We can best understand the behavior of the tunnel diode in the region with negative resistance if we imagine it as a self variable (dynamic) resistor R driven by a variable voltage source V – Fig. 1. If OP has a subtle sense of humor, I suggest we conduct this experiment in the form of a fun (but useful) game where he is the voltage source and I am the “tunnel diode”:)
Fig. 1. A setup for measuring the tunnel diode IV curve in the negative resistance region (Wikibooks)
The graphical representation – Fig. 2, is based on the fact that the voltage VA across the two elements is the same and the current IA flowing through them is the same as well. That is why their IV curves are superimposed on the same coordinate system. The IV curve of the variable resistor is a straight line (in orange) beginning from the coordinate origin and having a slope depending on the instant (static) resistance R. The IV curve of the input voltage source is a vertical line (in red) shifted to the right from the Y-axis. The intersection point A (aka operating point) represents the instant magnitudes of the current IA and the voltage VA.
Fig. 2. The graphical representation of the circuit operation as two superimposed IV curves (Wikibooks)
1. Low positive resistance region. When OP varies the input voltage from zero to the beginning of the negative resistance region, I keep a low constant resistance R… and OP (i.e., the voltage source) imagines it as a steep line (IV curve).
2. Negative resistance region. In the middle region, I decide to play a trick with OP and begin increasing R vigorously and simultaneously with the moderate voltage increase. As a result, in Ohm’s law I = V/R, both V and R change in opposite directions and with different rates… so the current decreases and OP sees a negative resistance. In Fig. 2, my (R, tunnel diode) IV curve begins rotating clockwise; the operating point A moves down and pictures the negative resistance part of the tunnel diode N-shaped IV curve.
3. High positive resistance region. After the negative resistance region, when OP increases the input voltage, my resistor possesses high positive resistance and OP imagines it as a sloping line (IV curve).