Circuit Symbols 3

*04 Electronics (160-187).qxd

2/2/07

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Circuit symbols 3 Semiconductor components

Key words diode light-emitting diode (LED) thermistor thyristor

transistor zener diode

Semiconductor components ● Diodes

and transistors are formed of semiconductor materials such as silicon and germanium crystals. The material is treated in one of two ways. Either an impurity such as arsenic is added, to give the material an excess of electrons, forming an n-type semiconductor, or an impurity such as boron is added to give the material fewer electrons (p-type semiconductor). ● A diode consists of an n-type layer fixed to a p-type layer. ● The connector from the p-type layer forms the anode and that from the ntype layer forms the cathode. When connected in a circuit in this way (forward bias), charge carriers in the diode move to the junction between the layers and current flows. If the diode is connected the opposite way, (reverse bias) either no current flows or, if a large voltage is applied, the diode breaks down. ● A bipolar transistor consists of three layers – either a layer of p-type material sandwiched between two ntype layers (n-p-n junction transistor) or a layer of n-type sandwiched between two p-type layers (p-n-p junction transistor). Each layer has a connector, they are called the base, the collector and the emitter.

p-n-p junction transistor

Light emitting diode (l.e.d)

n-p-n junction transistor

Diode/rectifier

field effect transistor (f.e.t) junction gate, n channel

Zener diode

Light sensistive diode

Thyristor

Variability of components The addition of variability to resistive, capacitative or inductive symbols is indicated as follows Symbol

indicates a variability of value which may, or may not, be continuous

indicates that the component may be pre-set to a particular value

Variability of components

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The function of many electronic circuits depends on small changes caused by sensors which operate within certain known ranges, such as thermistors, whose resistance falls with increased temperature. In order for circuits to function correctly in response to signals from sensors, some components, such as resistors can be adjusted to particular values.

Description of symbol when placed across the component symbol

indicates that it has an inherent non-linear variability

The following examples illustrate the use of these symbols

Variable resistor

Pre-set capacitor

Resistor with inherent variability (eg thermistor