Tunnel Diode Load Line Reading Log - Part 2/2
11.8 Tunnel diodes
The Tunnel diode is basically a very highly doped pn-junction (around 1019 to 1020 cm−3) that makes use of a quantum mechanical effect called tunneling. This type of diode is also known as an Esaki diode, after the inventor, Leo Esaki, who discovered the effect in 1957, a discovery for which he was awarded the Nobel Prize in Physics in 1973. As a consequence of the very high doping, a tunnel diode will have a very narrow depletion region, typically less than 10 nm.
We will not delve into the physics of tunneling, which is well covered in standard texts. The important point about the tunneling mechanism, from the engineering point of view, is that it gives rise to a region of negative resistance in the I-V characteristics, shown as region “B” in Figure 11.13.
In region “B,” an increase in forward voltage will result in a decrease in forward current, and vice versa. This is equivalent to saying that the device exhibits negative resistance in this region although, strictly speaking, we should call this negative dynamic resistance, as it refers to the negative slope of the V-I characteristics, not a physical “negative” resistor, which does not exist, of course.
Figure 11.13. Tunnel diode I-V characteristics.
Region “A” in Figure 11.13 is actually the region where tunneling occurs. Region “C” is the region of normal pn-junction behavior. In this sense, region “B” can be considered as the region of transition between region “A,” where the I-V characteristic is linear, and region “C” where the I-V characteristic obeys equation (11.4.5).
With reference to Figure 11.13 we can see that, as the bias voltage is increased from zero, the current increases linearly along curve “A” until a peak current is reached, at the bias voltage Vp.
This corresponds to the n-side conduction band becoming aligned with the p-side valance band in the device.
At this point tunneling stops, at a current level called the peak tunneling current, Ip in Figure 11.13. Ip is also known as the “Esaki current.”
We can analyze the circuit behavior of a tunnel diode with DC bias with the aid of Figure 11.14, from which, by inspection, we can write:
Figure 11.14. Tunnel diode circuit.
(11.8.1)
The current through the diode is then given by:
(11.8.2)
Equation (11.8.2) is in the form of a straight line current/voltage graph with slope (− 1/R) and an intercept on the current axis of (ID = VD/R). This is called a load line. As the voltage across the series combination of resistor plus diode increases, the load line is raised with its x-axis intercept at the applied voltage.
Figure 11.15 shows two possible load lines for the circuit in Figure 11.14, depending on the chosen value of R.
Figure 11.15. Tunnel diode characteristic with a load line.
With higher values of R, the load line will be the shallower load line-1 in Figure 11.15 that intersects the diode characteristic at three points, 1, 2, and 3, meaning that the circuit has three possible operating points.
Point 2 is an unstable operating point, as any perturbations in bias voltage will cause the diode to jump from point 2 to either point 1 or point 3 on the load line.
The circuit will therefore settle at either point 1 or point 3 depending on the history. It is in this mode that tunnel diodes are used as switched or memory devices. This mode is of little interest to the microwave circuit designer, however.
If the value of R, is reduced the load line will be much steeper, resembling load line-2 in Figure 11.15. In this case, the circuit has only one operating point, point 4.
The total differential resistance is negative (because R < |Rd|). In this mode, the diode can be made to oscillate at a microwave frequency dependent on the external L and C components.
Because the negative resistance phenomenon in tunnel diodes relies on a tunneling phenomenon, the currents generated are necessarily quite small and the amount of RF power generated by a tunnel diode oscillator is quite low.
Consequently, tunnel diodes are only suitable for low power applications and even in this arena they have been largely superseded by transistors.
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I would like to know what the trajectory of the operating point is when jumping from point 3 to point 1. Does it go through point 2?
ReplyDeleteI don't know, because I am just reading about "load line", and have not even started experimenting with load line. I am plotting the I-V curve in Region A (linear region) and also Region B (normal P-N junction region). But for Region B (Negative Dynamic Resistance (NDR), I don't know nothing, for two reasons: (1) I am just doing a DC sweep by hand, using a push button adjustable regulated power supply (PSU). I have also set up a triangular wave sign gen to do auto DC sweep and I will show you the wave form here later. But then I found that perhaps I can use the cheapy toy PCF8951, which is a 8-bit DAC and ADC. So now I am thinking of using Rpi4B Thonny python to plot point by point the tunnel diode 2SB3B curve. I can use the PCF8951 ADC to read the corresponding voltage points and translated the I/V pairs to a CVI table and use Excel to plot the curve. Perhaps I should stop here and first show you my by hand measurements to clarify things. Note: earlier I have uploaded some old Excel work in my GitHub site, but I found GitHub too professional and not very newbie friendly, so from now on I will only show my results in this tunneldiode.blotspot.com.
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