Applet Worksheet

Applet Worksheet
(Current Components in BJT Virtual Lab)

Objectives: Upon completion, students will be able to visualize the different physical current component in the transistor, be able to associate these current components (I_cn, I_b, I_e, ..) with the current Gain parameters (a &b), etc. That is, the student should become familiar with the different current components that flows into or out of the Base region of a BJT.
[To Instructors: This worksheet was used in a Computer Lab recitation session with EE Undergraduate juniors. This applet worksheet followed the Wide Base Narrow Base Worksheet and they togethor made slightly over 1 hour Recitation Session. Following this step by step instruction given below, students fill out this spreadsheet which is checked by TA's near the end of the Recitation session. The same instruction is available as Word document here.]

Procedure: Current Components in BJT Virtual Lab

For this Virtual Lab, visit the applet at http://jas.eng.buffalo.edu/education/bjt/bsim/index.html

Click the “Leakage” button in the applet and hide the leakage current components (I_ep0, I_cp0).
Click the “Recombination” and “I_ep” buttons to hide the corresponding current carriers in animation.

1) Find the current Gain parameters (a & b) using the current values in the applet.

2) Verify the relationship, b = a / (1-a), using a & b that you just found..
[Hint: I_c = I_cn (approximately); I_e = I_en+I_ep; I_b is given in the applet. b = I_c/I_b; anda =I_c/I_e]

Do either 2) or 3). You may do both (optional).

3) Collector Current

(i) Set the Voltage in the applet so that V_be = 0.74 Volts. Read I_c from the applet and calculate the I_s from I_c = I_s exp(V_be/V_T). I_s = ( ) Amp
(ii) Now, change the voltage so that V_be = 0.75 Volts. Use the I_s found above, and calculate I_c. Does this I_c agree with I_c that you read from the applet ?
I_c = ( ) by hand calc; I_c = ( ) from the Applet.

4) Base Current – The Recombination current portion

Now, turn on the “Recombination” current (the viertically dropping carriers in the applet) and the “I_ep” (the hole injection from Collector to Base).
(i) Set the voltage so that V_be = 0.66. Read from the applet and record I_b = (           ); I_ep = (        ) and the Minority electron concentration at the EBJ, n_p = (       ) cm^-3. Calculate the Recombination contribution to the Base current from I_rec = I_b – I_ep:    I_rec = (          ).
(ii) Repeat the above question for V_be = 0.75 Volts.   Read from applet: I_b = (           ); I_ep= (        ), and n_p = (       ) cm^-3. Calculate I_rec = (       )
(iii) Take the ratio of values between (i) and (ii) above for I_rec2/I_rec1 = (    )   and n_p2/n_p1 = (    ).   Do they agree with each other ?? Why should they be the same ?
Hint: Consider I_rec = Q_n / t_b where Q_n = 1/2 * A_e * W_b * n_p. A_e= Emitter junction area, W_b = Base width, and t_b = Minority carrier lifetime in the Base.