Tunnel Diodes and Field-Effect Transistors
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.
KeywordsValence Band Fermi Level Forward Bias Resonant Tunneling Negative Differential Resistance
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