BJT

Question 1

Which side of a BJT is the arrow on? What does the arrow point to? How does it relate to current?

Answer 1

The arrow is on the emitter side. The arrow always points to the N type material. Shows direction of output current.

Question 2

When is an NPN on?

Answer 2

When the voltage difference between the base and emitter is larger than 0.7V.

Question 3

When is a PNP on?

Answer 3

When the voltage difference between the base and emitter is less than -0.7V.

Question 4

What is gain? What are typical gains for NPN and PNP?

Answer 4

The output current will be some multiple of the input current. This number is the gain. Typical gain for NPN is 300 or more. Typical gain for PNP is 50 or less.

Question 5

What are some main differences between MOSFETs and BJTs?

Answer 5

1. MOSFET: simplicity in building logic gates. BJT very complex
2. MOSFET: very low power consumption BJT high power consumption.
3. MOSFET: faster and smaller BJT not much improvement in speed or geometries
4. MOSFET: relatively low current capability BJT very high current capability

Question 6

What is a buried layer? What is its purpose?

Answer 6

A layer of N+ sandwiched between N- and psub layers. Its purpose is to reduce the Collector resistance.

Question 7

What is a sinker layer? What is its purpose?

Answer 7

A deep N+ used to bridge the Collector and Burried layer together. Like the buried layer, provides a low resistance path for collector current to flow, improving the gain.

Question 8

What is a Schottky NPN?

Answer 8

An NPN with improved speed and power consumption.

Question 9

What is a vertical PNP?

Answer 9

A PNP which uses the psub as its collector.

Question 10

How do you build a regular diode out of a NPN?

Answer 10

Connect the the collector to the base. The diode borrows the Base-Emitter PN junction. The cathode is the emitter side.

Question 11

How do you build a Zener diode out of an NPN?

Answer 11

Connect the collector to the emitter. The diode borrows the Base-Collector PN junction. The cathode is the emitter / collector side.

Question 12

What are the breakdown voltages, speeds, operating modes, and materials of a regular diode, zener diode, and schottky diode?

Answer 12

Regular: 5-8V, regular speed, forward-biased, P+ (P-) to N+
Zener: 5V or >, regular speed, reverse-biased, P- to N-
Schottky: 5-8V, faster speed, forward-biased, M1 to N- or M1 to P+.

Question 13

What does LDD stand for?

Answer 13

Lightly Doped Drain (or Diffusion)

Question 14

What kind of PN junction is the Zener diode made of?

Answer 14

A graded PN junction, with LDD around the regular P+ or N+.

Question 15

What’s another name for the Schottky diode?

Answer 15

hot-carrier diode

Question 16

What three things characterize a Schottky diode, and what is it well suited for?

Answer 16

1. Fast switching times (low reverse recovery time)
2. low forward voltage drop (.25 to .4V)
3. Low junction capacitance.
   Well suited for high-frequency applications.

Question 17

What kind of junction are Schottky diodes made of?

Answer 17

metal-to-N rather than regular PN.