Bipolar Transistors - NOVA School of Science & Technology PDF

MICROELETRÓNICA: SISTEMAS E DISPOSITIVOS Transístores bipolares (BJTs)  Outline Transistors – The beginning – First bipolar transistor Bipolar transistors – Introduction – Fabrication – Operation principle – C...

MICROELETRÓNICA: SISTEMAS E DISPOSITIVOS Transístores bipolares (BJTs)  Outline Transistors – The beginning – First bipolar transistor Bipolar transistors – Introduction – Fabrication – Operation principle – Configurations – Common emitter – Operation regimes and current characteristics – Bipolar Transistor Amplifiers Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Transistors A 1955 AT&T publicity photo shows [in palm, from left] a phototransistor, a junction transistor, and a point-contact transistor. AT&T ARCHIVES AND HISTORY CENTER Check this article @ IEEE Spectrum: https://spectrum. ieee.org/transist or-history And this evolution video from Intel: https://www.youtube. com/watch?v=Z7M8et XUEUU Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Transistors The beginning – The transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research. – Picture shows the workbench of John Bardeen and Walter Brattain at Bell Laboratories. They were supposed to be doing fundamental research about crystal surfaces. – The experimental results hadn't been very good, though, and there's a rumor that their boss, William Shockley, came near to canceling the project. But in 1947, working alone, they switched to using tremendously pure materials. – It dawned on them that they could build the circuit in the picture. It was a working amplifier! John and Walter submitted a patent for the first working point contact transistor. – Originally made of germanium Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Transistors The beginning – Shockley was furious and took their work and invented the junction transistor and submitted a patent for it 9 days later. – The three shared a Nobel Prize in 1955. Bardeen and Brattain continued in research (and Bardeen later won another Nobel). – Shockley quit to start a semiconductor company in Palo Alto. It folded, but its staff went on to invent the integrated circuit (the "chip") and to found Intel Corporation. – By 1960, all important computers used transistors for logic, and ferrite cores for memory. Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Transistors The beginning – A gold foil was glued to a triangular insulating wedge – A narrow gap was cut with a razor blade to create emitter and collector. The gap was around 50 microns – Under forward bias the emitter-base junction minority carriers are injected into the base (strong bias required in contact transistor) – Most minority carriers are collected at reverse biased base-collector junction (base controls current between E and C) – Point-contact transistor is a surface-effect device (surfaces are easily contaminated, have defects, and suffer with mechanical stability) Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Transistors First bipolar transistor – W. Shockley invented the p-n junction transistor – The physically relevant region is moved to the bulk of the material Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Introduction – Bipolar transistors are one of the main ‘building-blocks’ in electronic systems – They are used in both analogue and digital circuits – They incorporate two p-n junctions and are sometimes known as bipolar junction transistors or BJTs – Often refer to them simply as bipolar transistors – While control in a FET is due to an electric field, control in a bipolar transistor is generally considered to be due to an electric current current into one terminal determines the current between two others – bipolar transistors are 3 terminal devices collector (c) base (b) emitter (e) – the base is the control input Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Introduction – two polarities: npn and pnp Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  For high-frequency applications, device Bipolar transistors dimensions are scaled both vertically and horizontally. Vertical scaling is mainly on the base width. Currently the base width can be smaller BJTs fabrication than 30 nm, and frequency >100 GHz has been obtained (THz with higher µ semicond, InP) Basic low performance structure. Lateral carrier flow, not ideal for BJTs Conventional structure. Vertical carrier flow, ideal for BJTs where current flows in the bulk emitter resistance is more important than the collector resistance, so contact is made directly over the junction, and the collector contact is via a buried n+-layer Modern BJTs configuration incorporation of poly-Si over the emitter junction. Allowing for fast and more precise control of dopant diffusion (can be controlled to less than 30 nm) In terms of performance, the poly-emitter had been found to yield higher current gain Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Encapsulation Heat sink Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors still under research THz operation with InP BJTs (Bozanic et al., Sensors 2019, https://doi.org/10.3390/s19112454 ) Miniaturized, high- mobility semiconductors Organic BJTs (Wang et al., Nature 2022, https://doi.org/10.1038/s41586-022-04837-4 ) fT=1.6 GHz, significantly larger than organic FETs (highly crystalline rubrene films; ultralow overlap capacitance due to vertical design) Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Operation principle – We will consider npn transistors – pnp devices are similar but with different polarities of voltage and currents (not used very often, µh< µe) – when using npn transistors: collector is normally more positive than the emitter VCE might be a few volts device resembles two back-to-back diodes – but has very different characteristics with the base open-circuit negligible current flows from the collector to the emitter Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Operation principle – When a positive voltage is applied to the base (with respect to the emitter) this forward biases the base-emitter junction – the base region is light doped and very thin because it is lightly doped, the current produced is mainly due to electrons flowing from the emitter to the base because the base region is thin, most of the electrons entering the base get swept across the base-collector junction into the collector this produces a collector current that is much larger than the base current – this gives current amplification Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors EB forward-bias lower EB potential barrier – electron injection in B and holes in E Electrons in B (minority carriers, InE): some recombine with holes, others diffuse through B and reach C. Electrons in C find the reverse-biased CB and fall down potential barrier (InC) In practice, IG, InCO and IpCO are small and combined as ICO (collector reverse-saturation current) This junction polarization scheme originates “active region of operation”. More on that later… Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Introduction – npn example Typical doping profile Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Configurations – transistors can be used in different configurations – most common is as shown: emitter terminal is common to input and output circuits this is a common-emitter configuration GAIN CONFIG Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Common base Common-base current gain (α) defined by current components crossing E and C junctions: 𝐼𝐶 − 𝐼𝐶𝑂 𝐼𝑛𝐶 𝛼≡ = = 𝛾𝛽𝑇 𝐼𝐸 𝐼𝑛𝐸 + 𝐼𝑝𝐸 + 𝐼𝑟𝑔 𝐼𝑛𝐸 𝐼𝑛𝐶 Where 𝛾 = and 𝛽𝑇 = 𝐼𝑛𝐸 +𝐼𝑝𝐸 +𝐼𝑟𝑔 𝐼𝑛𝐸 Emitter injection efficiency (portion of Base transport factor (portion of minority carriers injected into the base) surviving electrons) – diffusion length of – control of doping concentration minority carriers larger than B width −1 𝐷𝑝 𝑁𝑎 𝑥𝐵 𝐿𝑛 = 𝐷𝑛 𝜏𝑛 𝛾 = 1+ 𝑥𝐵2 𝐷𝑛 𝑁𝑑𝐸 𝑥𝐸 𝛽𝑇 = 1 − 2 2𝐿𝑛 Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors Common base 𝐼𝐶 = 𝛼𝐼𝐸 + 𝐼𝐶𝑂 1 − 𝑒 𝑉𝐵𝐶 /𝜙𝑇 If collector junction is reverse-biased (active region), VBC 5-10 times IBmin because  varies among components, with temperature and voltage and RB may change when current flows. Calculate the max IC and IB not to overcome device specifications. Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) BJTs as switch ton – rise time td – decay time (fall time) ts – storage time (time required to extract the excess carrier above required to maintain active mode). Is the limiting parameter in determining the switching speed Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) BJTs as amplifier – Common emitter mode – Linear Active Region – Significant current Gain – Example: Gain,  = 100 Assume to be in active region -> VBE=0.7V VBE = 0.7V Find if it’s in active region I E = I B + I C = (  + 1) I B VBB − VBE 5 − 0.7 IB = = = 0.0107 mA RB + RE *101 402 VCB>0 so the BJT is in active I C =  * I B = 100 * 0.0107 = 1.07 mA region VCB = VCC − I C * RC − I E * RE − VBE = = 10 − (3)(1.07) − (2)(101* 0.0107 ) − 0.7 = = 3.93V Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) BJTs “Q point” – Q point means Quiescent or Operating point – Very important for amplifiers because wrong ‘Q’ point selection increases amplifier distortion – Need to have a stable ‘Q’ point, meaning the operating point should not be sensitive to variation to temperature or BJT , which can vary widely By far best circuit for providing stable bias point Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) Common emitter BJTs “Q point” Applying KVL to the base-emitter circuit of the Thevenized Equivalent form: VB - IB RB -VBE - IE RE = 0 (1) Since IE = IB + IC = IB + IB= (1+ )IB (2) Replacing IE by (1+ )IB in (1), we get VB = VTH = Vcc R 2 VB − VBE R1 + R 2 IB = (3) R B + (1 + )R E R B = R TH = R1 R 2 R1 + R 2 If we design (1+ )RE  RB (say (1+ )RE  100RB) VB − VBE Thus we can setup a Q-point Then IB  (4) (1 + )R E independent of  (For Hence IC and IE example,  of 2N2222A, a VB − VBE NPN BJT can vary between becomeAnd IC = IE  (for large ) (5) 75 and 325 for IC = 1 mA and independent of ! RE VCE = 10V) Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) BJTs amplifiers – A simple transistor amplifier: RB is used to ‘bias’ the transistor by injecting an appropriate base current – C is a coupling capacitor and is used to couple the AC signal while preventing external circuits from affecting the bias – this is an AC-coupled amplifier – VB is set by the conduction voltage of the base- emitter junction and so is about 0.7 V – voltage across RB is thus VCC – 0.7 V, this voltage divided by RB gives the base current IB – the collector current is then given by IC = hFEIB – the voltage drop across RC is given by IC ×RC – the quiescent output voltage is therefore Vo = VCC - IC ×RC – output is determined by hFE which is very variable Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) BJTs amplifiers Negative feedback amplifiers Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) BJTs amplifiers Common-collector amplifier – unity gain – high input resistance – low output resistance – a very good (current) buffer amplifier Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) BJTs amplifiers Phase splitter Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)  Bipolar transistors (additional info only) BJTs in voltage regulators Voltage regulator Microeletrónica: Sistemas e Dispositivos Transístores bipolares (BJTs)