Semiconductor Devices.docx

SEMICONDUCTOR DEVICES

Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced thermionic devices (vacuum tubes) in most applications. They use electronic conduction in the solid state as opposed to the gaseous state or thermionic emission in a high vacuum.

1. VACUUM TUBE Alternatively referred to as an electron tube or valve and first developed by John Ambrose Fleming in 1904. The vacuum tube is a glass tube that has its gas removed, creating a vacuum. Vacuum tubes contain electrodes for controlling electron flow and were used in early computers as a switch or an amplifier.

WORKING The simplest vacuum tube, the diode, contains only a heater, a heated electron-emitting cathode (the filament itself acts as the cathode in some diodes), and a plate (anode). Current can only flow in one direction through the device between the two electrodes, as electrons emitted by the cathode travel through the tube and are collected by the anode. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grid or grid. Tubes with grids can be used for many purposes, including amplification, rectification, switching, oscillation, and display.

CONSTRUCTION A vacuum tube consists of two or more electrodes in a vacuum inside an airtight enclosure. Most tubes have glass envelopes with a glass-to-metal seal based on kovar sealable borosilicate glasses, though ceramic and metal envelopes (atop insulating bases) have been used. The electrodes are attached to leads which pass through the envelope via an airtight seal. Most vacuum tubes have a limited lifetime, due to the filament or heater burning out or other failure modes, so they are made as replaceable units; the electrode leads connect to pins on the tube's base which plug into a tube socket. Tubes were a frequent cause of failure in electronic equipment, and consumers were expected to be able to replace tubes themselves. In addition to the base terminals, some tubes had an electrode terminating at a top cap.

APPLICATIONS Modern scientific inventions have helped replace these tubes with solid state semiconductor devices, such as transistors and solid state diodes. These are usually smaller, cheaper, more efficient, and reliable. However, in some specialized applications, such as high power radio frequency transmitters and microwave ovens, they still find use even in modern times.

2. FIELD-EFFECT TRANSISTOR The field-effect transistor (FET) is a transistor that uses an electric field to control the electrical behaviour of the device. FETs are also known as unipolar transistors since they involve single-carrier-type operation.

CONSTRUCTION FETs can be majority-charge-carrier devices, in which the current is carried predominantly by majority carriers, or minority-charge-carrier devices, in which the current is mainly due to a flow of minority carriers.[3] The device consists of an active channel through which charge carriers, electrons or holes, flow from the source to the drain. Source and drain terminal conductors are connected to the semiconductor through ohmic contacts. The conductivity of the channel is a function of the potential applied across the gate and source terminals. The FET's three terminals are:

1.source (S), through which the carriers enter the channel. Conventionally, current entering the channel at S is designated by IS.

2.drain (D), through which the carriers leave the channel. Conventionally, current entering the channel at D is designated by ID. Drain-to-source voltage is VDS.

3.gate (G), the terminal that modulates the channel conductivity. By applying voltage to G, one can control ID.

WORKING The FET controls the flow of electrons (or electron holes) from the source to drain by affecting the size and shape of a "conductive channel" created and influenced by voltage (or lack of voltage) applied across the gate and source terminals. (For simplicity, this discussion assumes that the body and source are connected.) This conductive channel is the "stream" through which electrons flow from source to drain.

APPLICATIONS The most commonly used FET is the MOSFET. The CMOS (complementary metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where the (usually "enhancement-mode") pchannel MOSFET and n-channel MOSFET are connected in series such that when one is on, the other is off. In FETs, electrons can flow in either direction through the channel when operated in the linear mode. The naming convention of drain terminal and source terminal is somewhat arbitrary, as the devices are typically (but not always) built symmetrically from source to drain. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example. A common use of the FET is as an amplifier. For example, due to its large input resistance and low output resistance, it is effective as a buffer in common-drain (source follower) configuration. IGBTs are used in switching internal combustion engine ignition coils, where fast switching and voltage blocking capabilities are important.

3. HETEROSTRUCTURE BARRIER VARACTOR The heterostructure barrier varactor (HBV) is a semiconductor device which shows a variable capacitance with voltage bias, similar to a varactor diode. Unlike a diode, it has an antisymmetric current-voltage relationship and a symmetric capacitance-voltage relationship.

CONSTRUCTION: The HBV consists of two, back to back, anti-serially connected rectifying diodes (such as Schottky diodes for instance). The gap in the middle of the diode symbol represents the inherent capacitance of the device. The electrical characteristics of the HBV are realized by separating two layers of a semiconductor material (A) with a layer of another semiconductor material (B).

WORKING The band-gap of material (B) should be larger than for material (A). This results in a barrier for the carriers trying to travel through the layers (A)-(B)-(A). The (A) layers are usually n-doped which means that electrons are the majority carriers of this device. At different bias voltages the carriers are redistributed and the distance between the carriers on each side of the barrier (B) is different. As a consequence the HBV has electrical properties resembling the parallel plate capacitor with a voltage dependent plate distance d.

APPLICATIONS The main application for the HBV diode is to generate extremely high frequency signals from lower frequency input. This type of frequency multiplication is demonstrated as triplers (3× multiplication) at 100 GHz through 282 GHz and up to 450 GHz,and also as quintuplers (5× multiplication) at 175 GHz.

4. INSULATED-GATE BIPOLAR TRANSISTOR An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch which, as it was developed, came to combine high efficiency and fast switching.

CONSTRUCTION It consists of four alternating layers (P-N-P-N) that are controlled by a metal-oxidesemiconductor (MOS) gate structure without regenerative action. Although the structure of the IGBT is topologically the same as a thyristor with a MOS gate (MOS gate thyristor), the thyristor action is completely suppressed and only the transistor action is permitted in the entire device operation range. An IGBT cell is constructed similarly to a n-channel verticalconstruction power MOSFET, except the n+ drain is replaced with a p+ collector layer, thus forming a vertical PNP bipolar junction transistor. This additional p+ region creates a cascade connection of a PNP bipolar junction transistor with the surface n-channel MOSFET.

WORKING By connecting appropriate diodes, the current flow is allowed. When this transistor is switched on again, the current flowing in a diode at first works like a short. The voltage can be blocked by removing the stored voltage. This looks as a surplus current added to the load current which is called as the reverse recovery current of the diode ‘Irr’. The max of Irr occurs (di/dt = 0) when the amount of the sudden voltages through the IGBT & the diode matches the supply voltage. When the IGBT is turned ON, then the current changes which make an over-voltage point by the change in the current in the dependent inductances agreeing to ∆VCE = Lσ × di/d

APPLICATIONS This device is apt for several applications such as used in power electronics, particularly in PWM(Pulse Width Modulated), UPS (Un interruptible Power Supplies), SMPS (SwitchedMode Power Supplies), and other power circuits. It increases the efficiency, dynamic performance and reduces the level of the audible noise. It is similarly fitted in the of resonant mode converter circuits. Optimized IGBT is accessible for both low switching loss and low conduction loss.

5. INTEGRATED CIRCUIT An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, normally silicon.

CONSTRUCTION The semiconductors of the periodic table of the chemical elements were identified as the most likely materials for a solid-state vacuum tube. Starting with copper oxide, proceeding to germanium, then silicon, the materials were systematically studied in the 1940s and 1950s. Today, monocrystalline silicon is the main substrate used for ICs although some III-V compounds of the periodic table such as gallium arsenide are used for specialized applications like LEDs, lasers, solar cells and the highest-speed integrated circuits. It took decades to perfect methods of creating crystals without defects in the crystalline structure of the semiconducting material.

WORKING An electric circuit is like a pathway made of wires that electrons can flow through. A battery or other power source gives the force (voltage) that makes the electrons move. When the electrons get to a device like a light bulb, your computer, or a refrigerator, they give it the power to make it work.

APPLICATIONS The applications of an ICs includes the following 1.Radar 2.Wristwatches 3.Televisions 4.Juice Makers 5.PC 6.Video Processors 7.Audio Amplifiers 8.Memory Devices 9.Logic Devices and 10.Radio Frequency Encoders and Decoders