Comprehensive Study Guide on Diodes, Rectifiers, and Semiconductor Components

Classification of Solids and Energy Band Theory

Solids are classified into three primary categories based on their electrical conductivity. Conductors are substances like metals through which electricity can pass easily. Insulators are substances through which electricity cannot pass, with examples including wood, rubber, and mica. Semiconductors are substances such as germanium, silicon, and carbon, whose conductivity lies between that of conductors and insulators.

Energy bands in solids arise from interatomic interactions in a crystal. When atoms come together, the valence electrons of one atom are shared by more than one atom, causing the splitting of energy levels. The collection of these closely spaced energy levels is defined as an energy band.

The Valence Band is the energy band containing valence electrons; it may be partially or completely filled but is never empty. Electrons in this band cannot gain enough energy from an external electric field to participate in current conduction. In contrast, the Conduction Band contains conduction electrons and is either empty or partially filled. Electrons in this band are responsible for the conduction of current. The Forbidden Band is a completely empty region between the valence and conduction bands. The minimum energy required to shift an electron from the valence band to the conduction band is known as the band gap, denoted as $E_g$.

Types of Semiconductors and Doping

An intrinsic semiconductor refers to a semiconductor in its pure state. An extrinsic semiconductor is a semiconductor that has been doped with a suitable impurity to increase its conductivity. There are two types of extrinsic semiconductors based on the impurity used:

N-type Semiconductors are formed by doping with pentavalent impurities like Arsenic ($As$), Antimony ($Sb$), or Bismuth ($Bi$). In these materials, negatively charged electrons serve as charge carriers. Since every pentavalent impurity atom donates one electron to the crystal, it is called a donor atom.

P-type Semiconductors are created by doping with trivalent impurities like Aluminum ($Al$) or Boron ($B$). In these materials, positively charged holes act as the charge carriers. Every trivalent impurity atom tends to accept one electron, thus it is referred to as an acceptor atom.

The PN Junction and Potential Barriers

A PN-junction diode is formed by fusing p-type and n-type semiconductor materials. Upon joining, a large density gradient exists across the junction. Electrons move from the n-type silicon to the p-type silicon, leaving behind positively charged donor ions ($N_D$) on the n-side. Simultaneously, holes from the acceptor impurity migrate into the region of free electrons. This results in the p-type side along the junction filling with negatively charged acceptor ions ($N_A$), while the n-type side becomes positive. This charge transfer is known as diffusion.

This process creates a depletion layer, which is an area around the junction devoid of mobile charge carriers because donor atoms repel holes and acceptor atoms repel electrons. This creates a built-in potential difference across the junction. The open-circuit or zero-bias potential is given by the formula:

$E_0 = V_T \times \text{ln} \times \frac{N_D \times N_A}{n_i^2}$

In this equation, $E_0$ is the zero-bias junction voltage or potential barrier, $V_T$ is the thermal voltage ($26\,mV$ at room temperature), $N_D$ and $N_A$ are the impurity concentrations, and $n_i$ is the intrinsic concentration. At room temperature, the potential barrier for silicon is typically about $0.7\,V$, and for germanium, it is approximately $0.3\,V$.

Diode Biasing and I-V Characteristics

A PN junction diode allows current to pass in only one direction. There are three possible biasing conditions:

Zero Bias occurs when no external voltage potential is applied to the diode.

Reverse Bias occurs when the positive terminal of the voltage source is connected to the n-type material and the negative terminal to the p-type material. This attracts electrons and holes away from the junction, causing the depletion layer to grow wider and increasing impedance until it acts almost as an insulator. Practically zero current flows, though a very small leakage current (measured in micro-amperes, $\mu A$) flows due to minority carriers.

Forward Bias occurs when the positive terminal is connected to the p-type material and the negative terminal to the n-type material. If the external voltage exceeds the potential barrier ($0.7\,V$ for Si, $0.3\,V$ for Ge), the barrier is overcome. The external field weakens the potential barrier field, reducing the barrier width and allowing electrons and holes to cross the junction. This results in a forward current.

The static I-V characteristics of a diode show that in forward bias, current increases non-linearly and very slowly until it reaches the "knee" point, where a sudden increase occurs. In reverse bias, the diode generally does not conduct except for minor leakage. However, if the reverse voltage reaches a critical breakdown voltage, current increases rapidly. This breakdown voltage is a critical parameter for power devices.

The Ideal Diode Law expresses the current as a function of voltage:

$I = I_0 \times (e^{\frac{qV}{kT}} - 1)$

Where $I$ is the net current, $I_0$ is the dark saturation current (a measure of recombination in the device), $V$ is the applied voltage, $q$ is the absolute value of electron charge, $k$ is Boltzmann's constant, and $T$ is the absolute temperature in Kelvin ($K$).

Rectifiers: Conversion of AC to DC

A rectifier converts Alternating Current (AC) into Direct Current (DC). This process is necessary for the proper functioning of electronic equipment through DC power supplies. Rectifier performance is measured by efficiency (ratio of DC power output to AC input power), ripple factor (ratio of the RMS value of the AC component to the DC component in output), and Peak Inverse Voltage (PIV), which is the maximum reverse voltage a diode can withstand without damage.

In a Half-wave Rectifier (HWR), a single diode conducts only during the positive half-cycle of the AC input. During the negative half-cycle, the diode is reverse-biased and does not conduct. A transformer is used in this circuit to step up or step down voltage and to isolate the circuit to reduce shock risk.

Full-wave Rectifiers (FWR) convert the entire AC cycle into DC. There are two main types:

The Center-Tap Full-Wave Rectifier uses two diodes ($D1$ and $D2$) and a center-tapped secondary transformer winding. $D1$ conducts during the positive half-cycle, and $D2$ conducts during the negative half-cycle. While efficient, it is difficult to tap the transformer center, and diodes must withstand a high PIV (twice the maximum voltage across half the secondary winding).

The Full-wave Bridge Rectifier uses four diodes ($D1$, $D2$, $D3$, and $D4$) in a bridge arrangement. During the positive half-cycle, $D1$ and $D2$ are forward-biased; during the negative half-cycle, $D3$ and $D4$ are forward-biased. This eliminates the need for a center-tap transformer and provides an output double that of a center-tap rectifier for the same secondary voltage, with each diode experiencing only half the PIV of the center-tap circuit. However, it requires four diodes and is less suitable for very low voltage rectification due to the double voltage drop through two diodes in series.

Filter Circuits

Filter circuits are installed between a rectifier and a load to remove the AC component (ripple) from the rectifier output while allowing the DC component to pass. Capacitors ($C$) are used because they pass AC and block DC, while inductors or chokes ($L$) oppose AC and allow DC to pass.

A Capacitor Filter consists of a capacitor in parallel with the load. The capacitor charges to the peak voltage ($V_m$) during the rectifier's conduction and discharges through the load when the rectifier voltage decreases, maintaining the output near the peak value. This is high-performance, low-cost, and popular for small load currents up to $50\,mA$.

A Choke Input Filter uses an inductor in series with the rectifier output and a capacitor across the load. The choke blocks most AC, while the capacitor bypasses remaining AC to ground, leaving only DC for the load.

A Capacitor Input Filter (or $\pi$ filter) consists of a capacitor ($C1$) followed by a series choke ($L$) and a second capacitor ($C2$) across the load. $C1$ bypasses much of the AC, the choke blocks remaining AC, and $C2$ removes any AC the choke failed to block.

Zener Diodes and Breakdown Mechanisms

A Zener diode is a specifically doped crystal diode with a sharp, well-defined breakdown voltage. When forward-biased, it behaves like an ordinary diode. In reverse-bias, once the Zener voltage ($V_Z$) is reached, it enters the "On" state where the voltage remains constant even as current changes sharply. This allows it to function as a voltage regulator. If the reverse voltage is between $0\,V$ and $V_Z$, it is in the "Off" state and acts as an open circuit.

There are two breakdown mechanisms:

Avalanche Breakdown occurs in diodes with a wider depletion region at high reverse voltages. Charge carriers are accelerated to high speeds by the electric field, colliding with atoms to create new electron-hole pairs in a chain reaction. This process is generally destructive to the junction.

Zener Breakdown occurs in heavily doped diodes with narrow depletion regions. A strong electric field pulls valence electrons directly from the ions, creating a heavy reverse current. This is a temporary, non-destructive breakdown; the junction regains its original properties when the voltage is removed.

Light Emitting Diodes (LED) and Photodiodes

Light Emitting Diodes (LEDs) operate on the principle of electroluminescence. When forward-biased, electrons from the n-type material cross the junction and recombine with holes in the p-type material. As these electrons drop from the conduction band to the lower energy level of the valence band, they release energy as heat and light. Materials like Gallium Arsenide ($GaAs$) produce infrared radiation, Gallium Phosphide ($GaP$) produces red or green light, Gallium Arsenide Phosphide ($GaAsP$) produces red or yellow light, and Gallium Nitride ($GaN$) produces blue light. LEDs are energy-efficient ($80\%$ to $90\%$), have long lifetimes ($50,000$ hours), and operate at low voltages ($1.2\,V$ to $3.6\,V$) with current ratings between $10\,mA$ and $100\,mA$. A seven-segment display uses seven LEDs in a rectangular fashion to display digits, available as Common Cathode (CC) or Common Anode (CA) configurations.

Photodiodes are reverse-biased PN junctions where reverse current increases when the junction is exposed to light, based on the photoelectric effect. In the absence of light, a tiny "dark current" flows, and the device has a "dark resistance" defined as $R_R = \frac{V_R}{\text{Dark current}}$. Incident photons create electron-hole pairs, increasing the reverse current until it reaches saturation. The sensitivity ($m$) is the slope of the linear relationship between reverse current ($I_R$) and illumination ($E$). Photodiodes are used in smoke detectors, barcode scanners, and medical devices like pulse oximeters.

An Optoisolator (or optocoupler) uses light to couple a signal from a photoemitter (LED) to a photodetector (photodiode). The primary benefit is electrical isolation between input and output. The output voltage is given by $V_{\text{out}} = V_{SS} - I R_2$, where $I$ is the reverse current through the photodiode induced by light from the LED.

Questions & Discussion

Power Supply Design

1. Explain how to design a 5V regulated power supply from 230V, 50Hz mains supply. Draw the circuit diagram to support your explanation.

Diode and Related Concepts

2. How does the diode work during forward and reverse bias conditions?

3. What is a rectifier?

4. Give the difference between half-wave and center-tap full-wave rectifier with a neat labelled diagram.

5. Define the following: (i) Leakage current (ii) Ripple factor (iii) Rectifier efficiency (iv) Peak Inverse Voltage (v) Dark current

6. Give difference between forward and reverse biased PN junction diode with a neat labelled diagram.

7. Explain the following terms with a neat labelled diagram: (i) Zero bias (ii) Forward bias (iii) Reverse bias

8. Explain the working of a PN junction diode with a neat labelled diagram.

9. Draw and explain the V-I characteristics of a PN junction diode.

10. What is a zener diode? Draw and explain the characteristics of a zener diode.

11. Explain the difference between zener diode and photodiode with a neat labelled diagram.

LED and Photodiode

12. Explain the working of (i) LED, (ii) Photodiode.

13. How does LED differ from an ordinary diode? Give two applications of LEDs.

14. With a neat diagram/graph explain the characteristics of photodiode.

15. With a neat diagram explain the working of photodiode.

16. Explain the working of a LED. How does LED differ from an ordinary diode? Give two applications of LEDs.

17. Explain the working of optoisolator or optocoupler

Rectifiers

18. Describe a full-wave bridge rectifier using diodes.

19. Give difference between half-wave and full-wave bridge rectifier with a neat labelled diagram.

20. With a neat sketch, explain the working of centre-tap full-wave rectifier.

21. With a neat sketch, explain the working of full-wave bridge rectifier.

Filters

22. What is the use of a filter circuit along with a rectifier? Describe the action of the capacitor filter circuit with proper diagrams.

23. Describe the action of the choke input filter circuit with proper diagrams.

24. Describe the action of the capacitor input or pi filter circuit with proper diagrams.

25. What is the use of filter circuit along with rectifier? Describe action of any one filter circuit with proper diagrams.