Question 2.2: Find the value of VT in (2.1) at 20°C....

Recalling that absolute zero is -273°C, we write
V_T = \frac{kT}{q} = \frac{(1.38  ×  10^{-23})(273  +  20)}{1.6  ×  10^{-19}} = 25.27  \text{m V}

While (2.1) serves as a useful model of the junction diode insofar as dynamic resistance is concerned, Fig. 2-4 shows it to have regions of inaccuracy:

1. The actual (measured) forward voltage drop is greater than that predicted by (2.1) (due to ohmic resistance of metal contacts and semiconductor material).
2. The actual reverse current for -V_R ≤ v_D < 0 is greater than predicted (due to leakage current I_S along the surface of the semiconductor material).
3. The actual reverse current increases to significantly larger values than predicted for v_D < -V_R (due to a complex phenomenon called avalanche breakdown).

In commercially available diodes, proper doping (impurity addition) of the base material results in distinct static terminal characteristics.    A comparison of Ge- and Si-base diode characteristics is shown in Fig. 2-5.    If -V_R < v_D < -0.1  \text{V}, both diode types exhibit a near-constant reverse current I_R.    Typically, 1  μ\text{A} < I_R < 500   μ\text{A}

for Ge, while 10^{-3}  μ\text{A} < IR < 1  μ\text{A} for Si, for signal-level diodes (forward current ratings of less than 1  \text{A}). For a forward bias, the onset of low-resistance conduction is between 0.2  \text{and}  0.3  \text{V} for Ge, and between 0.6  \text{and}  0.7  \text{V} for Si.
For both Si and Ge diodes, the saturation current I_o doubles for an increase in temperature of 10°C; in other words, the ratio of saturation current at temperature T_2 to that at temperature T_1 is
\frac{(I_o)_2}{(I_o)_1} = 2^{(T_2 – T_1)/10}         (2.2)