Zener breakdown voltage, V Z • Minimum current to sustain breakdown, I Z min I z(min) I z(max) – V Z Forward bias region Constant zener voltage Zener breakdown region Knee voltage (c) I F (mA) – I R ( µ A) – V R (V) V F (V) mA (a) ( –12V) V – – – – R D µ A ( –15V) V – – – – R D (b) Figure . Zener diode (a) Forward bias (b) Reverse bias (c) V-I characteristics - - - - - - - - Unit electronics and Communication EXAMPLE . Find the current through the Zener diode when the load resistance is kΩ. Use diode approximation.
V Z = V R L = 2kΩ R S = kΩ I L V Solution Voltage across AB, V Z = 9V Voltage drop across R S = – = 6V Therefore current through the resistor R S , I = × mA Voltage across the load resistor, V AB V = Current through load resistor, V R L AB L = × . mA The current through the Zener diode, Z L = − mA mA mA . . Optoelectronic devices Optoelectronics deals with devices which convert electrical energy into light and light into electrical energy using semiconductors.
Optoelectronic device is an electronic device which utilizes light for useful applications. We will discuss some important optoelectronic devices namely, light emitting diodes, photo diodes and solar cells. i) Light Emitting Diode (LED) LED is a p-n junction diode which emits visible or invisible light when it is input voltage is applied to the circuit and constant output voltage V o is taken across the load resistance R L which is connected in parallel with Zener diode. The output voltage is maintained constant as long as the input voltage is greater than V Z .
V i R s I L R L I z V V – o z Figure