Tunnel Diodes (Esaki Diode) Esaki diodes was named Tunnel diode is the p-n junction after Leo Esaki, who in device that exhibits negative 1973 received the Nobel resistance. That means when the Prize in Physics for discovering the electron voltage is increased the current tunneling effect used in through it decreases. these diodes. Esaki reported the first paper on tunnel diodes in Physical Review in 1958 Regular p-n Diode Tunnel Diode 1 Part I Tunnel Diode principles Concept of Electron Tunneling Before contact After contact E EC CSiO SiO2 2 E EC E CSi EC Si CSi Si E EV E VSi EV Si VSi E Si EV VSiO SiO2 2 SiO Si SiO2 Si Si 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 EB can pass over the barrier E=EB Electron with energy less than EB cannot pass the barrier E=0 Si Si SiO2 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. Tunneling is caused by E=E the wave nature of E=E B electron B E=0 Si Si Si SiO2 Si SiO2 4 Newtonian Mechanics Quantum Mechanics 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 ENERGY DOES NOT CHANGE High doping Thick depletion layer Thin depletion layer EC EC EV EV 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 No) 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 EC EF EV 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 EC EV 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 EC EV 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. EC EV 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. EC EV No tunneling current; diffusion current starts growing 10 …continued…Operation of a Tunnel Diode Step 6: With further voltage increase, the tunnel diode I-V characteristic is similar to that of a regular p-n diode. EC EV 11 …continued…Operation of a Tunnel Diode Under Reverse 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-V is similar to the Zener diode with nearly zero breakdown voltage. EC EV 12 Part II Circuits with the Tunnel Diodes I R TD NDR region V Typical Tunnel Diode (TD) I-V characteristic has two distinct features: (1) it is STRONGLY non-linear (compare to the resistor I-V). Current - Voltage relationships for TDs cannot be described using the Ohm’s law (2) it has a negative differential resistance (NDR) region 13 Tunnel Diode I-V • The total current I in a tunnel peak diode is given by valley IIII=tun +diode + excess Ip • The p-n junction current, I ⎡⎛ V ⎞ ⎤ v Idiode I≈ s exp⎢⎜ ⎟ −1⎥ ⎜ ⎟ Vv ⎣⎝ ηVth ⎠ ⎦ Vp Is saturation current, η is the ideality factor and Vth = kT/q 14 Tunnel Diode I-V • The tunnel current, m V ⎡ ⎛ V ⎞ ⎤ peak I =exp⎢ −⎜ ⎟ ⎥ valley tun R ⎜ V ⎟ 0 ⎣⎢ ⎝ 0 ⎠ ⎦⎥ Typically, m = 1….3; V0 = 0.1….0.5 V Ip R0 is the TD resistance in the ohmic region The maximum |NDR| can be found as Iv ⎛⎞1+ m exp⎜⎟ Vv ⎝⎠m ||RR= V d max 0 m p The peak voltage Vp: 1 ⎛⎞1 m VVp = ⎜⎟ 0 ⎝⎠m 15 Tunnel Diode I-V • The excess current, peak valley ⎡ ⎤ V ⎛ − VV v ⎞ Iexcess = exp⎢⎜ ⎟⎥ R ⎜ V ⎟ v ⎣⎝ ex ⎠⎦ Ip Iv Iexcess is an additional tunneling current related to parasitic V tunneling via impurities. v V This current usually determines the p minimum (valley) current, Iv Rv and Vex are the empirical parameters; in high-quality diodes, Rv >> R0. Vex = 1…..5 V 16 NDR of the Tunnel Diode Tunnel Diode differential resistance is NEGATIVE in the voltage range 100 mV – 200 mV 17 Energy dissipation in resistors and Energy generation in Negative Resistors R + V - S Power = Voltage x Current = I2 R If current direction is from “-” toward “+”, then R =V/I is negative; For R<0, P <0, Positive power means energy dissipation (e.g. conversion into the Joule heat); Negative power corresponds to the power GENERATION (Energy supply); 18 Differential resistance and negative differential resistance ∂VVΔ Static resistance: Differential resistance: R =≈ R = V/I d ∂IIΔ I I R R ΔI R =cot(α ) I d α V V V ΔV For linear (“Ohmic”) components, R = Rd. For many semiconductor devices, R ≠ R : d I I α α I α3 α2 I α I R << R 1 Rd < R d V Rd2 < 0 V V V V Diode TD Zener Diode 19 (forward bias) (reverse bias) Transients in Negative Differential Resistance Circuits R C VS After turning the switch ON: V it()= S ×e-t/(RC) R i i R > 0 R < 0 t t 20 Tunnel Diode as a microwave oscillator Tunnel diode Cd Microwave cavity (LC- resonance circuit) Rd ~ RL us Load resistance is chosen so that RL <|Rd | in the NDR 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 Cd 0.8 0.6 0.4 0.2 Rd LC 0 R 0123456789 ~ L -0.2 us -0.4 -0.6 Rd >0 or -0.8 Rd<0 and RL > |Rd| The resonant circuit -1 with NDR can oscillate. Maximum frequency of 5 the TD-oscillator is 4 limited by the 3 2 characteristic tunneling 1 time: 0 -1 0123456789 fMAX ≤ (1/2π) (1/τtun) Tunneling time in TDs is -2 -3 extremely small: << 1 ps -4 R <0 and R < |R | F > 100 GHz -5 d L d MAX 22 -6 Tunnel Diode microwave oscillators After: M. Reddy et.al, IEEE ELECTRON DEVICE LETTERS, VOL. 18, NO. 5, MAY 1997 ~ 600 GHz oscillation frequencies has been achieved. 23 Nonlinear Circuit Analysis: Load Line technique V= V + IR s d Vd V V ⇒I = −d s + R R Vs R ⎛ 1 ⎞ ⎛ Vs ⎞ =I ⎜ − ⎟Vd + ⎜ ⎟ ⎝ R ⎠ ⎝ R ⎠ y ⎛V ⎞ y=mx + c ⎜ s ⎟ ⎝ R ⎠ slope= −1 X − saxiprctinete, Vs R V s i xprcteinte, cYa s −=R Vs Slope= , − m1 x R 24 Nonlinear Circuit Analysis: Load Line technique VVsd=+× IR Vd ⎛ 1 ⎞ ⎛ Vs ⎞ =I ⎜ − ⎟Vd + ⎜ ⎟ ⎝ R ⎠ ⎝ R ⎠ Vs R In the load line equation, I is the resistor current when the voltage across the diode is Vd On the other hand, when the voltage I ⎛V ⎞ ⎜ s ⎟ across the diode is Vd, the diode ⎝ R ⎠ current is given by the diode I-V curve slope= −1 R For example, when the diode voltage is Vd1 the diode current is Id1 Diode However, in this circuit, I must be equal d I-V Vs I . R I Hence the actual operating point is given d1 V by the load line – I-V intercept. Vd1 25 Load Line : example Vd Vd=0.78V Vs R 2V 500Ω Id=2.4 mA Id=2.4 mA V axis intercept, Vs = 2 V I axis intercept, (Vs/R) = 4 mA Vd=0.78V 26 Load Line : another example Vd Vd=0.76V Vs R 2.5V 1250Ω Id=1.4 mA Id=1.4 mA V axis intercept, Vs = 2.5 V I axis intercept, (Vs/R) = 2 mA Vd=0.76V 27 …continued… Load Line (Variation of R) Vd R Vs 2.0V R= 500Ω R= 750Ω R= 1000Ω 28 …continued… Load Line (Variation of Vs) Vd R V s 1000Ω V =3V Vs = 1V Vs =2V s 29 Circuit with the Tunnel Diode and Resistor V 8 I, mA d 6 3 2 4 TD Vs R 4 2 1 V, V 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Example 1: Vs = 0.7 V; R = 100 Ω; ⇒ Ιmax = 0.7V/100 Ω = 7 mA The circuit has three possible operating points.