Special Purpose Diodes and Their Applications Study Notes

Fundamentals of P-N Junction Diodes

Definition and Structure: A P-N junction is formed by mixing p-type and n-type semiconductor materials. It is also known as a diode because it has two distinct terminals: the anode (positive lead) and the cathode (negative lead).
Terminals: - Anode: The P-type region of the diode. In forward bias, it is connected to the positive terminal of the battery and charges the junction with holes. - Cathode: The N-type region of the diode, connected to the negative terminal in forward bias.
Physical Dimensions: The thickness of a P-N junction (specifically the depletion region) is on the order of $10^{-6}\,m$ .
The Depletion Region: - This region contains fixed donor and acceptor ions but no mobile charge carriers (electrons or holes). - It behaves as an insulator because the electric field sweeps out thermally generated electron-hole pairs, reducing charge carrier concentration to negligible levels.
Potential Barrier: - The electric field between donor and acceptor ions (with no external voltage) is called a barrier. - It is created by the diffusion of electrons and holes across the junction. - Material-Specific Barrier Potentials: - Germanium ( $Ge$ ): $0.3\,V$ (or $0.3\,eV$ ). - Silicon ( $Si$ ): $0.7\,V\, (0.7\,eV)$ .
Biasing Conditions: - Forward Bias: Positive terminal to P-side, negative terminal to N-side. This offers low resistance (ideally zero) and allows current to flow rapidly once the barrier voltage is exceeded. - Reverse Bias: Positive terminal to N-side, negative terminal to P-side. This causes holes and electrons to move away from the junction, widening the depletion region and offering very high resistance (acting as an insulator).
Current in Diodes: - Forward Current: Conducts in one direction only. Conventional current flows from P-side to N-side (opposite to electron flow). - Reverse Current: Primarily caused by the drift of charges (minority carriers). This current is extremely low, typically in the microampere ( $\mu A$ ), nanoampere ( $nA$ ), or picoampere ( $pA$ ) range. - Saturation Current ( $I_{s}$ or $I_{o}$ ): This current during reverse bias depends on temperature, doping level, and the physical size of the junction.

Zener Diodes and Breakdown Phenomena

Operational Principle: A Zener diode is specifically designed to operate in the breakdown region in reverse bias without sustaining damage.
Zener Breakdown Mechanism: The sudden increase in current is due to the rupture of many covalent bonds. This typically occurs at voltages below $6\,V$ .
Avalanche Breakdown: In normal P-N junction diodes, breakdown occurs via avalanche breakdown, which is distinct from Zener breakdown and typically happens at voltages above $6\,V$ .
Doping Level: Zener diodes are heavily doped to ensure the breakdown occurs at a lower specific voltage.
Voltage Regulation: - Zener diodes are used as shunt voltage regulators because they maintain a constant voltage across their terminals once in breakdown. - They are used to provide a reference voltage in DC power supplies.
Zener Diode Characteristics: - It possesses incremental or dynamic resistance, which is the inverse of the slope of its $I-V$ characteristics. - Temperature Coefficient (TC): - For Zener voltage less than $5\,V$ , TC is negative. - Around $5\,V$ , TC can be made zero. - For higher Zener voltages, TC is positive. - To achieve a zero temperature coefficient, a Zener diode with a negative TC (about $-2\,mV$ ) can be connected in series with a forward-biased diode (TC about $+2\,mV$ ).
Standard Zener Voltages: Typical values include $5.1\,V$ , $5.6\,V$ , $6.2\,V$ , and $9.1\,V$ . Note that $5.8\,V$ is not a standard value.
Series Connection: If two $15\,V$ Zener diodes are connected in series, the total regulated output voltage is $30\,V$ .

Light Emitting Diodes (LEDs)

Operational Principle: LEDs operate under forward bias. When electrons recombine with holes at the junction, energy is released in the form of photons, a process known as electroluminescence.
Material Requirements: - LEDs are heavily doped. - Semiconductors must have a bandgap of at least $1.8\,eV$ to produce visible light. - Gallium Arsenide ( $GaAs$ ): Has a bandgap of $1.4\,eV$ and is used to produce infrared LEDs. - Aluminium Alloys: Used to produce Red, Orange, and Yellow light.
Characteristics: - Fast action (low/no warm-up time). - Long life and low operational voltage. - The reverse breakdown voltage of LEDs is very low, typically around $5\,V$ ; exceeding this can fuse the device. - Intensity vs. Current: Increasing forward current increases light intensity only up to a certain maximum value; beyond that, intensity decreases. - I-V Relationship: In LED characteristics, as frequency increases, the voltage required for the same current increases. Higher wavelengths (lower frequency) require less voltage.

Rectification and Power Supplies

Rectification: The process of converting alternating current ( $AC$ ) into direct current ( $DC$ ). Rectifiers allow current to pass in only one direction.
Components of a Power Supply: - Transformer: Used to change the voltage level. - Rectifier: Converts $AC$ to pulsating $DC$ (containing both $DC$ and $AC$ harmonics/ripples). - Filter: Used to remove ripples ( $AC$ components) from the rectified output to prevent objective losses and improve efficiency. - Voltage Regulator: Stabilizes the $DC$ output voltage against variations in load or input.
Regulation Types: - Load Regulation: The fractional change of output voltage when load current increases from zero to its maximum value. - Line Regulation: Process of maintaining constant output voltage despite changes in input voltage.
Efficiency Facts: Commercial power supplies typically have voltage regulation within $1\%$ . In unregulated supplies, output voltage decreases as load current increases.
Modern Standard: Conventional Zener diode regulators are largely being replaced by Integrated Circuits (ICs).

Filter Circuits

Purpose: To eliminate the $AC$ component (ripples) from the output of a rectifier.
Types of Filters: - Shunt Capacitor Filter: Connected in parallel with the load. It stores electrical energy when the diode is conducting and delivers it when the diode is non-conducting. - Cut-in point: The instant conduction starts. - Cut-out point: The instant conduction stops. - Charge lost formula: $q = I_{DC} \times T$ (where $T$ is the non-conducting time). - RMS Ripple Voltage: $V_{rms} = \frac{I_{DC}}{2\sqrt{3}}$ . - Inductor (L) Filter: Connected in series with the load. It offers high impedance to $AC$ components and low resistance/impedance to $DC$ , thereby dampening the $AC$ signal. - LC Filter: The ripple factor remains constant regardless of the load current. - CLC (Pi) Filter: Consists of one inductor and two capacitors arranged like the letter ' $\pi$ '. - Offers very low ripple factor and smooth output. - The first capacitor does the majority of filtering because it offers very low reactance to the ripple frequency. - The inductor yields high reactance to $AC$ and zero resistance to $DC$ . - The output waveform of a CLC filter is superimposed with a sawtooth wave.

Semiconductor Physics

Intrinsic Semiconductors: Pure semiconductor crystals (undoped or i-type) where current flows due to the breakage of crystal bonds. The number of electrons in the conduction band equals the number of holes in the valence band.
Extrinsic Semiconductors: Doped semiconductors. - P-type: Created by adding trivalent impurities (acceptors) like Gallium ( $Ga$ ), Boron ( $B$ ), or Aluminum ( $Al$ ) to Germanium or Silicon. The majority charge carriers are holes (electron vacancies). - N-type: Created using donor impurities.
Identification: Diode leads are identified via color coding or a color band.

Uninterruptible Power Supply (UPS)

Purpose: Used for critical loads (like computers) where temporary power failure causes significant inconvenience.
Static UPS: Requires both an inverter (to convert battery $DC$ to $AC$ for the load) and a rectifier (to charge the battery from the mains).
Rotating UPS: Uses an alternator driven by a diesel engine to supply power when the mains fail.
Batteries: - Lead Acid: Commonly used because they are cheaper. - Nickel-Cadmium (NC): Better performance but $3$ to $4$ times more expensive than Lead Acid. - Lithium-Ion (Li-On): Also used in UPS systems.