fix symbol and text

src/14_doping_and_devices.md

@@ -430,7 +430,7 @@ annot = [
     y = 0.9,
     xref = "x", yref = "y",
     axref = "x", ayref = "y",
-    text = r'$\varphi$',
+    text = r'$\delta \varphi$',
     font = dict(size=40),
     showarrow=False),
     
@@ -556,10 +556,10 @@ As a result, a positive ionized donor dopant is not screened anymore and the $n$
 Similarly, $p$-type region obtains a negative charge.
 Therefore, an electric field develops across the junction.
 As the recombination process continues, a larger charge density $\rho$ develops and thus the electric field grows until it is large enough to prevent the electrons/holes from crossing the junction.
-If the resulting potential difference becomes $\delta \varphi \gg kT$, the density of electrons and holes drops exponentially fast.
+Inside the region, energy deviates by $\delta \varphi \gg kT$ from the bulk value and thus the density of electrons/holes drops exponentially fast.
 Therefore, we refer to the region as the **depletion region**.
 
-A schematic of a depletion region is shown below:
+The charge density $\rho$ distribution inside of a depletion region is shown below:
 
 ![](figures/pn_charge_density.svg)