Transistors: Bipolar and Field-Effect Devices
Transistor Fundamentals
BJT operation
A bipolar junction transistor uses two p-n junctions to control a large collector current with a small base current. In forward-active mode, a thin base region allows most injected carriers to reach the collector.
MOSFET operation
A metal-oxide-semiconductor field-effect transistor controls channel charge with an electric field from the gate. When the gate voltage exceeds threshold, an inversion layer forms and current can flow between source and drain.
Two control philosophies
BJTs are current-controlled devices using carrier injection, while MOSFETs are voltage-controlled devices using electrostatic channel formation.
BJT vs MOSFET
| Device | Control variable | Strengths |
|---|---|---|
| BJT | Base current | High transconductance and analog gain |
| MOSFET | Gate voltage | High input impedance and easy scaling |
| BJT | Carrier injection | Useful in certain high-speed analog roles |
| MOSFET | Channel modulation | Dominates digital integrated circuits |
What forms in a MOSFET channel when the gate voltage exceeds threshold?
The gate field attracts carriers to form a conductive inversion channel.
Correct answer: An inversion layer
What is the main control variable for a MOSFET?
The gate electric field modulates the channel charge and conductivity.
Correct answer: Gate voltage
Choosing between BJT and MOSFET reasoning
-
1
Decide whether current gain or voltage-controlled switching is needed.
-
2
Consider input impedance and power consumption.
-
3
Check speed, noise, and integration constraints.
-
4
Match the device family to the application.
Typical uses
BJTs
- Analog amplification
- Some high-current or specialized speed applications
MOSFETs
- Digital logic
- Power switching and CMOS circuits
Why do MOSFETs usually have very high input impedance?
The oxide insulates the gate, so almost no steady-state gate current flows.
Correct answer: The gate is insulated by an oxide layer
In a BJT, what region must be thin for efficient operation?
A thin base allows most carriers to traverse to the collector before recombining.
Correct answer: The base