Solid State Devices
Any semiconductor diode has an exponential voltage /current characteristic that gives an effective "knee" voltage sometimes used as a voltage reference. This voltage ranges from 0.3 V for germanium diodes up to about 3 volts for certain light emitting diodes. These devices have a strong temperature dependence, which may make them useful for temperature measurement or for compensating bias in analog circuits.
Zener diodes are also frequently used to provide a reference voltage of moderate stability and accuracy, useful for many electronic devices. An avalanche diode displays a similar stable voltage over a range of current. The most stable diodes of this type are made by temperature-compensating a Zener diode by placing it in series with a forward diode; such diodes are made as two-terminal devices, e.g. the 1N821 series having an overall voltage drop of 6.2 V at 7.5 mA, but are also sometimes included in integrated circuits.
The most common voltage reference circuit used in integrated circuits is the bandgap voltage reference. A bandgap-based reference (commonly just called a 'bandgap') uses analog circuits to add a multiple of the voltage difference between two bipolar junctions biased at different current densities to the voltage developed across a diode. The diode voltage has a negative temperature coefficient (i.e. it decreases with increasing temperature), and the junction voltage difference has a positive temperature coefficient. When added in the proportion required to make these coefficients cancel out, the resultant constant value is a voltage equal to the bandgap voltage of the semiconductor. In silicon, this is approximately 1.25V. Buried Zener references can provide even lower noise levels, but require higher operating voltages which are not available in many battery-operated devices.
Read more about this topic: Voltage Reference
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