ObjectiveGated Lateral BJT UsageLayout and Cross SectionEffect of the Gate: Surface vs. Subsurface CurrentEffect of the Substrate: Current SplittingEffect of Doping LevelsPrelabEbers-Moll Model Parameter ExtractionParameter Extraction Using the HP-4155Extraction of F, R, VAF, VARExtraction of ISParameter Extraction and Regions of Operation Using Circuit MeasurementsOptional ExperimentsCircuit Simulation (Basic)Experiment 6Gated Lateral BJTCharacteristicsW.T. Yeung, W.Y. Leung, S. Pimputkar,J.W.P. Chen, W. Belachew, and R.T. HoweUC Berkeley EE 105Fall 2003Contents1 Objective 12 Gated Lateral BJT Usage 22.1 Layout and Cross Section . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Effect of the Gate: Surface vs. Subsurface Current . . . . . . . . . . . . . . 42.3 Effect of the Substrate: Current Splitting . . . . . . . . . . . . . . . . . . . 52.4 Effect of Doping Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Prelab 54 Ebers-Moll Model Parameter Extraction 64.1 Parameter Extraction Using the HP-4155 . . . . . . . . . . . . . . . . . . . 74.1.1 Extraction of βF, βR, VAF, VAR. . . . . . . . . . . . . . . . . . . . 74.1.2 Extraction of IS. . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.2 Parameter Extraction and Regions of Operation Using Circuit Measurements 95 Optional Experiments 125.1 Circuit Simulation (Basic) . . . . . . . . . . . . . . . . . . . . . . . . . . 121 ObjectiveIn this lab, you will characterize a gated lateral BJT. After a brief introduction to gatedlateral BJT’s and how they differ from conventional pnp BJT’s, you will measure all theparameters needed to model the device using the full Ebers-Moll pnp BJT model, a large-signal model. This will be done both using the HP-4155 and more basic equipment. Next,1 of 12Gated Lateral BJT UsageFIGURE 1. Circuit Symbol for (a) the gated lateral BJT, and (b) the canonical pnp BJT.VSSECBG(a)EBC(b)for each region of operation, you will observe how the BJT operates and derive the corre-sponding simplified Ebers-Moll circuit model. Optionally, you will use the collected datato write a SPICE model for the BJT and use it run sample simulations. The key conceptsintroduced in this laboratory are:• Determination of the Ebers-Moll large signal parameters β, VA, and IS• The four regions of operation of the BJT• Determination of the region of operation from the voltages VEBand VCB2 Gated Lateral BJT UsageIn this lab, you will not be using a using a conventional BJT, but rather a gated lateral BJT;you will need to be aware of the differences between the two to do the lab. As you know,the vast majority of modern microchips (including the EE105 lab chip) are manufacturedin CMOS processes, which do not accomodate for bipolar transistors. Nevertheless, undercertain bias conditions, several second-order effects in the p-MOSFET become dominantand the pnp sandwich formed by source, n-well, and drain acts like a pnp BJT perturbedby nonidealities introduced by the gate and substrate which contact the pnp sandwich. Thisdevice is called a gated lateral BJT (or more commonly lateral BJT, if it is clear oneis refering to a CMOS process), and is given the circuit symbol shown in Fig. 1(a); thesymbol for a canonical BJT as covered in lecture is shown in Fig. 1(b) for comparison.2.1 Layout and Cross SectionTo better understand the structure, consider the gated lateral BJT layout and cross section,shown in Fig. 2. Most of the structure is needed simply to contact the various regions.At the center of the layout is a p-MOSFET whose gate has been fingered for compactnessfollowing common MOSFET layout practice. The four diffusion regions are alternatelypart of the source and drain (which will now serve as the emitter and collector respectively).Two small strips of layer 1 metal connect to the two source diffusion regions via threecontacts apiece; a fork-shaped layer 2 metal wire labeled E then connects to both metal 1strips through a via each. A similar structure is used for the collector, labeled C.As indicated by the circuit symbols in the cross section, the heart of the BJT is the pnpsandwich of MOSFET source diffusion, n-well, and drain diffusion. The n-well serves asthe base and is (Ohmically) contacted by the base wire through a ring-like n+diffusionExperiment 6 Gated Lateral BJT Characteristics 2 of 12Gated Lateral BJT UsageFIGURE 2. Gated Lateral BJT Layout and Cross SectionVSSp+,Cp+,Ep+,Cn, Bp−, VSSp+,EN−WellMetal 2Thin OxideMetal 1Contact ViaPolyVSSGE Bppparasitic vertical BJTn+ n+Cgated lateral BJTGECBp+ p+Experiment 6 Gated Lateral BJT Characteristics 3 of 12Gated Lateral BJT UsageFIGURE 3. MOSFET Cross Section with Indicated Paths for Potential PlotsS DYX’Y’BGXFIGURE 4. Potential Plots along Path XX’ in Fig. 3 for (a) zero emitter junction bias, VEB= 0, and (b)forward emitter junction bias, VEB> 0.φ(x) (n)basegate (p+)flatbandx0accumulationoxidedepletion(a)φ(x) (n)basegate (p+)VEBx0oxide(b)surrounding the MOSFET in the layout. As seen in the cross section, the substrate contactsthe n-well as well and represents a second parasitic collector, labelled VSS. It is necessaryto draw off the emitter current extracted by the substrate lest it disturb the surroundingcircuitry or even trigger latchup (a destructive situtation in which the junctions of parasiticpnpn sandwichs are locked into forward bias). Similar to the n-well/base, the substrate iscontacted by a ring-shaped diffusion surrounding the n-well, called a guard ring since itserves to guard against latchup.2.2 Effect of the Gate: Surface vs. Subsurface CurrentThe effect of the gate can be completely eliminated by biasing the MOS structue at flatbandor in accumulation, VGB≥ VFB. This is best understood by considering the potential curvesfor the MOS structure, shown in Fig. 4(a) for the caseVEB≡ VSB= 0 and in Fig. 4(b) for thecase VEB≡ VSB≥ 0 At flatband and in accumulation, the potential level is flat throughoutthe base. If there were no gate at all, this uniform potential would be interpreted as stemingfrom a uniform base doping, which is assumed in the 1-dimensional BJT model covered inlecture. In depletion, the gate bends down the potential curves in the base near the siliconsurface, corresponding to reduced base doping at the surface. While this low effectivebase doping can be used to advantage to produce BJT’s with extremely high values ofcommon-emitter current gain β (beyond 106), nonuniform base doping is not
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