diff --git a/electronics/src/builtin_voltage.rs b/electronics/src/builtin_voltage.rs
new file mode 100644
index 00000000..d8299197
--- /dev/null
+++ b/electronics/src/builtin_voltage.rs
@@ -0,0 +1,36 @@
+// Copyright (c) 2023 Xu Shaohua <shaohua@biofan.org>. All rights reserved.
+// Use of this source is governed by General Public License that can be found
+// in the LICENSE file.
+
+// TEMPERATURE (unit = K)
+pub const T: f64 = 300.0;
+pub const BOLTZMANN: f64 = 1.380_649e-23;
+pub const ELECTRON_VOLT: f64 = 1.602_176_634e-19;
+
+/// This function can calculate the Builtin Voltage of a pn junction diode.
+///
+/// This is calculated from the given three values.
+///
+/// # Parameters
+/// - `donor_conc` - donor concentration
+/// - `acceptor_conc` - acceptor concentration
+/// - `intrinsic_conc` - intrinsic concentration
+#[must_use]
+pub fn builtin_voltage(donor_conc: f64, acceptor_conc: f64, intrinsic_conc: f64) -> f64 {
+    debug_assert!(donor_conc > 0.0);
+    debug_assert!(acceptor_conc > 0.0);
+    debug_assert!(intrinsic_conc > 0.0);
+    debug_assert!(donor_conc > intrinsic_conc);
+    debug_assert!(acceptor_conc > intrinsic_conc);
+    BOLTZMANN * T * ((donor_conc * acceptor_conc) / intrinsic_conc.powi(2)).ln() / ELECTRON_VOLT
+}
+
+#[cfg(test)]
+mod tests {
+    use super::builtin_voltage;
+
+    #[test]
+    fn test_builtin_voltage() {
+        assert_eq!(builtin_voltage(1e17, 1e17, 1e10), 0.833370010652644);
+    }
+}
diff --git a/electronics/src/lib.rs b/electronics/src/lib.rs
index 1569889f..b01a123a 100644
--- a/electronics/src/lib.rs
+++ b/electronics/src/lib.rs
@@ -11,5 +11,6 @@
 )]
 
 //pub mod apparent_power;
+pub mod builtin_voltage;
 pub mod electric_power;
 pub mod ohms_law;