Sketch the energy band diagram for an n-p-n transistor in

Question

1. BJT FUNDAMENTALS

(A). Sketch the energy band diagram for an n-p-n transistor in equilibrium (all terminals grounded).

(B). Sketch the energy band diagram for n-p-n BJT in forward active mode.In forward active mode, state the bias condition (forward or reverse) of B-E junction and B-C junction. Also state the signs of V_BE and V_CE if the emitter is grounded (common-emitter setup).

2. SHORT AND LONG BASE TRANSISTOR

BJT is mostly used as an amplifying device. The current gain β characterizes the amplification ratio between output and input signals. Ideally β is a constant, i.e. the amplifier has a linear gain. In most cases we would like to maximize β.

(A). A symmetrical n-p-n BJT is biased in the common emitter configuration as shown in the circuit diagram in Figure. The B-E junction and B-C junction are forward biased at the same level (V_BE = V_BC , same doping in emitter and collector), and τ_n = τ_p in the base. Find the ratio of recombination currents between (i) base width W_B = 10L_n and (ii) W_B = 0.1L_n.

(B). Draw the minority carrier distribution in the base region for (i) and (ii) in (A).

(C). In order to maximize β, is it better to use short base or long base? Please explain in terms of the properties of the base.