16-08-2012, 02:44 PM
THE TUNNEL DIODE
TunnelDiode.ppt (Size: 471 KB / Downloads: 153)
NEGATIVE RESISTANCE DEVICE
It is a device which exhibits a negative incremental resistance over a limited range of V-I characteristic.
It is of two types :-
1. Current controllable type : V-I curve is a multi valued function of voltage and single valued function of current .eg:- UJT, p-n-p-n diode
2. Voltage controllable type : V-I curve is a multi valued function of current and single valued function of voltage. eg:- SCS, Tunnel diode
TUNNEL DIODE
It was introduced by Leo Esaki in 1958.
Heavily-doped p-n junction
Impurity concentration is 1 part in 10^3 as compared to 1 part in 10^8 in p-n junction diode
Width of the depletion layer is very small(about 100 A).
It is generally made up of Ge and GaAs.
It shows tunneling phenomenon.
Circuit symbol of tunnel diode is
WHAT IS TUNNELING
Classically, carrier must have energy at least equal to potential-barrier height to cross the junction .
But according to Quantum mechanics there is finite probability that it can penetrate through the barrier for a thin width.
This phenomenon is
called tunneling and
hence the Esaki Diode
is know as
Tunnel Diode.
AT HIGHER FORWARD VOLTAGE
No electrons on the n side are directly opposite to the empty states on the p side.
The tunneling current is zero.
The normal ideal diffusion current exists in the device.
TUNNEL DIODE EQUIVALENT CIRCUIT
This is the equivalent circuit of tunnel diode when biased in negative resistance region.
At higher frequencies the series R and L can be ignored.
Hence equivalent circuit can be reduced to parallel combination of junction capacitance and negative resistance.