Describe how a semiconductor is used as a halfwave rectifier.

Rectifier : The device that is used to convert alternating voltage or current into direct voltage or current is called a rectifier.
Half wave Rectifier :- i) A half wave rectifier can be constructed with a single p-n junction diode. The input A.C. signal to be rectified is applied across the primary of the transformer and out put D.C signal is taken across the load resistance (R_L) connected across secondary.
Circuit

ii) For all the positive half cycles of input A.C. signal, the diode D is in forward bias and hence it conducts current. So current passes through Load resistance (R_L).
iii) For all the negative half cycles of input A.C. signal, the diode D is in reverse bias and it does not conducts current. So no current flows through Load resistance (R_L).
iv) Only positive half cycle of input A.C. signal is converted into rectified D.C output.
v) The ratio of output D.C power to input A.C power is called efficiency of half wave rectifier.
$\mathrm{\eta }=\frac{\text{ output D.C power }}{\text{ input A.C power }}=\frac{0.406{\mathrm{R}}_{\mathrm{L}}}{{\mathrm{r}}_{\mathrm{f}}+{\mathrm{R}}_{\mathrm{L}}}$
Where R_L = Load Resistance, r_f = Forward resistance of diode
Maximum efficiency of half wave rectifier is 40.6%