Dear Vimal,
The Early voltage is a parameter describing the variation of the transistor collector or drain current in the active or the saturation region of operation with the VCE or VDS , respectively. An ideal transistor would operate as an ideal current source in this mode of operation where the the transistor is capable to amplify the signals applied on its inputs when it is biased to operate in the active region of operation.
Real transistors show a small increase of the their collector/drain current with the VCE OR VDS. This is due to the widening of the space charge region in the base of the bipolar transistor or the channel of the MOSFET transistor. In case of the bipolar transistor the gradient of the diffusion current increases leading to an increase in the collector current. While the the channel length becomes smaller as consequence of the drain space charge widening leading to smaller channel resistance and higher drain current.
In case of bipolar transistor it follows that:
IC =Icsat ( 1 + VCE/ VA) where VA is the Early voltage,
Differentiating IC with respect to VCE one obtains the large signal collector to emitter resistance rCA,
rCA= VA/ ICSAT,
It is clear that the early voltage physically is one of the determinants of the collector to emitter resistance of the transistor.
Similarly for MOSFET transistor,
r DS= VA/ IDsat,
Where rDS is the large signal drain to source resistance, IDsat is the drain saturation current and VA is the Early voltage of the MOSFET transistor.
For more information please follow the link: https://www.researchgate.net/publication/236003006_Electronic_Devices
wish you sucess
Book Electronic Devices
The Early effect is related to the dependence of the width of the space charge layer upon the bias across it. A space charge layer exists in the active region of a pn junction; this layer is normally a depletion layer, in which case the width of the layer increases with increasing reverse bias, and decreases with increasing forward bias. If the depletion layer increases at the collector-base junction of the bipolar transistor, the width of the neutral base decreases. A decreased neutral base results in a higher concentration gradient leading to a higher diffusion current. Finally, the collector current increases.
I have tried to find some physical sense in the quite artificial definition of the so-called "Early voltage" by asking a question, "Can the Early voltage be physically explained?":
https://www.researchgate.net/post/Can_the_Early_voltage_be_physically_explained
- Badges
- Science topic
- Similar topics
- Mathematical Sciences
- Control Theory
- Control Systems