SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

248 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

Fluid flow Fluid flow in a confined region

A potential energy profile created

to stop flow

OFF state

Constriction is controlled by a “gate”

OFF state

A reduced profile

allows flow

Opening in constriction

allows flow

⇒ Field effect transistors

channel constriction is

controlled by gate bias.

BJT, HBTs potential ⇒

energy profile is controlled

by base-emitter voltage

Controlling flow by potential

energy change

Controlling flow by altering

a constriction

Figure 6.2: Two different ways to control flow of a fluid. The bipolar and field effect transistors
use these two approaches to control current flow.

6.2 BIPOLARTRANSISTOR:ACONCEPTUALPICTURE.............

The bipolar junction transistor employs two back to backp−ndiodes which with clever
design rules can have a high amplification and can operate at high frequency. It can also act as a
digital device and as a microwave device.