Bipolar Transistor Function & Construction A bipolar transistor is a semi-conductor device that acts as a variable resistor Applications Amplifiers Voltage and Current Regulators Electronic Switches
Made up of three alternating layers, which form two P-N junctions.
Emitter: heavily doped region that supplies majority current carriers.
Base: Thin and lightly doped region that controls current flow through the transistor.
Collector: Large, lightly doped region used to collect current carriers from emitter.
Simplified Drawing & Schematic Symbol Construction drawing can be simplified as a “sandwich” of semiconductor materials. Can be either NPN or PNP transistor Schematic Symbol: Arrow always points between base and emitter Direction designates type NPN = Not Pointing iN PNP = Pointing iNProperly Also points in direction of conventional current flow Collector Base Emitter Collector Base Emitter
Biasing Requirements Base to Emitter Junction (VBE) Forward biased for majority current carriers to flow from emitter to base, where they become minority carriers Base to Collector Junction (VBC) Reverse biased to allow minority current carriers from the emitter flow from base to collector Collector VBC Q1 Base VBE Emitter
Steady-State Operation P N N With the Base to Emitter Junction forward biased, majority current carriers flow from emitter to base, where they become minority carriers This is emitter current (IE) Since the base is thin and lightly doped, very few current carriers recombine in the base This is base current (IB) The rest of the current carriers are swept across the base to collector junction This is collector current (IC) E C B IE IC IB Electron Current Flow Conventional Current Flow
Transient Operation (VBE) P N N Base to emitter junction is forward biased Low resistance, so small in VBE causes a large in IE The base is thin and lightly doped Few current carriers recombine to form more base current IB only slightly The rest of the current carriers are swept across the base to collector junction IC greatly E C B
Bipolar Transistor Characteristics Current Relationships Emitter current equals base current plus collector current Alpha (α): Ratio of Collector Current to Emitter Current Typically 0.95 – 0.98 Beta (β): Ratio of Collector Current to Base Current Ranges from 40 to >100 depending on application Gamma (γ): Ratio of Emitter Current to Base Current Slightly greater than β
Bipolar Transistor Characteristics Typical Characteristic Curve Shows relationship between IB, IC and VCE IC relatively constant for a given VCE IC dependant on IB Saturation IC no longer increases for an increase in IB IC is maximum VCE is minimum (VCE ≈ 0.2VDC) “ON” state when used as a switch Cutoff Transistor not biased to conduct IB, IC, IE = 0A VCE is maximum for circuit “OFF” state when used as a switch Saturation Cutoff
Bipolar Transistor Characteristics Load Line Straight line drawn between saturation and cutoff, representing range of operating points Quiescent Point (Q-Point) The steady-state values of IB, IC, and VCE with no AC input applied Determined by circuit design Saturation Q-Point Cutoff Load Line
Common Emitter Configuration +VCC Emitter is “common” to both input and output Input applied to base to change VBE Output taken from collector Gain Medium voltage gain and current gain High power gain Impedance Medium input and output impedance RC IB IC RB Vout C1 Vin Q1 IE