Tunnel Diodes and Field-Effect Transistors

Abstract

The concept of quantum mechanical tunneling is introduced. Degenerate and nondegenerate semiconductors are defined and distinguished. Possibility of carrier tunneling across extremely thin depletion regions is explained. Operation of a tunnel diode is described in terms of its energy band diagram. Current flow through the diode increases/decreases according to the availability/unavailability of vacant energy states in the valence band of the P-side that are aligned with respect to electron energy states on the N-side. Worthy of notice is the occurrence of negative resistance region in the current–voltage characteristic of a tunnel diode. The origin of such anomalous region is interpreted. The probability of resonant tunneling through a double barrier heterostructure is put in plain words on basis of the wave nature of electron. Acquisition of understanding of tunnel diode operation helps to bring out the dissimilarity between a tunnel diode and a resonant tunnel diode. Advantages, limitations and applications of resonant tunnel diodes in digital logic circuits and other areas are elaborated. The tunnel FET is proposed as an alternative to MOSFET. It is based on band-to-band tunneling for injection of carriers. It is a steep-slope switch offering the possibility of a subthreshold slope <60 mV/decade. This value is restricted in MOSFETs due to the tail of the Fermi distribution. Tunnel FETs cater to the very low power applications. They can operate at low voltages V _DS < 0.5 V. At such low voltages, CMOS performance is considerably worsened, which is a favorable aspect of tunnel FETs.

Review Exercises

• 12.1 Explain the idea of tunneling in quantum mechanics. How is tunneling explained from the wave nature of electron?

• 12.2 What do quantum mechanics and classical mechanics declare about the probability of penetrating a thick potential barrier? Under what condition, tunneling is possible across a depletion region?

• 12.3 What is the effect of a very high doping concentration on: (i) the position of Fermi level in a semiconductor, and (ii) the electrical properties of the semiconductor.

• 12.4 A P-type semiconductor is heavily doped? What is the location of the Fermi level?

• 12.5 An N-type semiconductor is doped with a high concentration of impurity atoms? Where is the Fermi level of the material situated?

• 12.6 How are voltage and current related in an ohmic resistor? How does an electronic component showing negative resistance differ from an ohmic resistor?

• 12.7 Define differential resistance . At a certain point of the current–voltage characteristic of a device, the slope of the curve was found to be negative. What is the sign of its differential resistance?

• 12.8 How does a tunnel diode differ from a conventional P–N junction diode in construction? State three salient features of the current–voltage characteristics of a tunnel diode.

• 12.9 A tunnel diode is subjected to increasing values of forward bias. Explain its current–voltage characteristics from energy band picture. Draw neat and labeled diagrams supporting your explanation.

• 12.10 What is the reason for high reverse leakage current in a tunnel diode? What is the reverse breakdown voltage of a tunnel diode?

• 12.11 Explain the phenomenon of resonant tunneling across two potential barriers arranged in a row.

• 12.12 What is a heterostructure? What technique is commonly applied for fabricating heterostructures?

• 12.13 Show on a diagram the different structural layers in a resonant tunnel diode. What are the typical thicknesses of quantum barriers and the quantum well?

• 12.14. Explain the resonance phenomenon in a double barrier quantum well structure using the ideas of interference of electron waves.

• 12.15 Mention five carrier transport mechanisms contributing to current flow in a resonant tunneling diode.

• 12.16 A resonant tunneling diode is connected in forward bias mode. How do the different segments in its current–voltage characteristics originate?

• 12.17 How does the reverse bias operation of a resonant tunneling diode differ from that of a tunnel diode?

• 12.18 Mention some applications of resonant tunneling diodes in digital and analog circuits.

• 12.19 Differentiate between a tunnel FET and conventional MOSFET from the point of view of: (a) device structure, (b) operating principle, and (c) subthreshold slope.

• 12.20 Draw the energy band diagrams of a tunnel FET in off- and on-states. Point out the differences between the two cases and their effects on carrier flow.