Mumbai University > Electronics ana telecommunication engineering > Sem 3 > Analog electronics 1

Marks: 5M

Years: Dec 2015

Effect of Temperature on JFET Parameters:

JFET offer better thermal stability as compared to BJTs. Increase in JFET temperature results in decrease in the depletion region width and decrease in the carrier mobility. Decrease in the width of depletion region results in increase in channel width, which in turn increases in $I_D$. This results in positive temperature coefficient for $I_D$.

Increase in $I_D$ with temperature results in increase in $V_GS$ (off) with temperature. $V_GS$ (off) also has a positive temperature coefficient of the order of 2.2 mV/°C.

Decrease in carrier mobility gives $I_D$ a negative temperature coefficient. Since both the mechanisms occur simultaneously, the effect of one mechanism compensates for the other.

Therefore, JFETs offer better temperature stability. A factor of proportionality between changes in $I_D$ and corresponding change in $V_GS$ isg_m, the Transconductance or Mutual conductance of JFET.

Hence, combining two effects that cause variation in $I_D$ in opposite directions, the condition for zero drain current drift is

$0.007|I_D|=0.0022g_m$

$\frac{|I_D|}{g_m}=0.314V$

Modulus sign in relation is used to cover both types of JFETs, i.e. n- and p-channel. It can be proved that condition for zero current drift in terms of $V_P$ is |$V_P$| - |$V_GS$|= 0.63 V. The proof involves making use of relations

$I_D=I_DSS(1-\frac{V_GS}{V_P})^2$

$g_m=g_mo(1-\frac{V_GS}{V_P})$

$g_mo=\frac{-2I_DSS}{V_P}$