Semiconductor Diodes: Operation and Characteristics

• A diode represents a two-electrode device that allows electric current to flow one way, blocking it the other way.
• This unilateral behavior is mainly used in switching and rectification.
• Diodes are the first electronic device invented
• They were initially bulky and slow vacuum tubes that needed a lot of power.
• Semiconductor diodes are small, require low power, and are very fast
• Semiconductor diodes are now in a wide range of applications.

Module 1 Learning Outcomes

• Explain the operation of a PN junction diode under different biasing conditions.
• Plot the I-V characteristics of the diode.
• Define static and dynamic resistance of the diode.
• Explain the breakdown phenomenon observed in diodes.
• Describe the working of a Zener diode and plot its I-V characteristics.
• Explain the diode's function as a capacitor.

Introduction

• Materials can be classified as metals, insulators, and semiconductors.
• Semiconductors, such as Germanium or Silicon, have electrical conductivity that lies between conductors and insulators.
• Semiconductors are basic materials in modern electronics.
• Examples: diodes, transistors, solar cells, LEDs, and integrated circuits.

Concept of a PN Junction

• P-type semiconductors have a large amount of holes.
• N-type semiconductors have a large amount of free electrons.
• When P-type joins with N-type materials, a gradient is created at the connection.
• Electrons move from the N-type to P-type, and holes move from the P-type material to N-type.
• Diffusion involves charge carriers moving from high to low concentration areas without an external electric field
• Diffusion will occur until equilibrium is established.
• At the junction, N-type becomes positive and P-type becomes negative.
• Regions on either side of the p-n interface lose neutrality and become charged.
• These charged regions are called the space charge region or depletion region since they lack mobile charge carriers.

PN Junction Under Bias

• Applying an external voltage to a diode is called biasing.
• Biasing conditions include no bias (zero bias), forward bias, and reverse bias.

Zero Bias

• Without bias voltage, the net flow of charge carriers in the semiconductor diode is zero.
• Minority carriers create a barrier in the depletion region, preventing flow and resulting in high impedance.
• Built-in potential ranges from 0.3 to 0.7V depending on the semiconductor material.

Forward Bias

• Applying negative voltage to the N-type and positive voltage to the P-type creates forward bias.
• If external voltage exceeds the barrier potential, current flows and the diode is "ON".

Reverse Bias

• A positive voltage to the N-type and negative voltage to the P-type creates a reverse biased condition.
• The positive voltage attracts electrons, widening the depletion layer.
• Increased barrier prevents forward current
• Applied potential favors minority carrier movement, causing reverse saturation current (I0 or IS).

I-V Characteristics of a Diode

• A practical diode in forward bias has current initially at zero, then increases sharply after cut-in voltage.
• A diode in forward bias behaves as a closed switch.
• In reversed bias, the diode acts as an open switch, with small current due to minority charge carriers, or reverse saturation current.
• At a particular reverse voltage, breakdown voltage, a sudden rise in current will be seen.

Forward Biased Characteristics

• Applying forward bias results in a thin depletion layer, offering low resistance for current flow.
• "Knee" point is where a sudden increase in current takes place.
• The "knee" marks the point on the static I-V characteristics where current rises sharply.

Reverse Biased Characteristics

• The PN junction offers high resistance.
• Almost zero current flows until the bias voltage.
• Leakage current which is very small, is of microamperes (µA ) for ordinary rectifier diodes.

Diode Current

• The current through the diode is calculated using a specific equation involving reverse saturation current.
• Applied bias voltage affects the diode current.
• Temperature equivalents factor into the diode current calculation.
• Germanium and silicon have different temperature equivalents.
• The current for large forward bias approximates to a simplified equation.

Effect of Temperature on Reverse Current

• The reverse saturation current doubles with every 10°C rise in temperature.

Static and Dynamic Resistance of a Diode

• There are two types of diode resistance: DC (static) and AC (dynamic) resistance.

DC or Static Resistance

• Static resistance is found by the values of VD and ID.
• DC resistance does not depend on curve shape but on the operating point or values of voltage and current.

AC or Dynamic Resistance.

• The dynamic resistance is curve tangent at the operating point.
• Dynamic resistance is calculated using a specified formula.

Ideal and Practical Diode

• An ideal diode has current flowing freely when forward biased and is known as an abrupt junction
• An ideal diode offers no resistance and has instantaneous material change, and will have zero voltage drop across the diode
• A practical diode will have some resistance to current flow when forward biased.
• Built-in potential leads to power dissipation.
• Practical diodes have current limits to avoid damage.
• A reverse biased diode has high resistance, but reverse bias can cause damage.

Equivalent Circuit of a Diode

• Diodes are replaced by equivalent circuits for analysis and design.
• Simplifications can be made to models for ideal diodes and more practical diodes

For an Ideal Diode

• An ideal diode has Vy = 0, RR = ∞, and RF = 0.
• For forward bias, the ideal diode is a short.
• For reverse bias, the ideal diode is an open.

Practical Silicon Diode

• The typical practical silicon diode has Vy = 0.7 V.
• RR < ∞, typically several MΩ.
• RF ≈ rd, typically less than 50 Ω.

Zener Diode Characteristics

• Zener diode, regulator diode, is used in voltage regulators.
• The voltage drastically increases when the reverse bias reaches Zener voltage Vz, which keeps voltage constant.
• Heavily doped, it allows reverse current flow when bias voltage is reached.
• The Zener diodes regulates voltage from 3V to hundreds of volts, with power range from 200mW to 100W.
• Zener diodes are used in "REVERSE" bias mode.
• Zener diodes maintain almost constant voltage through their reverse bias, regardless of current, which can be used to stabalize voltage.
• A Zener diode continues to regulate until the diode current drops below Iz.
• A Zener diode acts as an ordinary diode when forward biased.
• Zener diode is used to control voltage and provide constant voltage reference.
• IZK or IZmin is the minimum current to maintain breakdown.
• IZM or IZMax is the maximum current that can pass through the Zener diode safely.
• PZM or PZMax is the maximum power dissipation across Zener diode.

Diode as Capacitor

• Varactor diode is a capacitor that operates in reverse-bias.
• It is doped to maximize the inherent capacitance of the depletion region
• The region has nonconductive characteristics.
• The p- and n-regions act ascapacitor plates, affecting capacitance
• The capacitance is determined by plate area, dielectric constant, and plate separation.
• As reverse-bias voltage increases, plate separation increases, decreasing capacitance.
• Varactors are used in tuning circuits, such as VHF, UHF, and satellite receivers.
• As a parallel resonant circuit, the varactor acts as a variable capacitor, allowing adjustment of frequency via a voltage lever, and are also used in Frequency Modulation circuits.

Key summary points

• PN junction forward biased condition acts as closed switch; reversed biased acts as open switch.
• Reverse saturation current of a diode doubles for every 10°C rise in temperature.
• Zener diodes have two breakdown mechanisms: avalanche breakdown and Zener breakdown.
• Zener diodes are generally used in reverse breakdown region.
• A Zener diode has constant voltage across its over a range of currents.
• Varactor diodes operate in reverse bias and act as variable capacitance.

Application of Diodes

• Diode conducts when forward biased and is a closed switch.
• Diode turns goes off and is an open switch during reverse bias.
• Unilateral polarity is used to convert AC to DC, which is called rectification.

Module 2 learning outcomes

• Explain the need for AC to DC conversion
• Draw and explain the block diagram of a basic dc power supply unit.
• Discuss the importance of the various components used in the rectifier circuits.
• Discuss the working of a half wave and full wave rectifier circuits.
• Analyse the performance of rectifier circuits and compare them.
• Explain the working of rectifier circuits with capacitor filter.

Introduction to Diodes

• Modern life relies on electronic products such as phones or computers.
• DC voltage is supplied from the AC mains or batteries.
• Circuits convert AC mains to constant DC voltage

Signals

• AC power supply signal is sinusoidal.
• Defined by its peak amplitude and frequency.
• Peak amplitude is the maximum amplitude measured from zero.
• Frequency is the number of cycles per second, equal to the reciprocal of time taken to complete one cycle.
• Signals are expressed mathematically, and are plotted in figures.
• The DC value of a signal is the average value which is evaluated mathematically
• A pure sinusoidal signal has an average value of zero. The DC value of this signal is zero.

Basic Block Diagram of DC Power Supply

• A DC power supply converts AC to DC.
• A DC regulated power supply has: step-down transformer, rectifier circuits, and regulator.

Step Down Transformer

• Transformers alter voltage levels in circuits.
• A step-down transformer consists of primary and secondary wire coils.
• High voltage and low current is converted to low voltage high current, and have a large number of turns in primary coil compared to the secondary.
• The relationship between primary and secondary voltage depends on number of coils
• Transformers are used because they step down voltage and they provide electrical isolation for the components they supply

Rectifier Circuit

• The rectifier circuit is key to the DC power supply.
• They convert bidirectional AC signals to unidirectional signals, where current flows one way which is pulsating DC
• The process of converting AC signal to pulsating DC signal is called rectification and is commonly performed using semiconductor diodes.
• The AC component of a rectified output signal is not equal to zero. That is the rectified pulsating DC signal has both DC and AC components.

Filter Circuit

• The appliance needs pure DC voltage.
• Filters smooth pulsations or eliminate AC components to achieve constant voltage.
• Filters have capacitors that hold voltage, and then dissipate energy to the load when DC voltage drops.
• The is minimises the ac component and increases the DC value

Regulators

• Output of filter circuit is a DC signal with small AC component called ripples
• Regulation is the system's consistent supply capability, even with load and power fluctuations.
• A circuit that handles mains AC power variations or changing the load is called a voltage regulator

Half Wave Rectifier

• The half wave rectifier consists of a single semiconductor diode.
• The secondary voltage is fed to a rectifier circuit to measure outcome of the load resistance RL.
• For simple it is assumes that the diodes are perfect. The diodes are open when backwards biased and shorted when they have forward bias.

Half wave rectifier (HWR) circuit

• The diode conducts only during positive half cycle.
• The diode is referred to as Half Wave Rectifier
• Rectified voltage is pulsating DC and can be smoothened using filter circuits.
• The AC component of a rectified output signal is not equal to zero. That is the rectified pulsating DC signal has both DC and AC components.

HWR Circuit Parameters

• DC voltage is the average output across the load resistor.
• Ripple factor (γ) is the ratio of AC to DC components.
• Higher ripple factor indicates more AC content.
• Efficiency (η) is the ratio of DC output power to AC input power from the transformer.

PIV

• Efficiency shows outcome of the circuit to output DC power compared to AC input.
• During the negative half cycle, the diode is reverse biased.
• Maximum voltage is equal to the secondary peak voltage.
• PIV should be greater than Vm.

Full Wave Rectifier

• The transformer, is grounded.
• A center tapped transformer includes two input and three output nodes with end labeled as node A and node B.
• The center node is used as common reference voltage.
• Common ground in center tapped transformer is the center output node
• 2Vml Analysis of Center tap FWR circuit:

Secondary waveforms

• The secondary voltage observed between end nodes is a stepped down voltage.
• Magnitude of voltages between node A and ground or node B and center node is half.
• Phase shifts 180° when voltage is measured on node B.

Working of Centre Tapped Full Wave Rectifier Circuit

• During the first half cycle, voltage at node A is positive while voltage at node B is negative compared to ground.
• D1 gets forward biased and D2 reverse biased. Load resistor connects diodes' cathodes to ground.
• With ideal diodes, a transformer's secondary windings results in current flow
• Direction of current through load is towards ground .
• Measured voltage is equal to voltage at node A wrt ground.
• When node A's voltage is negative is a half cycle, the node B is positive compared to grnd.

PIV for center-tapped FWR

• The maximal voltage across the reversed diode, is twice the peak value of between either ends and centers' node.
• PIV of has PIV of 2Vm.

Components of the average or the DC are shown to be the value of FWR

Average are the equations to calculate average or the DC component and ripple factor

Bridge Wave Rectifiers

• Bridge rectifier has four diodes.
• Two parallel paths can shown, each with two diodes.
• All diodes are directed the same way
• Top path has diodes D3 and D1
• Bottom path has diodes D2 and D4 .
• Load has connected between the ends parallel paths.
• With correct secondary, the output is shown when assuming that diodes are perfect.

Operation of Bridge Wave Rectifiers

• The bridge rectifier performs rectification for both halves of AC power signal
• During the positive half cycle, D1 and D2 show forward bias, thus leading to closed path from node A. Through D1 the flows R-Load and D2 and from node B through the secondary coil. At the same time Dis and D4 are revered as shown in the diagram.
• Current flow through load is unidirectional for both half cycles.
• The the voltage is the peek voltage since it will be secondary peak voltage. To improve Vdc, Irms, Vrms Ripple should be used

Comparison

• The two kinds of wave have different advantages. Diodes PIV must be properly used to ensure the efficiency. Center type are more efficient

Filters

Rectifier with Filter

• The diode conducts only during positive half cycle, This known as Half Wave Rectifier, Rectified voltage requires filter type circuits
• During the positive half cycle, of transformer, the capacitor charges to peak and attempts to maintain maximum value and input fall to to zero.
• Load dissipates capacitor energy and the input raises the the capacitor voltage.

Voltage Regulators

• Zener diodes widely regulate voltage.
• In parallel connected with a variable voltage source, Zener diodes act as open circuit until they voltage reaches the diodes maximum and voltage reaches it.
• At the breakdown voltage the level diode remains same. Zener good voltage reference is is used in figure shown

Voltage Regulate Module outcomes:

1.Describe the operation of the Zener as a voltage regulator 2.Understand the functions of the IC based based voltage regulator

Regulator with Zener

• Zener can be the load. This circuits sustain constant voltage over diode levels. Extra diode is directed into diod
• Diode does not damage the power.
• Output can be found on diagram for for proper power for the diode
• For proper Load Diagram find diagrams for that proper function

Calculating Resister Z(Max

• The proper R should grant that that the minimum Zener should provide enough power to allow the diode to do maximum output where Pz does not does provide less current
• For line regulation and load to be at its their proper range

Applications

Common Diode applications are Voltage regulator, Voltage Limiter, protection and reference

IC

Diagram shows proper voltage. Diagram are versatile to to provide variety for applications. Diagram can be used to fix adjustable regulators for current and voltage. Includes fixed regulators