Junction Capacitance Of Pn Junction Diode

In the previous Posts we discussed about the Pn Junction Diode And VI Characteristics and Diode Parameters And Applications And today we going to discuss about the Junction Capacitance Of Diode

U can Also Read Our Posts Regarding PN Junction Diode

Zener Breakdown And Avalanche Breakdown

Junction Capacitance :

i) Transition Capacitance (CT) or Space-charge Capacitance

When a P-N junction is reverse-biased, the depletion region acts like an insulator or as a dielectric material essential for making a capacitor. The P- and N-type regions on either side have low resistance and act as the plates. We, therefore, have all the components necessary for making a parallel-plate capacitor. This junction capacitance is called transition or space charge capacitance (Cpn or CT). It may be calculated by the usual formula C = ∈A/d. Its typical value is 40 pF. Since thickness of depletion (or transition) layer depends on the amount of reverse bias, capacitance CT can be controlled with the help of applied bias. This property of variable capacitance possessed by a reverse biased P-N junction is used in the construction of a device known as varicap or varactor. This capacitance is voltage dependent as given by the relation

CT =K/(Vk+Vr)n

where

Vk = knee voltage ;

Vr = applied reverse voltage

K = constant depending on semiconductor material

n = 1/2 – for alloy junction and

= 1/3 –for diffused junction

Diffusion Or Storage Capacitance (CD)

This capacitive effect is present when the junction is forward-biased. It is called diffusion capacitance to account for the time delay in moving charges across the junction by diffusion process.  Due to this fact, this capacitance cannot be identified in terms of a dielectric and plates. It varies directly with the magnitude of forward current as explained below in more details.

 Consider a forward-biased junction which is carrying a forward current IF. Suppose the applied voltage is suddenly reversed, then increases suddenly but leaves lot of majority charge carriers in the depletion region. These charge carriers must get out of the region which, to their bad luck, becomes wider under the reverse bias. Hence, it is seen that when a forward-biased P-N junction is suddenly reverse-biased, a reverse current flows which is large initially but gradually decreases to the level of saturation current I0. This effect can be likened to the discharging current of a capacitor and is, therefore, rightly represented by a capacitance called diffusion capacitance CD. Since the number of charge carriers left in depletion layer is proportional to forward current, CD is directly proportional to IF. Its typical value is 0.02 µF which is 5000 times CT. The capacitance assumes great significance in the operation of devices which are required to switch rapidly from forward to reverse bias. If CD is large, this switchover cannot be rapid. It will delay both the switch-on and the switch-off. This effect of CD is variously known as recovery time or carrier storage.

CD=dQ/dV

= τI / ηVT

Diode Parameters And Applications

Today I am going to post a topic on the Diode Parameters And Diode Applications


Diode Parameters :

1) Bulk Resistance (rB)

It is the sum of the resistance values of the P-and N-type semiconductor materials of which the
diode is made of.
∴ rB = rP + rN 

Itually, it is very small. It is given by

rB = (VF −VB)/IF

It is the resistance offered by the diode well above the barrier voltage i.e. when current is large. Obviously, this resistance is offered in the forward direction.


2) Junction Resistance (rj) 

Its value for forward-biased junction depends on the magnitude of forward dc current.


rj = 25 mV/IF (mA) – for Ge
   

   = 50 mV/IF (mA) – for Si



Obviously, it is a variable resistance.


3) Dynamic Or Ac Resistance :

rac or rd = rB + rj


For large values of forward current, rj  is neg￾ligible. 
Hence, 
rac = rB. 
For small values of IF, rB is negligible as compared to rj

. Hence rac = r

4) Forward Voltage Drop

It is given by the relation
forward voltage drop = power dissipated

forward dc current

5. Reverse saturation current (I0).


6. Reverse breakdown voltage (VBR).


7. Reverse dc resistance RR 


Applications :

The main applications of semiconductor diodes in modern electronic circuitry are as under :
1. As power or rectifier diodes. They convert ac current into dc current for dc power supplies
of electronic circuits.
2. As signal diodes in communication circuits for modulation and demodulation of small
signals.
3. As Zener diodes in voltage stabilizing circuits.
4. As varactor diodes–for use in voltage-controlled tuning circuits as may be found in radio
and TV receivers. For this purpose, the diode is deliberately made to have a certain range of

junction capacitance. The capacitance of the reverse-biased diode is given by C = K/sqrt ( VR)
where VR is the reverse voltage.

5. In logic circuits used in computers

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PN-Junction Diode & VI Characteristics

Pn Junction Diode :

Construction

It is a two-terminal device consisting of a P-N junction formed either in Ge or Si crystal. Its circuit symbol is shown in Fig. The P-and N-type regions are referred to as anode and cathode respectively. In Fig arrow head indicates the conventional direction of current flow when forward-biased. It is the same direction in which hole flow takes place. Commercially available diodes usually have some means to indicate which lead is P and which lead is N. Standard notation consists of type numbers preceded by
‘IN’ such as IN 240 and IN 1250. Here, 240 and 1250 correspond to colour bands. Fig.shows typical
diodes having a variety of physical structures whereas Fig.  illustrates terminal identifications. Also refer to the picture of two commercial diodes shown in Fig 

The low-current diodes whose body is about 3 mm long can carry a forward current of about 100 mA, have saturation current of 5 µA at room temperature (25ºC) and can withstand a reverse voltage of 75 V without breaking down. The medium-current diodes can pass a forward current of about 500 mA and can withstand a reverse voltage of 250 V. The high-current diodes or power diodes can pass a forward current of many amperes and can survive several hundred volts of reverse voltage.


Working :

A P-N junction diode is one-way device offering low resistance when forward-biased  and behaving almost as an insulator when reverse-biased Hence, such diodes are mostly used as rectifiers i.e. for converting alternating current into direct current.

Forward Characteristics :

When the diode is forward-biased and the applied voltage is increased from zero, hardly any current flows through the device in the beginning. It is so because the exter￾nal voltage is being opposed by the inter￾nal barrier voltage VB whose value is 0.7 V for Si and 0.3 V for Ge. As soon as VB is neutralized, current through the diode increases rapidly with increasing applied battery voltage. It isbfound that as little a voltage as 1.0 V produces a forward current of about 50 mA. A burnout is likely to occur if forward voltage is increased beyond a
certain safe limit.



Reverse Characteristics :

When the diode is reverse-biased, majority oarriers are blocked and only a small current (due to minority carriers) flows through the diode. As
the reverse voltage is increased from zero, the reverse current very quickly reaches its maximum or saturation value I0 which is also known as leakage current. It is of the order of nanoamperes (nA) for Si and microamperes (µA) for Ge. The value of I0 (or Is
) is independent of the applied reverse voltage but depends on (a) temperature, (b) degree of doping and (c) physical size of the junction.
As seen from Fig. 52.4, when reverse voltage exceeds a certain value called break-down voltage  (or Zener voltage Vz), the leakage current suddenly and sharply increases, the curve indicating zero resistance at this point. Any further increase in voltage is likely to produce burnout unless protected by a current-limiting resistor. When P-N junction diodes are employed primarily because of this breakdown property as voltage

regulators, they are called Zener diodes

Drift And Diffusion Currents In Semi Conductor

Hello All It's Long Time 
Because Of My Exams....Now I am Back With

Drift And Diffusion Currents

The flow of charge or Currents through the semiconductor material are of two types

1) Drift Current
2) Diffusion Current

1) Drift Current
When an electric field applied across the semiconductor ,then the charge carriers will attain the some velocity called as drift velocity(Vd) Which is equal to the product of the mobility of the charge carriers and the applied electric field intensity [E]. The Holes Moves To wards the Negative Terminal And The Electrons Move Toward The Positive Terminal. this combined effet is known as the drift current.

The drift current may be defined as the flow of electric current in the presence of the electric field .

The drift bcurennt density (Jn) is given by


2) Diffusion Current :

• It is possible to flow the current in the semiconductor in the obsence of the electric supply
• In a region The HOles or electrons any one is greater it will recombined with the another pair in the other region 
• then the the electron or hole moves  this movement of charge is called as the  diffusion current

ZENER BREAKDOWN

When the P and N regions are heavily dopped, direct rupture of the covalent bonds takes place because of the electric field. At the junction of the PN junction diode the new electron -hole pairs will be created increase the reverse current takes place at a constant reverse bias voltage typically 6v for heavily dopped diodes . As a result of hevy dopping the diplition region is reduced then the field across the diplition is very large 10^7 this is called as the zener breakdown

AVALANCHE BREAKDOWN

AVALANCHE BREAKDOWN :-

As the applied rever4se bias increases , the field across the junction increases correspondingly.

Thermally generated carriers while traversing the junction acquire large amount of kinetic energy from this field. As a result the velocity of these carriers will increase.these electrons disrupt covalent bonds by colliding the immobile ions and create new electron -hole pairs. these new carriers again acquire sufficient energy from the field and collide with the other immobile ions there by generating further electron-hole pairs.

This process commulatiove in nature and results generation of avalanche of the charge carriers within ashort time.

This mechanism of carier generation is known as the Avalanche Multiplication. This prcess resuts flow of large amount of current at the same value of the reverse bias

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ZENER DIODE EXPLANATION

ZENER DIODE :-

We know that the working of the  PN Jucnction Diode, It is very similar to the PN junction in Forward bias condition.

But in the Reverse bias condition It have a special feature thats why a Zener Diode is different from a Normal diode

When the reverse voltage reaches breakdown vltae in Normal PN Junction Diode, the current throuh the junction and the power dissipated i the junction is very high.Such an operation is distructive and the diode will gets damaged.

Whereas diodes can designed with adequate power dissipation capabilities to operate in the break down region. Such diode is known as the ZENER DIODE.Zener diode is heavily dopped  thnan the Normal Diode

The diode will have two conditions to work
1) Aalanche Breakdown
2)Zener Breakdown

A zener diode should be work betweeen the zener and avalanche breakdown regions.

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What is PN junction

Lets Know about Pn Junction

PN Junction

It is possible to manufacture a single piece of a semiconductor
material half of which is doped by P-type impurity and the other half
by N-type impurity ,The plane dividing the
two zones is called junction. Theoretically, junction plane is assumed
to lie where the density of donors and acceptors is equal. The P-N
junction is fundamental to the operation of diodes, transistors and
other solid-state devices.
Let us see if anything unusual happens at the junction. It is found
that following three phenomena take place :
1. A thin depletion layer or region (also called space-charge
region or transition region) is established on both sides of the junction and is so called
because it is depleted of free charge carriers. Its thickness is about 10−6m.
2. A barrier potential or junction potential is developed across the junction.
3. The presence of depletion layer gives rise to junction and diffusion capacitances

TYPES OF SEMICONDUCTORS- Pn Junction Diode

Hello today I am going to discuss about types of semi conductors

Semiconductors are classified into 

1) Intrinsic Semiconductors (Pure Semiconductors)
2) Extrinsic Semiconductors (impure Semiconductors)

These Extrinsic Semiconductors Are again Classified into two types
1)N-Type
2)P-type

Intrinsic Semiconductors
A pure Semiconductor is called as Intrinsic Semiconductor.
In a Semiconductor at room temperature some of the valence electrons
May acquire sufficient energy to enter into the conduction band to form free electrons.

Under the influence of electric field,these electrons constitute electric current
.the missing electron leaves the empty place in the valency band that is called as Hole
*Holes also contribute electric current

In Intrinsic Semiconductors, even at room temperature ,electron Hole pairs will be created,the the conduction will takes place in the Semiconductor

Extrinsic Semiconductors

Due to poor performance of Intrinsic Semiconductor at room temperature it is not useful in electronic devices. 

The Semiconductor are added with some impurities into Intrinsic Semiconductors to increase the capability of current conduction.
That Semiconductors are called as Extrinsic Semiconductors.

N-Type Semiconductors :

 This type of semiconductor is obtained when a pentavalent
material like antimonty (Sb) is added to pure germanium crystal. , each
antimony atom forms covalent bonds with the surrounding four germanium atoms with the help  of our of its five electrons. The fifth electron is superfluous and is loosely bound to the antimony atom. Hence, it can be easily excited from the valence band to the conduction band by the application of electric field or increase in thermal energy. Thus,
practically every antimony atom introduced
into the germanium
lattice, contributes one
conduction electron
into the germanium
lattice without creating
a positive hole. 

Antimony is called donor impurity and makes the pure germanium an N-type (N for
negative) extrinsic semi-conductor. As an aid to memory, the student should associate the N in doNor
with N in the N-type material and N in Negative charge carrier.
It may be noted that by giving away its one valence electron, the donor atom becomes a positively￾charged ion. But it cannot take part in conduction because it is firmly fixed or tied into the crystal
lattice. It will be seen that apart from electrons and holes intrinsically available in germanium, the
addition of antimony greatly increases the number of conduction electrons. Hence, concentration of
electrons in the conduction band is increased and exceeds the concentration of holes in the valence
band. Because of this, Fermi level shifts upwards towards the bottom of the conduction band as
shown in Fig because the number of charge carriers has become more in conduction band
than in valence band.
In terms of energy levels, the fifth antimony electron has an energy level (called donor level)  just
below the conduction band. Usually, the donor level is 0.01 eV below conduction band for germanium
and 0.054 eV for silicon
It is seen from the above description that in N-type semiconductors, electrons are the majority
carriers while holes constitute the minority carriers. Hence, N-type semiconductor conducts principally by electrons in the nearly empty conduction band and the process is called ‘excess’ conduction.
Another point worth noting is that even though N-type semiconductor has excess of electrons,
still it is electrically neutral. It is so because by the addition of donor impurity, number of electrons available for conduction purposes becomes more than the number of holes available intrinsically. But the total charge of the semiconductor does not change because the donor impurity brings in as much negative charge (by way of electrons) as positive charge (by way of protons in its nucleus)


P-Type Semiconductors :

This type of semiconductor is obtained when traces of a
trivalent like boron (B) are added to a pure germanium crystal.
In this case, the three valence electrons of boron atom form covalent bonds with four surrounding
germanium atoms but one bond is left incomplete and gives rise to a hole as shown in Fig. 51.25 (a).
Thus, boron which is called an acceptor impurity causes as many positive holes in a germanium
crystal as there are boron atoms thereby producing a P-type (P for positive) extrinsic semiconductor.
As an aid to memory, the student should associate the P in accePtor with P in P-type material and P
with Positive charge carrier.
In this type of semiconductor, conduction is by the movement of holes in the valence band.
Accordingly, holes form the majority carriers whereas electrons constitute minority carriers. The

process of conduction is called ‘deficit’ conduction.