biosensors lecture 6- temperature sensors, diodes and transistors

Thermocouple

• oldest and simplest temperature sensor

• heat flows along the conductors if there is a temperature difference

• heat flow causes a flow of electrons: seebeck effect

material choice for thermocouples

materials which have a large and constant seebeck coefficient over a large temperature range

thermistor

• thermal resistor

• resistance of thermistor is inversely related to the temperature

• voltage drop across the thermistor is also inversely related to the temperature

• superior sensitivity

• limited temperature range

semiconductors

• silicon is the most popular semiconductor material

• semiconductor becomes conductive with small external energy

• adding impurities (holes or electrons) increases conductivity

Doping

adding small impurities to a silicon crystal

Forward bias PN junction diode

positive to P (holes)

negative to N (electrons)

current flows

Reverse bias PN junction diode

negative to P

positive to N

current can not flow

rectifier diode

converts AC to DC

Zener Diode

• special type of diode

• zener voltage is much smaller than the breakdown voltage of typical diodes

• no permanent damage happens with breakdown

• process is entirely reversible

• acts as a voltage regulator in a circuit

transistors

• 3 element device

1. Emitter

2. base

3. collector

• either NPN or PNP

Zener diode temperature sensor

• zener voltage is linearly proportional to the temperature

• acts as a transducer for temperature sensors

Transistor temperature sensor

• voltage drop across base-emitter is linearly proportional to the temperature