Budapest University of Technology and Economics Department of Electron Devices Microelectronics, BSc course Operation of PN junctions: Characteristics

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Diode characteristics ► Ideal characteristic ► Secondary effects

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET The ideal diode characteristic

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET The ideal diode characteristic I 0 is proportional with the minority carrier concentrations I0I0

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET The ideal diode characteristic Problem Saturation current of Si diode: I 0 = A. What is U F, if I F is 10 mA? Problem How much should we increase the forward voltage if we want to increase the current 10x ?

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Secondary effects ► Series resistance ► Generation current ► Breakdown phenomena (a bit later) ► Recombination current (just mention)

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Secondary effects The series resistance Appears at high current levels. Reason: Solution: epitaxial structure

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Secondary effects Calculate the series resistance according to the 100 o C characteristic! Problem The series resistance

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Secondary effects The generation current In reverse region, in theory: that would result in pA only The experince is:

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Secondary effects The recombination current Phenomenon appearing in the forward region Can be well described by the Shockley-Read-Hall model for semiconductors with indirect band UFUF log I F ~ exp(U/U T ) ~ exp(U/2U T ) m: non-ideality factor, between 1..2

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Capacitances of a diode ► Space charge capacitance ► Diffusion capacitance

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Capacitances of the diode Space charge capacitance dominates in the reverse region Diffusion capacitance in forward region only Interpretation as a differential at a given forward voltage/current charge

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET charge Capacitances of the diode The space charge capacitance

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET electron injection injected electrons recombination access holes Capacitances of the diode The diffusion capacitance Where are the opposite charges?

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Capacitances of the diode The diffusion capacitance Harmful! Slows down the operation. Reduction: decrease , narrow base diode

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Capacitances of the diode Problem Let us calculate the space charge capacitance of a Si diode if the width of the depletion layer is 0.33  m and the cross-sectional area is 0.02 mm 2 ! Let us calculate the diffusion capacitance in the operating point of I=1 mA if  =100 ns!

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Capacitances of the diode Orders of magnitude: C Sp 1-10 pF C D nF-s (for a small power diode) Utilization C Sp tuning oscillators, microwave amplification C D -- CDCD C Sp

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Operating point ► Finding the DC operating point ► Linearization in the operating point, small signal operation ► Differential resistance, capacitance ► Models

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET ► Characteristics: defines the current-voltage pairs that may occur during the operation. ► During the real operation the diode or any nonlinear element works in one point of the characteristics, that is the operating point, or quiescent point. ► This is determined also by the surrounding elements. The operating point

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Finding the operating point The problem: a linear element and a non-linear element connected in series: Graphical solution U+U+ I U U + /R UDUD URUR I=U R /R U+U+ R UDUD

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Finding the operating point How does the operating point change if the U + supply voltage is increased? The operating line is shifted in parallel U+U+ I U U + /R UDUD URUR U+U+ R UDUD I=U R /R

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Finding the operating point How does the operating line change if we change R? It turns around point U + - its slope will change U+U+ I U U+/RU+/R U+/R2U+/R2 U+/R1U+/R1 U+U+ R UDUD I=U R /R

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Large signal model of the diode The computer simulation model also looks like this. Also needed: model equations (e.g. I=I 0 (exp(U/U T )-1) model parameters (e.g.. I 0, r s, etc.) C Sp (U)+C D (U)

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET linearize For small changes we can linearize the characteristics Linearization in the operating point U+U+ I U U + /R UDUD URUR I=U R /R U+U+ R UDUD

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET O Small signal operation of diodes The operating point IOIO IOIO I U u(t) u1u1 t i1i1 i1i1 i(t) t Small signal: linearized analysis, for the alternating current component What does small signal mean? r diff operating point dependent

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Differential resistance of the diode Forward region, I >> I 0 : If we consider the series resistance as well:

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Differential resistance of the diode Problem The series resistance of a diode is 2 . Let us calculate its differential resistance in the I=1 mA, 10 mA, 100 mA operating points!

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Small signal model of the diode Element values are operating point dependent! C Sp +C D Recall:

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Temperature dependence

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Temperature dependence ► The characteristics shows strong temperature dependence ► Reason: temperature dependence of the minority carriers  Forward voltages: V F at I F decreases with about 2mV for 1  C increase linear temperature dependence in a large range  appropriate for temperature measurements  Reverse voltages: I R at U R decreases with  7-10% for 1  C (that means doubling at each 10  C)

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Temperature dependence Forward region: Reverese region: For a Si diode: I R ~ n i   1,15  1,075  7,5 %/ o C

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Temperature dependence Forward region: Problem If U=700 mV, what is dU/dT for a Si diode? Compare with the characteristics!

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Temperature dependence IOIO UDUD I U IMIM UDUD UDUD cca. -2mV/ o C In case of a forced current the forward voltage of a pn junction is an excellent temperature sensor... The sensitivity slightly depeneds on the I O current

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET The diode in switching mode

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Diodes as rectifiers The diode was considered to be ideal! What if this is not the case? u in u out U out (t)= 0, if U in (t)  0 U in (t), if U in (t) < 0

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Diodes as rectifiers Relation of U in and U out u in u out U in U out ideal actual Transfer characteristic

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Diodes as rectifiers diode characteristic u in u out piecewise approximation U in U out without U d compensation witht U d compensation

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Transient phenomena Abrupt switching from forward to reverse voltage: due to its capacitances, the diode is open for some time. This is called reverse recovery.

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Reverse recovery Reverse recovery time t rr (2-3 ns for a quick diode)

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Transient behavior of the diode The diffusion equation: We calculate n(x,t) from this Simplification: instead of n(x,t) we calculate with Q(t) net charge

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Transient behavior of the diode The charge equation The current is spent on maintaining recombination depleting/supplying diffusion charge

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Break-down phenomena ► Avalanche ► Zener

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Break-down phenomena Reason: either of Avalanche mechanism Zener punch- throug Voltage [V]

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Avalanche break-down M – multiplication factor U L depends on the less doped side: Electrons Holes Ionization coefficient

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET The Zener effect Physical reason: tunnelling tunnelling current of electrons

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Break-down phenomena In case of Si: below 6V – Zener, above this – avalanche. U L ~N -1 U L ~N -0.7 Comparison of the two phenomena Zener avalanche

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Break-down phenomena Application: a Zener-diode Voltage reference Voltage stabilizer (at low power consumption) U in U out  U in  U out

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Break-down phenomena Temeperature dependence of Zener-diodes The best: diodes around 5V (Si diode)

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Practical issues ► Packaging ► Data sheets

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Samall current Large current (IC realizations will be discussed later) Actual realization of diodes diode chip glass Si chip copper lead

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Data sheets

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Data sheets

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Data sheets

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET  100 o C 6.5  1200 nA (1200/6.5)^0.01= %/ o C Data sheets

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET Data sheets