A valence electron (say at M) from a nearby covalent bond comes to fill the hole at L. As we will see, if a pentavalent impurity (with 5 valence electrons) is added to a semiconductor, a large number of free electrons are produced in the semiconductor.
Charge on n-type and p-type Semiconductors
Since the holes are positively charged, therefore, they are directed towards the negative terminal which is known as hole current. It may be noted that in p-type conductivity the valence electrons move from one covalent bond to another, unlike n-type where the current conduction takes place by free electrons.
Majority and Minority Carriers
Properties of pn Junction
Applying D.C. Voltage Across pn Junction or Biasing a pn Junction
Once the potential barrier is eliminated by the forward voltage, the junction resistance becomes almost zero and a low resistance path is created for the entire circuit. The potential barrier decreases and at some forward voltage (0.1 to 0.3 V), it is eliminated altogether. ii).
Current Flow in a Forward Biased pn Junction
No current flows in the circuit due to the establishment of high resistance path. On the other hand, with forward bias to the junction, a low resistance path is created and therefore current flows in the circuit. iii).
Volt-Ampere Characteristics of pn Junction
At a certain forward voltage (0.7 V for Si and 0.3 V for Ge), the potential barrier is completely removed and current begins to flow in the circuit. However, in practice, a very small current (on the order of µA) flows in a reverse-biased circuit, as shown on the back.
Important Terms
This is because the applied external voltage is used to overcome the potential barrier. However, when the external voltage exceeds the potential barrier voltage, the pn junction behaves as a common conductor.
Limitations in the Operating Conditions of pn Junction
MULTIPLE-CHOICE QUESTIONS
The leakage current across a pn junction is i) minority carriers (ii) majority carriers (iii) junction capacitance (iv) none of the above. In an intrinsic semiconductor, the number of free electrons is .. i) equal to the number of holes (ii) is greater than the number of holes (iii) is less than the number of holes (iv) none of the above.
Answers to Multiple-Choice Questions
At room temperature, an intrinsic semiconductor has .. ii) few free electrons and holes (iii) only many free electrons (iv) no holes or free electrons. At absolute temperature, an intrinsic semiconductor has .. i) few free electrons (ii) many holes (iii) many free electrons (iv) no holes or free electrons.
Chapter Review Topics
With forward bias in a pn junction, the width of the depletion layer .. i) decreases (ii) increases (iii) remains the same. iv) none of the above. At room temperature, an internal silicon crystal acts roughly as .. i) a battery (ii) a conductor (iii) an insulator. iv) a piece of copper wire.
Discussion Questions
Semiconductor Diode 6.3 Resistance of Crystal Diode
Output Frequency of Half-Wave Rectifier
Full-Wave Rectifier 6.13 Full-Wave Bridge Rectifier
Zener Diode as Voltage Stabiliser 6.29 Crystal Diodes versus Vacuum
INTRINTR
INTRODUCTION ODUCTION ODUCTION ODUCTION ODUCTION
Semiconductor Diode
Semiconductor Diode
A crystal diode has two terminals. in a circuit, one thing to decide is whether the diode is forward or reverse biased. For this purpose, there are the following methods: i) Some manufacturers actually paint the symbol on the body of the diode eg. Crystal diodes BY127, BY114 manufactured by BEL [See Fig. ii) Sometimes, red and blue markings are used on the body of the crystal diode.
Crystal Diode as a Rectifier
On the other hand, if the conventional current tries to flow opposite to the arrowhead, the diode is reverse biased. i) If arrowhead of diode symbol is positive w.r.t. bar of the symbol, the diode is forward biased. ii). In diode D1, the conventional current flows in the direction of arrowhead and thus this diode is forward biased.
Resistance of Crystal Diode
In practice, however, the reverse resistance is not infinite, because for any value of reverse bias, a small leakage current does exist. It can be emphasized here that reverse resistance is very large compared to the forward resistance.
Equivalent Circuit of Crystal Diode
An ideal diode is one that behaves as a perfect conductor when forward biased and as a perfect insulator when reverse biased. It may be mentioned here that though ideal diode is never found in practice, yet diode circuit analysis is made on this basis.
Crystal Diode Equivalent Circuits
The peak current through the diode will occur at the moment when the input voltage reaches its positive peak, i.e. It is clear from the above example that the output voltage is almost the same whether the current diode is used or the diode is considered ideal.
Important Terms
In rectifier service it must be ensured that reverse voltage across the diode does not exceed its PIV during the negative half cycle of input AC. It can be noted that the reverse current is usually very small compared to forward current.
Crystal Diode Rectifiers
If the reverse voltage across a diode exceeds this value, the reverse current increases significantly and breaks the junction due to excessive heat. For example, the forward current for a typical diode can be as high as 100mA while the reverse current can be only a few µA - a ratio of many thousands between the forward and reverse currents.
Half-Wave Rectifier
It may be noted that the output across the load is pulsating DC. These pulsations in the output are further smoothed using filter circuitry discussed later. For example, if the input frequency of the sine wave applied to a half-wave rectifier is 100 Hz, the frequency of the output wave will also be 100 Hz.
Efficiency of Half-Wave Rectifier
For example, if the input frequency of the sine wave applied to a half-wave rectifier is 100 Hz, then the output wave frequency will also be 100 Hz. 89. This shows that in half-wave rectification, a maximum of 40.6% of a.c. power for a half-wave rectifier is 100 watts. The output power received is 40 watts. i).
Full-Wave Rectifier
Centre-Tap Full-Wave Rectifier
Assume that Vm is the maximum voltage across the half-secondary winding. 6.25 shows that the circuit at the moment when the secondary voltage reaches its maximum value in the positive direction. Consequently, the peak reverse voltage is twice the peak voltage in the half-secondary winding, i.e. i).
Full-Wave Bridge Rectifier
Since the diodes are considered ideal, diodes D1 and D3 can be replaced by wires as shown in Fig. Similarly, during the next half cycle, D2 and D4 are forward biased while D1 and D3 will be reverse biased.
Output Frequency of Full-Wave Rectifier
Efficiency of Full-Wave Rectifier
6.33 (ii) show center-tapped and bridge-type circuits having the same load resistance and transformer turns ratio. For this to happen, the turns ratio of the transformers must be as shown in fig.
Faults in Centre-Tap Full-Wave Rectifier
Faults in Transformer. The transformer in a rectifier circuit can develop the following faults
If either winding measures a very high resistance, the winding is open. iii)If any of the windings shorts to the transformer case, the primary fuse will blow. This error can be checked by measuring the resistances of the winding wires to the transformer housing.
Faults in Rectifier Diodes. If a fault occurs in a rectifier diode, the circuit conditions will indicate the type of fault
- Nature of Rectifier Output
- Ripple Factor
- Comparison of Rectifiers
- Filter Circuits
- Types of Filter Circuits
- Voltage Multipliers
- Half-Wave Voltage Doubler
- Voltage Stabilisation
- Zener Diode
- Equivalent Circuit of Zener Diode
- Zener Diode as Voltage Stabiliser
- Solving Zener Diode Circuits
- Crystal Diodes versus Vacuum Diodes
For the circuit in Fig. i) the output voltage (ii) the voltage drop across series resistance (iii) the current through zener diode. The input voltage is constant at 12V and the minimum zener current is 10 mA. The minimum zener current will occur when the load current is maximum.
Problems
What PIV rating is required for the diodes in a bridge rectifier that produces an average output voltage.
Transistors
Transistor
Some Facts about the Transistor 8.5 Transistor Symbols
Transistor Connections
Characteristics of Common Base Connection
Measurement of Leakage Current
Common Collector Connection 8.15 Commonly Used Transistor
Transistor Load Line Analysis 8.19 Practical Way of Drawing CE
Performance of Transistor Amplifier
Power Rating of Transistor 8.25 Semiconductor Devices
Transistors Versus Vacuum Tubes
INTRINTRINTR
INTR INTRODUCTION ODUCTION ODUCTION ODUCTION ODUCTION
Naming the Transistor Terminals
The middle section is called the base and forms two junctions between emitter and collector. The middle section that forms two pn junctions between emitter and collector is called the base.
Some Facts about the Transistor
The section on one side that supplies charge carriers (electrons or holes) is called the emitter. The emitter diode is always forward biased, while the collector diode is always reverse biased. v) The resistance of emitter diode (forward biased) is very small compared to collector diode (reverse biased).
Transistor Action
The connection between emitter and base can be called emitter-base diode or simply the emitter diode. The input circuit (i.e., the emitter-base junction) has low resistance due to the forward bias, while the output circuit (i.e., the collector-base junction) has high resistance due to the reverse bias.
Transistor Symbols
There are two basic types of transistor: the bipolar junction transistor (BJT) and the field-effect transistor (FET). As we shall see, these two types of transistors differ both in their operating characteristics and in their internal construction.
Transistor Circuit as an Amplifier
Thus, a weak signal applied in the input circuit appears in the amplified form in the collector circuit. The collector-base junction is reverse biased and has a very high resistance of the order of mega-ohms.
Common Base Connection
These relationships further show that the collector current of a transistor can be controlled by the emitter or base current. This is the leakage collector current (i.e. the collector current when the emitter is open) and is indicated by ICBO.
Characteristics of Common Base Connection
It is the ratio of the change in base collector voltage (ΔVCB) to the resulting change in collector current (ΔIC) at constant emitter current, i.e. This is not surprising because the collector current changes very little with the change in VCB.
Common Emitter Connection
It is the curve between collector current IC and collector base voltage VCB at *constant emitter current IE. In general, the collector current is taken along the y-axis and the collector-base voltage along the x-axis. This is consistent with the theory that the emitter current flows almost entirely to the collector terminal.
- Measurement of Leakage Current
- Characteristics of Common Emitter Connection
- Common Collector Connection
- Comparison of Transistor Connections
- Commonly Used Transistor Connection
- Transistor as an Amplifier in CE Arrangement
- Transistor Load Line Analysis
- Operating Point
- Practical Way of Drawing CE Circuit
- Output from Transistor Amplifier
- Cut off and Saturation Points
- Power Rating of Transistor
- Determination of Transistor Configuration
- Semiconductor Devices Numbering System
- Transistor Lead Identification
- Transistor Testing
- Applications of Common Base Amplifiers
This is the collector cut-off current (ie the collector current that flows when the base is open) and is denoted by ICEO. The useful output is the voltage drop across the collector load RC due to alternating current. The purpose of zero-signal collector current is to ensure that the emitter-base connection is forward-biased at all times.
MULTIPLE-CHOICE QUESTIONS
- Principle and Working of JFET 19.5 Importance of JFET
- JFET as an Amplifier 19.9 Salient Features of JFET
- Parameters of JFET 19.15 Variation of
- JFET Biasing by Bias Battery 19.19 JFET with Voltage-Divider Bias
- Symbols for D-MOSFET 19.31 D-MOSFET Transfer Character-
- D-MOSFET Biasing 19.35 D-MOSFETs Versus JFETs
The power gain of a transistor connected in .. i) common emitter (ii) common base (iii) common collector (iv) none of the above. The phase difference between the input and output voltages of a transistor connected in common emitter arrangement is.
INTRODUCTION
The field effect transistor (FET) has, due to its construction and bias, large input impedance which can be more than 100 megohms. The rapidly expanding FET market has led many semiconductor mar- 19.1 Types of Field-Effect Transistors.
Field Effect Transistors
Types of Field Effect Transistors
A bipolar junction transistor (BJT) is a current-controlled device, that is, the output characteristics of the device are controlled by base current and not by base voltage. However, in a field effect transistor (FET), the output characteristics are controlled by input voltage (i.e. electric field) and not by input current.
Junction Field Effect Transistor (JFET)
Note that in each case the voltage between the gate and the source is such that the gate is reverse biased. This means that the device has a high input impedance. source draining current ID flows from source to drain. iii) In all JFETs the source current IS is equal to the drain current ie.
Principle and Working of JFET
The drain and source terminals are interchangeable, i.e. each end can be used as a source and the other end as a drain. If the reverse voltage VGS on the gate is continuously increased, a condition is reached when the two depletion layers touch each other and the channel is cut off.
Schematic Symbol of JFET
If the channel is n-type, the arrow on the gate points towards the channel as shown in the figure. In the case of a p-type channel, the arrow on the gate points from the channel to the gate [see fig.
Importance of JFET
Difference Between JFET and Bipolar Transistor
However, for the p-type channel, the arrow on the gate points from the channel to the gate [See Fig. iv). A bipolar transistor uses a current in its base to control a large current between the collector and the emitter, while a JFET uses the voltage at the 'gate' terminal (= base) to control the current between the drain (= collector) and the source ( = issuers).
JFET as an Amplifier
Output Characteristics of JFET
The following points can be noted from the characteristics: i) At first, the drain current ID increases rapidly with the drain-source voltage VDS, but then becomes constant. Therefore, the increase in discharge current is very small with VDS above the turn-off voltage.
Salient Features of JFET
The drain-source voltage above which the drain current becomes constant is known as pinch-off voltage. ii) After pinch-off stress, the channel width becomes so narrow that depletion layers almost touch each other.
Important Terms
If the discharge voltage exceeds VDS (max), the JFET will break down as shown in Fig. As long as VDS is kept within this range, ID will remain constant for a constant value of VGS.
- Expression for Drain Current (I D )
- Advantages of JFET
- Parameters of JFET
- Relation Among JFET Parameters
- Variation of Transconductance (g m or g fs ) of JFET
- JFET Biasing
- JFET Biasing by Bias Battery
- Self-Bias for JFET
- JFET with Voltage-Divider Bias
- JFET Connections
- Practical JFET Amplifier
- D.C. Load Line Analysis
- Voltage Gain of JFET Amplifier
- Voltage Gain of JFET Amplifier (With Source Resistance R S )
- JFET Applications
- Metal Oxide Semiconductor FET (MOSFET)
- Types of MOSFETs
- Symbols for D-MOSFET
- Circuit Operation of D-MOSFET
- D-MOSFET Transfer Characteristic
- Transconductance and Input Impedance of D-MOSFET
- D-MOSFET Biasing
- Common-Source D-MOSFET Amplifier
- D-MOSFETs Versus JFETs
- E-MOSFET
- E-MOSFET Biasing Circuits
- D-MOSFETs Versus E-MOSFETs
If the gate of a JFET is made less negative, . width of the conducting channel.. i) remains the same (ii) decreases (iii) increases (iv) none of the above. An n-channel D-MOSFET with a positive VGS operates in .. i) the depletion mode (ii) the enhancement mode (iii) cutoff (iv) saturation.
Answers to Multiple-Choice Questions
At cutoff, the JFET channel is .. ii) completely closed by the depletion region. iii) extremely narrow (iv) reverse biased. If the source bypass capacitor is removed, .. i) voltage gain will increase (ii) transconductance will increase (iii) voltage gain will decrease (iv) point Q will shift.
Discussion Questions
Sketch the transfer curve for a p-channel JFET with IDSS = 4 mA and VP = 3 V. Determine in which mode each D-MOSFET in Fig. i) Depletion (ii) Enhancement (iii) Zero bias].
