Basics of Semiconductor Devices

Semiconductors are tetravalent atoms called intrinsic semiconductors with conductivity between conductors and insulators. Adding impurities creates P-type and N-type semiconductors, a process called doping. Doped semiconductors are extrinsic. Current flows through both electrons and holes. 

N type & P type Semiconductor Device 

 N-Type: These semiconductors are doped with pentavalent atoms, making electrons the majority charge carriers and holes the minority carriers. These donor atoms provide extra electrons. Common impurities used are antimony, arsenic, and phosphorus. 

P-type: These semiconductors are doped with trivalent atoms, resulting in holes as the majority charge carriers and electrons as the minority carriers. These acceptor atoms create holes. Typical impurities include aluminium, boron, indium, and gallium. 

PN Junction Diode & Barrier Potential 

A semiconductor diode, like a valve diode, is formed by joining P-type and N-type semiconductors, creating a P-N junction. The P-type, with majority holes, acts as the anode, while the N-type, with majority electrons, acts as the cathode.  

When a P-N junction forms, some electrons from the N-region cross over and fill holes in the P-region, creating positive ions in the N-region and negative ions in the P-region. This forms a depletion region with a barrier potential that opposes further electron and hole movement. The barrier potential is 0.7V for silicon diodes and 0.3V for germanium diodes. 

Forward & Reverse Bias of PN Junction Diode 

In forward bias, the P-N junction diode conducts with low resistance and almost acts like a closed switch.  The negative terminal repels electrons in the N-region toward the junction, overcoming the potential barrier (0.7V for silicon, 0.3V for germanium). Electrons then move through the P-region and are collected by the positive terminal, while holes are attracted to the negative terminal, facilitating current flow.  

This is another way of connecting diode, P- region (anode) to negative terminal  and N-region (cathode) to positive terminal of the battery, and then it is called as 'reverse bias'.  In this condition the width of potential barrier increases and restricts the flow of majority carriers. A very small amount of current flows due to minority carriers and is called leakage current.  

 The above explanation concludes that current flow through diode is possible only in one direction, in forward bias and not in reverse bias.  

Half Wave Rectifier & It’s Working (Applications of diode)   

AC Voltage consists of two half cycles, as shown in Fig . The upper half cycle is known as the positive half cycle, and the lower cycle is known as the negative half cycle.  

A half-wave rectifier is a simple and low-cost rectifier circuit. It is used where high-quality DC is not required, for example, to operate a nightlight or radio circuit. 

 Working:  

i) Refer to the circuit diagram and waveforms. In the first half cycle of AC voltage, when terminal A of the transformer is positive relative to terminal B (fig. 1.27a), the diode becomes forward-biased and acts as a closed switch. Current flows from terminal A through RL to terminal B, producing an output voltage across RL like the positive half cycle of the AC input

ii) In the negative half cycle, terminal B is positive relative to terminal A (fig. 1.27b). The diode becomes reverse biased, acting as an open switch, and blocking current flow.

Zener Diode (Types of diodes) 

A Zener diode provides stable voltage in reverse bias and is available in voltages from 2 to 200V. In reverse bias, as voltage increases, the current remains minimal until reaching the breakdown voltage (Vz). At this point, current increases sharply, but the voltage across the diode stays constant, even with further voltage increases, ensuring stable voltage regulation. Zener diodes are used in DC power supplies as a voltage regulator or in special circuits to stabilize the voltage. 

Light Emitting Diode (LED) & Photo Diode 

LEDs, made from Gallium, Arsenic, and Phosphorus, emit light in forward bias instead of heat like normal diodes. Recombination of holes and electrons at the P-N junction produces light. LEDs come in red, green, and yellow. LEDs are used in 7-segment displays, watches, calculators, digital meters, and power supply indicators. A series resistor is needed to limit current and prevent damage. 

Photo diode conducts when light is incident on it. Photodiodes, made with cadmium selenide (CdSe), cadmium sulphide (CdS), germanium, and silicon, operate in reverse bias. They generate electron-hole pairs and current when exposed to light, with current depending on light intensity. It is used in computer punch card; light detectors, sound reproduction of film projector and light operated switches.