Electronics Devices and Circuits Notes

PN Junction Theory

• A PN-junction is formed when an N-type material is fused together with a P-type material, creating a semiconductor diode.

• N-type semiconductor: Silicon doped with Antimony.

• P-type semiconductor: Silicon doped with Boron.

PN Junction Formation

• Individual N-type and P-type materials are electrically neutral and do little on their own.

• Joining them creates a PN Junction with distinct behavior.

• A large density gradient exists at the junction.

• Free electrons from the N-type material migrate to fill holes in the P-type material, creating negative ions.

• Electrons moving from N-type to P-type leave behind positively charged donor ions (N_D).

• Holes from the P-type material migrate to the N-type region with many free electrons.

• The P-type region along the junction becomes filled with negatively charged acceptor ions (N_A).

• The N-type region along the junction becomes positively charged.

• This charge transfer is called diffusion.

• The width of P and N layers depends on acceptor density (NA) and donor density (ND), respectively.

Equilibrium and Depletion Layer

• The diffusion process continues until a large enough electrical charge prevents further charge carriers from crossing the junction.

• Equilibrium is reached, resulting in a "potential barrier" around the junction.

• Donor atoms repel holes, and acceptor atoms repel electrons.

• The regions around the PN Junction become depleted of free carriers, forming the Depletion Layer.

Charge Neutrality

• The total charge on each side of the PN Junction must be equal and opposite for a neutral charge condition.

• If the depletion layer has a distance D, it penetrates into the silicon by a distance of Dp for the positive side and Dn for the negative side.

• Relationship for charge neutrality (equilibrium): Dp * NA = Dn * ND

Built-in Potential Difference

• N-type material becomes positive with respect to the P-type material as electrons are lost from the N-type and holes are lost from the P-type.

• Impurity ions on both sides of the junction create an electric field with the N-side at a positive voltage relative to the P-side.

• Free charges require extra energy to overcome the potential barrier to cross the depletion region.

• The electric field creates a "built-in potential difference" across the junction.

Zero Bias Junction Voltage

• Open-circuit (zero bias) potential is given by:
  E_o

• Where:

  • E_o is the zero bias junction voltage

  • V_T is the thermal voltage (26mV at room temperature)

  • ND and NA are the impurity concentrations

  • n_i is the intrinsic concentration

• A positive voltage (forward bias) can supply free electrons and holes with the extra energy needed.

• The external voltage required depends on the semiconductor material and its temperature.

• Typically, at room temperature:

  • Silicon: 0.6 – 0.7 volts

  • Germanium: 0.3 – 0.35 volts

• This potential barrier exists even without an external power source.

Significance of Built-in Potential

• The built-in potential opposes the flow of holes and electrons across the junction, hence the term "potential barrier."

• A PN junction is formed within a single crystal rather than joining separate pieces.

• Results in rectifying current–voltage (IV or I–V) characteristics.

• Electrical contacts are fused onto either side for external circuit connection.

• The resulting device is called a PN junction Diode or simply Signal Diode.

PN Junction Diode

• Formed when a p-type semiconductor is fused to an n-type semiconductor, creating a potential barrier voltage across the diode junction.

• Connecting to a battery source provides additional energy to overcome the potential barrier.

• This allows free electrons to cross the depletion region.

• The behavior of the PN junction produces an asymmetrical conducting two-terminal device: the PN Junction Diode.

Diode Characteristics

• A PN Junction Diode passes current in one direction only.

• Unlike a resistor, a diode's behavior is non-linear with respect to applied voltage.

• It has an exponential current-voltage (I-V) relationship, not described by Ohm’s law.

• Applying a suitable positive voltage (forward bias) supplies extra energy to cross the junction, decreasing the depletion layer width.

• Applying a negative voltage (reverse bias) pulls free charges away from the junction, increasing the depletion layer width.

• This changes the effective resistance of the junction, allowing or blocking current flow.

• The depletion layer widens with increased reverse voltage and narrows with increased forward voltage.

• The differences in electrical properties cause physical changes, resulting in rectification (asymmetrical current flow with altered bias voltage polarity).

Junction Diode Biasing

• Operating Regions: Two operating regions.

• Biasing Conditions: Three possible biasing conditions

  1. Zero Bias: No external voltage is applied.

  2. Reverse Bias: Voltage is negative (-ve) to P-type and positive (+ve) to N-type, increasing the depletion width.

  3. Forward Bias: Voltage is positive (+ve) to P-type and negative (-ve) to N-type, decreasing the depletion width.

Zero Biased Junction Diode

• Zero Bias: No external potential energy is applied.

• Shorting the terminals allows a few holes (majority carriers) with enough energy to overcome the potential barrier to move across the junction ("Forward Current", I_F).

• Holes generated in the N-type material (minority carriers) move across the junction in the opposite direction ("Reverse Current", I_R).

• This transfer of electrons and holes is called diffusion.

Equilibrium in Zero Bias

• The potential barrier discourages diffusion of more majority carriers.

• The potential barrier helps minority carriers to drift across the junction.

• "Equilibrium" is established when majority carriers are equal and moving in opposite directions, resulting in zero net current.

• The junction is in a state of "Dynamic Equilibrium."

• Minority carriers are constantly generated due to thermal energy.

• Raising the temperature increases minority carrier generation, increasing leakage current, but no electric current flows without a connected circuit.

Reverse Biased PN Junction Diode

• Reverse Bias: Positive voltage applied to the N-type material and negative voltage to the P-type material.

• Positive voltage attracts electrons away from the junction, and negative voltage attracts holes away from the junction.

• The depletion layer widens due to a lack of electrons and holes.

• This creates a high impedance path (almost an insulator) and a high potential barrier, preventing current flow.

Reverse Bias Characteristics

• High resistance value with practically zero current flow.

• A very small reverse leakage current flows (measured in micro-amperes, μA).

• If the reverse bias voltage (V_r) is increased excessively, it causes the diode's PN junction to overheat and fail due to the avalanche effect.

• This may cause a short circuit, leading to maximum circuit current flow.

Avalanche Effect and Zener Diodes

• The avalanche effect has applications in voltage stabilizing circuits.

• A series limiting resistor is used with the diode to limit the reverse breakdown current to a preset maximum value, producing a fixed voltage output.

• These diodes are known as Zener Diodes.

Forward Biased PN Junction Diode

• Forward Bias: A negative voltage is applied to the N-type material, and a positive voltage is applied to the P-type material.

• If the external voltage exceeds the potential barrier (approx. 0.7V for silicon, 0.3V for germanium), the barrier is overcome, and current starts to flow.

• The negative voltage pushes electrons towards the junction, and the positive voltage pushes holes towards the junction.

• This results in zero current flow up to the "knee" point on the static curves, then a high current flow with little increase in external voltage.

Forward Bias Characteristics

• Forward biasing results in a thin and narrow depletion layer, representing a low impedance path.

• The "knee" point on the static I-V characteristics curve represents the sudden increase in current.

Forward Bias Behavior

• Represents a low resistance path through the PN junction, allowing large currents to flow with a small increase in bias voltage.

• The potential difference across the junction is kept constant by the depletion layer at approximately 0.3V for germanium and 0.7V for silicon diodes.

• Resistors are used in series with the diode to limit current flow because the diode can conduct "infinite" current above the knee point, effectively becoming a short circuit.

• Exceeding the maximum forward current specification causes the device to dissipate excessive power as heat, leading to failure.

Key Characteristics of PN Junction Diodes

• Semiconductors contain two types of mobile charge carriers: "Holes" and "Electrons."

• Holes are positively charged, and electrons are negatively charged.

• N-type doping: Semiconductors are doped with donor impurities (e.g., Antimony), containing primarily mobile electrons.

• P-type doping: Semiconductors are doped with acceptor impurities (e.g., Boron), containing primarily mobile holes.

• The junction region has no charge carriers and is known as the depletion region.

• The junction (depletion) region has a physical thickness that varies with the applied voltage.

Diode Biasing Summary

• Zero Bias:

  • No external energy source is applied.

  • A natural Potential Barrier is developed across a depletion layer (approx. 0.5 to 0.7V for silicon diodes, approx. 0.3V for germanium diodes).

• Forward Bias:

  • The depletion region thickness reduces.

  • The diode acts like a short circuit, allowing full circuit current to flow.

• Reverse Bias:

  • The depletion region thickness increases.

  • The diode acts like an open circuit, blocking any current flow (only a small leakage current flows).

Power Diodes

• Small signal diodes are used in low-power, low current (less than 1-amp) rectification and power supply applications.

• For larger forward bias currents or higher reverse bias blocking voltages, power diodes are required.

• The power semiconductor diode has a much larger PN junction area, resulting in a high forward current capability (up to several hundred amps (KA)) and a reverse blocking voltage (up to several thousand volts (KV)).

• Power diodes are generally unsuitable for high frequency applications above 1MHz (although special high frequency, high current diodes are available).

• For high frequency, low voltage rectifier applications, Schottky Diodes are generally used because of their short reverse recovery time and low voltage drop in their forward bias condition.

• Power diodes have a forward “ON” resistance of fractions of an Ohm and a reverse blocking resistance in the mega-Ohms range.

• Larger power diodes are often “stud mounted” onto heatsinks to reduce their thermal resistance (0.1 to 1 oC/Watt).

AC Voltage and Power Diodes

• When an alternating voltage is applied across a power diode:

  • Positive half cycle: the diode conducts, passing current.

  • Negative half cycle: the diode does not conduct, blocking current flow.

• Conduction only occurs during the positive half cycle (unidirectional, i.e., DC).

Halfwave Rectification

• Rectifier Definition: A circuit that converts Alternating Current (AC) input power into Direct Current (DC) output power.

• Simplest rectifier circuit: Half Wave Rectifier.

• The power diode passes just one half of each complete sine wave of the AC supply.

• It is called a "half-wave" rectifier because it passes only half of the incoming AC power supply.

Half-Wave Rectifier Operation

• Positive Half Cycle:

  • The diode is forward biased (anode is positive with respect to the cathode), resulting in current flowing through the diode.

  • The DC load is resistive (resistor, R), the current is proportional to the voltage (Ohm’s Law), and the voltage across the load resistor is the same as the supply voltage, Vs (minus Vf).

  • Vout = Vs

• Negative Half Cycle:

  • The diode is reverse biased (anode is negative with respect to the cathode).

  • NO current flows through the diode or circuit.

  • No voltage appears across the load resistor, so V_{out} = 0.

• The current on the DC side flows in one direction only, making the circuit Unidirectional.

Equivalent DC Voltage

• The load resistor receives a positive half of the waveform, zero volts, a positive half of the waveform, zero volts, etc.

• The value of this irregular voltage is equal to an equivalent DC voltage of:

  • 0.318 * V_{max} of the input sinusoidal waveform

  • 0.45 * V_{rms} of the input sinusoidal waveform

• The equivalent DC voltage, V_{DC}, across the load resistor is calculated as follows.

Power Diode Example

Calculate the voltage drop V{DC} and current I{DC} flowing through a 100Ω resistor connected to a 240 V_{rms} single phase half-wave rectifier. Also, calculate the average DC power consumed by the load.

V{DC} = 0.318 V{MAX} = 0.318 * (240 * 1.414) = 108 Volts

I{DC} = \frac{V{DC}}{R} = \frac{108V}{100Ω} = 1.08 Amps

Power = I^2R = 1.08^2 * 100 = 116 Watts

Half-wave Rectifier with Smoothing Capacitor

• A smoothing capacitor is connected in parallel with the load resistor to reduce ripple voltage.

• During the positive half cycle, the capacitor charges, storing energy.

• During the negative half cycle (when the diode is OFF), the capacitor discharges through the load resistor, maintaining the output voltage.

Capacitor Smoothing and Ripple

• For a given capacitor value, a greater load current (smaller load resistance) will discharge the capacitor more quickly (RC Time Constant) and increases the ripple obtained.

• For single phase, half-wave rectifiers, reducing the ripple voltage by capacitor smoothing alone is generally impractical.

• "Full-wave Rectification" is a more practical solution.

Disadvantages of Half-Wave Rectifiers

• The output amplitude is less than the input amplitude.

• There is no output during the negative half cycle, so half the power is wasted.

• The output is pulsed DC, resulting in excessive ripple.

Fullwave Rectifier

• Like the half wave circuit, a full wave rectifier circuit produces an output voltage or current which is purely DC or has some specified DC component.

• Full wave rectifiers have some fundamental advantages over their half wave rectifier counterparts. The average (DC) output voltage is higher than for half wave, the output of the full wave rectifier has much less ripple than that of the half wave rectifier producing a smoother output waveform.

• In a Full Wave Rectifier circuit two diodes are now used, one for each half of the cycle. A multiple winding transformer is used whose secondary winding is split equally into two halves with a common centre tapped connection, (C).

Full-Wave Rectifier Circuit Operation

• Employs two diodes, one for each half of the AC cycle.

• Utilizes a multiple winding transformer with a secondary winding split equally into two halves with a common center-tapped connection (C).

• Each diode conducts in turn when its anode terminal is positive with respect to the transformer center point C, producing an output during both half-cycles.

Full-Wave Rectifier Details

• The full wave rectifier circuit consists of two power diodes connected to a single load resistance (RL) with each diode taking it in turn to supply current to the load.

• When point A of the transformer is positive with respect to point C, diode D1 conducts in the forward direction as indicated by the arrows.

• When point B is positive (in the negative half of the cycle) with respect to point C, diode D2 conducts in the forward direction and the current flowing through resistor R is in the same direction for both half-cycles.

• As the output voltage across the resistor R is the phasor sum of the two waveforms combined, this type of full wave rectifier circuit is also known as a “bi-phase” circuit.

Full-wave Rectifier Output Waveform

• The spaces between each half-wave developed by each diode is now being filled in by the other diode the average DC output voltage across the load resistor is now double that of the single half-wave rectifier circuit and is about 0.637Vmax of the peak voltage, assuming no losses.

• Where: VMAX is the maximum peak value in one half of the secondary winding and VRMS is the rms value as: VRMS = 0.7071VMAX. The DC current is given as: IDC = VDC/R.

The Full Wave Bridge Rectifier

• Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop “bridge” configuration to produce the desired output.

• The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below. The Diode Bridge Rectifier

Full-Wave Bridge Rectifier Operation

• Components: Uses four diodes (D1 to D4) arranged in a bridge configuration.

• Positive Half Cycle:

  • Diodes D1 and D2 conduct in series.

  • Diodes D3 and D4 are reverse biased.

  • Current flows through the load.

• Negative Half Cycle:

  • Diodes D3 and D4 conduct in series.

  • Diodes D1 and D2 switch “OFF” (reverse biased).

  • Current flowing through the load is in the same direction as before.

Transistor Bipolar Transistor

• The Bipolar Junction Transistor is a semiconductor device which can be used for switching or amplification

• Unlike semiconductor diodes which are made up from two pieces of semiconductor material to form one simple pn-junction.

• The bipolar transistor uses one more layer of semiconductor material to produce a device with properties and characteristics of an amplifier.

• If we join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series which would share a common Positive, (P) or Negative, (N) terminal.

• The fusion of these two diodes produces a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT for short.

Transistor Basics

• Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor’s ability to change between these two states enables it to have two basic functions: “switching” (digital electronics) or “amplification” (analogue electronics).

• Then bipolar transistors have the ability to operate within three different regions

  • Active Region – the transistor operates as an amplifier and Ic = β*Ib

  • Saturation – the transistor is “Fully-ON” operating as a switch and I_c = I(saturation)

  • Cut-off – the transistor is “Fully-OFF” operating as a switch and I_c = 0

Bipolar Transistor Terminals

• Bipolar Transistor Construction The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter (E), the Base (B) and the Collector (C) respectively.

• Bipolar Transistors are current regulating devices that control the amount of current flowing through them from the Emitter to the Collector terminals in proportion to the amount of biasing voltage applied to their base terminal, thus acting like a current-controlled switch.

• As a small current flowing into the base terminal controls a much larger collector current forming the basis of transistor action.

Bipolar Transistor Configurations

• As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output signals.

• Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement.

The Common Base (CB) Configuration

• As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal.

• The input signal is applied between the transistors base and the emitter terminals, while the corresponding output signal is taken from between the base and the collector terminals as shown.

• The base terminal is grounded or can be connected to some fixed reference voltage point.

Common Base Amplifier

• The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency (Rƒ) amplifiers due to its very good high frequency response.

Voltage Gain = \frac{Ic}{Ie} * \frac{RL}{R{in}}

• Where:

  • Ic/Ie is the current gain, alpha (α)

  • RL/R{in} is the resistance gain.

The Common Emitter (CE) Configuration

• In the Common Emitter or grounded emitter configuration, the input signal is applied between the base and the emitter, while the output is taken from between the collector and the emitter as shown.

• This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the “normal” method of bipolar transistor connection.

• The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations.

• This is mainly because the input impedance is LOW as it is connected to a forward biased PN-junction, while the output impedance is HIGH as it is taken from a reverse biased PN-junction.

Common Emitter Amplifier

• In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + I_b.

• As the load resistance (RL) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib. A transistors current gain is given the Greek symbol of Beta, (β).

• As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/I_e is called Alpha, given the Greek symbol of α. Note: that the value of Alpha will always be less than unity.

• Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current (Ib), will result in a much larger change in the collector current (Ic).

Common Collector (CC) Configuration

• In the Common Collector or grounded collector configuration, the collector is connected to ground through the supply, thus the collector terminal is common to both the input and the output.

• The input signal is connected directly to the base terminal, while the output signal is taken from across the emitter load resistor as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit.

• The common collector, or emitter follower configuration is very useful for impedance matching applications because of its very high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance.

Bipolar Transistor Configurations

Common Collector Amplifier Example

A common collector amplifier is constructed using an NPN bipolar transistor and a voltage divider biasing network. If R1 = 5k6Ω, R2 = 6k8Ω and the supply voltage is 12 volts. Calculate the values of: VB, VC and VE, the emitter current IE, the internal emitter resistance r’e and the amplifiers voltage gain AV when a load resistance of 4k7Ω is used.

1. Base biasing voltage, V_B

2. Collector voltage, VC. As there is no collector load resistance, the transistors collector terminal is connected directly to the DC supply rail, so VC = V_{CC} = 12 volts.

3. Emitter biasing voltage, V_E

4. Emitter Current, lÊ

\frac{5.8}{4700}
= 0.00123 = 1.23 mA

5. AC Emitter Resistance, r'e

\frac{25mV}{1.23mA}
20.3Ω

6. Voltage gain, Av

A₁ = \frac{RE}{r₂ + RE} = \frac{4700}{20.3 + 4700} = 0.996 or 99.6\%