Physics of Bipolar Junction Transistor Section

Schedule 92/11TuesdayPhysics of a BJT L2/11TuesdayMeasure Beta of a transistor 102/13Thursday Small signal model, PNP [homework: small eq. circuit, (PNP)] /18TuesdayBJT in saturation mode4.5 L2/18Tuesday BJT in saturation/BJT implementation of an NAND gate 122/20ThursdayTransistor bias, [homework assigned]

Overview

Outline Schematic Symbol Review – Forward Bias Diode – Reverse Bias Diode Energy Band Diagram – Intrinsic Semiconductor under an Applied Voltage – Forward Biased Diode – Reverse Biased Diode Apply KCL to a BJT

Voltage and Current Polarities of NPN and PNP transistors A “fat” voltage between collector and emitter voltage places a transistor in the active region! A “skinny” voltage between collector and emitter voltage places a transistor in the active region!

Structure and Circuit Symbol (emits charge carriers) (collects charge carriers) (controls the carriers that make the journey from E to C) Two PN junctions (BE and BC) C and E cannot be swapped.

A Bipolar Transistor in the Active Region VBE=0.8 V VBC=-0.2 V BE Junction: Forward Bias BC Junction: Reverse Bias Over Simplified View of BJT In the Active Region (not necessarily the right way to understand circuit)

Review: Forward Bias Diode Depletion region shrinks due to charges from the battery. The electric field is weaker. Majority carrier can cross via diffusion; Greater diffusion current. Current flows from P side to N side

EquilibriumForward Biased Diode Majority carriers cross the junction via diffusion. Minority carriers increased on both sides of the junction. Review: Forward Bias Diode

Energy Band Diagram of a Semiconductor Under an Applied Voltage Electrons roll downhill like stones. Holes float up like bubles!

Forward Biased Diode

Graphical Illustration

Review: PN Junction under Reverse Bias Reverse: Connect the + terminal to the n side. Depletion region widens. Therefore, stronger E. Minority carriers cross the PN junction easily through diffusion. Current is composed mostly of drift current contributed by minority carriers. n p to the left and p n to the right. Current from n side to p side, the current is negative. E

Energy Band Diagram of a Reverse Biased PN Junction Stronger E field in the depletion region npnp pnpn npnp pnpn

Injection of Electrons into Depletion Region Into depletion region on the p side. Outcome: The electron is swept to the n side by E.

An Overview Electrons are injected into the BC junction Electrons are injected into the B; holes to the E. Electrons are swept across the reversed biased BC

Thin Base Region The base region is made thin in order to reduce recombination as electrons travel from BE junction to BC junction.

Highly Doped Emitter In order to emphasize the current contribution due to the electrons (which can cross the BC junction), the emitter is heavily doped by N type materials.

Electrons in the Base Electrons injected into the base; high electron density at x1. Electrons are swept Into the collector; low electron density at x2 The electron gradient allows electrons to travel through diffusion.

Structures of BJT Transistors (NPN transistor)(PNP transistor)

BJT Current Assumption: BEJ: Forward Biased BCJ: Reverse Biased

Base Current The proportional of hole current and electron current is determined by dopants (N D and N A ). Even though the presence of holes are minimized, a small number holes still must enter through the base.

Recombination Base must supply holes that will enter the emitter and for recombination with the electrons.

KCL

Optional Slides

Emitter Area

Determine the Output Voltage