Analog Electronics - Diode Types & Its Applications Complete Study Notes

Ideal Diode

The ideal diode may be considered the most fundamental non-linear circuit element. It is two terminal devices having the circuit symbol of figure 1(a) and the characteristic in figure 1(b).

Fig (1): The ideal diode (a) diode circuit symbol (b) i-v characteristics.

The characteristics of the ideal diode can be explained as below:

If the negative voltage (relative to the reference direction indicated in figure 1.a) is applied to diode, no current flows and the diode behave as an open circuit as shown in fig 1(c). Diode operated in this mode are said to reverse biased.

An ideal diode has zero current when operated in the reverse direction and is said to be cut-off and simply OFF.

If a positive current (relative to the reference direction indicated in fig 1.a) is applied to the ideal diode, zero voltage drop appear across the diode. Diode operated in this mode are said to forward biased.

An ideal diode behaves as a short circuit in forward direction as shown in fig 1(d), it passes any current with zero voltage drop. Forward biased diode is said to turn ON or simply ON.

Load line Analysis

The simplest diode configuration is shown in figure 3(a) which have characteristics as shown in figure 3(b)

Applying Kirchhoff’s law to the series circuit of figure 3(a)

E – V_D – V_R = 0

E = V_D + I_DR ……………………(i)

If V_D = 0V (for finding point A)

E = I_D R

∴

If I_D = 0A (for finding point B)

E = V_D = 0.R

A straight line drawn between the two points A & B is known as dc load line of diode circuits.
If the level of load resistance is change then intersection on the vertical axis will also change. This result in the change of slope of load line and a different point of intersection between the load line and device characteristics.
Load line can be defined by a network and characteristic curve defined by the device. The point of intersection between the two is known as point of operation for this circuit. It is usually called as quiescent point or Q-point.
The solution obtained at the intersection of the two curves is same that would be obtained by a simultaneous mathematical solution of

Short Trick for Diode ‘ON’ or ‘OFF’ testing

Zener Diode

Figure : Zener diode

Zener diode is a special purpose diode designed to operate under reverse bias in the breakdown region.
A Zener diode has higher doping than conventional diodes
In a Zener diode the depletion layer is very thin and electric field strength at the junction is very high even for a small reverse voltage.

Application of Zener Diode as a Voltage Regulator

At the instant when the applied reverse biased voltage on the Zener diode is equal to the Zener breakdown voltage, further increase in reverse voltage makes the electric field at the p-n junction significantly high.
The electric field is high enough to pull the electrons that are beyond the junction on the n side towards the p-side, significantly increasing the current.
The increased current allows a wide range of current to flow through the diode in the breakdown region such that the reverse voltage has no significant change.

Figure V-I Characteristics of Zener Diode

When V_RB < V _Br, the current through Zener diode is practically zero and is thus non-conducting.
When V_RB = V _Br, the current suddenly shoots up due to the breakdown phenomena and Zener starts conducting.
When V_RB > V _Br, more and more current will be passing through Zener diode but the voltage drop across the device will be maintained almost a constant and it is around the V _Br

Dynamic resistance of Zener Diode

It is the ratio of change in voltage across the diode to the change in current through the diode.

For an ideal Zener diode, R_Z = 0

Zener Breakdown Phenomenon

It is due to large electric-field intensity.
It is due to tearing-off or rupturing-off of covalent bonds within the depletion layer.
Break down voltages are below 6 V.
Zener break-down voltages V _Br decreases with temperature (NTC)

Avalanche Breakdown Phenomenon (Chain or cumulative Action)

Figure Avalanche breakdown

It is due to multiple collisions between electrons and ions within the depletion layer.
It is due to electron multiplication.
It is due to impact ionization.
Electrons comes from p-side to depletion layer and collide with –ve ion of depletion layer and its leads into multiple collision.
Always operated under Reverse bias.
Breakdown voltage is always greater than 6V.
Avalanche breakdown voltage increase with temperature (P.T.C) because of positive temperature coefficient.

TUNNEL DIODE

A tunnel diode is a high conductivity two terminal p-n junction diode doped heavily about 1000 times higher than a conventional junction diode.

Tunnelling

In a tunnel diode, many carriers punch through the junction even when they do not have enough energy to overcome the potential barrier (0.3 V for Ge and 0.7 V for Si). Consequently, large forward current is produced even though the applied bias is much less than 0.3 V or 0.7 V. The phenomenon is known as tunnelling.

Current-Voltage Characteristic

Figure 1 shows the current-voltage characteristic of a tunnel diode. If the tunnel diode is reversed biased, then it acts like a good conductor, i.e. the reverse current increases with increasing reverse voltage.

Figure: Current-Voltage characteristic of Tunnel Diode

We must note the following points about the tunnel diode with reference to the characteristic curve shown in Figure .

Between points A and B, the current decreases with increases in voltage. This shows that the tunnel diode has a negative resistance in this region. The portion AB constitute the most important property of tunnel diode which makes it useful in high frequency oscillations.
For voltages above V_v (valley voltage), the current starts increasing as in case of conventional diode.

iii. The reverse current increases with increasing reverse voltage.

If we take currents between I_v and I_pand draw perpendiculars to current axis, they cut the curve corresponding to three different applied voltages, one corresponding to curve OA, other at V_Pand the third with respect to curve AB. Thus, each current can be obtained at three different applied voltages. This feature makes the tunnel diode useful in pulse and digital circuits.

Tunnel Diode Parameters

The two important parameters of tunnel diode are

Negative Resistance:

This is the resistance offered by tunnel diode in negative region. We denote the negative resistance by R_n and define as

Where is the change in forward voltage between any two points lying within negative resistance region of V-I characteristic, and is the corresponding change in forward current.

Current Ratio:

This is the ratio of peak current (I_P) to valley current I_V, i.e.

This parameter is important in high speed switching circuits.

Tunnel Diode Equivalent Circuit

Figure 2 shows the equivalent circuit of a tunnel diode that consists of the following four elements:

Series Resistance (R_S):

This is the resistance due to leads, contacts and semiconductor material.

Series Inductance (L_S):

This is the induction due to lead lengths.

iii. Junction Capacitance (C):

This is due to diffusion capacitance and applied voltage.

Negative Resistance (-R_n):

This is the resistance offered by tunnel diode between peak point A and valley point B.

Figure : Equivalent Circuit of a Tunnel

PIN DIODE

A PIN (Positive-Intrinsic-Negative) diode is schematically shown in Figure 3. In a PIN diode, a high resistivity intrinsic layer is sandwiched between p+ and n+ regions. Due to increased separation between p and n regions, the capacitance decreases. Therefore, the PIN diode has fast response time at high frequencies.

Figure: Schematic Construction of PIN Diode

Characteristic of PIN Diode

Some important characteristic of PIN diodes are:

When a PIN diode is forward biased, it offers a variable resistance.
When a PIN diode is reversed biased, it offers infinite resistance in the reverse direction.

iii. PIN diode has highly improved switching time in comparison with a PN diode.

Applications of Pin diodes

Some important applications of PIN diodes are:

PIN diodes can be used in construction of phase modulator and amplitude modulator.
It can be used as alternator.

iii. It is used as constant impedance device.

It can be used as phase shifter.
It can be used as T-R switch in radar applications.

VARACTOR DIODE

A varactor diode is specially manufactured p-n junction with suitable impurity concentration profile and operated under reverse-biased conditions so as to yield a variable junction capacitance.

Figure : Varactor Diode (a) Symbol (b) Equivalent Circuit

Characteristic of Varactor Diode

In terms of applied reverse bias V, the transition capacitance of a varactor diode is approximated by the expression,

where,

V_kis the volt equivalent temperature;

V is the reverse bias applied in volts;

n = 1/2 for alloyed junctions;

n = 1/3 for diffused junctions

Applications of Varactor Diode

Following are some important applications of varactor diode:

Used in parametric amplifier.
Varactor diode is used in automatic frequency control.

iii. It is used in tuning circuits.

Used in adjustable band pass filter.

LIGHT EMITTING DIODE (LED)

LED will emit the light when properly forward biased.
PRINCIPLE: ELECTRO-LUMINESCENSE (conversion of electrical energy into light energy).
In LED light is emitted due to a large number of recombination in the depletion region.
LED i.e. generally fabricated with DBGSC.
Popularly used material is GaAs.
LED can emit the light either in the VISIBLE SPECTRUM or INVISIBLE SPECTRUM OF LIGHT depending on DOPENTS.
In the invisible spectrum of light, LED emits INFRARED LIGHT.
IRLED is widely used as a remote-control transmitter.
The colour of light given by LED depends on

Wavelength and frequency of emitted light.
Type and concentration of dopants.

LED fabricated with GaAs emits infrared light.
LED materials are

GaAs
GaAsP
GaP←Highly unstable material (unreliable, unpredictable). Belongs to IBGSC. Also, since material is unstable, but then also under controlled doping it is made used to work as LED. Material is forced to emit light; under controlled doping.

Modern LED’s are fabricated with some of the DBGSC and also some of the IBGSC under “controlled doping”.
Always operated under forward bias.
When Reverse Biased, LED will be working as normal diode & it cannot emit any light.
The function of limiting resistance in the LED

To limit the forward current.
To limit the light output.

The efficiency of LED

Cutting Voltage; depending on dopant.
Power dissipation in mW.
When compared to LCD, the disadvantage of LED is higher power dissipation.
LED has longer operating life.
LED is relatively faster in operation when compared to LCD because of smaller response time (in us).