P-N Junction Diode - 2 | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) PDF Download

Zener Diode

A Zener diode which is also called a Breakdown diode works in reverse bias conditions. An electrical breakdown occurring in the reverse-biased condition of the PN junction diode is called the Zener effect. In this condition when the electric field increases to a high value it enables the tunnelling of electrons from the valence band to the conduction band of a semiconductor, which suddenly increases the reverse current.

Zener Diode

Avalanche breakdown

1. This breakdown occurs in the P-N junction diode with a reverse bias applied.
2. The doping is much greater than that used in the ordinary PN junction diode but is smaller than that of a diode in which Zener breakdown occurs.

 i.e. D_zener> D_avalanche > D_ordinary.
3. The reverse bias voltage applied relatively higher of the order of 8 to 10 V depending upon doping.
4. The width of the depletion layer is relatively more than that in the diode with Zener breakdown.
5. Since the reverse bias voltage applied is relatively large, a free charge carrier say an electron acquires high K.E and strikes an atom.
The covalent bond is broken and other free-electrons are liberated so that now there are two free electrons available for conduction.
6. Since the applied voltage is maintained high, there two electrons again acquire high K.E, striking other atoms and liberating two more free electrons and so on.
7. At a given voltage called the avalanche breakdown voltage, a number of free charge carriers are available to avalanche multiplication as shown so that the conductivity of the region suddenly increases.

Therefore the current suddenly increases maintaining a constant potential to the junction.

Variation Vz with temperature

Dynamic Resistance r =
= Very large for V > Vz
= Very small for V>Vz

8. The VI characteristic is similar to that of a diode with Zener breakdown.

Tunnel Diode/ESAKI DIODE

• If the concentration of impurity atom is greatly increased from its normal value of 1 part in 108 to 1 part in 103-device characteristics are totally change.
• It is fabricated from Ge or GaAs.
• The width of the junction barrier varies as 1 /n where n = impurity concentration, due to high doping thin barrier results. There is a large probability that an electron will penetrate through the barrier even the barrier potential is not able to prevent it.

This is called tunneling and these high impurity density diodes are called tunnel diodes.

• A tunnel diode is an excellent conductor in the reverse direction also.
• Its reverse breakdown voltage is very small almost approaching zero.

Use:

1. Tunnel diode can act as a very fast and high-speed switch. This is because tunneling takes place at the speed of light. Switching time as low as 50 ps.

2. It can be an element in a high frequency (microwave oscillator).

 3.

4. It can be used in pulse and digital circuits (memory storage).

Photo-Diode:

It is a reverse bias semiconductor diode in which the electron-hole pair is produced in or near the depletion region. As a result of the high electric field in the depletion region, the carrier generated by incident light separate quickly into the majority of regions before they recombine. Thus, most of the electrons generated pairs contribute to the photocurrent dark current as low as reverse saturation current.

Avalanche Photo-diode: These are operated at high Reverse-Bias voltage such that avalanche multiplication takes place.
These internal gain as 104 and response time 0.1 ns.

Photo Devices: 

The amount of absorption of light depends upon the frequency of the incident light and the intrinsic semiconductor material.
The energy carried by photon, E = h_f Band energy = E_G When h_f >> E_G. Then sharp rise in absorption takes place.

When the photon energy
E = h_f becomes larger than E_G, Electron hole pairs are created by absorbing light, and heroics conduction of current is possible.

•  In semiconductors, E_G decreases with a rise in temperature.
• When hf is sufficiently increased then the electron is freed from the solid by the direct transition from the valence band to vacuum. This is called the photoelectric effect.
• The frequency which creates electron-hole pair is, Ge and GaAs are in the infrared region.

Varactor diode

• Under Reverse Bias conditions the depletion region width increases and thus the junction capacitance decreases.
• Varactor diode is a special and PN junction that yield variable junction capacitance with applied Reverse Bias voltages
• The doping concentration decreases towards the junction.

Material: Si, Ge, GaAs
Most superior material: Ga As
Cut off frequency for Si: 250 Ghz Ga As ® 900 GHz.

Application: 

1) Low noise Parametric amplifier
2) Harmonic frequency generator
3) Frequency convector or mixer

Regular Voltage regulator using Zener Diode

Condition for regulations is

Case:1 When V_in (max) and R_L max (no load)

Case:2 When V_in (min) and R_L (min) (maximum load current) this worst-case

R_max = R_s2 =

 Practically Iz (min) = 0.2 of Iz (rated) Iz(max) = 0.81Z (rated)

Series regulator is preferable for
® High voltage
® High output currents
® Relatively varying loads.

Shunt regulator is preferred for :
1. Medium to low voltage
2. High currents
3. Relatively constant loads.

Emitter follower voltage regulators are preferred for

1. Low output voltage

It has poor ripple suppression and poor regulation with respect to variation in the input voltage variation.

 Regulation Factor

At Constant temperature DT=0, regulation should be low for good regulation.

Pulse Shaping Circuit and Waveform Generator

Diode clipping circuits 

Clipping circuits are used to select for transmission that part of an arbitrary waveform which lies above or below some particular reference voltage level. These typical clipping with input-output waveform are shown.

Clamping

Wave Form 

Consider the given circuit which consist of signal source Vi of neglecting output impedance a capacitor and a diode D.

Assuming the ideal diode i.e. Vy = 0 and Rf = 0.
The input is sinusoidal. During the first quarter cycle of the input signal, the capacitor gets charged as the diode is forward biased, with no output voltage.
o V=0
At the end of the first quarter cycle Vc = Vm. After the diode comes reverse bias. The input signal reduces but the capacitor is not able to discharge due to diode Vo = Vi =Vm. During succeeding cycles, the positive extreme of the signal just barely reaches zero. The diode need never again conduct and the positive extremity of the signal has been clamped or restore to zero.
If the amplitude of the input signal increases, the diode will conduct for the first quarter cycle. The capacitor will again get charged and the positive extremity will get clamped to zero voltage.
However, If the amplitude of the input signal decreases. The signal does not remain clamped to zero. The reason being that capacitor does not get a path to discharge.

To permit a decrease in capacitor voltage it is necessary to shunt a resistor across c or to shunt a resistor across the diode.

The circuit with shunting diode is given below

Now the capacitor gets a discharge path. when the input suddenly rate.
Vo = Vi =Vmi 

Even if the input does not decrease to 3V the capacitor continues to discharge and diode get forward bias during the peak of input voltage and capacitor charge during peaks. The RC time constant is very important. It must be very large in comparison to the period of the signal. if capacitor loses too much charge when diode is not conducting, then it may be that it does not charge during the small-time internal when the diode conducts.
Clamping circuit taking source and diode resistance into account.
The realistic clamping circuit is given below: Here source resistance Rs and diode forward resistance takes into account.