Basic Device Questions

(a) BJT Question: Identify the direction of carriers (electrons and holes) and the resulting depletion regions for the PNP transistor drawn below.  Indicate by arrows the direction of the current flow into or out of each terminal for forward active operation.   Also, indicate the polarities of the applied voltages (i.e. either positive or negative). 

What is the typical relationship between the emitter doping (N_E), the base doping (N_B), and the collector doping (N_C), in a BJT transistor?  Why are these choices made for a good analog BJT device operating in the forward active regime?

N_E >> N_B >> N_C.
N_E >> N_B for high Beta (most carriers injected from emitter to base),
N_B >> N_C for high Early voltage.

 

(b) MOSFET Cross-section: Sketch the cross-section from a typical CMOS process.  Assume an n-well process, and include both an nFET and pFET device.

 

(c) Basic Equations: Identify equations with model descriptions.  Choose the most correct answer in each case.

____ Diode Model Equation ____ MOSFET Model: Subthreshold Operation,                       Saturation ____ MOSFET Model: Above Threshold Operation,                      Ohmic, k = 1. ____ BJT Model: Forward Active ____ MOSFET Transconductance: Above Threshold

I = Ith exp( Vgs / UT) ( 1 +  ( Vds / VA) ) I = Is exp( VBE / UT) -  (Is / aR) exp( VBC / UT) I = (K/2)  (Vgs - VT ) 2 I = Is exp( V / UT) I / UT I = Ith exp( (kVg - Vs) / UT) exp( Vds / VA) 2 I / (Vg - VT ) I = K ( (Vgs - VT )Vds – Vds2/2) I = Is exp( VBE / UT) exp( VCE / VA)

Answers: 4, 6, 8, 2, 7