Tunnel Diode

A tunnel diode can amplify a signal using only a few milliamps of current because it has a built-in negative resistance region in its I-V curve. This property also makes it the basis for microwave oscillators that operate at frequencies where ordinary transistors fail.

Core Concept

A tunnel diode is a heavily doped p-n junction where both p and n regions are doped to degeneracy (above 10^19 cm^-3). This extreme doping makes the depletion layer extremely thin, around 10 nm. At such small widths, charge carriers can cross the barrier by quantum mechanical tunneling without needing enough thermal energy to climb over it.

As forward voltage increases from zero, tunneling current first rises to a peak at the peak voltage V_p, then falls to a minimum at the valley voltage V_v. In this region between V_p and V_v, current decreases as voltage increases. This is the negative differential resistance (NDR) region. After V_v, normal diode forward conduction takes over and current rises again.

The Esaki diode (named after Leo Esaki who won the 1973 Nobel Prize for this work) uses germanium or gallium arsenide. GaAs tunnel diodes like the 1N3712 operate up to 100 GHz. They were once used in early computer memory but are now used mainly in microwave oscillators and fast-switching circuits.

Key Equations

Negative differential resistance: -r_d = -(V_v - V_p) / (I_p - I_v) where V_p and I_p are peak voltage and current, V_v and I_v are valley voltage and current. The magnitude gives the NDR value in ohms.

Peak-to-valley current ratio: PVCR = I_p / I_v a higher PVCR means a more pronounced NDR region. Germanium tunnel diodes achieve PVCR of 8. GaAs devices reach PVCR of 15 or more.

Oscillation frequency in an LC circuit with tunnel diode: f_osc = 1 / (2π √(L × C)) the NDR must exceed the positive resistance losses in the circuit for oscillation to be sustained.

Given:
  Peak voltage V_p = 60 mV = 0.06 V
  Peak current I_p = 10 mA = 0.01 A
  Valley voltage V_v = 350 mV = 0.35 V
  Valley current I_v = 1 mA = 0.001 A

Why this formula:
  Negative differential resistance is the slope of the NDR region (must be negative).

Formula:
  r_d = ΔV / ΔI = (V_v - V_p) / (I_v - I_p)
  NDR magnitude = |r_d|

Substitution:
  r_d = (0.35 - 0.06) / (0.001 - 0.01)
      = 0.29 / (-0.009)

Calculation:
  r_d = -32.2 Ω
  NDR magnitude = 32.2 Ω

  PVCR = I_p / I_v = 10 / 1 = 10

Final Answer:
  Negative differential resistance = -32.2 Ω
  Peak-to-valley current ratio = 10
  This diode can sustain oscillation if circuit losses are below 32.2 Ω.

Exam Tip: GATE problems on tunnel diodes almost always ask you to identify the NDR region and calculate r_d = ΔV/ΔI between the peak and valley points. Remember that r_d is negative in the NDR region. If you compute a positive value, you have subtracted in the wrong order. Also note: tunnel diode doping is 10^19 to 10^20 cm^-3, far higher than a normal p-n junction at 10^15 to 10^17 cm^-3. This distinction appears in theory questions.

Key Properties

• Doping concentration is 10^19 to 10^20 cm^-3, making the depletion layer only 5 nm to 10 nm wide, thin enough for tunneling.
• GaAs tunnel diodes like 1N3712 operate up to 100 GHz, far beyond silicon bipolar transistors in the same era.
• Peak voltage V_p for germanium is about 60 mV to 100 mV. For GaAs it is around 150 mV.
• PVCR of 8 for Ge, 15 for GaAs. Higher PVCR means stronger NDR and more useful oscillator performance.
• The NDR region has no shot noise contribution from diffusion current, making tunnel diode oscillators very low-noise at microwave frequencies.
• Tunnel diodes are two-terminal devices. They cannot be isolated (no gate), which limits circuit flexibility compared to transistor oscillators.

Quick Revision

• Tunnel diode uses quantum mechanical tunneling through an ultra-thin depletion layer.
• Heavy doping (10^19 to 10^20 cm^-3) creates the thin barrier needed for tunneling.
• NDR region is between peak point (V_p, I_p) and valley point (V_v, I_v).
• r_d = ΔV/ΔI is negative in the NDR region.
• PVCR = I_p/I_v: Ge gives 8, GaAs gives 15 or more.
• Used in microwave oscillators, fast switching, and low-noise amplifiers.
• GaAs tunnel diodes operate beyond 100 GHz.
• Exam trap: Students calculate r_d as a positive number by flipping the subtraction. In the NDR region, ΔI is negative when ΔV is positive, so r_d = ΔV/ΔI must come out negative. A positive answer means an arithmetic sign error.