SEMICONDUCTOR DEVICES

Diodes as a semiconductor devices Symbol and Structure Diodes is made by joining p-types and n- types semiconductor materials. Diodes are undirectional devices that allow current to flow through them only in one direction.

Biasing Diode There are two operating regions and three possible "biasing" conditions for the standard Junction Diode and these are: Zero Bias - No external voltage potential is applied to the PN- junction. Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN-junction width. Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN-junction width.

Zero Biased The potential barrier exists discourages the diffusion of any more majority carriers across the junction. The potential barrier helps minority carriers (few free electrons in the P- region and few holes in the N- region) to drift across the junction. Then an "Equilibrium" or balance will be established when the majority carriers are equal and both moving in opposite directions, so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of "Dynamic Equilibrium".

Reverse Biased N material is connected to the positive terminal of the voltage source, V and the P material is connected to the negative terminal of the voltage source,V.The effect is that charge carriers in both sections are pulled away from the junction.Free electrons on the N side are attracted away from the junction because of the attraction of the positive terminal of the voltage source,V.Likewise,hole in the P side are attracted away from the junction because of the attraction of the negative terminal of the voltage source,V.Its will increases the depletion region.

Forward Biased N material is connected to the negative terminal of the voltage source, V and the P material is connected to the positive terminal of the voltage source,V.The voltage source must be large enough to overcome the internal barrier potential, VB. The voltage source repels free electrons in the N side across the depletion zone and into the P side. Once on the P side, the free electron falls into a hole. The electron will then travel from hole to hole as it is attracted to the positive terminal of the voltage source, V. Its will decreases the depletion region.

I-V Characteristic Curve for silicon Diode

Zener Diode Zener Diode symbol and I-V Characteristics

Zener Diode is used in its i. reverse bias or reverse breakdown mode  Diodes anode connects to the negative supply.  From the I-V characteristics its show that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max). This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply or load variations.Zener diode will continue to regulate the voltage until the diodes current falls below the minimum IZ(min) value in the reverse breakdown region.

Light Emitting Diodes LED are the most visible type of diode, that emit a fairly narrow bandwidth of either visible light at different coloured wavelengths, invisible infra-red light for remote controls or laser type light when a forward current is passed through them. LED are semiconductor devices that convert electrical energy into light energy Light emitting diodes are available in a wide range of colours with the most common being RED(GaAsP), AMBER(GaAsP, YELLOW (GaAsPN) and GREEN (AlGaP)

Biased Voltage Biased voltage applied to transistor to operate successfully. There are two type of biased voltage supplied to the transistor as a signal for it operations. Two main condition in biasing a transistor are: i.E-B(emitter-base) junction must be forward biased ii.C-B (Collector –base )junction must be reverse biased N PN P NP E B C E B C

Bipolar Junction Transistor (BJT) Like diode, BJTs are formed by P and N region and as we are already the point at which P and N region join is known as a junction. The bipolar junction transistor is a three terminal current operated semiconductor device. The terminals are called the emitter, base and collector. The emitter and collector is made up of the same type of semiconductor material. Figure above shows the structure and schematic symbol for NPN and PNP transistor:

The arrow at the transistor symbol shows the direction of conventional current when it is operates. Either NPN or PNP, the base is very thin.Contains less majority current carrier. Emitter functions as charge provider or majority current carrier in transistor. Collector has to collect charge for circuit operates. Base is the junction which is control the current flow.

Transistor Operation N P N E B C IEIE IBIB ICIC E-B junction has been forward biased by V1, E-B junction will flow the current because of electrons at emitter (N-type) will move by negative potential supply V1. These electrons tried to go to positive potential V1 through base (P-type). But because of base only a thin layer and have small amount of hole, not all electrons can flows. Only little electrons current can flow at the base known as base current, IB. The others electrons will collect at base layer in large amount. Because of collector layer has been connected to positive terminal of V2, the positive potential will attracted that electrons flow through collector as current collector, IC. IE = IC + IB