Transistors

The transistor was invented at Bell Laboratories in 1947. Today there are two main types of transistors. The bipolar junction transistor and the field-effect transistor (FET). The bipolar junction transistor or BJT is used in mainly linear amplifiers or as an electronic switch. The term bipolar means both electrons and holes are used as current carriers.

The BJT is constructed from three doped semiconductors joined together, thus forming two pn junctions. The arrangement of doped semiconductors is either pnp or npn. These three doped semiconductors are called emitter, base and collector as shown in the symbol below for an npn transistor:

Transistor Symbol

 The base region is lightly doped and thin, compared to emitter and collector regions.

Diodes2, Special purpose diodes

• Current Regulator Diode
  A current regulator diode maintains a constant current in forward-biased. This is opposed to a zener diode which maintains constant voltage in reverse bias operation. The constant forward current is called the regulator current.
• Schottky Diode

A schottky diode is not a pn junction device. Rather it is formed by joining a n type material with a metal such as gold, silver or platinum. Its symbol is

So instead of a pn junction there is a metal-to-semiconductor junction. Schottky diodes are used mainly in fast-switching and high-frequency applications. They can be used in digital circuits to decrease switching times.

• PIN Diode

 A pin diode is made of heavily doped p and n regions with an intrinsic i region in between p and n regions. In reverse-bias position it acts like a constant capacitance while in forward bias position it acts like a current-controlled variable resistance. This low forward resistance decreases with increasing current.

The pin diode is used in dc-controlled microwave switch operated by rapid changes in bias. It is also used as a modulating device which uses variable forward-resistance characteristic. Some types of pin diodes are used as photo-detectors in fibre-optic systems.

• Step Recovery Diode

The step-recovery diode uses graded doping where the doping level of the semiconductor materials is reduced as the pn junction is approached. This produces an abrupt turn-off time by allowing a fast release of charge during switching from forward to reverse bias. A fast re-establishment of forward current is also achieved during switching from forward to reverse bias.

The step-recovery diode is used in VHF (very high frequency) and fast-switching applications.

• Tunnel Diode

The tunnel diode has a negative resistance characteristic. That is, an increase in voltage results in a decrease in current. This is opposite to Ohm's law, where an increase in voltage results in an increase in current. The negative resistance property of tunnel diode is useful in oscillator and microwave amplifier applications.  Its symbol is

• Laser Diodes

LASER (Light Amplification by Stimulated Emission of Radiation). Laser diodes made of doped gallium arsenide, emits a light that is monochromatic, which means that it consists of a single colour and not a mixture of colours. It is also a coherent light made up of only a single wavelength.

The length of a pn junction bears a precise relationship with the wavelength of the light. There is a highly reflective surface at one end of the junction and a partially reflective surface at the other end produced by polishing the ends. Laser diodes are used in forward biased operation.

Diodes1

In the following notes we will give an outline and salient features of various electronic devices. These notes are suitable for undergraduate courses taught in many engineering colleges and universities.
Following are some of the diodes used in practice:

• General-purpose and Rectifier Diodes

The general-purpose and rectifier diodes are the most common form. These diodes are essentially a PN junction, made from doped semiconductor. They are used to convert alternating current into direct current. The symbol of a diode is given below.

However there are general-purpose diodes used in limiting or clamping circuits. Limiting circuits are sometimes used to clip off parts of input voltages above or below certain levels. A clamper circuit is used to add a dc voltage level to an input signal voltage. Clamping action is used in  voltage multiplier circuits to increase peak voltages in rectifier circuits without increasing the input transformer's voltage rating. A multiplication of two, three or four may be used. These multipliers are found in TV receivers.

• Zener Diodes

Zener diodes are used for voltage regulation in dc power supplies. The zener diode is also a silicon PN junction device. It differs from rectifier diodes because it is designed to operate reverse biased in the reverse breakdown region. Its symbol is,

Zener diodes with breakdown voltages of less than 5 V operate in zener breakdown. Those with breakdown voltages greater than 5 V operate in avalanche breakdown. Zeners are available with breakdown voltages of 1.8 V to 200 V.

Zener diodes have the ability to keep the voltages across its terminals constant which is a key feature of the zener diode. A zener diode operating in breakdown acts as a voltage regulator because it maintains a nearly constant voltage across its terminals over a specified range of reverse-current values.

• Veractor Diodes

Veractor diodes also called variable capacitance diodes because the junction capacitance varies with the amount of reverse-bias voltage. These devices are commonly used in tuning circuits used in communication systems. Its symbol is

We see that the depletion region in a PN junction is widened by reverse bias and acts as a capacitor dielectric because of its nonconductive characteristic. The p and n region are conductive and can be thought as the capacitor plates.

Since depletion region widens with reverse-bias voltage, the plate separation increases, dielectric thickness increases thus decreasing the capacitance. A decrease in reverse-bias voltage narrows the depletion region and increases capacitance.

An important parameter of varactor diodes is tuning ratio which is the ratio of the diode capacitance at a minimum reverse voltage to the diode capacitance at a maximum reverse voltage. For example if the minimum and maximum reverse voltages are 4 V to 60 V and the corresponding diode capacitances are 6.8 pF and 2.3 pF, the tuning ratio is 6.8/2.3 = 2.9.

• Optical Diodes

The light-emitting diode (LED) and the photodiode are two types of optical diodes. The photodiode is also called a light detector as opposed to the LED. The symobl of LED is,

The LED operates in forward-biased operation. The wavelength of the emitted light is established by adding impurities during the doping process. The wavelength indicates the color of light and whether it is visible or invisible (infrared). LEDs are made of gallium arsenide (GaAs), gallium phosphide (GaP), or gallium arsenide phosphide (GaAsP). Silicon and germanium which are heat-producing materials are not used. GaAs LEDs produce invisible infrared radiation, while GaAsP produces red or yellow light, and GaP produces red or green light.

The photodiode or light-detecting diode is a PN junction device that operates in reverse-bias condition. Its symbol is

A photodiode has a small window that allows light to strike the PN junction. When the PN junction of a photodiode is exposed to light, the reverse current increases with the light intensity.