positive than the anode) and is particularly known for its stable breakdown voltage. Unlike standard diodes, which typically block current in reverse bias, the Zener diode allows current to flow when the reverse voltage reaches a specific level, known as the Zener voltage (VZ). This unique property makes Zener diodes very useful in applications that require voltage regulation, protection, and reference sources. 1) Zener Diodes Functionality: In forward bias, the Zener diode behaves like a standard diode, conducting current with a typical forward voltage drop. In reverse bias:  For voltages lower than the Zener voltage (VZ), the diode remains non-conductive, blocking current like a normal diode. Mme K.Djelid 11   When the reverse voltage reaches VZ , the diode enters the breakdown region, allowing current to flow. Despite this, the voltage across the diode stays at approximately VZ , providing a stable reference voltage, making it ideal for voltage regulation. 2) Zener Diode Applications : a. Voltage Regulation The most common use of Zener diodes is in voltage regulator circuits to maintain a constant output voltage despite variations in the input voltage or load conditions. The Zener diode is connected in reverse bias across the load, with a series resistor to limit current. When the input voltage is higher than VZ , the Zener diode conducts, keeping the voltage across the load stable at VZ. The series resistor limits the current through the diode, preventing damage. b. Overvoltage Protection (Clamping) Zener diodes are used to protect sensitive electronic components from voltage spikes. The Zener diode is placed in parallel with the component to be protected. When a voltage spike occurs, exceeding VZ , the diode conducts and clamps the voltage, protecting the component. Mme K.Djelid 12 c. Voltage Reference Zener diodes provide a stable voltage reference in various circuits. The stable VZ is used as a reference for analog-to-digital converters (ADCs), voltage regulators, and precision measurement devices. d. Signal Clipping and Limiting In signal processing, Zener diodes are used to clip or limit the amplitude of an AC signal. Two Zener diodes are placed in reverse parallel across the output signal. For an input signal higher than VZ, one of the diodes conducts, clipping the signal to the Zener voltage level. 3) Important Parameters of Zener Diodes  Zener Voltage (VZ): The reverse breakdown voltage at which the diode starts conducting in reverse bias.  Power Dissipation (Pmax): The maximum power the Zener diode can dissipate, given by P=VZ×IZ.  Zener Impedance (ZZ): The dynamic resistance of the diode in the breakdown region, affecting the stability of the voltage.  Temperature Coefficient: Indicates how VZ changes with temperature. Lower-voltage Zener diodes (