The three most important aspects of this characteristic curve are as follows:
The forward current increase to a peak current (IP) with a small, applied
The decreasing forward current with an increasing forward bias to a minimum
valley current (IV).
The normal increasing forward current with further increases in the bias
The portion of the characteristic curve between IP and IV is the region of negative
resistance. An explanation of why a tunnel diode has a region of negative resistance is best
understood by using energy levels as in the previous explanation of the Zener effect.
3-19. Simply stated, the theory known as quantum-mechanical tunneling is an electron
crossing a PN junction without having sufficient energy to do so otherwise. Because of the
heavy doping, the width of the depletion region is only one-millionth of an inch. You
might think of the process simply as an arc-over between the N- and the P-side across the
3-20. Figure 3-6 shows the equilibrium energy level diagram of a tunnel diode with no
bias applied. In view (A), the valence band of the P-material overlaps the conduction band
of the N-material. The majority electrons and holes are at the same energy level in the
equilibrium state. If there is any movement of current carriers across the depletion region
due to thermal energy, the net current flow will be zero because equal numbers of current
carriers flow in opposite directions. In view (B), the zero net current flow is marked by a
"0" on the current-voltage curve.
Figure 3-6. Tunnel Diode Energy Diagram With No Bias
3-21. Figure 3-7, view (A), shows the energy diagram of a tunnel diode with a small
forward bias (50 millivolts) applied. The bias causes unequal energy levels between some
of the majority carriers at the energy band overlap point, but not enough of a potential
difference to cause the carriers to cross the forbidden gap in the normal manner. Since the
valence band of the P-material and the conduction band of the N-material still overlap,
current carriers tunnel across at the overlap and cause a substantial current flow. The
amount of current flow is marked by point 2 on the curve in view (B). In view (A) the
amount of overlap between the valence band and the conduction band decreased when
forward bias was applied.
23 June 2005