Resonant Tunneling Diodes

1. Resonant Tunneling Diodes Johnny Ling, University of Rochester December 16th, 2006

2. Outline • Motivation • Introduction to normal tunneling diode • Resonant tunneling diode • Advantages and Limitations • Conclusion

3. Motivation • An increasing number of applications that require signal sources at very high frequencies (300-1500GHz) • Ultimate limit on the current trend of down-scaling transistors and integrated circuits to achieve faster speeds and lower power consumption • The highest frequency conventional transistor oscillator built today is only about 215 GHz.

4. P-N diode with heavy doping (1020 cm-3) in both regions (Degenerately doped) The depletion region is very narrow (<10nm) High concentration of electrons in the conduction band of N-type and holes in the valence band of P-type material Tunneling diodes (TD)

5. Tunneling Diodes (cont.) • Apply increasing forward bias voltage • Starting at zero bias:

6. Tunneling Diodes (cont.) • Electrons in N-region conduction band are energetically aligned to the holes in the valence band of P-region. Tunneling occurs. Forward current is produced.

7. Tunneling Diodes (cont.) • As you increase the bias voltage, a maximum current will be produced when all electrons are aligned with the holes

8. Tunneling Diodes (cont.) • As bias voltages continues to increase, current will decrease because less electrons are aligned with the holes

9. Tunneling Diodes (cont.) • As the bias voltage continues to increase, electrons are no longer energetically aligned with the holes and the diffusion current dominates over tunneling

10. Tunneling Diodes (cont.) • Reverse bias voltage – breakdown • High leakage current, not a good rectifier

11. Resonant Tunneling Diode (RTD) • Electrons must have a certain minimum energy above the energy level of the quantized states in the quantum well in order for tunneling to occur. Once the bias voltage is big enough to provide enough energy, RTDs looks like a normal TD • In reverse bias, RTDs do not have large leakage current

12. Negative Differential Resistance(NDR) • Characterized by the current peak to valley ratio (PVR=I/V) • To achieve maximize dynamic range, high PVR is desired. • To obtain maximum output power from RTD, high current density is required • Decrease the thickness of the quantum well barrier • Increase emitter doping level • However, PVR will be decreased and leakage will increase

13. Advantages and Limitations • RTDs is considered among the fastest devices because tunneling is very fast and is not transit-time limited as in CMOS technology, etc. • RTDs provide a low leakage current when a reverse bias is applied. • Large dynamic range within a small input voltage range • However, the output current and power of RTDs is very limited compared to CMOS.

14. Conclusion • RTDs is much faster than any other conventional transistor. • Very important alternative as transistor technology continues to scale down to the nanometer range • Very good rectifier – low leakage current • Much research needs to be done to improve the output power and also to integrate them with conventional transistors