Working of diode and MOSFET

  What is a diode?

A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode, and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other semiconducting materials such as gallium arsenide and germanium are also used.

· Types of Diode:

Zener diode

PN junction diode

Tunnel diode

Varactor diode

Schottky diode

Photodiode

PIN diode

Laser diode

Avalanche diode

Light-emitting diode

· Working of a diode:

The operation of a diode involves unbiased, forward biased, and reverse biased condition.

We will discuss the above-mentioned condition in detail. Let us start with the unbiased condition.

· The unbiased condition of a diode:

When no external potential or voltage is provided to the device. Then it is known as the unbiased condition of a diode.

The figure given below will help you to have a better understanding of the unbiased condition of a diode.

Here, the p-type material is fused with an n-type material. This fusion creates a junction. When no voltage is applied across the diode then, the majority charge carriers i.e., holes from the p side and electrons from the n side get combined at the junction. These charge carriers on combining generate immobile ions that deplete across the junction. Due to this a depletion region is formed at the junction.

It is to be noted here that the flow of charge carriers across the cross-section area is known as diffusion. Hence the current at no bias condition is known as diffusion current.

The potential difference at the depletion region generates an electric field across it. Due to this electric field, no further movement of majority charge carriers is allowed. This is why the width of the depletion region is fixed. The potential at the depletion region acts as a barrier for further movement hence known as a barrier or built-in potential. However, still, minority carrier drifts across the depletion region and a negligible current flows. This very small current due to minority carriers is known as drift current.

· The forward biased condition of a diode:

In the forward biased condition, the p side of the device is connected with the positive terminal of the supply. And n side is connected with the negative battery potential. Thereby causing the junction to be forward biased.

Below a figure is given that represents the diode arrangement with positive biasing:

When forward biasing is applied. The holes in the p side experience a repulsive force from the positive terminal. Similarly, electrons experience a repulsion from the negative terminal of the supply provided. However, initially, the majority of carriers from both sides do not move across the junction due to barrier potential.

But, as the barrier potential is exceeded, the majority charge carrier now shows movement across the junction. This movement of charge carriers after overcoming the barrier potential generates a current. This current is known as the majority current. The moment this barrier is removed, the resistance offered by the junction becomes automatically 0. Thus, a forward current now starts to flow through the device.

It is noteworthy here that the barrier potential offered by silicon is 0.7V and for germanium is 0.3V. So, after overcoming the respective potential in the case of both the materials, forward current starts flowing through the device.

· The reverse biased condition of Diode:

When we externally provide the potential to the device in such a way that the p side is connected to the negative terminal of the supply. And n side is connected with the positive terminal. Then the device is said to be reverse biased.

The figure below shows the reverse-biased arrangement of a PN junction diode:

When a reverse potential has been applied the holes from the p side experience attraction from the negative terminal. And electrons on the n side experience attraction from the positive terminal of the supply provided. Due to this, the majority of carriers present in both the side move in the direction away from the junction. This broadens the width of the depletion region and hence the potential barrier is increased.

This takes the device to a non-conducting state. However, due to the minority carriers present in both p and n side, a very small current flows. This small current through the device is known as reverse leakage current. This reverse current is independent of barrier potential and depends only on the temperature and construction of the device.

· What is a MOSFET

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of insulated-gate field-effect transistor that is fabricated by the controlled oxidation of a semiconductor, typically silicon. The voltage of the covered gate determines the electrical conductivity of the device; this ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals.

The MOSFET was invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959, and first presented in 1960. It is the basic building block of modern electronics, and the most frequently manufactured device in history, with an estimated total of 13 sextillions (1.3×1022) MOSFETs manufactured between 1960 and 2018. It is the dominant semiconductor device in digital and analog integrated circuits (ICs), and the most common power device. It is a compact transistor that has been miniaturized and mass-produced for a wide range of applications, revolutionizing the electronics industry and the world economy, and being central to the digital revolution, silicon age, and information age. MOSFET scaling and miniaturization have been driving the rapid exponential growth of electronic semiconductor technology since the 1960s and enables high-density ICs such as memory chips and microprocessors. The MOSFET is considered the "workhorse" of the electronics industry.

A key advantage of a MOSFET is that it requires almost no input current to control the load current when compared with bipolar junction transistors (BJTs). In an enhancement mode MOSFET, the voltage applied to the gate terminal can increase the conductivity from the "normally off" state. In a depletion-mode MOSFET, the voltage applied at the gate can reduce the conductivity from the "normally on" state. MOSFETs are also capable of high scalability, with increasing miniaturization, and can be easily scaled down to smaller dimensions. They also have a faster switching speed (ideal for digital signals), much smaller size, consume significantly less power, and allow much higher density (ideal for large-scale integration), compared to BJTs. MOSFETs are also cheaper and have relatively simple processing steps, resulting in high manufacturing yield.

· Types of MOSFET

PMOS and NMOS logic

Complementary MOS (CMOS)

Depletion-mode

Metal–insulator–semiconductor field-effect transistor (MISFET)

Floating - gate MOSFET (FGMOS)

Power MOSFET

Double-diffused metal–oxide–semiconductor (DMOS)

MOS capacitor

Thin-film transistor (TFT)

Bipolar–MOS transistors

MOS sensors

A multi-gate field-effect transistor (MuGFET)

Quantum field-effect transistor (QFET)

Radiation-hardened-by-design (RHBD)

· Working of MOSFET

In general, the MOSFET works as a switch, the MOSFET controls the voltage and current flow between the source and drain. The working of the MOSFET depends on the MOS capacitor, which is the semiconductor surface below the oxide layers between the source and drain terminal. It can be inverted from p-type to n-type, simply by applying positive or negative gate voltage respectively. The below image shows the block diagram of the MOSFET.

When a drain-source voltage (VDS) is connected between the drain and source, a positive voltage is applied to the Drain, and the negative voltage is applied to the Source. Here the PN junction at the drain is reverse biased and the PN junction at the Source is forward biased. At this stage, there will not be any current flow between the drain and the source.

If we apply a positive voltage (VGG) to the gate terminal, due to electrostatic attraction the minority charge carriers (electrons) in the P substrate will start to accumulate on the gate contact which forms a conductive bridge between the two n+ regions. The number of free electrons accumulated at the gate contact depends on the strength of positive voltage applied. The higher the applied voltage greater the width of the n-channel formed due to electron accumulation, this eventually increases the conductivity and the drain current (ID) will start to flow between the Source and Drain.

When there is no voltage applied to the gate terminal, there will not be any current flow apart from a small amount of current due to minority charge carriers. The minimum voltage at which the MOSFET starts conducting is called the threshold voltage.