Tunnel Diodes (Esaki Diode) Tunnel diode is the p-n junction device that exhibits negative resistance. That means when the voltage is increased the current through it decreases. Esaki diodes was named after Leo Esaki, who in 1973 received the Nobel Prize in Physics for discovering the electron tunneling effect used in these diodes. Esaki reported the first paper on tunnel diodes in Physical eview in 1958 egular p-n Diode Tunnel Diode 1
Part I Tunnel Diode principles Concept of Electron Tunneling Before contact After contact E CSiO 2 E CSiO 2 E CSi E CSi E CSi E CSi E Si E Si E Si E Si E SiO 2 E SiO 2 Si SiO 2 Si Si SiO 2 Si 2
continued Concept of Electron Tunneling For thick barrier, both Newtonian and Quantum mechanics say that the electrons cannot cross the barrier. It can only pass the barrier if it has more energy than the barrier height. Electron with energy greater than E B can pass over the barrier E=E B Electron with energy less than E B cannot pass the barrier E=0 Si SiO 2 Si 3
continued Concept of Electron Tunneling For thin barrier, Newtonian mechanics still says that the electrons cannot cross the barrier. However, Quantum mechanics says that the electron wave nature will allow it to tunnel through the barrier. E=E B Tunneling is caused by the wave nature of electron E=E B E=0 Si SiO 2 Si Si SiO 2 Si Newtonian Mechanics Quantum Mechanics 4
Electron Tunneling in p-n junction When the p and n region are highly doped, the depletion region becomes very thin (~10nm). In such case, there is a finite probability that electrons can tunnel from the conduction band of n-region to the valence band of p-region During the tunneling the particle ENEGY DOES NOT CHANGE Thick depletion layer High doping Thin depletion layer E C E C E E Electrons tunnel through the thin barrier p n p n 5
Tunnel Diode Operation When the semiconductor is very highly doped (the doping is greater than N o ) the Fermi level goes above the conduction band for n-type and below valence band for p- type material. These are called degenerate materials. Under Forward Bias Step 1: At zero bias there is no current flow E C E F E 6
continued Operation of a Tunnel Diode Step 2: A small forward bias is applied. Potential barrier is still very high no noticeable injection and forward current through the junction. However, electrons in the conduction band of the n region will tunnel to the empty states of the valence band in p region. This will create a forward bias tunnel current E C E Direct tunneling current starts growing 7
continued Tunnel Diode Operation Step 3: With a larger voltage the energy of the majority of electrons in the n-region is equal to that of the empty states (holes) in the valence band of p-region; this will produce maximum tunneling current E C E Maximum Direct tunneling current 8
continued Tunnel Diode Operation Step 4: As the forward bias continues to increase, the number of electrons in the n side that are directly opposite to the empty states in the valence band (in terms of their energy) decrease. Therefore decrease in the tunneling current will start. E C E Direct tunneling current decreases 9
continued Tunnel Diode Operation Step 5: As more forward voltage is applied, the tunneling current drops to zero. But the regular diode forward current due to electron hole injection increases due to lower potential barrier. E C E No tunneling current; diffusion current starts growing 10
continued Operation of a Tunnel Diode Step 6: With further voltage increase, the tunnel diode I- characteristic is similar to that of a regular p-n diode. E C E 11
continued Operation of a Tunnel Diode Under everse Bias In this case the, electrons in the valence band of the p side tunnel directly towards the empty states present in the conduction band of the n side creating large tunneling current which increases with the application of reverse voltage. The TD reverse I- is similar to the Zener diode with nearly zero breakdown voltage. E C E 12
Part II Circuits with the Tunnel Diodes I TD ND region Typical Tunnel Diode (TD) I- characteristic has two distinct features: (1) it is STONGLY non-linear (compare to the resistor I-). Current - oltage relationships for TDs cannot be described using the Ohm s law (2) it has a negative differential resistance (ND) region 13
Tunnel Diode I- The total current I in a tunnel diode is given by I = I + I + tun diode I excess peak valley The p-n junction current, I p I diode I s exp 1 ηth I v v I s saturation current, η is the ideality factor and th = kt/q p 14
Tunnel Diode I- The tunnel current, I tun = 0 exp 0 m peak valley Typically, m = 1.3; 0 = 0.1.0.5 I p 0 is the TD resistance in the ohmic region The maximum ND can be found as I v = d max 0 1+ m exp m m p v The peak voltage p : p 1 m 1 = m 0 15
Tunnel Diode I- The excess current, I excess = v exp ex v I p peak valley I excess is an additional tunneling current related to parasitic tunneling via impurities. I v v This current usually determines the minimum (valley) current, I v p v and ex are the empirical parameters; in high-quality diodes, v >> 0. ex = 1..5 16
ND of the Tunnel Diode Tunnel Diode differential resistance is NEGATIE in the voltage range 100 m 200 m 17
Energy dissipation in resistors and Energy generation in Negative esistors + - S Power = oltage x Current = I 2 If current direction is from - toward +, then =/I is negative; For <0, P <0, Positive power means energy dissipation (e.g. conversion into the Joule heat); Negative power corresponds to the power GENEATION (Energy supply); 18
Differential resistance and negative differential resistance Static resistance: = /I Differential resistance: d Δ = I ΔI I I ΔI I α d = cot( α ) For linear ( Ohmic ) components, = d. Δ For many semiconductor devices, d : I α I α I α 2 α 3 I d < I d << α 1 Diode (forward bias) Zener Diode (reverse bias) d2 < 0 TD 19
Transients in Negative Differential esistance Circuits S C After turning the switch ON: i S it ()= e -t/(c) i > 0 < 0 t t 20
Tunnel Diode as a microwave oscillator Tunnel diode C d Microwave cavity (LC- resonance circuit) ~ L u s d Load resistance is chosen so that L < d in the ND region At the TD operating point, the total circuit differential resistance is negative 21
Tunnel Diode as a microwave oscillator Transient in resonant cavity after turning the bias voltage ON 1 C d 0.8 u s d ~ L LC The resonant circuit with ND can oscillate. Maximum frequency of the TD-oscillator is limited by the characteristic tunneling time: f MAX (1/2π) (1/τ tun) Tunneling time in TDs is extremely small: << 1 ps F MAX > 100 GHz 0.6 0.4 0.2 0 0-0.2 1 2 3 4 5 6 7 8 9-0.4-0.6-0.8 5 4 3 2 1-2 -3-4 -5-6 -1 d >0 or d <0 and L > d 0-1 0 1 2 3 4 5 6 7 8 9 d <0 and L < d 22
Tunnel Diode microwave oscillators After: M. eddy et.al, IEEE ELECTON DEICE LETTES, OL. 18, NO. 5, MAY 1997 ~ 600 GHz oscillation frequencies has been achieved. 23
Nonlinear Circuit Analysis: Load Line technique s I = = d d + I s + d 1 I = d + s s y = mx + c y X axis intercept, s s slope = 1 Y axis intercept, c = s s Slope, m = 1 x 24
Nonlinear Circuit Analysis: Load Line technique = + I s d 1 = s I d + In the load line equation, I is the resistor current when the voltage across the diode is d On the other hand, when the voltage across the diode is d, the diode current is given by the diode I- curve For example, when the diode voltage is d1 the diode current is I d1 However, in this circuit, I d must be equal I. Hence the actual operating point is given by the load line I- intercept. s I d1 I Diode I- s d1 d slope = 1 s 25
Load Line : example d s 2 d =0.78 I d =2.4 ma 500Ω I d =2.4 ma axis intercept, s = 2 I axis intercept, ( s /) = 4 ma d =0.78 26
Load Line : another example s d d =0.76 2.5 1250Ω I d =1.4 ma axis intercept, s = 2.5 I d =1.4 ma I axis intercept, ( s /) = 2 ma d =0.76 27
continued Load Line (ariation of ) d s 2.0 = 500Ω = 750Ω = 1000Ω 28
continued Load Line (ariation of s ) d s 1000Ω s = 1 s =2 s =3 29
Circuit with the Tunnel Diode and esistor 8 I, ma d 6 4 2 3 2 4 1 TD, s 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Example 1: s = 0.7 ; = 100 Ω; Ι max = 0.7/100 Ω = 7 ma The circuit has three possible operating points. Point 2 is typically unstable (depending on parasitic L and C components. The circuit will operate at the point 1 or point 3 depending on the history. Example 2: s = 0.3 ; 10 Ω; Ι max 30 ma The circuit has only one operating point - point 4. The total differential resistance is NEGATIE (because < d ). Depending on the L and C components, the circuit can be stable (amplifier) or unstable (oscillator) 30