Class 12 Physics Notes Chapter 7 (Chapter 7) – Lab Manual (English) Book
Detailed Notes with MCQs of Chapter 7 from your Physics Lab Manual, which deals with Semiconductor Electronics. These experiments are fundamental for understanding diodes and transistors, concepts frequently tested in various government exams. Pay close attention to the principles, circuit diagrams, characteristic graphs, and precautions.
Chapter 7: Semiconductor Electronics - Experiments
This chapter primarily covers the experimental study of the characteristics of key semiconductor devices.
1. Experiment: p-n Junction Diode Characteristics
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Aim: To study the Voltage-Current (V-I) characteristics of a p-n junction diode in forward bias and reverse bias.
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Apparatus: p-n junction diode (e.g., IN4007), variable DC power supply (0-3V for forward, 0-30V for reverse), DC Voltmeter (0-3V or 0-1V range for forward, 0-30V range for reverse), DC Milliammeter (0-100mA range for forward), DC Microammeter (0-100µA range for reverse), a current limiting resistor (~100Ω), Rheostat (optional, if power supply is not continuously variable), connecting wires, breadboard/circuit board, key.
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Theory:
- A p-n junction diode allows current to flow easily when forward biased (p-side connected to positive terminal, n-side to negative) and offers high resistance when reverse biased (p-side to negative, n-side to positive).
- Forward Bias: The applied voltage opposes the barrier potential. When the applied voltage exceeds the barrier potential (Knee Voltage), the depletion region width decreases significantly, resistance becomes low, and current (due to majority carriers) increases rapidly.
- Reverse Bias: The applied voltage supports the barrier potential. The depletion region width increases, resistance becomes very high, and only a small current (due to minority carriers), called reverse saturation current (in µA range), flows. If the reverse voltage is increased excessively, breakdown (Avalanche or Zener) occurs, leading to a sharp increase in current which can damage the diode.
- Knee Voltage (Cut-in Voltage): The forward voltage at which the diode current starts increasing rapidly (approx. 0.3V for Ge, 0.7V for Si).
- Breakdown Voltage: The reverse voltage at which the junction breaks down and reverse current increases sharply.
- Dynamic Resistance (AC Resistance): The resistance offered by the diode under AC conditions. It's the reciprocal of the slope of the V-I characteristic curve.
- Forward Dynamic Resistance: r_f = ΔV_f / ΔI_f (low value, few ohms)
- Reverse Dynamic Resistance: r_r = ΔV_r / ΔI_r (very high value, MΩ range)
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Circuit Diagrams:
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Forward Bias:
(+) Supply --- Resistor --- Milliammeter (mA) --- Diode (P->N) --- (-) Supply | | Voltmeter (V) across Diode(Note: Voltmeter in parallel across diode, Milliammeter in series)
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Reverse Bias:
(+) Supply --- Microammeter (µA) --- Diode (N<-P) --- (-) Supply | | Voltmeter (V) across Diode(Note: Voltmeter in parallel across diode, Microammeter in series, Diode polarity reversed)
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Procedure Highlights:
- Connect the circuit carefully, ensuring correct polarity of the diode and meters.
- Start with minimum voltage.
- Forward Bias: Increase voltage gradually in small steps (e.g., 0.1V), noting the corresponding current. Take more readings near the knee voltage.
- Reverse Bias: Increase reverse voltage gradually, noting the very small current. Do not exceed the breakdown voltage specified for the diode unless studying breakdown specifically (like for Zener).
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Observations & Graph:
- Record voltage (V) and current (I) readings in a table for both biases.
- Plot V-I graph: Voltage on X-axis, Current on Y-axis. Forward bias in the first quadrant, reverse bias in the third quadrant.
- Graph Shape: Forward bias shows negligible current initially, then a sharp exponential rise after the knee voltage. Reverse bias shows a very small, almost constant saturation current, until breakdown (if reached).
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Calculations: Calculate dynamic resistance (r_f and r_r) from the slope of the linear parts of the graph (after knee voltage for forward, before breakdown for reverse).
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Precautions:
- Ensure correct polarity connections.
- Use appropriate ranges for meters (mA for forward, µA for reverse).
- Include a current limiting resistor in forward bias.
- Do not exceed the voltage/current ratings of the diode.
2. Experiment: Zener Diode Characteristics
- Aim: To study the V-I characteristics of a Zener diode and determine its reverse breakdown voltage (Zener Voltage, Vz).
- Apparatus: Zener diode (e.g., 5.1V or 6.2V Zener), variable DC power supply (0-15V or higher, depending on Vz), DC Voltmeter (0-15V range), DC Milliammeter (0-50mA range), Resistor (~1kΩ), connecting wires, breadboard, key.
- Theory:
- A Zener diode is a specially designed p-n junction diode that operates in the reverse breakdown region.
- It is heavily doped compared to a normal diode, resulting in a thin depletion region.
- Zener Breakdown: Occurs at relatively lower voltages (typically < 6V) due to strong electric field causing electrons to be pulled from covalent bonds (field emission).
- Avalanche Breakdown: Occurs at higher voltages (typically > 6V) where minority carriers gain enough energy to ionize atoms, creating more carriers (carrier multiplication).
- Voltage Regulation: The key property is that after breakdown, the voltage across the Zener diode remains almost constant (equal to Vz) even if the current through it changes significantly. This makes it useful as a voltage regulator.
- Circuit Diagram: (Usually studied only in reverse bias for its main application)
(Note: Connected in reverse bias. Resistor limits current after breakdown)(+) Supply --- Resistor --- Milliammeter (mA) --- Zener Diode (N<-P) --- (-) Supply | | Voltmeter (V) across Zener Diode - Procedure Highlights:
- Connect the circuit with the Zener diode in reverse bias.
- Increase the reverse voltage gradually from zero.
- Note the voltage across the Zener diode and the current through it.
- Take more readings near the expected breakdown voltage, observing the sharp increase in current while the voltage stabilizes.
- Observations & Graph:
- Record reverse voltage (Vr) and reverse current (Ir) in a table.
- Plot Vr-Ir graph (Voltage on X-axis, Current on Y-axis, usually plotted in the third quadrant, but often shown with positive axes for clarity).
- Graph Shape: Shows negligible current initially, then a very sharp increase (almost vertical line) at the Zener breakdown voltage (Vz). The voltage across the diode remains nearly constant at Vz after breakdown.
- Calculations: Determine Vz from the graph (the voltage where the current increases sharply).
- Precautions:
- Connect the Zener diode with correct reverse polarity.
- Use a suitable series resistor to limit the current after breakdown, preventing damage.
- Do not exceed the maximum current rating (Iz_max) of the Zener diode.
3. Experiment: Transistor Characteristics (Common Emitter Configuration)
- Aim: To study the input and output characteristics of an n-p-n (or p-n-p) transistor in Common Emitter (CE) configuration and determine the current gain (β).
- Apparatus: n-p-n Transistor (e.g., BC547, BC107), two variable DC power supplies (0-3V for input Base-Emitter, 0-15V for output Collector-Emitter), DC Microammeter (0-100µA range for Base current, Ib), DC Milliammeter (0-50mA range for Collector current, Ic), DC Voltmeters (0-1V range for Base-Emitter voltage, Vbe; 0-15V range for Collector-Emitter voltage, Vce), Resistors (e.g., 100kΩ for base circuit, 1kΩ for collector circuit), connecting wires, breadboard, keys.
- Theory:
- A transistor (BJT - Bipolar Junction Transistor) has three terminals: Emitter (E), Base (B), and Collector (C).
- CE Configuration: Emitter is common to both input (Base-Emitter) and output (Collector-Emitter) circuits. Input signal is applied between Base and Emitter, output is taken across Collector and Emitter.
- Input Characteristics: A graph between Base Current (Ib) and Base-Emitter Voltage (Vbe) at a constant Collector-Emitter Voltage (Vce). It resembles the forward bias characteristic of a p-n junction diode.
- Input Dynamic Resistance: r_i = (ΔVbe / ΔIb) at constant Vce.
- Output Characteristics: A graph between Collector Current (Ic) and Collector-Emitter Voltage (Vce) at a constant Base Current (Ib). The graph shows different regions:
- Cut-off Region: Both junctions reverse biased (or zero biased). Ic is almost zero. (Below Ib=0 curve).
- Active Region: Input junction forward biased, output junction reverse biased. Ic increases slightly with Vce and is largely controlled by Ib. This is the region for amplification.
- Saturation Region: Both junctions forward biased. Ic increases sharply with Vce and is nearly independent of Ib. (Region near the Vce axis).
- Current Gain (β): Ratio of change in collector current to the change in base current in the active region.
- DC Current Gain: β_dc = Ic / Ib
- AC Current Gain: β_ac = (ΔIc / ΔIb) at constant Vce. (β is usually large, >50).
- Circuit Diagram (n-p-n CE):
(Note: R_B and R_C are current limiting/load resistors. Vbb biases input, Vcc biases output.)(+) Vbb Supply --- R_B --- Microammeter (Ib) --- Base | | Transistor (E common) (+) Vcc Supply --- R_C --- Milliammeter (Ic) --- Collector | | (-) Vbb & Vcc Supplies ------------------------- Emitter --- Ground/Common Voltmeter (Vbe) across Base-Emitter Voltmeter (Vce) across Collector-Emitter - Procedure Highlights:
- Input Characteristics: Set Vce to a constant value (e.g., 2V). Vary Vbe (using Vbb) and note the corresponding Ib. Repeat for another constant Vce (e.g., 4V).
- Output Characteristics: Set Ib to a constant value (e.g., 20µA) by adjusting Vbb/R_B. Vary Vce (using Vcc) from 0 upwards and note the corresponding Ic. Repeat for other constant Ib values (e.g., 40µA, 60µA).
- Observations & Graphs:
- Record readings in separate tables for input and output characteristics.
- Plot Input Characteristics: Vbe (X-axis) vs Ib (Y-axis) for different constant Vce values.
- Plot Output Characteristics: Vce (X-axis) vs Ic (Y-axis) for different constant Ib values.
- Calculations:
- Calculate Input Resistance (r_i) from the slope of the input characteristic curve.
- Calculate Output Resistance (r_o = ΔVce / ΔIc at constant Ib) from the slope of the output characteristic curve in the active region.
- Calculate AC Current Gain (β_ac) from the output characteristics by taking the ratio of change in Ic to change in Ib for a fixed Vce in the active region.
- Precautions:
- Identify E, B, C terminals correctly.
- Connect with correct polarities (for n-p-n: Base positive w.r.t Emitter, Collector positive w.r.t Emitter). Reverse for p-n-p.
- Do not exceed voltage/current/power ratings of the transistor.
- Use appropriate meter ranges.
4. Identification of Components & Multimeter Use
- Resistor: Typically cylindrical with colour bands indicating resistance value and tolerance (BBROY Great Britain Very Good Wife - Black Brown Red Orange Yellow Green Blue Violet Grey White).
- Capacitor: Various types (ceramic disc, electrolytic, polyester). Value and voltage rating often printed. Electrolytic capacitors have polarity (longer lead is usually positive, or a stripe indicates negative).
- Diode: Usually cylindrical, a band indicates the Cathode (n-side).
- LED (Light Emitting Diode): Longer lead is Anode (+), shorter lead is Cathode (-).
- Transistor: Typically has 3 leads. Shape (plastic/metal casing) and markings help identify type (n-p-n/p-n-p) and pinout (E, B, C) using a datasheet or multimeter test.
- IC (Integrated Circuit): Black rectangular package with multiple pins. A notch or dot indicates pin 1.
- Multimeter:
- Functions: Measures Voltage (DC/AC), Current (DC/AC), Resistance (Ω). May also test continuity (beeps if resistance is very low), diodes (shows forward voltage drop), transistor hFE (β).
- Connections: Voltmeter connected in PARALLEL across the component. Ammeter connected in SERIES with the component. Ohmmeter used on component removed from the circuit.
- Range Selection: Always start with the highest range and decrease as needed for better precision.
- Polarity: Observe correct polarity for DC measurements.
Multiple Choice Questions (MCQs)
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In the forward bias characteristic curve of a p-n junction diode, the 'knee voltage' typically represents:
a) The breakdown voltage
b) The voltage at which reverse current flows
c) The approximate barrier potential voltage
d) The peak inverse voltage -
When measuring the current through a forward-biased silicon diode, which meter and range would be most appropriate?
a) Microammeter (µA range)
b) Milliammeter (mA range)
c) Voltmeter (V range)
d) Ohmmeter (kΩ range) -
A Zener diode is primarily used in electronic circuits as a:
a) Current amplifier
b) Voltage regulator
c) Rectifier
d) Oscillator -
In the reverse bias characteristics of a Zener diode, when the Zener breakdown occurs:
a) The voltage across it increases sharply, and current becomes zero.
b) The voltage across it remains almost constant, while the current increases sharply.
c) Both voltage and current decrease.
d) The diode is destroyed immediately. -
To study the output characteristics (Ic vs Vce) of an n-p-n transistor in CE configuration, which parameter is kept constant for each curve?
a) Collector Current (Ic)
b) Collector-Emitter Voltage (Vce)
c) Base Current (Ib)
d) Base-Emitter Voltage (Vbe) -
The AC current gain (β_ac) of a transistor in CE configuration is defined as:
a) ΔIc / ΔIe at constant Vce
b) ΔIb / ΔIc at constant Vce
c) ΔIc / ΔIb at constant Vce
d) ΔVce / ΔIc at constant Ib -
While connecting a voltmeter to measure the voltage drop across a resistor in a circuit, it should be connected:
a) In series with the resistor
b) In parallel with the resistor
c) Before the resistor in the circuit path
d) After the resistor in the circuit path -
The band on a typical p-n junction diode indicates:
a) The p-side (Anode)
b) The n-side (Cathode)
c) The Emitter terminal
d) The Collector terminal -
A crucial precaution while determining the characteristics of a p-n junction diode or transistor is:
a) To use only AC power supply
b) To connect the ammeter in parallel
c) To ensure correct polarity of bias voltages and meters
d) To always keep the voltage at maximum -
The input dynamic resistance (r_i) of a transistor in CE configuration is calculated from the slope of the:
a) Output characteristics (Ic vs Vce)
b) Input characteristics (Vbe vs Ib)
c) Transfer characteristics (Ic vs Ib)
d) Reverse bias diode characteristics
Answer Key for MCQs:
- c
- b
- b
- b
- c
- c
- b
- b
- c
- b
Study these notes thoroughly, focusing on understanding the concepts behind the procedures and graphs. Good luck with your preparation!