Tunnel Pulser Project:
In 1958, Leo Esaki, a Japanese scientist, discovered that if a semiconductor junction diode is heavily doped with impurities, it will have a region of negative resistance. A tunnel diode is a semiconductor with a negative resistance region that results in very fast switching speeds , up to 5 GHz. The operation depends upon a quantum mechanic principle known as "tunnelling" wherein the intrinsic voltage barrier (0.3 Volt for Germanium junctions) is reduced due to doping levels which enhance tunnelling.
Noble Prize winner Dr. Leona Esaki.
Tunnel Diode (Tektronix)
Transparent Sound try to achieve a complete technical and historical document about an interesting product as a tunnel diode and his possibilities, for example a Tunnel Diode Pulser. This is a very simple circuit based on the performance of a Tunnel diode. The circuit is useful for calibrating oscilloscopes or environments where Slew Rate is a major item.
The theory about this phenomena is very difficult and almost not to understand for normal electronic engineers. With this collection of interesting articles, URL links and some comment of my side I hope to explain the working of a tunnel diode.
Different Barriers of Tunnel Diode-1
Different Barriers of Tunnel Diode-2
The normal junction diode uses semiconductor materials that are lightly doped with one impurity atom for ten-million semiconductor atoms. This low doping level results in a relatively wide depletion region. Conduction occurs in the normal junction diode only if the voltage applied to it is large enough to overcome the potential barrier of the junction.
In the Tunnel
Diode, the semiconductor materials
used in forming a junction are doped to the extent of one-thousand impurity
atoms for ten-million semiconductor atoms. This heavy doping produces an
extremely narrow depletion zone similar to that in the Zener diode. Also because
of the heavy doping, a tunnel diode exhibits an unusual current-voltage
characteristic curve as compared with that of an ordinary junction diode.
Because of the narrow depletion zone, the reverse characteristic starts at 0Volt. By the lack of a breakdown voltage we can speak of resistor behaviour in this region. The dynamic resistance in that area will be a couple of ten Ohm. The characteristic curve for a tunnel diode is illustrated in figure below.
|Tunnel diode curve|
A practical application is a Tunnel-Pulser circuit, this circuit is very useful for calibrating oscilloscopes and testing Amplifiers on their Slew-Rate.
Tunnel-Pulser Tegam 67068101
Tunnel-Pulser Transparent Sound
Tunnel-Pulser Transparent Sound
(Bottom open view)
Photos Tunnel-Pulser Transparent Sound made by www.a-artfotografie.nl
The circuit in figure-1 (here below) is build around a 0386GE tunnel diode from Tektronix. Diode D1 is protecting diode D2 against negative input voltage, Variable resistor P1 is for adjusting the circuit for the right slope. Resistor R1 is for current limiting and resistors R2, R3 are for creating a precise 50W output resistance.
If the input signal is a 100kHz, 5V block signal, the tunnel diode reacts on the positive flank of the input signal. The output signal will be around 300mV but with a very fast positive flank. This flank is so fast, that the scope in this case will be slower and therefore the visual flank (Slew-Rate) is the Slew-Rate of the scope. Here below some pictures of the flank of a tunnel pulser, in this case the flank of the HAMEG HM-1507 oscilloscope. The noise contribution of a tunnel diode is very low because of the low temperature sensitivity of the tunnel barrier. I've used two 100W output resistors to keep the impedance exact 50W. One disadvantage is the possibility to reduce the output signal by using extended feed through attenuators. This could be handy for different input sensitivity sections. My goal for this items was to realise a small peace of tool in one housing.
<Pictures will be coming soon>
specifications 1N3720 tunnel diode:
A replacement for the 0386GE Tektronix Tunnel diode is the 1N3720
|Ipeak Max. (A)||22m|
|Peak Curr. Tol.||2.0m|
|Total Cap. (F)||150p|
|Fwd Volt @Ipeak||500m|
|Resist. Cutoff Freq||1.6G|
|Series Induct. (H)||.50n|
|Maximum Operating Temp (шC)||100ю|
Replacement circuit for the tunnel diode in the negative differential area:
The parasitic serial inductor, originate from the connection leads
The parasitic serial resistor, value 1,5W -- 4W
The negative differential resistor, value 10W -- 120W
|Cd:||The parasitic diode capacitance, value 1,5pF – 20pF|
Interesting links of the tunnel diode: