Autotronics - Transistor

Introductions


- Active electronic devices used as switches and amplifier for electrical signal.

- Can usually be found as part of electronic switches in the application of communication technique, power electronics and computer system.

- Nowadays transistors are widely applied in integrated circuit especially in microelectronic industries.

- Made of silicon material with at least 3 terminals for connection to an external circuit.

- A voltage or current applied to one pair of transistor’s terminals changes the current flowing through another pair of terminals.

- There are 2 types of transistors which are bipolar transistor and field effect transistor.

- Both of them can be differentiated by the way signal is controlled.

- Bipolar transistor :-

      o The most commonly used transistor

      o Become the transistor of choice for many analogue circuits due to their great linearity and ease of manufacture.

- MOSFET

      o Widely used in digital circuits because of their utility in low-power devices

      o Usually in CMOS configuration.



Simple Operation

- The common usage as amplifier by controlling its output in proportion to the input signal.

- This action is due to its gain properties which enable it to use a small signal applied between one pair of its terminal to control a much larger signal at another pair of terminals.

- Another common application of transistor is as switch.

- Used to turn current on or off in a circuit as an electrically controlled switch.



a) Bipolar transistor

- Control electron (negative charge carrier) and hole (positive charge carrier) by manipulating the principal of generation and recombination of electron in a crystal.

- The principal is basically same as diode whereby electron and hole play a big role.

- A bipolar transistor has terminals labelled as base, collector and emitter.

- A small current at the base terminal(that is flowing from the base to the emitter) can control or switch a much larger current between the collector and the emitter terminals.

- Controlled by manipulating the electrical current.

- The contacts are Base, Emitter and Collector.

- Small control current is applied at the Base-Emitter path.

- The characteristic of NPN-transistor and PNP-transistor can be differentiated by the structure of the transistor.

- Bipolar transistor is basically a self block whereby without control from the small current at the Base-Emitter path, current can’t flow through the Collector-Emitter path.





Field Effect Transistor

- Field Effect Transistor (FET)

- Also known as uni polar transistor is controlled by manipulating voltage across it.

- The contacts are known as Gate, Drain and Source.

- In metal oxide semiconductor FET (MOSFET) there is an extra contact which is called as Bulk, where it is connected to Source.

- The resistance and also the current across the Drain-Source path are controlled by the voltage and also electric field at gate.

- The controlled current in the Drain-Source Canal can flow in both position rather than Collector current in bipolar transistor which can flow only in one direction.



Junction Field Effect Transistor (JFET)

- Normally JFET is a self conductive transistor.

- When there is no voltage at gate, the path between Source and Drain become conductive.

- But if voltage is applied at the Gate the conductivity between Source and Drain will reduce.

- There are 2 types of JFET i.e. N-Canal and P-Canal.

- N-Canal transistor can be recognized by referring to the arrow symbol which direction is showing into the transistor.

- For P-Canal the arrow will point in the opposite direction.

- JFET is normally used in special application such as microphone amplifier due to the complexity of the control.



Metal-Oxide Semiconductor – Field-Effect-Transistor

- This transistor is known as MOSFET due to the structure of semiconductor layer at the Gate.

- But nowadays poly silicon is used as the material at gate.

- MOSFET is normally used for charge transport which is suitable for use at very high frequencies e.g. microwave frequencies.



Special Type of Transistor

- Beside the normal type of transistors there are other types of transistors for special usage such as Bipolar transistor with isolated Gate electrode.

- This type of transistor is widely used in power electronic area where by it is actually a combination of MOS- and bipolar technology in a same package.

- Due to its capability it has been used in power electronic area to replace thyristor.

- Fototransistor is actually an optically sensitive bipolar transistor which has same application as other opto-coupler devices.

- The control of the transistor is not by the Base-emitter current but it is controlled by light.

- Light act similar as the Base current at the PN-junction.

- In some of the LCD display monitor, thin film transistors are used to control every pixel on the monitor.

- These FET is actually transparent. They are used to control the contrast, colour and the brightness of the monitor for each pixel.

- Therefore for each monitor more than million thin film transistors are used for this purpose.

- In programmable storage disk such as EPROMs and EEPROMs special MOSFET (floating gate) are used as prime storage element.

- By manipulating the electrical charge which are stored in the floating gate, the transistor can be switched on and off and also information of one bit can also be stored.



Applications


Digital Circuit

- Here transistors are used mainly in the making of integrated circuit in the RAM-Storage, Flash-Storage, microcontroller, microprocessor and logic gate.

- Normally there are about 1 billion of transistors on a substrat which are made of silicon.


Analogue Circuit

- In the analogue circuit transistors are used as operational amplifier, signal generator and also as reference voltage source.

- To convert between analogue to digital or vice versa Analogue-digital converter and digital-analogue converter are used frequently.

- The number of transistors used are about 100 to 10000.