Semiconductor Diodes

P type and N type semiconductors, taken separately are of very limited use. If we join a piece of P type material to a piece of N type material such the

crystal structure remains continuous at the boundary, A PN JUNCTION is formed.

The diode is a two terminal semiconductor device. Just like resistor, diode has two terminals. Unlike resistor, it has nonlinear current-voltage characteristics. Diode can function as Rectifier (High current diodes,

Switching (low current devices), Voltage Regulator (Zener diodes), and other operations in electronic circuits.

A PN junction cannot be produced by simply pushing two pieces together or by welding etc, because it gives rise to discontinuities across the crystal

structure. Special fabrication techniques are adopted to form a P N junction A PN junction is a device formed by joining p-type (doped with B, Al) with n-

type (doped with P, As, Sb) semiconductors and separated by a thin junction is called PN Junction diode or junction diode. Fig. 4.1 shows such a PN Junction Diode.

Fig. 4.1: PN Junction Diode

Graphic Symbol of pn-Junction Diode

Fig.4.2: Symbol of pn-Junction Diode

Fig. 4.2 shows the graphic symbol of pn-Junction Diode. The arrowhead

indicates the conventional direction of current flow when the diode is forward biased. The p-side of the diode is always the positive terminal for

forward bias and is termed anode. The n-side, called the cathode, is the negative terminal when the device is forward biased.

V-I characteristic for the Ideal Diode

Conducting in one direction and not in the other is V-I characteristic of the diode. Forward biasing voltage makes diode to turn on. Reverse biasing voltage makes it turn off.

Diode as open Switch Diode as closed Switch

Fig.4.3: V-I Characteristics of Ideal Diode

Formation of depletion layer (NO external connections):

Fig.4.4: PN-Junction Diode without battery connections

Fig. 4.4 shows a PN-Junction Diode. Here, the excess electrons in the N region cross the junction and combine with the excess holes in the P region.

N region loses its electrons and becomes positively charged. P region accepts the electrons and becomes negatively charged. At one point, the migratory action is stopped. Additional electrons from the N region are repelled by the

net negative charge of the p region. Similarly, additional holes from the P region are repelled by the net positive charge of the n region. Net result is a

creation of a thin layer of each side of the junction, which is depleted (emptied) of mobile charge carriers. This is known as DEPLETION LAYER.

Thickness of depletion region is of the order of 10-6meter. The depletion layer contains no free and mobile charge carriers but only fixed and

immobile ions. Its width depends upon the doping level of semiconductor layer. Heavy doped semiconductor results in thin depletion layer and lightly doped results in thick depletion layer. The electrons in the N region have to

climb the potential hill in order to reach the P region. Electrons trying to cross from the N region to P region experience a retarding field of the battery and therefore repelled. Similarly for holes from P region will get repelled.

The Potential thus produced is called potential barrier. For germanium, potential barrier is 0.3 V and it is 0.7V for Silicon.

PN junction can basically work in two modes (A battery is connected to the diode). Forward bias mode (positive terminal connected to p-region and

negative terminal connected to n region) and reverse bias mode (negative terminal connected to p-region and positive terminal connected to n region)

Forward biased PN junction

Fig.4.5: Forward biased PN Junction

Forward bias across the diode forces the majority charge carriers to move across the junction decreasing the width of the depletion layer. This in turn

decreases the built-in potential and lowers the barrier height. The number of carriers able to diffuse across the barrier will increase. Diffusion current

increases. Drift current remains the same. The drift current is essentially constant, as it is dependent on temperature. Hence, Current flows from p to n region.

Reverse biased PN junction

Fig.4.6: Forward biased PN Junction

Fig.4.6 shows a reverse biased PN Junction. If the positive of the battery is connected to the n-type and the negative terminal to the p-type, the free electrons and free holes are attracted back towards the battery, hence back from the depletion layer, hence the depletion layer grows. This in turn Increases the built-in potential, increase the barrier height. Decrease the

number of carriers able to diffuse across the barrier. Diffusion current decreases. Drift current remains the same. Almost no current flows. Reverse leakage current, IS, is the drift current, flowing from N to P. Thus a reverse

biased pn junction does not conduct current. Only the minority carriers cross the junction constituting very low reverse saturation current. This

current is of the order of micro ampere. V-I Characteristics of PN Junction Diode

Fig.4.7: V-I Characteristics of PN Junction Diode

Fig.4.7 shows V-I Characteristics of PN Junction Diode. The Forward-Bias

Region is determined by v > 0, The Reverse-Bias Region, determined by –Vzk

<v<0 and The Breakdown Region, determined by v< Vzk.

In forward bias, the PN junction has a “turn on” voltage based on the “built-in” potential of the PN junction. Turn on voltage is typically in the range of 0.5V to

0.8V.

In reverse bias, the PN junction conducts essentially no current until a critical breakdown voltage is reached. The breakdown voltage can range from 1V to 100V. Breakdown mechanisms include avalanche and zener tunneling.

When diode is in forward bias, no current flows until the barrier voltage (0.3V

for Ge) is overcome. Then the curve has a linear rise and the current increases, with the increase in forward voltage like an ordinary conductor. Above 3V, the majority carriers passing the junction gain sufficient energy to knock out the

valence electrons and raise them to the conduction band. Therefore, the forward current increases sharply.

With reverse bias, potential barrier at the junction increased. Junction resistance increases and prevents current flow. However, the minority carriers

are accelerated by the reverse voltage resulting in a very small current (REVERSE CURRENT) and it is in the order of microamperes.

With reverse bias, when reverse voltage is increased beyond a value, called breakdown voltage, the reverse current increases sharply and the diode shows

almost zero resistance. It is known as avalanche breakdown. Reverse voltage above 25 V destroys the junction permanently.

PN Junction under Forward-Bias Conditions

I-V characteristic equation is given by

Which is exponential relationship and nonlinear.

Is is called saturation current, strongly depends on temperature.

n =1 or 2， in general n=1

VT is thermal voltage.

)1（ TnVv

s eIi

Turn-on voltage A conduction diode has approximately a constant voltage drop across it. It’s

called turn-on voltage. VD(On) = 0.7 V for silicon and it is equal to 0.3 V for germanium. Diodes with different current rating will exhibit the turn-on voltage

at different currents. Negative TC -2mv/oC,

Diode Parameters

Reverse Voltage VR

The maximum reverse DC voltage that can be applied

across the diode.

Reverse Current IR

The maximum current when the diode is reverse-biased

with a DC voltage.

Forward Current IF

The maximum average value of a rectified forward current.

Maximum operation frequency fM

The maximum operation frequency of the

diode.

Diode Approximations

Depending on the application, diode can be modeled in one of three approximations as shown in Fig.4.8.

Fig.4.8: Diode Approximations

The First Approximation The first approximation of a diode is simply a switch. If the diode is forward biased, the switch is closed. The diode will behave as a conductor, with a 0

ohm of resistance.

Fig.4.9:Diode as closed switch

If the diode is reverse biased, the switch is open. The diode will behave as open, with infinite resistance.

Fig.4.10:Diode as open switch

The Second Approximation

The second approximation requires 0.7 volts of forward bias to overcome the barrier potential.

When forward biased, 0.7 volts appears across the diode and current flow.

Fig.4.11: Second approximation when diode is forward biased

When reverse biased, no current will flow

Fig.4.12: Second approximation when diode is reverse biased

The Third Approximation

The third approximation includes both the 0.7 volts barrier potential and the internal resistance of the diode (called bulk resistance).

Fig.4.13: Third Approximation

When are the different approximations are used?

First Approximation: This is used primarily in troubleshooting. Is the diode conducting or not?

Second Approximation: This is used when a more accurate determination of load current and load voltage is required. Third Approximation: This is used during original design of diode circuit

DC Load Analysis of Diode Circuits

Let’s us consider a very simple circuit, which is shown in Fig.4.14.

Fig. 4.14: Simple Circuit We will assume that the diode is forward biased. Using KVL

VDD = IR + VD 1

From the Characteristic equation for the diode

I = Is (ev/nVT -1) 2

Assuming n, Is and VT are known, we have two equations for the two unknown quantities VD and I. Substituting 2 in 1

Which is a transcendental equation for VD. There is no simple analytical

solution to this equation. Begin with diode V-I characteristic curve shown in Fg.4.15.

Fig. 4.15: V-I Characteristic

We rearrange the equation 1 in terms of I

I = VDD /R + VD /R which is an equation for straight line y= b +mx as shown in Fig.4.16.

)1R（ TnVv

sDDD eIVV

Fig. 4.16: Straight Line

We call this straight line as load line. Now plot both these curves on the same

graph as shown in Fig. 4.17.

Fig. 4.17: Intersection of V-I Curve and straight line

The point where these curves intersect is the simultaneous solution to two

equations 1 and 2. The graphical method is impractical solution method for all

but simple circuits. However, it is useful for a qualitative understanding of

these circuits.

For example: What happens when VDD increases

Fig. 4.18: Effect of VDD

A load line is used in graphical analysis of nonlinear electronic circuits,

representing the constraint other parts of the circuit place on a non-linear device, like a diode or transistor. It is usually drawn on a graph of

the current vs the voltage in the nonlinear device, called the device's characteristic curve. A load line, usually a straight line, represents the response of the linear part of the circuit, connected to the nonlinear device in

question. The points where the characteristic curve and the load line intersect are the possible operating point(s) (Q points) of the circuit; at these points the current

and voltage parameters of both parts of the circuit match.

Problem 1: Draw the dc load line for the circuit, on the diode forward

characteristics.

Fig.4.19: Diode Circuit

Applying KVL, VDD = (I R) + VD

Substituting I = 0 gives, VDD = (I R ) + VD = 0 + VD or VD = VDD = 5 V

Plot point A on the diode characteristics at I = 0 and VD = 5 V Now substitute VD = 0 in to equation for VDD or VDD = (I R ) + 0 giving I = VDD/R

= 5 / 100 = 50 mA Plot point B on the diode characteristics at I = 50 mA and VD = 0

Draw the DC load line through points A and B. The intersection of DC load line

and V-I characteristics of diode provides intersection point, which is called Q point. From the Q point, ID = 40 mA and VD = 1 V, which are called DC conditions. From the Q point, ID = 40 mA and VD = 1 V, which are called DC

conditions.

Fig.4.20: Q Point

Application of Diodes

An important application of diode is regulated power supply.

Fig.4.21: Regulated Power Supply

Rectifier is one of most widely used electronic circuit to convert AC voltage to

DC voltage. Since the rectifier circuit uses diodes to convert ac voltage to dc, it’s also called a converter circuit. All power that supply to a modern factory is

alternating current, so it is important to have circuit that can convert the ac power to dc power since most solid-state device require a source of dc power to operate.

Rectifier is a circuit that converts ac voltage of main supply into pulsating dc

voltage using one or more pn junction diodes. There are 3 types of rectifiers, Half Wave Rectifier, Full Wave Rectifier; Center Tap Rectifier and Bridge Rectifier.

Fig.4.22: Half Wave Rectifier

Fig. 4.22 shows a half wave rectifier. The rectifier will conduct each time its

anode is positive with respect to its cathode. So when the end of the secondary winding shown + is positive, the diode acts as a short-circuit and the + appears

across the load. Current flows around the secondary circuit for the time that the diode is conducting. One-half cycle of the AC from the transformer is conducted by the rectifier, one half cycle is stopped. This is pulsating DC -

half-wave rectified.

Fig.4.23: Equivalent Circuit of Half Wave Rectifier

Fig.4.24: Waveforms of Half Wave Rectifier

Disadvantage of HWR

The ripple factor of half wave rectifier is 1.21, which is quite high.

The output contains lot of ripples The maximum theoretical efficiency is 40% and the practical value will be quite less than this. This indicates that HWR is quite inefficient.

Centre Tap Rectifier

Fig. 4.25 shows a centre tap Rectifier. This is two half-wave rectifiers combined - it uses a center-tapped secondary winding and one additional diode. Each

side of the centre-tap has the same number of turns and each "works" for half the cycle as our half-wave rectifier did.

Fig.4.25 : Centre tap Rectifier

Current Flow during the positive half of the input cycle

The "top half" of the secondary works with one diode like the half-wave

circuit. When the polarity of the secondary changes, the upper diode shuts off

and the lower diode conducts. The result is that the lower diode "fills in"

another half-cycle in the waveform when the upper diode is not conducting.

Fig.4.26: Centre tap Rectifier during positive half cycle

Current Flow during the negative half of the input cycle:

Fig.4.26: Centre tap Rectifier during negative half cycle

Fig.4.27: Centre tap Rectifier Waveforms

Peak Inverse Voltage

Fig.4.28: PIV of Centre tap Rectifier

Advantages of Centre tap Rectifier

Efficiency is higher. The large dc power output

The ripple factor is less

Disadvantages of Centre tap Rectifier

PIV rating of diode is higher. Higher PIV diodes are larger in size and costlier. The cost of center tap transformer is high.

Full Wave Bridge Rectifier

Fig. 4.29 shows full wave bridge rectifier.

Fig.4.29: Full Wave Bridge Rectifier

Fig.4.30: Full Wave Bridge Rectifier during FB

Fig.4.30: Full Wave Bridge Rectifier during RB

The full wave rectifier, shown in Fig.4.30, uses one single winding as the secondary and four diodes - two are conducting at any one time. Parallel-side

diodes conduct at the same time. When the polarity changes, other two diodes conduct. The output waveform is the same as the full-wave rectifier.

Advantages of Bridge Rectifier Uses a simpler transformer. It does not need center tap transformer (no centre-

tap and no extra winding). PIV rating of diodes in bridge rectifier is Vm. Hence the diodes for use in Bridge rectifier are smaller and cheaper.

Disadvantages of Bridge Rectifier

It requires four diodes, two of which conduct in alternate half cycles. This creates a total voltage drop of 1.4V (if Si diodes are used). A two diode voltage drop of 1.4V becomes significant, if low dc voltage is required.

Comparison of HWR, FWCR and FWBR

Parameter HWR FWCR FWBR

Ripple Factor 1.21 0.48 0.48

Conversion Efficiency 40% 81.2% 81.2%

PIV Rating Vm 2Vm Vm

Filter Circuits (Basics)

The ripple means variations in impure DC voltage. Due to ripples the quality of

DC voltage is reduced. Thus, the ripple factor is defined as the ratio of rms value of AC voltage to DC voltage. The ripple factor is independent of RL .The inductor can also store electricity in the form of PD across it, for a short time.

In inductor current lags voltage by 900. The inductor allows DC current and

produces high opposition (XL) to AC. Thus, it allows steady current but opposes charging current.

Capacitor Filter Circuit

When filter circuit is used, load must be connected across the output. When first half cycle arrives, the capacitor charges up to VP. Thus, it maintains

constant voltage. Its filtration decreases as RL decreases.

Fig.4.31: Capacitor Filter

Voltage Regulators

A load means anything which we connect across the output of a circuit. When

load is connected, loading effect is produced. Due to loading effect, the output voltage drops. This loaded voltage is called apparent voltage. The apparent voltage is always less than true voltage.

Load regulation is defined as the change in output voltage when load current

changes from minimum to maximum. Line regulation is defined as change in output voltage for a specified range of line voltage.

Voltage regulator has two important facilities-It has over voltage protection i.e. when line voltage is abnormal it is switched off. It has over load protection i.e. when load current exceeds the limit, it is switched off.

Shunt Zener Regulator

Fig.4.32: Shunt Zener Regulator

Fig.4.32 shows a shunt Zener Regulator. It uses Zener diode. The zener diode is a special PN junction diode. Its doping concentration decides the zener

voltage. When a is +ve, k is –ve, its forward voltage is 0.7V. It cannot be used in rectification, since it conducts in both directions.

The circuit stabilizes the output voltage, when RL = constant, as follows- When Vin increases, IS increases, IZ increases but IL = constant, So (Vin- VZ )

increases and VZ = VO = Constant. When Vin decreases IS decreases IZ decreases but IL = constant, So (Vin- VZ) decreases and VZ = VO = constant.

The circuit stabilizes the output voltage when Vin = constant, as follows-

When RL increases, IL decreases but IS = constant, So IZ increases and keeps Vo = constant When RL decreases IL increases but IS = constant, So IZ decreases and keeps Vo

constant

The circuit stabilizes the output voltage When Vin = constant, as follows When RL increases IL decreases but IS = constant So IZ increases and keeps VO = constant.

When RL decreases IL increases but IS = constant So IZ decreases and keeps VO = constant.

Introduction BJT

The basic of electronic system nowadays is semiconductor device. The famous and commonly use of this device is BJTs (Bipolar Junction Transistors). It can

be used as an amplifier and logic switches. As shown in Fig.5.1, BJT consists of three terminal: collector : C, base: B and emitter : E. There are Two types of BJT : pnp and npn and 3 layer semiconductor device consisting: 2 n- and 1

p-type layers of material (npn transistor) or 2 p- and 1 n-type layers of material (pnp transistor)

The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material. A single pn junction

has two different types of bias: forward bias, reverse bias. Thus, a two-pn-junction device has four types of bias. Base is located at the middle and thinner from the level of collector and emitter. The emitter and collector

terminals are made of the same type of semiconductor material, while the base of the other type of material.

Fig.5.1: BJT

Fig.5.2: BJT symbol

As shown in Fig.5.2, IC is the collector current, IB is the base current and IE=

the emitter current. The arrow is always drawn on the emitter. The arrow always point toward the n-type. The arrow indicates the direction of the emitter current: pnp: E B npn: B E.

By imaging the analogy of diode, transistor can be constructed like two diodes

that connected together, as shown in Fig.5.3. It can be concluded that the work of transistor is based on work of diode.

Fig.5.3: Analogy of BJT (a) PNP (b) NPN

Transistor Operation Both biasing potentials have been applied to a pnp transistor and resulting

majority and minority carrier flows indicated. Majority carriers (+) will diffuse

across the forward-biased p-n junction into the n-type material. A very small

number of carriers (+) will through n-type material to the base terminal.

Resulting IB is typically in order of microamperes. The large number of majority

carriers will diffuse across the reverse-biased junction into the p-type material

connected to the collector terminal. Majority carriers can cross the reverse-

biased junction because the injected majority carriers will appear as minority

carriers in the n-type material.

Fig.5.4: Forward-biased junction of a pnp transistor

Fig.5.5: Reverse-biased junction of a pnp transistor

Applying KCL to the transistor: IE = IC + IB

It comprises of two components – the majority and minority carriers IC = ICmajority + ICOminority

ICO – IC current with emitter terminal open and is called leakage current.

Common-Base Configuration

Fig. 5.5 shows a common base configuration of BJT. Common-base terminology is derived from the fact that the base is common to both input and

output of the configuration. Base is usually the terminal closest to or at ground potential.

All current directions will refer to conventional (hole) flow and the arrows in all electronic symbols have a direction defined by this convention. Note that the

applied biasing (voltage sources) are such as to establish current in the direction indicated for each branch.

Fig.5.6: BJT in common Base Configuration

To describe the behavior of common-base amplifiers requires two set of characteristics: Input or emitter characteristics

Fig.5.7: Input Characteristics of BJT in common Base Configuration

Output or collector characteristics The output characteristics have 3 basic regions: Active region –defined by the biasing arrangements , Cutoff region – region where the collector current is 0A

and Saturation region- region of the characteristics to the left of VCB = 0V

Fig.5.8: output Characteristics of BJT in common Base Configuration

The curves (output characteristics) clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by IC ≈IE. Once a

transistor is in the ‘on’ state, the base-emitter voltage will be assumed to be VBE = 0.7V as shown in Fig.5.9.

Fig.5.9: VBE Vs IE

In the dc mode, the level of IC and IE due to the majority carriers are related by

a quantity called alpha = IC / IE.

E

C

I

I

IC = IE + ICBO

It can then be summarize to IC = IE (ignore ICBO due to small value)

For ac situations where the point of operation moves on the characteristics

curve, an ac alpha defined by = IE/IC

Alpha, a common base current gain factor shows the efficiency by calculating

the current percent from current flow from emitter to collector. The value of

is typical from 0.9 ~ 0.998. Proper biasing CB configuration in active region by

approximation IC IE (IB 0 uA)

Common-Emitter Configuration

Fig.5.10 shows a common Emitter Configuration of BJT. It is called common-emitter configuration since emitter is common or reference to both input and output terminals. emitter is usually the terminal closest to or at ground

potential. Almost amplifier design is using connection of CE due to the high gain for current and voltage.

Two set of characteristics are necessary to describe the behavior for CE; input (base terminal) and output (collector terminal) parameters.

Fig.5.10: Common Emitter Configuration of BJT

Input characteristics for a CE NPN transistor

Fig. 5.11 shows input characteristics of CE transistor. IB is uA compared to ma

of IC. IB will flow when VBE > 0.7V for si and 0.3V for ge Before this value IB is very small and no IB. Base-emitter junction is forward bias. Increasing VCE will

reduce IB for different values.

Fig.5.11: Input characteristics for a CE NPN transistor

Output Characteristics for a CE npn transistor: Fig. 5.12 shows Output Characteristics for a CE npn transistor. For small VCE (VCE < VCESAT, IC increase linearly with increasing of VCE. VCE > VCESAT IC not

totally depends on VCE constant IC. IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC. IB=0 A ICEO occur. Noticing the

value when IC=0A. There is still some value of current flows.

Fig. 5.12: Output Characteristics of a CE npn transistor

Beta () or amplification factor: The ratio of dc collector current (IC) to the dc

base current (IB) is dc beta (dc ) which is dc current gain where IC and IB are

determined at a particular operating point, Q-point (quiescent point). It’s

defined by the following equation: 30 < dc < 300 2N3904. On data sheet,

dc=hFE with h is derived from ac hybrid equivalent cct. FE are derived from forward-current amplification and common-emitter configuration respectively.

For ac conditions an ac beta has been defined as the changes of collector current (IC) compared to the changes of base current (IB) where IC and IB are

determined at operating point. On data sheet, ac=hfe . It can defined by the following equation:

Relationship between α and β

Common – Collector Configuration:

Fig.5.13 showsa common collector configuration of BJT. The common collector configuration is also called emitter-follower (EF). It is called common-emitter configuration, since both the signal source and the load share the collector

terminal as a common connection point. The output voltage is obtained at emitter terminal. The input characteristic of common-collector configuration is

similar with common-emitter. configuration. Common-collector circuit configuration is provided with the load resistor connected from emitter to ground. It is used primarily for impedance-matching purpose since it has high

input impedance and low output impedance.

Fig. 5.13: Common Collector Configuration

Fig. 5.13: Input characteristics of Common Collector Configuration

Many BJT transistor used as an amplifier. Thus it is important to notice the limits of operations. At least 3 maximum values is mentioned in data sheet.

There are: a) Maximum power dissipation at collector: PCmax or PD

b) Maximum collector-emitter voltage: VCEmax sometimes named as VBR(CEO) or VCEO.

c) Maximum collector current: ICmax

There are few rules that need to be followed for BJT transistor used as an amplifier. The rules are: transistor need to be operate in active region! IC < ICmax

PC < PCmax