p[η_]:=Coth[η]-(1/η)

(* ((Em pm)/((Subscript[k, B])T))*)

p[x]

Plot[p[η],{η,0,40},PlotRange->All]

Plot[p[(1/T)],{T,0,40},PlotRange->All]

Limit[p[x],x->0]

p1[x_]:=D[p[x],{x,1}]

Limit[p1[x],x->0]

Series[p[x],{x,0,5}]

<<PhysicalConstants`
<<Units`

pm=Convert[ElectronCharge 0.2 Nano Meter,Coulomb Meter]

Em= Convert[((10 Volt)/(1 Milli Meter)),(Newton/Coulomb)]

T=300 Kelvin

Convert[BoltzmannConstant T,ElectronVolt]

Em= Convert[((10 Volt)/(1 Milli Meter)),(Newton/Coulomb)]
T=1 Kelvin
Convert[((Em 10pm)/(BoltzmannConstant T)),1]