- A photodiode operates in reverse bias, and its current increases when light falls on it.
- Light creates electron–hole pairs in the depletion region, increasing current.
- The reverse current increases with increasing light intensity.
- It has a fast response and works over a wide range of wavelengths.
- It is used in alarms, sensors, optical communication, and safety devices.
Definitions [46]
Define α.
The dc common-base current ratio or current gain (αdc) is defined as the ratio of the collector current (Ic) to emitter current. (Ig)
`alpha_"dc" = "I"_"C"/"I"_"E"`
Define β
The dc common-emitter current ratio or current gain (βdc) is defined as the ratio of the collector current (IC) to base current (IB).
`beta_"dc" = "I"_"C"/"I"_"B"`
Define peak value of alternating signal.
The maximum value of an alternating current or emf in either direction is known as the peak value.
Definition: Photodiode
A special type of p–n junction diode that converts light energy into electrical energy is called a photodiode.
Definition: Output Dynamic Resistance
The ratio of change in collector–emitter voltage to the corresponding change in collector current at constant base current is called output dynamic resistance (ro).
\[r_o=\frac{\Delta V_{CE}}{\Delta I_C}\]
Definition: Amplifier
A device used for increasing the amplitude of an alternating signal (voltage, current or power) is called an amplifier.
Definition: AC Current Gain
The ratio of AC collector current to AC base current is called AC current gain (βAC).
\[\beta_{AC}=\frac{i_C}{i_B}\]
Definition: Bipolar Junction Transistor
A semiconductor device having two p–n junctions and three terminals (emitter, base and collector) is called a Bipolar Junction Transistor (BJT).
Definition: Input Dynamic Resistance
The ratio of change in base–emitter voltage to the corresponding change in base current at constant VCE is called input dynamic resistance (ri).
\[r_i=\frac{\Delta V_{BE}}{\Delta I_B}\]
Definition: DC Current Gain
The ratio of collector current to base current in DC operation is called DC current gain (βDC).
\[\beta_{DC}=\frac{I_C}{I_B}\]
Definition: DC Current Amplification Factor
The ratio of collector current to emitter current in DC operation is called DC current amplification factor (αDC).
\[\alpha_{DC}=\frac{I_C}{I_E}\]
Definition: Input Characteristic
The variation of base current (IB) with base–emitter voltage (VBE) at constant collector–emitter voltage is called input characteristic.
Definition: Output Characteristic
The variation of collector current (IC) with collector–emitter voltage (VCE) at constant base current is called output characteristic.
Definition: Voltage Gain
The ratio of output voltage to input voltage in an amplifier is called voltage gain (AV).
AV = \[-\frac{\beta_{AC}R_L}{r_i}\]
Definition: Digital Electronics
An electronic circuit that handles only a digital signal is called a digital circuit, and the branch of electronics which deals with digital circuits is called digital electronics.
Definition: Rectification
The conversion of AC voltage into a DC voltage is called rectification.
Definition: Full-Wave Rectifier
A rectifier circuit that uses both half cycles of the AC input voltage to produce DC output is called a full-wave rectifier.
Definition: Half-Wave Rectifier
The process of converting alternating current (AC) into direct current (DC) using only one diode, in which current flows only during one half cycle of the AC input is called a half-wave rectifier.
Definition: Ripple
The unwanted AC component present in the rectified DC output is called ripple.
Definition: Ripple Factor
The ratio of the root mean square (r.m.s.) value of the AC component to the DC component in the rectifier output is called the ripple factor.
Mathematically,
Ripple Factor = \[\frac{\text{r.m.s. value of AC component}}{\mathrm{DC~value}}\]
Definition: Zener Voltage
The voltage at which a Zener diode operates in the reverse breakdown region is called Zener voltage (VZ).
Definition: Filter Circuit
A circuit that removes the AC component (ripple) from the rectifier output and allows only the DC component is called a filter circuit.
Definition: Unregulated Power Supply
A power supply whose output voltage changes when a load is connected across it is called an unregulated power supply.
Definition: Maximum Zener Current
The maximum current that can flow through a Zener diode at its rated voltage is called the maximum Zener current (IZ(max)).
IZ(max) = \[\frac {P_Z}{V_Z}\]
Definition: Logic Gate
A digital circuit with one or more input signals but only one output signal is called a logic gate.
Definition: Boolean Expression
The mathematical statement that provides the relationship between the input and the output of a logic gate is called a Boolean expression.
Definition: Regulated Power Supply
A power supply whose output voltage remains constant even after connecting a load across it is called a regulated power supply.
Definition: Zener Voltage Regulator
A voltage regulator circuit that commonly uses a Zener diode to maintain constant output voltage is called a Zener voltage regulator.
Definition: Junction Breakdown
The sudden increase in reverse current of a p–n junction diode when the reverse voltage exceeds a critical value is called junction breakdown.
Definition: Breakdown Voltage
The critical reverse voltage at which breakdown occurs in a diode is called breakdown voltage.
Definition: Zener Diode
A p–n junction diode specially designed to operate in the breakdown region is called a Zener diode.
Definition: Dark Resistance
The resistance of a photodiode when it is not illuminated is called dark resistance.
Definition: Zener Breakdown
The breakdown occurring in a heavily doped p–n junction due to strong electric field breaking covalent bonds is called Zener breakdown.
Definition: Voltage Regulator
A circuit that maintains a constant output voltage despite variations in input voltage or load current is called a voltage regulator.
Definition: Zener Resistance
The opposition offered to the current flowing through a Zener diode in its operating region is called Zener resistance (Zener impedance).
Definition: Dark Current
The reverse current that flows through a photodiode even when it is not illuminated is called dark current.
Definition: Power Rating
The maximum power that a Zener diode can safely dissipate is called power rating.
PZ = VZ × IZ(max)
Definition: Saturation Current
The constant maximum reverse current reached at higher illumination levels is called the saturation current.
Definition: Solar Cell
A device that converts solar energy directly into electrical energy using a p–n junction is called a solar cell or photovoltaic cell.
Definition: Light-Generated Current
The current produced in a solar cell due to incident light is called light-generated current or photo-generated current.
Definition: Short-Circuit Current
The current flowing through a solar cell when its terminals are shorted is called short-circuit current (ISC).
Definition: Open-Circuit Voltage
The voltage across a solar cell when no external load is connected is called open-circuit voltage (VOC).
Definition: Light Emitting Diode
A p–n junction diode that emits light when forward biased is called a Light Emitting Diode (LED).
Definition: Cut-in Voltage
The minimum forward voltage at which an LED starts conducting and emitting light is called the cut-in voltage.
Definition: Electroluminescence
The recombination of electrons and holes releasing energy in the form of photons is called electroluminescence.
Definition: Power Gain
The product of current gain and voltage gain in an amplifier is called power gain (AP).
AP = βAC × AV
Formulae [2]
Formula: Dark Resistance
Rd = \[\frac{\text{Maximum reverse voltage}}{\text{Dark current}}\]
Formula: Power Delivered to the Load
PL = V × I
At maximum power point:
PL = VOC × ISC
Key Points
Key Points: p-n Junction Diode as o Rectifier
- A diode allows current to flow in one direction, so it converts AC to DC.
- A half-wave rectifier uses only one half-cycle of AC, so its efficiency is low (40.6%).
- A full-wave rectifier uses both half-cycles, resulting in higher efficiency (81.2%) and less ripple.
Key Points: Logic Gates
- A NOT gate has one input and one output; it produces the opposite (complement) of the input.
- An OR gate gives output 1 when at least one of its inputs is 1.
- An AND gate gives output 1 only when all its inputs are 1.
- A NAND gate is the complement of an AND gate; its output is 0 only when all inputs are 1.
- A NOR gate is the complement of an OR gate; its output is 1 only when all inputs are 0.
- An XOR (Exclusive-OR) gate gives output 1 when the two inputs are different (odd number of 1’s).
Key Points: Photo Diode
Key Points: Light Emitting Diode
- LED emits light when forward-biased due to the recombination of electrons and holes at the junction.
- The intensity of light emitted increases with an increase in forward current.
- LEDs are made from compound semiconductors, and different materials produce different colours.
- LEDs are energy-efficient, long-lasting, fast-responding, and environmentally friendly.
- LEDs are widely used in indicators, display systems, lighting, optical communication, and seven-segment displays.
Important Questions [11]
- What is a rectifier?
- Draw the circuit diagram of a half-wave rectifier. Hence explain its working.
- Draw a neat diagram of a full-wave rectifier and explain it’s working.
- An LED emits visible light when it’s ______.
- Solar cell operates on the principle of ______.
- What is a Light Emitting Diode?
- Draw Light Emitting Diode circuit symbol.
- Define α
- Define β
- Derive the relation between α and β.
- Explain the working of the n-p-n transistor in a common base configuration.
