Basic Electronics - Semiconductor Devices Complete Study Notes

P-N Junction Diode

Majority carrier electrons in the n-region will begin diffusing into the p-region and majority carrier holes in the p-region will be diffusing into the n-region. If we assume there are no external excitation to the semiconductor, then this diffusion process cannot continue indefinitely. As electrons diffuse from the n-region, positively charged donor atoms are left behind. Similarly, as holes diffuse from the p-region, they uncover negatively charged acceptor atoms. The un-neutralized ions in the neighbourhood of the junction are referred to as uncovered charges. The general shape of the charge density ‘ρ’ depends upon how the diode is doped. Since the region of the junction is depleted of mobile charges, it is called depletion region, the space-charge region, or the transition region.

                                                                      Figure 1

The net positive and negative charges in ‘n’ and ‘p’ regions induce an electric fields in the region near the metallurgical junction, in the direction from the positive to the negative charge, or from the n to the p region.

Density gradients still exist in the exist in the majority carrier concentrations at each edge of the space charge region and producing a “diffusion force” that acts on the majority carriers as shown in Figure 1. The electric field in the space charge region produces another force on the electrons and holes which in the opposite direction to the diffusion force for each type of particle. In thermal equilibrium, the diffusion force and the field force exactly balance each other.

Symbol

                                                     Figure 2: Symbol of P-N Junction Diode

Volt-Ampere Characteristics of a p-n Junction Diode:

Below figure indicates the characteristic curve consisting of three distinct regions

                                  Figure 3: Volt-Ampere Characteristics of a p-n Junction Diode

Zener Diode

• Basically a p-n junction with little increase in doping level (1:10⁵) aria fabricated only with Si.
• Generally designed with normal junction and popularly known as constant voltage device.
• It can be used as reference voltage device.
• Major application is as a voltage regulator circuit and can be used as a clipper.
• Always operated under reverse bias.
• When forward bias it will be working as a normal diode with cut-in voltage 0.6 V or 0.7 V.
• Zener diode is specified in terms of breakdown voltage and maximum power dissipation.
• Zener diodes are commercially available with breakdown voltages in range of 2.5 V – 300 V.

                                              Figure 4 : Symbol of Zener Diode

Characteristics of Zener Diode

• When Zener diode is in reverse bias below the breakdown voltage, the current passing through the Zener diode is practically zero (nano amperes) and Zener diode is not conducting and Working as a normal diode.
• When reverse voltage equal to breakdown voltage, the current through the Zener diode suddenly increases to I_ZK and this is due to the breakdown phenomenon.
• When reverse voltage exceed breakdown voltage, more current will be passing into the Zener diode but the voltage drop across it will be maintained almost a constant and it is around its breakdown voltage Hence Zener diode is known as constant voltage device.

                                       Figure 5: Current-Voltage characteristic of Zener Diode

Equivalent Circuit of Zener Diode

Case-I: When Zener diode is in forward bias

                              Figure 6: (a) Zener diode in forward bias and (b) Equivalent circuit

Forward bias zener diode can be replaced by a cut-in voltage

Case-II: When Zener diode is in reverse bias

                         Figure 7: (a) Zener diode in reverse bias (b) & (c) Equivalent circuit

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 8: 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 1.

1. 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.
2. 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.

1. If we take currents between I_v and I_p and draw perpendiculars to current axis, they cut the curve corresponding to three different applied voltages, one corresponding to curve OA, other at V_P and 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.

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 9: Schematic Construction of PIN Diode

Characteristic of PIN Diode

Some important characteristic of PIN diodes are:

1. When a PIN diode is forward biased, it offers a variable resistance.
2. 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:

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

iii. It is used as constant impedance device.

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

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

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

• LED fabricated with GaAs emits infrared light.
• LED materials are

1. GaAs
2. GaAsP
3. 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

1. To limit the forward current.
2. 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).

                                                            Figure 10: Symbol of LED

Applications

1. As Remote-Control Transmitter.
2. As a display device.

iii. In designing of Opto Couplers.

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