RAJIV GANDHI COLLEGE OF ENGINEERING AND TECHNOLOGY

PONDY-CUDDALORE MAIN ROAD, KIRUMAMPAKKAM-PUDUCHERRY

DEPARTMENT OF ECE

EC T34 – ELECTRONIC DEVICES AND CIRCUITS

II YEAR – Mr.L.ARUNJEEVA., AP/ECE

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UNIT I

SEMICONDUCTOR DIODES

PN JUNCTION DIODE

In a piece of semiconductor material if one half is doped by p- type impurity and the the

other half is doped by n-type impurity, a PN junction is formed. The plane dividing the two

halves is called PN junction.

The N-type material has high concentration of free electrons while p- type material has

high concentration of holes. At the junction, there is a tendency for the free electrons to diffuse

over to P-side and holes to N-side. This process is called diffusion.

As the free electrons move across the junction from N-type to P-type, the donor ions

becomes positively charged. Hence a positive charge is built on the N-side of the junction. The

free electrons that cross the junction uncover the negative acceptor ions by filling in the holes.

Therefore a net negative charge is established on the p-side of the junction. This net

negative charge on the p side prevents further diffusion of electrons onto the p-side. Similarly

the net positive charge on the N-side repels the hole crossing from P-side to N-side. Thus a

barrier is set-up near the junction which prevents further movement of charge carriers i.e.

electrons and holes.

As a consequence of the induced electric field across the depletion layer, an

electrostatic potential difference is established between P & N – regions, called potential

Barrier, junction barrier, diffusion potential or contact potential.(Vo)

Note:

Vo = 0.3 V for Ge = 0.72 V for Si

UNDER FORWARD BIAS CONDITION

When positive terminal of the battery is connected to the P-type and negative terminal to

the N-type of the PN junction diode, the bias applied is known as Forward Bias.

OPERATION

As shown in the figure, the applied potential with external battery acts in opposition to

the internal potential barrier and disturbs the equilibrium.

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Under the forward bias condition, the applied positive potential repels the holes in the p-type

region so that the holes move towards the junction and the applied negative potential repels the

electrons in the N-type region and the electrons move towards the junction.

Eventually, when the applied potential is more than the internal barrier potential the depletion

region and the internal potential barrier disappear.

VI CHARACTERISTICS OF A DIODE UNDER FORWARD BIAS

As the forward voltage (Vf) is increased, for Vf<Vo, the forward current If is almost zero

because the potential barrier prevents the holes from P-region and electrons from N-region to

flow across the depletion region in the opposite direction.

For Vf>Vo, the potential barrier completely disappears and hence, the holes cross the junction

from P-type to N-type and the electrons cross the junction in the opposite direction, resulting in

relatively large current flow.

UNDER REVERSE BIAS CONDITION

When the negative terminal of the battery is connected to the P-type and positive

terminal of the battery is connected to N-type, the bias applied is known as reverse bias.

OPERATION

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Under applied reverse bias, holes from P-side move towards the negative terminal of the

battery and electrons from N-side are attracted towards the positive terminal of the battery.

Hence, the width of the depletion region increases.

Hence, the resultant potential barrier is increased which prevents the flow of majority

carriers in both directions; the depletion width (W) is proportional to √ Vo under reverse bias.

But a very small current (µA) flows under reverse bias as shown in the characteristics curve.

Electrons forming covalent bonds of the semiconductor atoms in P and N- type regions

may absorb sufficient energy from heat, causing breaking of some covalent bonds. Hence

electron-hole pairs are produced in both regions.

Thus holes in p- regions are attracted towards the negative terminal and electrons in the n

region are attracted towards the positive terminal of the battery.

Consequently, the minority carriers, electrons in P-region and holes in N-region, wander over to

the junction and flow towards their majority carrier side giving rise to small reverse current,

called Reverse Saturation Current, Io.

THEORY OF DIODE CURRENT EQUATION

Ideal Diodes

The diode equation gives an expression for the current through a diode as a function of

voltage. The Ideal Diode Law, expressed as:

where: I = the net current flowing through the diode; I0 = "dark saturation current", the diode leakage current density in the absence of light; V = applied voltage across the terminals of the diode; q = absolute value of electron charge; k = Boltzmann's constant; and T = absolute temperature (K).

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The "dark saturation current" (I0) is an extremely important parameter which differentiates one

diode from another. I0 is a measure of the recombination in a device. A diode with a larger

recombination will have a larger I0. Note that:

I0 increases as T increases; and I0 decreases as material quality increases.

For actual diodes, the expression becomes:

where: n = ideality factor, a number between 1 and 2 which typically increases as the current decreases.

TEMPERATURE EFFECTS

Temperature plays an important role in determining the characteristic of diodes. As

temperature increases, the turn-on voltage, vON, decreases. Alternatively, a decrease in

temperature results in an increase in vON. This is illustrated in figure below where VON varies

linearly with temperature which is evidenced by the evenly spaced curves for increasing

temperature in 25 °C increments.

The temperature relationship is described by equation

VON(TNew ) – VON(Troom) = kT(TNew – T room)

Dependence of iD on temperature versus vD for real diode (kT = -2.0 mV /°C)

where,

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Troom= room temperature, or 25°C.

TNew= new temperature of diode in °C.

VON(Troom ) = diode voltage at room temperature.

VON (TNew) = diode voltage at new temperature.

kT = temperature coefficient in V/°C.

Although kT varies with changing operating parameters, standard engineering practice

permits approximation as a constant. Values of kT for the various types of diodes at room

temperature are given as follows:

kT= -2.5 mV/°C for germanium diodes

kT = -2.0 mV/°C for silicon diodes

The reverse saturation current, IO also depends on temperature. At room temperature, it

increases approximately 16% per °C for silicon and 10% per °C for germanium diodes. In other

words, IO approximately doubles for every 5 °C increase in temperature for silicon, and for every

7 °C for germanium. The expression for the reverse saturation current as a function of

temperature can be approximated as

where Ki= 0.15/°C ( for silicon) and T1 and T2 are two arbitrary temperatures.

DIODE RESISTANCE

DC or STATIC RESISTANCE

The resistance of the diode at the operating point can be found by using

. The DC

resistance levels at the knee and below will be greater than the resistance levels obtained for

the vertical rise section of the characteristics. Resistance in reverse bias will be quite high.

AC or DYNAMIC RESISTANCE

A straight line drawn tangent to the curve through the q-point will define a particular

change in voltage and current that can be used to determine the ac or dynamic resistance for

this region of the diode characteristics.

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Note: keep the change in voltage and current as small as possible and equidistant to either side

of Q-point.

AVERAGE AC RESISTANCE

The average ac resistance is the resistance determined by a straight line drawn between the

two intersections established by the maxi mum and minimum values of input voltage.

( )

DIODE EQUIVALENT CIRCUITS

An equivalent circuit is a combination of elements properly chosen to best represent the actual

terminal characteristics of a device, system etc.

(i) Piecewise-Linear Equivalent Circuit

One technique for obtaining an equivalent circuit for a diode is to approximate the

characteristics of the device by straight-line segments, as shown below. The

resulting equivalent circuit is naturally called the piecewise-linear equivalent

circuit. But the straight-line segments do not result in an exact duplication of the

actual characteristics, especially in the knee region. For the sloping section of the

equivalence, the average ac resistance is included next to the actual device. It

defines the resistance level of the device when it is in the “on” state. The ideal diode

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is included to establish that there is only one direction of conduction through the

device, and a reverse-bias condition will result in the open-circuit state for the device.

Since a silicon semiconductor diode does not reach the conduction state until VD

reaches 0.7 V with a forward bias, a battery VT opposing the conduction direction

must appear in the equivalent circuit as shown below. VT represents the horizontal

offset in the curve.

Defining the piece wise-linear equivalent circuit using straight-line segments to

approximate the characteristic curve.

Components of the piecewise-linear equivalent circuit

(ii) Simplified Equivalent Circuit

For most applications, the resistance rav is sufficiently small to be ignored in

comparison to the other elements of the network. The removal of rav from the

equivalent circuit is the same as implying that the characteristics of the diode appear

as shown in Figure below. The reduced equivalent circuit appears in the same figure.

It states that a forward-biased silicon diode in an electronic system under dc

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conditions has a drop of 0.7 V across it in the conduction state at any level of diode

current.

Simplified equivalent circuit for the silicon semiconductor diode

(iii) Ideal Equivalent Circuit

0.7-V level can be ignored when compared to the applied voltage level. In this

case the equivalent circuit will be reduced to that of an ideal diode as shown in

Figure below with its characteristics.

Ideal diode and its characteristics

SPACE CHARGE (or) TRANSISTION CAPACITANCE CT OF DIODE

Reverse bias causes majority carriers to move away from the junction, thereby creating

more ions. Hence the thickness of depletion region increases. This region behaves as the

dielectric material used for making capacitors. The p-type and n-type conducting on each side of

dielectric act as the plate. The incremental capacitance CT is defined by

Since

Therefore,

where, dQ is the increase in charge caused by a change dV in voltage. CT is not constant, it

depends upon applied voltage and therefore it is defined as dQ / dV.

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When p-n junction is forward biased, then a capacitance is defined called diffusion

capacitance CD (rate of change of injected charge with voltage) to take into account the time

delay in moving the charges across the junction by the diffusion process.

If the amount of charge to be moved across the junction is increased, the time delay is greater,

it follows that diffusion capacitance varies directly with the magnitude of forward current.

Alloy junction

The junction in which there is an abrupt change from acceptor ions on one side to donor

ions on the other side is called alloy or fusion junction.

Since net charge = 0,

= Acceptor concentration

Donor concentration.

If << ;

Neglected and assume that that the entire barrier potential VB appears across the

uncovered acceptor ions.

By poisson’s equation,

=

ε – premitivity of the semiconductor

ε =

where, relative dielectric constant

permitivity of free space

Integrating the above equation,

At x = Wp ≈ W , V= VB, and if VB = V0 – V

V0 – V =

Thus the thickness of depletion layer increases with applied voltage ie. W = √ VB

If A = Area of junction, then Q =

Therefore

=

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Hence CT =

DIFFUSION CAPACITANCE

If the bias is in the forward direction the potential barrier at the junction is lowered and

holes from the p- side enter the n side. similarly the electrons from the n side move into the p-

side. This is the process of minority carrier injection, where the excess hole density falls off

exponentially with the distance.

The diffusion or storage capacitance (CD) is defined as the rate of change of injected charge

with applied voltage.

Expression for CD:

Assume : one side of the diode i.e p-type is heavily doped compared with n-type and current

due to electrons crossing the junction from n type to p type is zero. Therefore, total diode

current crossing the junction is the hole current moving from p side to n side.

Ie. I = Ipn(o)

The excess minority charge Q is given by,

Q = ∫ ( )

Wher A – diode cross section

E – charge of an electron

Q = [ ( )

]

And CD =

= Ae LP ( )

The hole current Ipn(x), at x= 0 is I =

( )

Differentiating Pn(o) with respect to voltage is and simplifying,

CD =

PN DIODE SWITCHING TIMES

When a diode is driven from the reversed condition to the forwardstste or in the

opposite direction, the diode response is accompanied by a transient and an interval of time

before the diode recovers to its steady state.

Forward Recovery time tfr

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It is the time difference between the 10% point of the diode voltage and the time when

this voltage reaches and remains within 10% of this final value.

Diode Reverse Recovery time

When an external voltage forward biases PN junctions, the number of minority carriers is

very large. They are supplied from the other side of the junction.

As the minority carriers approach the junction they rapidly swept across and the density

of minority carriers diminishes to zero at the junction.

If the external voltage is suddenly reversed in a diode in FB, the diode current will not

immediately fall to its steady state reverse voltage value.

The current cannot attain its steady – state value until the minority carrier distributionat

the moment of voltage reversal had the form(a) reduces to form (b).

Storage and transition times

Consider the voltage in (b) is applied to the diode resistor circuit in figure (a).Upto time t1,

the voltage vi = vf, forward biases the diode. Then the current is i =

At t1 , the vi = -vr, but

the current does not drop to zero, but instead reverses and remains at the value i =

until t = t2.

At t = t2 as in ©, the injected minority carrier density at x= 0 has reached its equilibrium

state.If the diode ohmic resistance is Rd, at t1 the diode voltage falls of slightly, but does not

reverse.

At t = t2, the diode voltage begins to reverse and the magnitude of the diode current

begins to decrease.

Storage time ts

The interval t1 to t2, for the stored minority charge to become zero, is called the storage

time ts.

Transition Time t1

The time which elapses between t2 and the time when the diode has nominally

recovered is called the transistion time tt.This recovery interval will be completed when the

minority carriers at some distance from the junction have diffused to the junction and crossed it

and CT has charged to voltage VR.

Reverse recovery time (trr)

The reverse recovery time (or) turn – off time is the interval from the current reversal at t = t1

until the diode has recovered to a specified extent in terms of diode current or resistance ie. trr =

ts + tt

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ZENER DIODES

When the reverse voltage reaches breakdown voltage in normal PN junction diode, the

current through the junction and power dissipated at the junction will be high. Such an operation

is destructive and diode gets damaged.

Diodes which are designed with adequate power dissipation capabilities to operate in the

breakdown region may be employed as voltage – reference or constant – voltage devices. Such

diodes are known as avalanche, breakdown or zener diodes.

Under forward biased condition, the zener works in similar with the PN diode. Under

reverse bias, breakdown of the junction occurs and voltage remains constant and current varies

largely.

Two mechanisms of diode breakdown for increasing reverse voltage are:

Avalanche Breakdown

The thermally generated electrons and holes acquire sufficient energy from the applied

reverse potential to produce new carriers by removing valence electrons. These new carriers, in

turn produce additional carriers again through the process of disrupting bonds. This cumulative

process is referred to as avalanche multiplication. It results in the flow of large reverse currents.

The diode is said to be in avalanche breakdown region.

Zener Breakdown

When the P and N regions are heavily doped, direct rupture of covalent bonds takes

place because of strong electric fields at the junction of PN diode. The new electron – hole

pairs so created increase the reverse current in a reverse biased PN diode.

The decrease in current takes place at a constant value of reverse bias ie. below 6V for

heavily doped diodes.

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