1. Derivation of fT And fMAX In Bipolar And MOSFETs
C.-F. Huang
2004/04/10
National Taiwan University

2. Derivation of fT (MOSFETs)
S → CE short

3. Derivation of fT (MOSFETs) (Continued)
Assume the zero can be neglected.

4. Derivation of fT (Bipolar)
For Bipolar Transistors,
CDE is due to minority carriers caused by FB

5. Derivation of fT (Bipolar) (Continued)
QE = minority holes stored in emitter
QB = minority electrons stored in base
QBE = electrons induced by the current
through the depletion region of BE-junction
QBC = electrons induced by the current
through the depletion region of BC-junction

6. Derivation of fT (Bipolar) (Continued)
Width of Neutral Region
Width of Depletion Region
if drift current is considered.
is greater than because of reverse-biasing.

7. Derivation of fT (MOSFETs) (RS and RD are included)

8. Derivation of fT (MOSFETs) (Continued) (RS and RD are included)
Miller’s Theorem

9. Derivation of fT (Bipolar) (RS and RD are included)
For bipolar, the result is similar.
The only difference is that the term must be included.

10. Derivation of fMAX (MOSFETs)

11. Output and Input Impedance
For high frequency,
Rg is independent of RL

12. Derivation of fMAX (MOSFETs) (Continued)
For the matching conditions,

13. Derivation of fMAX (MOSFETs) (Continued)
See p.3

14. Derivation of fMAX (Bipolar)
For bipolar transistors, there is no term.

15. Derivation of fMAX (MOSFETs)(Continued) (RS and RD are included)
For high frequency condition, Cgs → short
Replace Rg by Rg+Rs
w/ (RS+RD) term
w/o (RS+RD) term