Resonant Tunneling Resonant Tunneling DiodesDiodes

Johnny Ling, University of RochesterJohnny Ling, University of Rochester

December 16December 16thth, 2006, 2006

OutlineOutline

►MotivationMotivation

► Introduction to normal tunneling diodeIntroduction to normal tunneling diode

► Resonant tunneling diodeResonant tunneling diode

► Advantages and LimitationsAdvantages and Limitations

► ConclusionConclusion

MotivationMotivation

► An increasing number of applications that An increasing number of applications that require signal sources at very high require signal sources at very high frequencies (300-1500GHz) frequencies (300-1500GHz)

► Ultimate limit on the current trend of down-Ultimate limit on the current trend of down-scaling transistors and integrated circuits to scaling transistors and integrated circuits to achieve faster speeds and lower power achieve faster speeds and lower power consumptionconsumption

► The highest frequency conventional The highest frequency conventional transistor oscillator built today is only about transistor oscillator built today is only about 215 GHz.215 GHz.

Tunneling diodes (TD)Tunneling diodes (TD)

► P-N diode with heavy P-N diode with heavy doping (10doping (102020 cm cm-3-3) in ) in both regions both regions (Degenerately doped)(Degenerately doped)

► The depletion region is The depletion region is very narrow (<10nm) very narrow (<10nm)

► High concentration of High concentration of electrons in the electrons in the conduction band of N-conduction band of N-type and holes in the type and holes in the valence band of P-valence band of P-type materialtype material

Tunneling Diodes (cont.)Tunneling Diodes (cont.)► Apply increasing forward bias voltageApply increasing forward bias voltage

► Starting at zero bias:Starting at zero bias:

Tunneling Diodes (cont.)Tunneling Diodes (cont.)

► Electrons in N-region conduction band are Electrons in N-region conduction band are energetically aligned to the holes in the energetically aligned to the holes in the valence band of P-region. Tunneling occurs. valence band of P-region. Tunneling occurs. Forward current is produced.Forward current is produced.

Tunneling Diodes (cont.)Tunneling Diodes (cont.)

►As you increase the bias voltage, a As you increase the bias voltage, a maximum current will be produced maximum current will be produced when all electrons are aligned with the when all electrons are aligned with the holesholes

Tunneling Diodes (cont.)Tunneling Diodes (cont.)

►As bias voltages continues to increase, As bias voltages continues to increase, current will decrease because less current will decrease because less electrons are aligned with the holeselectrons are aligned with the holes

Tunneling Diodes (cont.)Tunneling Diodes (cont.)

►As the bias voltage continues to As the bias voltage continues to increase, electrons are no longer increase, electrons are no longer energetically aligned with the holes energetically aligned with the holes and the diffusion current dominates and the diffusion current dominates over tunnelingover tunneling

Tunneling Diodes (cont.)Tunneling Diodes (cont.)

►Reverse bias voltage – breakdownReverse bias voltage – breakdown►High leakage current, not a good High leakage current, not a good

rectifierrectifier

Resonant Tunneling Diode Resonant Tunneling Diode (RTD)(RTD)

► Electrons must have a certain minimum energy above Electrons must have a certain minimum energy above the energy level of the quantized states in the the energy level of the quantized states in the quantum well in order for tunneling to occur. Once quantum well in order for tunneling to occur. Once the bias voltage is big enough to provide enough the bias voltage is big enough to provide enough energy, RTDs looks like a normal TDenergy, RTDs looks like a normal TD

► In reverse bias, RTDs do not have large leakage In reverse bias, RTDs do not have large leakage currentcurrent

Negative Differential Negative Differential Resistance(NDR)Resistance(NDR)

►Characterized by the current peak to valley ratioCharacterized by the current peak to valley ratio

(PVR=I/V)(PVR=I/V)►To achieve maximize dynamic range, high PVR is To achieve maximize dynamic range, high PVR is desired.desired.►To obtain maximum output power from RTD, highTo obtain maximum output power from RTD, high

current density is requiredcurrent density is required►Decrease the thickness of the quantum well Decrease the thickness of the quantum well barrierbarrier►Increase emitter doping levelIncrease emitter doping level►However, PVR will be decreased and leakage will However, PVR will be decreased and leakage will increaseincrease

Advantages and LimitationsAdvantages and Limitations

► RTDs is considered among the fastest RTDs is considered among the fastest devices because tunneling is very fast and is devices because tunneling is very fast and is not transit-time limited as in CMOS not transit-time limited as in CMOS technology, etc.technology, etc.

► RTDs provide a low leakage current when a RTDs provide a low leakage current when a reverse bias is applied.reverse bias is applied.

► Large dynamic range within a small input Large dynamic range within a small input voltage rangevoltage range

►However, the output current and power of However, the output current and power of RTDs is very limited compared to CMOS.RTDs is very limited compared to CMOS.

ConclusionConclusion

► RTDs is much faster than any other conventional RTDs is much faster than any other conventional transistor. transistor.

► Very important alternative as transistor Very important alternative as transistor technology continues to scale down to the technology continues to scale down to the nanometer rangenanometer range

► Very good rectifier – low leakage currentVery good rectifier – low leakage current►Much research needs to be done to improveMuch research needs to be done to improve the output power and also to integrate them with the output power and also to integrate them with conventional transistors conventional transistors